Biology Notes
1
Contents
Chapter:
Page:
1)
Cells
3
2)
Movement of Substances
6
3)
Nutrients
8
4)
Enzymes
15
5)
Nutrition in Humans
18
6)
Nutrition in Plants
25
7)
Transport in Humans
31
8)
Transport in Plants
43
9)
Respiration in Humans
47
10)
Excretion in Humans
55
11)
Homeostasis
63
12)
Nervous System
67
13)
Human Eye
73
14)
Hormones
78
15)
Cell Division
81
16)
Reproduction in Plants
86
17)
Reproduction in Humans
91
18)
Heredity
98
19)
Molecular Genetics
104
20)
Ecology
108
21)
Our Impact on the Ecosystem
113
2
Chapter 1: Cells
-
A cell is a​ basic​ unit of life
Cell Structure
Animal cell
Plant cell
Plant vs Animal Cells
Part/structure
Animal cell
Plant cell
Cellulose
Absent
Present
Vacuole
Small and numerous vacuole
A large and central vacuole
Chloroplasts
Absent
Present
Centriole
Present
Absent generally
3
Parts and their Functions
Part
Nucleus
Function
Nuclear envelope: separates the content of the nucleus
from the rest of the cytoplasm.
Nucleoplasm: dense material within the nucleus.
Nucleolus: plays a part in the making of proteins in the
cell.
Chromatin: long strands of DNA.
Cytoplasm
Forms the larger part of the cell, made up of 90% water
and contains dissolved ​protein, sugars and enzymes.
Also contains larger suspended ​particles of fats​ and
many structures called ​organelles​. Site of chemical
reactions in the cell.
Mitochondria
Rod-shaped structures that carry out cellular respiration
to release energy.
Energy used to perform cellular activities
(membrane visible under a electron scanning
microscope)
Plasma membrane
Cell wall
Plasma membrane is a membrane that surrounds the
entire cell. It is ​partially permeable​ allowing certain
substances to pass through.
Plant cells are supported by a ​cellulose cell wall​.
Rough Endoplasmic Reticulum
(RER)
Consist of a network of flattened spaces lined with a
membrane. Have ribosomes attached to the membrane.
Responsible for the synthesis of protein
Smooth Endoplasmic Reticulum
(SER)
Responsible for the synthesis of fats, steroids and sex
hormones.
Vacuoles
Ribosomes
Small numerous and temporary spaces that stores
substances.
Place for synthesising proteins. Can be
attached/unattached to the endoplasmic reticulum.
Chloroplasts
Only found in the cells of green plants. Carries out
photosynthesis.
(membrane visible under electron scanning microscope)
Golgi apparatus
Consists of stacks of flattened membranous sacs and
vesicles involved in processing packaging and secretion
of substances out of the cell.
4
Movement of Substances out of the Cell
1) Vesicles ​transport substances​ within the cell. Small vesicles containing substances made by
the ER are ​pinched off​ the ER.
2) These vesicles then ​fuse with the Golgi apparatus ​and ​release their conten​t into the Golgi
apparatus. The substance made by the ER may be ​modified​ inside the Golgi apparatus.
3) Secretory vesicles containing these substances are ​pinched off​ from the Golgi apparatus.
They ​move​ to the cell surface membrane.
4) The secretory vesicle ​fuse​ with the cell surface membrane and their contents are ​released
out ​of the cell.
Specialised Cells, Tissues, Organs and Systems
-
Differentiation is the process by which a cell becomes specialised for a specific function.
Cells In A Multicellular Organism
- A tissue is a group of cells with similar structures which work together to perform a specific
function.
1) Cells
- Cells are the basic unit of life.
2) Simple Tissue
- Cells of the same kind may group together to form a simple tissue.
3) Complex Tissue
- Complex tissues contains more than one type of cells grouped together.
4) Organ
- An organ contains more than one type of tissue, all working together for a specific
function.
5) Organ System
- An organ system consists of several organs working together for a common purpose.
6) Organism
- Various systems together make up the entire body of an organism.
5
Chapter 2: Movement of Substances
Diffusion
-
Diffusion is the net movement of molecules from a region of higher concentration to
a region of lower concentration.
Concentration gradient is the difference in concentration between two regions.
It is a passive mode of transportation as energy is not required for the movement of
molecules from one place to another.
When the molecules have spread evenly (equilibrium) between the two regions, the
concentrations are the same. As a result, there will be no net movement of
molecules between the regions as the concentration gradient is no longer present.
As a result, there will be no net movement of molecules between these region.
Importance of Diffusion
- Diffusion enables living organisms, unicellular or multicellular, to ​survive​ by allowing the
exchange of nutrients, ​ gases and waste substances.
Osmosis
-
Osmosis is the net movement of water molecules from a region of ​higher water potential​ to
a region of ​lower water potential​ through a ​partially permeable membrane​.
Water potential is the measure of the tendency of water to move from one place to another.
A dilute solution has more water molecules per unit volume than a concentrated solution.
Hence it has a higher water potential.
Water always moves from an area of higher water potential to an area of lower water
potential naturally by osmosis.
Application to living cells
- Cell membranes act as a partially permeable membrane that allows some substances to pass
through but not others.
- The concentration of dissolved substances is usually higher than their surroundings, thus
allow water to enter by osmosis.
6
Type of cells
Effect in concentrated solution
Effect in dilute solution
Leaf cell
Plasmolysis occurs​ when cells are
placed in concentrated solution.
Water​ leaves the cells
Vacuole​ decreases in size
Cytoplasm​ shrinks away from the
cell wall.
Turgor pressure​ results in
when cells are placed in a
dilute solution.
Water​ enters the cell
Vacuole​ increase in size
Cellulose​ cell wall prevents
over expansion of the cell
membrane by exerting an
opposing pressure.
When cell is fully​ turgid further
entry of water is prevented.
Red blood cell
Crenation ​occurs when cells are
placed in a concentration solution
Water ​leaves the cell
Membrane​ of the cell forms l​ ittle
spikes
Cell shrinks, ​becomes dehydrated
and dies.
Haemolysis​ occurs when cells
are placed in a dilute solution.
Water​ enters the cell
Animal cells ​do not have walls
to prevent​ over expansion of
the cell membrane.
Active transport
-
-
Active transport is the movement of substances across a cell membrane against a
concentration gradient, from a region of lower concentration to a region of higher
concentration.
Amount of mitochondria affects the rate of active transport
Surface area to volume ratio
-
The surface area to volume ratio is the amount of surface area per unit volume of an object.
-
In living organisms, it is an important factor that affects the rate of diffusion of substances
across the cell membrane. Nutrients, oxygen and waste substances rely on the diffusion of
substances across the cell membrane in order to enter or leave the cells.
-
Cells with large surface area to volume ratio enable nutrients and oxygen to diffuse into the
cell quickly and allows the waste to diffuse out of the cell quickly. If the cells have a small
surface area to volume ratio, the nutrients will take longer to diffuse as compared to a cell
with a larger surface area to volume ratio.
7
Chapter 3: Nutrients
Importance Of Nutrients In The Body
Nutrients provide energy for vital activities
- Green plants undergo photosynthesis using the light energy from the sun to convert raw
materials from their surroundings into food.
- These food substances contain chemical energy and can be used for bodily processes.
Food provides raw materials to make new protoplasm
- Protoplasm is synthesised from certain substances found in food.
- Protoplasm is used for growth, reproduction, and repair of body parts.
Food helps organisms stay healthy
- Substances in food such as vitamins and minerals help to keep the body healthy.
Water
-
Water is an essential component of all body tissues. About 70% of our body weight is water.
Functions of water
- A solvent for chemical reactions, required in photosynthesis
- A key component of tissues
- Regulation of blood temperature
- Transportation of substances by acting as a solvent
8
Carbohydrates
-
-
Carbohydrates are organic molecules made up of the elements carbon, hydrogen and
oxygen. The hydrogen and oxygen atoms are present in the ratio 2:1
- Glucose has the formula C​6​H​12​O​6
- Sucrose has the formula C​12​H​22​O​11
Carbohydrates can be classified as single sugars, double sugars or complex carbohydrates.
Single sugars / monosaccharides
- Single sugars or monosaccharides are small molecules. They cannot be further digested into
smaller molecules. They can pass through cell membranes and be absorbed into the cells.
- Three common single sugars, all having the chemical formula C​6​H​12​O​6 ​, are:
- Glucose
- Found in plants and animals
- Fructose
- Common in plants but rare in animals
- Galactose
- Present in milk sugar in mammals
Double sugars / disaccharides
- Double sugars, or disaccharides, have two molecules of single sugars bonded together.
- Three common double sugars, all having the same chemical formula C​12​H​22​O​11​, are:
- Maltose
- Maltose occurs in germinating grains. It consists of two glucose molecules
bonded together.
- Glucose + Glucose -----Condensation Reaction-----> Maltose + Water
- A condensation reaction is a chemical reaction in which two simple
molecules are joined together to form a larger molecules with the removal
of one molecule of water.
- A double sugar can be split into two single sugar molecules by using an
enzyme.
- Maltose + Water -----Maltase, Hydrolytic Reaction-----> Glucose + Glucose
- Hydrolysis or a hydrolytic reaction is a reaction in which a water molecule is
needed to break up a complex molecule into smaller molecules
- Sucrose
- Only sugar among the single and double sugars to not be a reducing sugar
- Lactose
- Found in milk of all mammals
9
Complex carbohydrates / polysaccharide
- A ​complex carbohydrate​, or ​polysaccharide​, consists of ​many monosaccharide​ molecules
joined together​. Polysaccharides are produced by the ​condensation(polymerisation) ​of
many similar molecules to form a long molecule.
Polysaccharide
Structure
Role
Occurrence
Starch
A starch molecule is
made up of ​several
thousand glucose
molecules joined
together.
It is a ​storage ​form of
carbohydrates in ​plants​. When
needed, it can be broken down
to glucose to provide energy for
cell activities.
Found in ​storage
organs, e.g.
potato ​tubers
and ​tapioca​.
Cellulose
A cellulose molecule is
made up of many glucose
molecules joined
together. The bonds
between the glucose
units are different from
that in starch.
The cellulose cell wall ​protects
plant cells from bursting​ or
damage. Cellulose cannot be
digested in our intestines. They
serve as dietary fibres that
prevent constipation.
Present in cell
walls ​of plants.
Glycogen
Glycogen is a branched
molecule. It is made up of
many glucose molecules
joined together.
It is a ​storage form​ of
carbohydrates in mammals.
When needed, it is broken
down to glucose to provide
energy for cell activities.
Stored in the ​liver
and ​muscles ​of
mammals.
10
Fats
-
Fats are ​organic compounds containing carbon​. ​Hydrogen ​and ​oxygen​, with much less
oxygen as compared to hydrogen.
Properties and functions of fats:
-
-
-
-
-
-
-
Compact and insoluble in water
- Fats are ​stored as droplets​ inside cells because they are ​insoluble ​and do not affect
water potential in cells.
Energy Store
- Triglyceride(fat) contains a greater number of Carbon-Hydrogen bond per gram than
starch or glycogen, therefore ​one gram of triglyceride yield about twice as much
energy compared to one gram of carbohydrates.
- Triglyceride has about half the mass of carbohydrates for an equivalent amount of
energy stored.
- Carbohydrates are still the most direct source of energy in living things as they are
mobilised much more quickly compared to fats.
Common food store in animals living in cold climates.
- Hibernating animals store fats as food reserve as it is difficult for animals to hunt for
food in cold seasons.
Conduct heat slowly
- Mammals have specialised cells for storing fat under their skin; cells are grouped
together to form adipose tissue.
- Function as an excellent heat insulator against heat loss from deeper regions of the
body to the outside.
Less dense than water
- Large animals that live in cold seas such as whales and seals, often have very thick
layers of adipose tissue called blubber.
- Gives buoyancy to aquatic animals.
- Both the heat insulation and buoyancy help these animals to survive in this
environment.
Absorb shock
- Acts as a protective layer around delicate and vital organs.
Important component of myelin sheath in nerve cells and cell membrane.
- Act as an electrical insulator, allowing rapid transmission of electrical impulses along
myelinated neurons.
- An essential component of the cell membrane.
Acts as a solvent
- Acts as a solvent for fat-soluble vitamins and other vital substances.
11
Proteins
-
Proteins are made up of the elements carbon, hydrogen, oxygen and nitrogen. Sulfur is
sometimes present.
Each type of protein had a unique three dimensional shape thus proteins have diverse
functions.
When heated, the weak bonds in the proteins are broken, and the protein is denatured.
Proteins are built up from amino acids.
Structure of Amino Acids
- Amino acids are monomers of the protein
- There are 20 naturally occurring amino acids.
Properties of amino acid
- Amino acids are colourless and crystalline solids with relatively high melting point.
- Soluble in water where they form ions but generally insoluble in organic solvents.
Formation of polypeptide
- Amino acids join together to form a polypeptide through condensation reaction with the
removal of water molecules.
- The covalent bond that is formed is a peptide bond.
Functions of Proteins
- Proteins are used in the synthesis of new cells, for growth and repair of worn-up cells.
- Proteins are used as biological catalyst to speed up chemical reactions, e.g. enzymes
- Proteins serve as chemical messengers, e.g. hormones such as insulin
- Proteins serve a transport function, e.g. haemoglobin is used to transport oxygen in red
blood cells
- Proteins perform a structural function, e.g. collagen is a component of skin, bones while
keratin is a component of hair, nails, and feathers.
- Proteins are used for the defense of the body, e.g. antibodies which recognise and combine
with foreign substances such as bacteria.
- Proteins can be a source of energy during starvation and are oxidised after all the
carbohydrates and lipids are used up.
12
Food Tests
Benedict’s Test
- Tests for the presence of reducing sugar.
- Sucrose is the only common non-reducing sugar among the monosaccharides and
disaccharides.
1)
2)
3)
4)
Add equal volume of Benedict’s solution to 2 cm​3 ​of solution to be tested.
Shake the mixture
Place test tube in boiling water bath for five minutes.
Observe precipitate formation and colour changes.
Results:
Blue solution turns:
Blue (absent) → Green (little amount) → Yellow (moderate amount) → Orange → Brick-red (most)
Iodine Test
- Tests for the presence of starch.
1) Place food substance on a white tile. Solid foods may need to be chopped up to smaller
pieces.
2) Add 2-3 drops of dilute iodine solution to substance to be tested.
3) Observe colour changes, if any.
Results:
Solution changes colour from yellowish-brown to blue-black colouration → Starch present
Solution remains yellowish-brown
→ Starch absent
Biuret Test
- Tests for the presence of protein
1) Add 1 cm​3 ​sodium hydroxide solution to 2 cm​3 ​ food solution.
2) Shake thoroughly.
3) Add 1% copper (II) sulfate solution, drop by drop, shaking after every drop until a colour
change is observed.
Results:
Solution changes colour from blue to violet → Protein present
Solution remains blue
→ Protein absent
13
Ethanol Emulsion Test
- Tests for the presence of fats/oil
For liquid food mixture:
1) Add 2 cm​3 ​of ethanol to the substance in a dry test tube.
2) Shake the mixture thoroughly.
3) Add 2cm​3 ​of water to mixture.
For solid food mixture:
1) Chop solid food into small pieces and place into a dry test tube.
2) Add 2 cm​3​ of ethanol and shake thoroughly.
3) Allow the solid particles to settle.
4) Decant the ethanol into another test tube containing 2cm​3 ​of water.
Results:
White emulsion formed
→ Fats/oil present.
Solution remains colourless → Fats/oil absent.
14
Chapter 4: Enzymes
-
An enzyme is a protein that functions as a biological catalyst. They can alter or speed up
chemical reactions. They remain chemically unchanged at the end of the reaction.
Enzymes can build up or break down complex substances.
Enzyme Action and Specificity
Lock and Key hypothesis
- The active site of an enzyme has a specific shape into which the substrate(s) fit exactly. The
shape of the substrate is complementary to the shape of the active site of the enzyme.
- The substrate acts as a “key” while the enzyme is a “lock”.
- The substrate binds to the active site of the enzyme, forming an enzyme-substrate complex.
- Once the products are formed, they no longer fit into the active site of the enzyme and are
released back into the surrounding medium.
Chemical Reaction and Activation Energy
-
In a typical chemical reaction, the reactants have to collide with each other in the correct
orientation and amount of energy, known as effective collisions, if not no reaction would
occur.
Activation energy
- Activation energy is the energy required to make substances react.
- It also represents the energy barrier that has to be overcome before a reaction can take
place to form products.
- The greater the activation energy, the slower the reaction at any particular temperature.
- Activation energy of a reaction can be supplied by heating the reactants and the reaction will
then proceed at a faster rate that without heating.
How enzymes lower activation energy by effective collision.
- Effective collision is the collision between substrate and enzyme at the correct orientation.
- When substrate molecules bind to the enzyme molecules at the active site it forms an
enzyme-substrate complex.
- The formation of enzyme-substrate complex lowers the activation energy as enzyme
molecule holds the substrate molecule(s) in an arrangement that forces them together in
the correct orientation.
- Substrate molecules react with each other.
- Once the reaction has occurred, the enzyme-substrate complex dissociates to release the
products and the chemically unchanged enzyme molecule is ready for another cycle of
reaction.
15
Characteristics of Enzymes
Enzymes speed up chemical reactions
- Enzymes alter the rates of chemical reactions that occur in a cell. Enzymes speed up a
chemical reaction by lowering the activation energy needed to start a reaction.
Enzymes are required in minute amounts
- Enzymes remain chemically unchanged after catalysing a reaction. The same enzyme
molecule can be used over and over again, thus a small amount of enzyme can catalyse a
large number of chemical reactions.
Enzymes are specific in action
- Due to their active sites being complementary to only one type of substrate, enzymes are
highly specific in action. Due to this enzyme specificity, each chemical reaction inside a cell is
catalysed by a unique enzyme.
Factors Affecting Rate of Enzyme-Catalysed Reaction
Temperature
- Enzymes have an optimum temperature at which the enzyme activity is at its maximum.
Different enzymes have different optimum temperatures, with most ranging from 37-45℃.
At low temperature:
- Rate of enzyme activity is low.
- Enzymes are inactive at low temperatures.
- Reversible condition - Inactive enzymes can be made active by increasing temperature.
From low temperature to optimum temperature:
- As temperature increases, kinetic energy of substrate and enzyme molecules increases.
- Substrate and enzyme molecules collide more often. This increases the chances of effective
collisions and formation of enzyme-substrate complex and thus rate of reaction increases.
- Reaction rate doubles for every 10℃ rise in temperature until optimum temperature is
reached.
- Reaction rate is its maximum at optimum temperature.
Past optimum temperature:
- As temperature increases beyond optimum temperature, reaction rate starts to decrease.
- Enzyme is denatured. The enzyme loses its 3D shape and active site is unable to bind to the
substrate. Once an enzyme is denatured, it is irreversible and it cannot regain its function
even when temperature is lowered.
- As temperature continues to increase, more enzyme molecules become denatured, which
causes the rate of reaction to decrease further.
16
pH
-
Enzymes have an optimum pH, which is the pH at which enzymes activity is at its maximum.
Different enzymes have different optimum pH.
Any pH that deviates from the optimum pH will cause the rate of reaction to decrease. At
extreme pH, enzyme loses its 3D shape and active site is unable to bind to the substrate.
Denaturation is an irreversible process and once an enzyme is denatured, it cannot regain its
function even when pH is back to optimum.
Enzyme concentration
- Limiting factor.
- At low enzyme concentrations, adding more enzyme increases the rate of reaction.
- With more enzymes present, there are more active sites for effective collisions to take place.
- The rate of reaction is directly proportional to the enzyme concentration until enzyme
concentration can no longer increase the rate of reaction.
- At this point, substrate concentration becomes the limiting factor.
- The rate of reaction becomes constant and reaches a plateau.
Substrate concentration
- Limiting factor.
- At low substrate concentration, few substrate molecules are present, hence there are many
available active sites for effective collisions to occur.
- The rate of reaction increases proportionally with an increase in substrate concentration.
- Substrate concentration is the limiting factor.
- At higher substrate concentrations, increasing the amount of substrate cannot increase the
rate of reaction.
- All enzyme molecules are being made use of - enzyme molecules are saturated and the
amount of product formed per unit time remains the same.
- Enzyme concentration becomes the limiting factor.
- Rate of reaction becomes constant and reaches a plateau.
17
Chapter 5: Nutrition in Humans
Nutrition
-
Nutrition is the process by which organisms obtain food and energy for growth, repair and
maintenance of the body.
In humans, nutrition consists of:
- Feeding or Ingestion
- Food is taken into the body
- Digestion
- Large food molecules are broken down into smaller, soluble molecules that
can be absorbed into the body cells.
- Physical digestion: Mechanical break up of food into smaller particles.
- Chemical digestion: Breakdown of large food molecules into smaller soluble
molecules catalysed by digestive enzymes through hydrolytic reactions.
- Absorption
- Digested food substances are absorbed into the body cells.
- Assimilation
- Some of the absorbed food substances are converted into new protoplasm
or used to provide energy.
Human Digestive System
Mouth and buccal cavity
- Food enters the body through the mouth, which leads into the buccal cavity. The mouth
contains:
- Teeth
- Chewing action of the teeth breaks down large pieces of food into smaller
pieces. This increases the surface area of the food so enzymes can act on it
more efficiently.
- Salivary glands
- Secretes saliva into the mouth. Saliva flows into the buccal cavity via tubes
called salivary ducts.
- Tongue
- Helps to mix food with saliva. Taste buds on the tongue help to identify and
select suitable foods.
Pharynx
- The pharynx is the part of the gut which connects the buccal cavity to the oesophagus and
the larynx. The pharynx also leads to the trachea.
- The larynx has a slit-like opening called the glottis.
18
Oesophagus
- The oesophagus or gullet is a narrow, muscular tube. It passes through the tharos and the
diaphragm to join the stomach.
- The diaphragm is a sheet of muscle which separates the thorax from the abdomen.
- The wall of the oesophagus contains two layers of muscles. These muscles are present along
the whole gut from the oesophagus to the rectum.
- The two layers of muscles are:
- Longitudinal muscles, on the outer side of the gut wall
- Circular muscles, on the inner side of the gut wall.
- Both sets of muscles produce long, slow contractions that move food along the gut by
peristalsis.
Peristalsis
- Peristalsis is the rhythmic, wave-like muscular contractions in the wall of the alimentary
canal.
- Peristalsis enables food to be mixed with the digestive juices, and also pushes or propels the
food along the gut.
- The circular muscles constrict the lumen whereas the longitudinal muscles shorten and
widen the lumen.
- The circular and longitudinal muscles are antagonistic muscles.
- When the circular muscles contract, the longitudinal muscles relax. As a result, the
wall of the gut constricts, and the gut becomes narrower and longer. The food is
squeezed or pushed forwards.
- When the longitudinal muscles contract, the circular muscles relax. The gut dilates,
and it becomes wider and shorter. This widens the lumen for food to enter.
Stomach
- The stomach is a distensible muscular bag, with thick and well-developed muscular walls.
- When the stomach is fully distended, it sends signals to the brain that it is full.
- The stomach wall has numerous pits that lead to gastric glands that secrete gastric juice into
the stomach cavity.
- The pyloric sphincter is located at the place where the stomach joins the small intestine.
When the muscle contracts, the entrance to the small intestine closes. When the ring
relaxes, the entrance opens, allowing food to pass from the stomach into the small intestine.
Liver
-
The liver is dark red and is made up of five lobes.
The liver secretes bile, which is stored in the gall bladder.
Gall bladder
- Bile is stored temporarily in high concentration in the gall bladder
- Bile flows into the duodenum via the bile duct.
Small intestine
- The small intestine consists of the U-shaped duodenum, the jejunum, and the ileum.
- The lining of the walls of the small intestine contains glands which secrete digestive
enzymes.
- The wall os the small intestine is adapted to absorb digested food products and water.
19
Large intestine
- The large intestine is shorter but broader than the small intestine.
- It consists of the colon and the rectum.
- At the junction between the colon and small intestine are the caecum and the appendix. In
humans, these have no specific functions.
- Faecal matter is temporarily stored in the rectum. When the rectrum contracts, the faeces is
expelled through the anus.
- The main function of the colon is to absorb water and mineral salts from the undigested
food material. No digestion occurs in the large intestine.
Digestive Processes
-
Digestion is the process whereby large food molecules are broken down into smaller, soluble
molecules that can be absorbed into the body cells.
Digestive processes in the mouth
- Food in the mouth stimulates the salivary glands to secrete saliva
- Saliva is mixed with food. Mucus in saliva helps to soften the food.
- Saliva contains an enzyme called salivary amylase which digests starch to maltose.
- The pH of the saliva is neutral. Salivary amylase is most active at this pH.
- Chewing breaks the food up into smaller pieces. This increases the surface area to volume
ratio for salivary amylase to work on.
- The tongue rolls the food into small, slippery, round masses or boli.
- The boli are swallowed and passed down into the oesophagus via the pharynx.
- Peristalsis in the walls of the oesophagus pushes each bolus of food down into the stomach.
Digestive processes in the stomach
- The presence of food in the stomach stimulates the gastric glands to secrete gastric juices
into the stomach cavity.
- Peristalsis in the stomach wall churns and breaks up the food. Peristalsis also mixes the food
well with gastric juices.
- Gastric juice is a dilute solution of hydrochloric acid and pepsin.
- The dilute hydrochloric acid:
- stops the action of salivary amylase by denaturing it.
- changes the inactive form of the enzyme pepsinogen, in the gastric juice to the
active pepsin.
- provides a slightly acidic medium suitable for the actions of pepsin
- kills certain potentially harmful microorganisms in food.
- Protease (pepsin) digests proteins to polypeptides.
- Food normally remains in the stomach for about three to four hours. The partly digested
food becomes liquefied, forming chyme.
- Chyme passes in small amounts into the duodenum when the pyloric sphincter relaxes and
opens.
20
Digestive processes in the small intestine
- Chyme enters the small intestine. It stimulates:
- the pancreas to secrete pancreatic juice. Pancreatic juice contains the enzymes
pancreatic amylase, pancreatic protease (trypsin), and pancreatic lipase. The
pancreatic juice passes through the pancreatic duct into the duodenum.
- the gall bladder to release bile. Bile does not contain enzymes so it cannot digest
good, but bilt salts sped up the digestion of fats. Bile passes through the bile duct
into the duodenum.
- the epithelial cells in the small intestine to produce the enzymes maltase, peptidases
and lipase.
- Food comes into contact with pancreatic juice, bile and intestinal juice.
- All three fluids are alkaline. They:
- neutralise the acidic chyme
- provide a suitable alkaline medium for the action of the pancreatic and intestinal
enzymes.
Digestion of Foods
Carbohydrate digestion
- Starch is digested by amylases.
- Starch digestion starts in the mouth. However, only a little starch is digested as food does
not remain long in the mouth.
- No digestion of starch occurs in the stomach.
- In the small intestine:
- Starch is digested by pancreatic amylase to form maltose.
- Maltose is digested by maltase to form glucose.
- Lactose is digested by lactase to form glucose and galactose.
- Sucrose is digested by sucrase to form glucose and fructose.
- The end-products of carbohydrate digestion are simple sugars which can be absorbed into
the bloodstream.
Protein digestion
- Proteins are digested by proteases.
- Some protein digestion begins in the stomach, where pepsin digests proteins to
polypeptides.
- The undigested proteins which enter the small intestine are digested by trypsin to
polypeptides.
- The polypeptides produced are further digested to amino acids by peptidases.
- The end-products of protein digestion are amino acids.
Fat digestion
- Fats are digested by lipases.
- In the small intestine, bile salts emulsify fats. They break down the fats into tiny fat droplets,
which increases surface area to volume ratio for increased rate of digestion.
- Emulsified fats are digested by pancreatic and intestinal lipases to fatty acids and glycerol.
- The end-products of fat digestion are fatty acids and glycerol.
21
Absorption
-
Absorption is the process where digested food substances are absorbed into the body cells.
Adaptations in the small intestine
Adaptations
Functions
Small intestine is long.
Provide sufficient time for absorption to take
place.
Small intestine is lined with many villi, each
possessing numerous microvilli.
To increase surface area to volume ratio for
absorption of digested food particles.
The epithelium of the villus is one cell thick.
To reduce the distance for digested products to
diffuse into the blood vessels.
The small intestine consists of a dense network
of blood capillaries and lacteal or lymphatic
capillaries within the intestinal walls and villi.
Continuous transport of digested food
substances maintains the steep concentration
gradient for fast absorption of digested
particles.
Transport processes for food substances
Substance
Process
Monosaccharides
Diffusion, active transport
Amino acids
Diffusion, active transport
Fatty acids and glycerol
Diffusion
Water
Osmosis
Mineral salts
Diffusion, active transport
Undigested and unabsorbed matter
- Undigested and unabsorbed matter is stored temporarily in the rectum.
- They are discharged as faeces through the anus through egestion.
22
Transport and Assimilation of Absorbed Nutrients
-
Assimilation is the process whereby some of the absorbed food substances are converted
into new protoplasm or used to provide energy.
Glucose and amino acids
- Blood capillaries from the small intestine join together to form the hepatic portal vein, which
transports nutrients to the liver.
- Glucose is assimilated and then oxidised during tissue respiration to release energy for the
vital activities of the cells.
- Excess glucose is returned to the lover and stored as glycogen. Insulin stimulates the liver
cells to convert excess glucose into glycogen. Glucagon stimulates the liver to convert
glycogen back into glucose.
- Amino acids which enter the cells are converted into new protoplasm that is used for growth
and repair of worn-out parts of the body.
- Amino acids are used to form enzymes and hormones.
- Excess amino acids are deaminated in the liver.
Fats
-
Fats are absorbed into the lymphatic capillaries. The lymphatic capillaries join to form larger
lymphatic vessels, which discharge fats into the bloodstream.
Under normal conditions when there is a sufficient supply of glucose, fats are not broken
down, but are used to build protoplasm.
When glucose is in short supply, fats are broken down to provide energy needed by the
body.
Excess fats are stored in the adipose tissues beneath the skin and around the heart and
kidneys. They act as shock absorbers.
Functions of the Liver
-
The liver is the main organ involved in regulating the levels of certain substances in the body.
Regulation of blood glucose content
- The liver plays a key role in carbohydrate metabolism by keeping the amount of glucose in
the blood constant, especially after a heavy meal or during fasting.
- Insulin stimulates liver cells to convert glucose into glycogen.
- Glucagon stimulates liver cells to convert glycogen into glucose.
Production of bile
- The live secretes bile, which emulsifies fats.
- Bile is temporarily stored in the gall bladder.
Iron storage
- The liver breaks down the haemoglobin in red blood cells, and stores the iron that is
released. Bile pigments are formed from the breakdown of the haemoglobin.
23
Protein synthesis
- The liver synthesis protein found in blood plasma from amino acids in the diet. These plasma
proteins include prothrombin and fibrinogen which are essential for clotting of blood.
24
Deamination of amino acids
- Deamination is the process by which amino groups are removed from amino acids and
converted to urea.
- Excess amino acids are transported to the liver, where they are deaminated and converted
to urea.
- Urea is removed from the body in urine.
Detoxification
- Detoxification is the process of converting harmful substances into harmless substances.
- Alcohol is broken down in the liver. Liver cells contain alcohol dehydrogenase, which breaks
down alcohol to acetaldehyde. Acetaldehyde can be further broken down to compounds
that can be used in respiration to provide energy for cell activities.
Effects of Excessive Alcohol Consumption
Harmful effects on the digestive system
- Alcohol stimulates acid secretion in the stomach. Excess stomach acid increases the risk of
gastric ulcers.
- Prolonged alcohol abuse may lead to cirrhosis of the liver, where liver cells are destroyed
and replaced with fibrous tissue, making the liver less able to function.
- Patients with alcoholic cirrhosis may haemorrhage or have bleeding in the liver.
- This can lead to liver failure, and subsequently, death.
Harmful effect on the nervous system
- Depressant
- Alcohol is a depressant. It slows down some brain functions. Its effects vary from
one person to another.
- Reduced self-control
- Under the influence of alcohol, a person becomes carefree as alcohol takes away his
inhibitions. His self-control is reduced. Some may take social or personal liberties
which they may regret after the effects of alcohol have worn off.
- Increased reaction times
- As the person drinks more alcohol, other observable effects of intoxication such as
slurred speech occur.
- Blurred vision and poor muscular coordination make him clumsy and unable to walk
steadily. His judgement deteriorates and he tends to underestimate speed.
- As a driver, he may drive faster and with less caution, and his reactions become
slower.
Social implications
- When a person drinks alcohol frequently he can become addicted. He is unable to stop
drinking until he is drunk and his body becomes dependent on alcohol.
- Alcoholics tend to:
- neglect their work and families, and exhibit violent behaviour especially towards
family members.
- commit more crimes compared to people who are sober.
25
Chapter 6: Nutrition in Plants
Photosynthesis
-
Photosynthesis is the process in which light energy absorbed by chlorophyll is converted into
chemical energy. The chemical energy is used to synthesise glucose from water and carbon
dioxide. Oxygen is released as a product.
Word equation
Carbon dioxide + Water —
Chlorophyll
Light Energy
→ Glucose + Oxygen + Water
Chemical equation
Chlorophyll
6CO​2​ + 12H​2​O — Light Energy → C​6​H​12​O​6 ​ + 6O​2​ + 6H​2​O
Simplified equation
Chlorophyll
6CO​2​ + 6H​2​O — Light Energy → C​6​H​12​O​6 ​ + 6O​2
Light-dependant stage
- Light energy is absorbed by chlorophyll and then converted into chemical energy.
- Light energy is also used to split water molecules into oxygen and hydrogen atoms, also
known as photolysis of water. 12 molecules of water are split to yield 6 molecules of oxygen
and 24 atoms of hydrogen. Oxygen is released out of the lead by the stomata.
Light-independent stage
- Hydrogen released during photolysis is used to reduce carbon dioxide to glucose. The energy
needed for this process comes from the light-dependant stage.
- 24 hydrogen atoms are used to reduce 6 molecules of carbon dioxide to form 1 molecule of
glucose.
26
Comparing Light-dependent and Light-independent stages
Light-dependent stage
Enzymes
Light-independent stage
Needed in both cells
Site of occurence in
chloroplast
Thylakoids
Stoma
Requirements
Light energy, water
Carbon dioxide, hydrogen
Energy
Light energy converted into ATP
Comes from light-dependent stage
(ATP)
Reactions
Photolysis of water
Reduction of carbon dioxide
Products formed
Oxygen gas, hydrogen atoms
Glucose, water
What happens to glucose formed during photosynthesis?
- Glucose is used for respiration to provide ATP for cellular activities.
- Glucose is used to form cellulose cell wall.
- When rate of photosynthesis is higher than the rate of respiration, excess glucose is
converted into starch for storage. When photosynthesis stops, starch is converted back to
glucose for usage.
- Glucose is also converted into triglycerides (fat) and lipids for storage and synthesis of cell
membrane.
- Glucose is converted to sucrose for transport to other parts of the plant via the phloem
- Glucose reacts with nitrates and other mineral salts from the soil to form amino acids in
leaves. Amino acids form proteins for new cellular materials.
Factors Affecting Rate of Photosynthesis
Light Intensity
- Compensation point is the point where rate of photosynthesis equals the rate of respiration.
- As light intensity increases during the morning and fades during the evening, there will be a
time when the rate of photosynthesis exactly matches the rate of respiration.
- At this point, there will be no net intake or output of carbon dioxide or oxygen. This is the
compensation point.
- The glucose produced by photosynthesis exactly compensates for the glucose broken down
by respiration.
- As light intensity increases, the rate of the light-dependent reaction, and thus
photosynthesis, increases proportionally.
- As light intensity increases beyond a certain point, the rate of photosynthesis is limited by
some other factors such as temperature and carbon dioxide concentration.
27
Carbon Dioxide concentration
- An increase in the carbon dioxide concentration increases the rate of photosynthesis up to a
certain point.
- Beyond this point, when the concentration of carbon dioxide increases, the rate of
photosynthesis plateaus.
- Rate of photosynthesis is limited by another factor such as light intensity or temperature.
Carbon dioxide is no longer the limiting factor.
- Under normal circumstances, carbon dioxide is an important limiting factor since
atmospheric carbon dioxide remains constant at about 0.03%
Temperature
- Photosynthesis is dependent on temperature as it is a reaction catalysed by enzymes.
- When temperature increases, kinetic energy increases which increases the frequency of
effective collision between enzymes and substrates, thus increasing the rate of
photosynthesis.
- As the enzymes approach their optimum temperatures, the overall rate of photosynthesis
reaches maximum.
- When the temperature increases beyond the enzymes’ optimum temperatures, the rate of
photosynthesis begins to decrease until it stops. Enzymes are denatured.
Importance of Photosynthesis
Photosynthesis makes chemical energy available to animals and other organisms
- Sunlight is the ultimate source of energy for living organisms. Photosynthesis enables the
light energy to be converted into chemical energy which is then stored within the
carbohydrate molecules. Fats, proteins and other organic compounds can be formed with
carbohydrates. All these substances eventually become the food of other organisms. They
thus obtain this chemical energy directly or indirectly from plants.
Photosynthesis removes carbon dioxide and provides oxygen
- Photosynthesis removes carbon dioxide from the air and at the the same time, produces
oxygen. The oxygen released is used by living organisms in respiration to release energy for
cell activities. This also maintains a constant level of oxygen and carbon dioxide in the
atmosphere.
Energy is stored in fossil fuels through photosynthesis
- All the energy in fossil fuels like coal, oil and gas, came from the sun, captured through
photosynthesis. Burning of fossil fuels release energy for human activities.
28
External Features of a Leaf
-
A typical green leaf consists of a lamina and petiole.
Lamina
- The lamina had a large flat surface compared to its volume. This enables it to obtain the
maximum amount of sunlight for photosynthesis. A large, thin lamina also means that
carbon dioxide can rapidly reach the inner cells of the leaf.
Petiole
- The petiole holds the lamina away from the stem so that the lamina can obtain sufficient
sunlight and air. In some leaves, for example grasses and maize, the petiole is absent as they
have lone laminae.
Network of veins
- Veins carry water and mineral salts to the cells in the lamina and carry manufactured food
from these cells to other parts of the plant. In a simple leaf, there is a main vein giving off
branches repeatedly, forming a network of fine veins.
Leaf arrangement
- Leaves are always organised around the stem in a regular pattern. In general, leaves grow
either in pairs or singly in an alternate arrangement to ensure that the leaves are not
blocking one another from sunlight and that each leaf receives sufficient light.
Internal Structure of the Lamina
Cuticle
- Waxy and transparent layer.
- Reduced water loss through evaporation from the leaf and prevents the invasion of bacteria
or viruses.
- Does not contain chloroplasts.
Epidermis (upper and lower)
- Protects the inner cells and allows light to pass through.
- Single layer of closely packed cells.
- Does not contain chloroplasts.
Palisade mesophyll
- Long and cylindrical cells which contain the largest number of chloroplasts for
photosynthesis.
- They are nearest to the upper epidermis and closely packed together.
- Filled with chloroplasts.
29
Spongy mesophyll
- Irregularly shaped cells which contain chloroplasts for photosynthesis.
- Loosely packed with large intercellular air spaces among the cells to increase surface area for
gaseous exchange.
- Cells are covered with a thin film of moisture.
- Contains vascular bundle.
- Contains less chloroplasts that the palisade mesophyll cells.
Vascular bundle
- Contains the transport tissues xylem and phloem.
Intercellular air spaces
- Allow circulation of air inside leaf for photosynthesis and respiration.
- Interconnecting system of air spaces in the spongy mesophyll allows for rapid diffusion of
gases into and out of the cells.
Guard cells
- Contains chloroplasts and regulate the size of the stomata for gaseous exchange and
transpiration.
- Cell wall near the stoma is thicker than elsewhere in the cell.
- In the day:
- Guard cells photosynthesise
- Chemical energy is used to pump potassium ions into the guard cells from
neighbouring epidermal cell.
- Concentration of K​+​ ions increases in the guard cells.
- Water potential in guard cell is lowered.
- Water from neighbouring cells enter guard cells by osmosis.
- Guard cells swell and becomes turgid.
- Due to difference in thickness of cell wall in guard cells, one side expands
more than the other.
- In the night:
- Potassium ions move out of the guard cells via diffusion
- Water potential in guard cells increases.
- Water moves out of the guard cells.
- Guard cells become flaccid.
- Stoma closes.
30
Adaptations in the Leaf for Photosynthesis
Adaptation
Petiole
Thin, broad lamina
Function
Holds leaf in position to absorb maximum light energy.
Thin lamina provides a short diffusion distance for gases and enables
light to reach all mesophyll cells.
Broad lamina provides a large surface area for maximum absorption
of light.
Waxy cuticle on upper
and lower epidermis
Reduces water loss through evaporation from the leaf, transparent
for light to enter leaf.
Stomata present in
epidermal layers
Open in presence of light, allowing carbon dioxide to diffuse in and
oxygen to diffuse out of the leaf.
Chloroplasts containing
chlorophyll in all
mesophyll cells
Chlorophyll absorbs and transforms light energy to chemical energy
used in the manufacture of sugars.
More chloroplasts in
upper palisade tissue
Interconnecting system of
air spaces in the spongy
mesophyll
Veins containing xylem
and phloem situated
close to mesophyll cells
More light energy can be absorbed near the leaf surface.
Allows rapid diffusion of carbon dioxide and oxygen into and out of
the mesophyll cells.
Xylem transports water and mineral salts to mesophyll cells.
Phloem transports sugars away from the leaf.
Carbon Dioxide Entering the Leaf
1) In daylight when photosynthesis occurs, the carbon dioxide in the leaf is rapidly used up. The
carbon dioxide concentration in the leaf becomes lower than that in the atmospheric air, so
a diffusion gradient exists. Therefore, carbon dioxide diffuses from the surrounding air
through the stomata into the air spaces in the leaf.
2) The surfaces of the mesophyll cells are always covered by a thin film of water so that carbon
dioxide can dissolve in it.
3) The dissolved carbon dioxide then diffuses into the cells.
Water and Mineral Salts Entering the Leaf
1) The xylem transports water and dissolved mineral salts to the leaf form the roots.
2) Once out of the veins, the water and mineral salts move from cell to cell right through the
mesophyll cells of the leaf.
31
Chapter 7: Transport in Humans
The Need for a Transport System
-
-
In a simple unicellular organism, the movement of materials into and from the cell occurs by
diffusion as no part of the cell ios afr form the external environment.
In a complex multicellular organism, a transport system is needed to carry materials from
one part of the body to another, as cells are located deep into the body, far from the
external environment.
Mammals have developed a transport system consisting of blood vessels, blood and a heart.
Double Circulation and its Advantages
Complete separation of oxygenated and deoxygenated blood
- Ensures only oxygenated blood can reach tissue cells.
- Efficiency of transport of oxygenated blood.
Blood passes through the heart twice in one complete circuit
- Pulmonary circulation at lower pressure
- Blood enters the lungs at a lower pressure compared to blood leaving the heart. This ensures
that blood flows more slowly through the lungs, allowing sufficient time for the blood to be
well oxygenated before it returns to the heart.
- Systemic circulation at higher pressure
- Oxygenated blood is distributed to the rest of the body tissues more quickly. Helps to
maintain the high metabolic rate in mammals.
32
Structure and Composition of Blood
Plasma
- Plasma is a pale yellowish liquid. It is about 90% water and the rest is a complex mixture of
various dissolved substances such as:
- Soluble proteins such as fibrinogen, prothrombin and antibodies. Fibrinogen and
prothrombin play an important part in the clotting of blood. These proteins are
made in the lever. Antibodies help to fight diseases.
- Dissolved mineral salts, for example, hydrogencarbonates, chlorides, sulfates and
phosphates of calcium, sodium and potassium. All these occur as ions in the plasma.
Calcium is essential for blood clotting.
- Food substances, for example, glucose, amino acids, fats and vitamins.
- Excretory products, for example, urea, uric acid and creatinine. Carbon dioxide is
present as hydrogencarbonate ions.
- Hormones, for example, insulin.
- Plasma transports all these substances, together with the blood cells, around the body. The
amount of soluble proteins, mineral salts and glucose in the blood plasma and kept relatively
constant.
Red blood cells
- There are about 5 million red blood cells in each cubic millimetre of blood. Each red blood
cell:
- Contains the pigment haemoglobin which combines reversibly with oxygen. It
enables red blood cells to transport oxygen from the lungs to all cells in the body.
- Has a circular, flattened biconcave disc. The centre of the cell is thinner than its
edge. This increases the cell’s surface area to volume ratio. The cell can thus absorb
and release oxygen at a faster rate.
- Does not possess a nucleus, enabling it to carry more haemoglobin and thus more
oxygen.
- Is elastic and can turn bell-shaped in order to squeeze through blood vessels smaller
than itself in diameter.
- Red blood cells are produced by the bone marrow. Each red blood cell lives for about 3-4
months, before they are destroyed by the spleen.
White blood cells
- White blood cells are larger than red blood cells but are fewer in number. For each white
blood cell, there are 700 red blood cells. Each white blood cell:
- Is colourless and does not contain haemoglobin
- Is irregular in shape and contains a nucleus
- Can move, change its shape and squeeze through the walls of the thinnest blood
capillaries into the spaces among tissue cells.
- There are two main kinds of white blood cells: lymphocytes and phagocytes. They play a vital
role in keeping the body healthy by fighting diseases.
33
-
-
Lymphocytes
- Each lymphocyte has a large, rounded nucleus and a relatively small amount of
non-granular cytoplasm. Lymphocytes tend to be nearly round in shape and only
show limited movements. They produce antibodies that may protect the body from
disease-causing microorganisms.
Phagocytes
- Phagocytes ingest and digest foreign particles such as bacteria.
Blood platelets
- Blood platelets or thrombocytes are not true cells. They are membrane-bound fragments of
cytoplasm from certain bone marrow cells. They play a part in the clotting of blood
Blood Groups
-
-
-
Red blood cells have antigens on the surface of their cell membranes.
Blood plasma contains antibodies which recognise and bind to specific antigens on the red
blood cells of another person.
Transfusion of wrong type of blood causes agglutination or clumping of red blood cells. This
could lead to death as the clumps may block up small blood vessels and prevent the flow of
blood.
Blood group O is known as the universal donor as there are no antigens on the donor’s red
blood cells and thus the recipient’s antibodies would not cause agglutination of the donor’s
blood.
Blood group AB is also known as the universal acceptor ass there are no antibodies in the
plasma of the recipient which could cause agglutination of the donor’s blood.
Blood group
Antigen present on red blood cells
Antibody present in plasma
A
Antigen A
Antibody b
B
Antigen B
Antibody a
O
No antigen
Antibodies a & b
AB
Antigens A & B
No antibodies
Blood Transfusion
-
A transfusion is the transfer of whole blood or blood components into the bloodstream of
another person.
Main purpose is to increase blood volume or to improve immunity.
Incompatible transfusions can result in agglutination - binding of antibodies in recipient’s
plasma to antigens on donated RBC’s
Reaction between donor’s antibodies in the plasma and the recipient’s antigens on the RBCs
is not significant. Donor’s antibodies are diluted in the recipient’s plasma.
34
Functions of Blood
Transport function
- Respiratory gases - oxygen and carbon dioxide. Blood transports oxygen from the lungs to
the cells of the body and carbon dioxide form the body cells to the lungs for removal.
- Nutrients from alimentary canal to body cells - glucose and amino acids pass through the
liver for processing before entering the general circulation.
- Metabolic waste from sites of production to sites of removal - urea produced in liver is sent
to the kidneys for excretion, lactate produced in muscles is sent to liver to be broken down.
- Hormones produced by endocrine glands are transported to target organs - Insulin stimulate
the conversion of excess glucose to glycogen in liver and muscle.
Substances transported
Digested food products
Carried from
Carried to
Small intestines
Liver, all parts of the
body
Nitrogenous waste
All parts of the body
Kidney
Carbon dioxide
All parts of the body
Lungs
Hormones
Glands
Target cells or organs
Heat
Respiring body tissue
All parts of the body
Oxygen
Lungs
All parts of the body
Excretory products
Protective function
- Clotting mechanism
- Protect against blood loss
- Prevent the entry of pathogens
- Involves platelets, plasma protein and other plasma factors.
- Blood clotting occurs to seal a wound and prevent entry of bacteria and further loss
of blood.
- Blood clotting mechanism:
1) Damaged tissues and platelets produce Thrombokinase.
2) Thrombokinase converts the protein prothrombin into thrombin in the
presence of calcium ions.
3) Thrombin catalyses the conversion of the soluble protein to insoluble fibrin
threads.
4) Fibrin threads entangle blood cells and forms a clot, sealing the wound.
- Undamaged blood vessels contain heparin, an anti-clotting substance. When
thrombokinase is released in damaged tissues, it neutralises heparin so clotting can
take place.
- Defense mechanism
- White blood cells protect against disease-causing organisms.
- Phagocytes carry out phagocytosis to engulf disease-causing organisms.
- Lymphocytes produce and secrete specific antibodies against disease-causing
organisms.
35
-
-
-
Phagocytosis
- Phagocytosis is the process of engulfing and ingesting foreign particles.
- In the process of fighting bacteria, some phagocytes are killed with the
bacteria, forming pus.
Production of antibodies
- When pathogens or disease-causing organisms enter the bloodstream, they
stimulate lymphocytes to produce antibodies. Antibodies protect our body
from diseases by:
- Destroying bacteria.
- Causing bacteria to clump together and be engulfed by
phagocytosis.
- Neutralising the toxins produced by bacteria.
- Immunisation or vaccination directly induces lymphocytes to produce
antibodies by exposing a person to dead or weakened forms of the
pathogen.
Organ transplant and tissue rejection
- A damaged or diseased organ can be replaced by a healthy organ from a
donor.
- The immune system of the organ recipient may treat a transplanted organ as
a foreign body. This induces production of antibodies against the organ and
results in tissue rejection.
Regulation of pH, water potential and temperature
- Circulating blood maintains homeostasis of body fluids by:
- Maintaining an optimum pH in the tissues through the use of buffers.
- Maintaining the water potential of body fluid.
- Blood solutes affect the water potential of the blood.
- Water potential gradient between the blood and the tissue fluid is affected.
- The water potential is largely due to sodium ions and plasma proteins.
- The blood solutes level regulates the movement of water between blood
and tissues.
- Water in blood plasma plays a part in the distribution of heat between:
- The heat-producing areas such as the liver.
- Areas of heat loss such as the skin.
36
The Circulatory System
37
38
Tissue Fluid
- The tiny spaces between tissue cells contain a colourless liquid, the tissue fluid. The tissue
cells are bathed with tissue fluid which carried substances in solution between the tissue
cells and the blood capillaries.
- Dissolved food substances and oxygen diffuse from the blood in the blood capillaries into the
tissue fluid and then into the cells. Metabolic waste products diffuse from the cells into the
tissue fluid and then through the blood capillary walls into the blood. The blood transports
these to the excretory organs for removal.
- Since blood capillaries are narrow, the red blood cells can only move through the lumen of
the blood capillaries in a line, one behind the other. The red blood cells may become
bell-shaped as they pass through narrow blood capillaries. The advantages of this are:
- Diameter of red blood cell is reduced so that it can easily pass through the lumen of
the capillaries.
- The cell increases its surface area to speed up absorption or release of oxygen.
- Rate of blood flow is reduced, giving more time for, and thus increasing the
efficiency of, exchange of materials between the blood and the tissue cells.
The Heart
Structure of the heart
- In human beings, the heart is about the size of a clenched fist. It lies in the thorax behind the
chest bone and between the two lungs.
- The whole heart is surrounded by a “bag” called the ;pericardium. The pericardium is made
up of two layers of membrane. The inner membrane is in contact with the tissues making up
the heart. Between the two pericardial membranes is a fluid which helps to reduce friction
when the heart is beating.
- The mammalian heart has four chambers.
- Atria
- The atria have comparatively thin muscular walls since they only force blood
into the ventricles and this does not require high pressure.
- Ventricles
- Ventricles have comparatively thick muscular walls especially the left
ventricle, since it had to pump blood around the whole body and this
requires high pressure.
- The right ventricle has thinner walls than the left ventricle since it only
pumps blood to the lungs, which is close to the heart.
- Median septum
- The right and left sides of the heart are separated by a muscular wall called
the median septum.
- The median septum prevents the mixing of deoxygenated blood in the right
side with the oxygenated blood in the left side.
- Mixing of deoxygenated blood with oxygenated blood will reduce the
amount of oxygen carried to the tissue cells.
39
Path of blood through the Heart
1) Deoxygenated blood from various parts of the body is returned to the right atrium. Blood
from the head, neck and arims is returned to the right atrium by the superior vena cava.
Blood from the other parts of the body is brought back by the inferior vena cava. The
superior and inferior vena cava are collectively called the canae cavae.
2) When the right atrium contracts, blood flows into the right ventricle. Between the right
atrium and the right ventricle is the tricuspid valve, which opens when the pressure in the
right ventricle becomes lower than the pressure in the right atrium. It consists of three flaps.
These flaps are attached to the walls of the right ventricle by cord-like tendons called
chordae tendineae. The flaps point downwards to permit easy flow of blood from the atrium
into the ventricle.
3) When the right ventricle contracts, the blood pressure forces the flaps of the tricuspid valve
to close. This prevents backflow of blood into the atrium. The chordae tendinae prevent the
flaps from being reverted into the atrium when the right ventricle contracts. Blood leaves
the right ventricle through the pulmonary arch. The pulmonary arch leaves the heart and
divides into two pulmonary arteries, one to each lung. Semi-lunar valves in the pulmonary
arch prevent backflow of blood into the right ventricle.
4) The blood in the pulmonary arteries is at a lower pressure than the blood in the aorta. This
slows down the rate of blood flow to give more time for gas exchange in the lungs.
5) Oxygenated blood from the lungs is brought back to the left atrium by the pulmonary veins.
When the left atrium contracts, the blood pressure in the left atrium becomes higher than
that in the left ventricle. This causes the bicuspid valve to open and blood enters the left
ventricle. The bicuspid valve separates the left atrium from the left ventricle. This is similar in
structure and function to the tricuspid valve except it has two flaps instead of three. When
the left ventricle contracts, blood leaves through a large artery, the aorta.
6) From the aorta, blood is distributed to all parts of the body except the lungs. The aorta
curves upwards from the left ventricle as a U-shaped tube. It also possesses semi-lunar
valves to prevent backflow of blood into the left ventricle. Blood entering the aorta is at a
very high pressure. Two small coronary arteries emerge from the aorta. They bring oxygen
and nutrients to the heart muscles.
The cardiac cycle
1) The atria contract, forcing blood into the relaxed ventricles.
2) After a short pause, the ventricles contract. The rise in pressure causes the atrio-ventricular
valves to close to prevent backflow of blood into the atria. This produces a loud ‘lub’ sound.
The semi-lunar valves open. Blood flows from the right ventricle and left ventricle into the
pulmonary arch and aortic arch respectively.
3) As the ventricles contract, the atria relax. The right atrium receives blood from the venae
cavae while the left atrium receives blood from the pulmonary veins.
4) The ventricles relax. The fall in pressure causes the semi-lunar valves to close to prevent
backflow of blood from the two arches into the ventricles. This produces a softer ‘dub’
sound. The AV valves also open and blood flows from the atria into the ventricles.
5) The atria contract again and the whole cycle repeats.
40
Pressure changes in the Heart
1) A slight increase in the ventricular pressure due to the contraction of the left atrium, forcing
blood into the relaxed ventricle.
2) The ventricle begins to contract, the bicuspid valve closes and the pressure increases.
3) The pressure in the ventricle continues to rise as it contracts.
4) The pressure in the left ventricle becomes higher than that in the aorta. The semi-lunar valve
in the aorta opens.
5) The ventricle begins to relax. The aortic valve closes to prevent backflow of the blood into
the ventricle.
6) The pressure in the ventricle continues to decrease as it relaxes.
7) The bicuspid valve opens as the pressure in the ventricle becomes lower than that in the
atrium.
8) The pressure in the ventricle gradually increases as blood continues to enter the ventricle
from the atrium.
9) The cycle repeats.
41
Main Arteries and Veins of the Body
Coronary Heart Disease
-
The most common heart disease is coronary heart disease.
The coronary arteries branch out from the aorta to provide oxygen and nutrients to the
heart muscles to sustain it for contractions.
Buildup of cholesterol and fatty deposits in the coronary artery wall results in
atherosclerosis.
Plaque narrows lumen of arteries, resulting in less oxygen and nutrients being supplied to
heart muscles.
Patients with coronary heart disease may experience angina - chest pain or discomfort in the
area of heart that does not get sufficient blood.
When the coronary arteries are completely blocked, myocardial infarction (heart attack) may
occur.
- When heart tissue does not get any blood flow, the tissue dies, resulting in damage
to the heart.
- Infarction may disrupt the conduction system of the heart and cause sudden death
to the patient.
42
Factors increasing risk of getting a heart attack
- Family history
- Being a male
- Age
- Smoker
- High intake of saturated fats and salt
- Lack of exercise
- High blood pressure
- Intake of excessive sugars
- High alcohol intake
Preventive measures against coronary heart disease
- A proper diet is important in reducing the risk of atherosclerosis. Polyunsaturated plant fats
should substitute animal fats as they do not stick to the inner surface of the arteries. Such a
diet may also lower the cholesterol level in the blood.
- Proper stress management helps to reduce the risk of a heart attack.
- Smoking is harmful to the body and should be avoided. Cigarette smoke contains nicotine
and carbon monoxide that increase the risk of coronary heart disease.
Regular physical exercise has long-term beneficial effects on the circulatory system. It
strengthens the heart and maintains the elasticity of the arterial walls. The risk of high blood
pressure or hypertension can be greatly reduced.
43
Chapter 8: Transport in Plants
Xylem
Structure
Made up of long and hollow
tubes.
Made up of dead cells.
Inner walls of vessels are
strengthened with lignin
deposits.
Function
Structural adaptation for
function
Transport water and mineral
salts from the roots to the
stems and leaves in one
direction.
Empty lumen without any
cytoplasm, organelles and
cross-walls/end walls (cell
walls in between cells). This
reduces resistance to the flow
of water.
Provide mechanical support
for the plant.
Walls are strengthened with
lignin. Prevents the collapse of
the vessel and allows it to
provide mechanical support
for the plant.
Lignin can deposit in different
patterns.
Phloem
Structure
Function
Structural adaptation for
function
Made up of sieve tubes
(consisting of elongated and
thin-walled sieve tube cells)
and companion cells.
Transport manufactured food
Pores in the sieve plates allow
substances (sucrose and amino faster flow of manufactured
acids) to other parts of the
food substances between cells.
plant.
Sieve tube cells after maturing,
wil lose its vacuole, nucleus
and most organelles. It now
has degenerate cell contents.
It also has a degenerate
cytoplasm.
Process of transporting
manufactured food substances
(sucrose and amino acids) in
phloem is known as
transportation.
Cross-walls have lots of small
pores and are called sieve
plates.
Companion cells, which lie
next to sieve tube cells,
contain many mitochondria.
Mitochondria provides energy
needed for active transport
and for metabolic processes.
44
Vascular Bundle
Stem
-
-
In a dicotyledonous stem, the xylem and phloem are grouped together to form vascular
bundles.
The vascular bundles are arranged in a ring around a central region called the pith.
The phloem lies outside the xylem with a tissue called the cambium between them.
Cambium cells can divide and differentiate to form new xylem and phloem tissues, giving
rise to a thickening of the stem.
The region between the pith and epidermis is the cortex. Both the cortex and the pith serve
to store up food substances, such as starch.
The stem is covered by a layer of cells called the epidermis. The epidermal cells are
protected by a waxy, waterproof cuticle that greatly reduces evaporation of water from the
stem.
Roots
- In a dicotyledonous root, the xylem and phloem are not bundled together. Instead, they
alternate with each other.
- The cortex of the root is also a storage tissue.
- The epidermis of the root is the outermost layer of the cells. It bears many root hairs. It is
also called the piliferous layer.
- Each root hair is a tubular outgrowth of an epidermal cell. This outgrowth increases the
surface area to volume ratio of the root hair cell. The absorption of water and mineral salts is
increased through this adaptation.
Entry of Water and Mineral Salts into the Plant in the Roots
Water
- Each root hair grows between the soil particles, coming into close contact with the soil
solution surrounding them.
- The cell sap in the root hair cell is relatively concentrated with sugars and mineral salts. Thus
it has a lower potential than the soil solution. Water enters the root hair cells by osmosis.
- The entry of water dilutes the sap. The root hair cell no has a higher water potential than the
next cell in the cortex. Hence, water move from root hair cell to the next cell by osmosis.
- Water then travels from cell to cell by osmosis until it reaches the xylem.
Ions and mineral salts
- Ions and mineral salts are absorbed by active transport, when the concentration of ions in
the soil solution is lower than that in the root hair cell sap.
- Ions and mineral salts are absorbed by diffusion, when the concentration of certain ions in
the soil solution is higher than that in the root hair cell.
Adaptations of root hair cell
- The root hair is long and narrow. This increases the surface area to volume ratio which in
turn increases the rate of absorption of water and mineral salts by the root hair cell.
- Cell surface membrane prevents the cell sap from leaking out. The cell sap contains sugars,
amino acids and salts. It has a lower water potential than the soil solution. This results in
water entering the root hair by osmosis.
45
-
The root hair cell contains many mitochondria. Aerobic respiration in the mitochondria
releases energy for the active transport of ions into the cell.
46
Moving Water against Gravity
Root pressure
- The respiring cells around the xylem vessels in the roots use active transport to pump
mineral salts into the vessels. This lowers the water potential in the xylem vessels, which
causes water to move into the xylem vessels by osmosis. This pushes water into the xylem
vessels and upwards.
Capillary action
- Water tends to move up very narrow tubes (capillary tubes) due to forces of cohesion
(forces of attraction among water molecules) and adhesion (forces of attraction between
water molecules and the inner walls of the tube).
Transpiration
-
Transpiration is the loss of water vapour from a plant, mainly through the stomata of the
leaves.
Transpiration pull
- The evaporation of water from the leaves removes water from the xylem vessels. This results
in a suction force which pulls water up the xylem vessels.
- Main force in drawing water and mineral salts up the plant.
1) Water continuously moves out of the mesophyll cells to form a thin film of moisture over
their surfaces.
2) Water evaporates from this thin film of moisture and moves into the intercellular air spaces.
Water vapour accumulates in the large air spaces near the stomata (sub-stomatal air
spaces).
3) Water vapour then diffuses through the stomata to the drier air outside the leaf.
4) As water evaporates from the mesophyll cells, the water potential of the cell sap decreases.
The mesophyll cells begin to absorb water by osmosis from the cells deeper inside the leaf.
These cells, in turn, remove water from the xylem vessels.
5) This results in transpiration pull, a suction force which pulls the whole column of water up
the xylem vessels.
Importance of transpiration
- Transpiration pull draws water and mineral salts from the roots to the stems and leaves.
- Evaporation of water from the cells in the leaves removes latent heat of vaporisation. This
cools the plant, preventing it from being scorched by the hot sun.
- Water transported to the leaves can be used in photosynthesis, to keep cells turgid, and to
replace water lost by the cells. Turgid cells keep the leaves spread out widely to trap sunlight
for photosynthesis.
47
Factors affecting rate of transpiration
- Humidity
- Higher humidity decreases the rate of transpiration while lower humidity increases
the rate of transpiration.
- Intercellular air spaces in the leaf are normally saturated with water vapour. There is
a water vapour concentration gradient between the leaf and the atmosphere. The
drier or less humid the air outside the leaf, the steeper this concentration gradient
is, thus the rate of transpiration will be faster. Increasing the humidity of the air will
decrease the water vapour and concentration gradient between the leaf and the
atmosphere. Thus rate of transpiration decreases as humidity increases.
- Drier air causes this concentration gradient to become steeper, increasing rate of
diffusion of water vapour to the surrounding air, increasing transpiration rate.
- Wind
- Higher wind speed increases the rate of transpiration while low/no wind decreases
the rate of transpiration.
- Wind blows away water vapour that accumulates outside the stomata. This
maintains the water vapour concentration gradient between leaf and atmosphere.
Thus stronger wind results in higher rate of transpiration.
- In still air, water vapour that diffuses out of the leaf makes the air around the leaf
more humid, making the water vapour concentration gradient less steep and
decreasing the rate of transpiration.
- Temperature
- Higher surrounding temperature increases the rate of transpiration while lower
temperature decreases the rate of transpiration.
- Assuming that other factors remain constant, a rise in the temperature of the
surroundings increases the rate of evaporation of water from the cell surfaces. Thus,
the rate of transpiration is greater at higher temperatures.
- Light intensity
- Higher light intensity increases the rate of transpiration while lower light intensity
decreases the rate of transpiration.
Wilting
-
Water lost via transpiration has to be replaced by absorption from the roots. Wilting occurs
when the rate of transpiration exceeds the rate of absorption at the roots.
If the rate of transpiration is less than the rate of water absorption, plant cells become turgid
and plant becomes firm and upright.
If the rate of transpiration is more than the rate if water absorption, plant cells become
flaccid and plant wilts.
Advantages of wilting
- When the leaves fold up, the leaves droop and less leaf surface is exposed to the sun and
thus the rate of photosynthesis decreases.
Disadvantages of wilting
- When the plant wilts, the leaves droop and less leaf surface is exposed to the sun and thus
rate of photosynthesis decreases.
48
Chapter 9: Respiration in Humans
-
Respiration is the breakdown of food substances with the release of energy in living cells.
Aerobic Respiration
-
Aerobic respiration is the ​complete​ breakdown of food substances in the ​presence​ of oxygen
with the release of a ​large​ amount of energy. Carbon dioxide and water are released as
waste products.
Word equation:
Glucose + Oxygen → Carbon Dioxide + Water + Large amount of energy released
Chemical equation:
C​6​H​12​O​6​ + 6O​2​ → 6CO​2​ + 6H​2​O + E​ nergy
Uses for energy in the body
- Muscular contraction
- Protein synthesis
- Cell division
- Active transport
- Passage of nerve impulses
- Maintenance of a constant body temperature
Anaerobic Respiration
-
Anaerobic respiration is the ​partial​ breakdown of food substances in the ​absence​ of oxygen.
Anaerobic respiration releases ​less​ energy than aerobic respiration.
Word equation (yeast):
Glucose → Ethanol + Carbon Dioxide + Small amount of energy
Chemical equation (yeast):
C​6​H​12​O​6​ → 2C​2​H​5​OH + 2CO​2​ + Energy
Word equation (muscle cells):
Glucose → Lactic Acid + Small amount of energy
Chemical equation (muscle cells)
C​6​H​12​O​6​ → 2C​3​H​6​O​3​ + Energy
49
Effect of Lactic Acid on Muscles During Exercise
Anaerobic respiration in muscle cells
- During vigorous muscular contractions, the muscle cells first respire aerobically.
- Breathing rate increases to remove carbon dioxide and take in oxygen at a faster rate. Heart
rate will also increase so that the oxygen can be transported to the muscles at a faster rate.
However, there is a limit to the increase in the rate of breathing and heartbeat.
- In such cases, muscle cells also respire anaerobically for short durations in order to meet the
energy demands of the activity (in addition to the aerobic respiration still taking place).
- The ​extra​ energy released by anaerobic respiration supplements the energy released by
aerobic respiration to allow the muscles to continue contracting.
- When anaerobic respiration occurs, there is a buildup of ​lactic acid​ in the muscle cells.
- Since there is insufficient oxygen to meet the demands of the vigorous muscular
contractions, the muscles are said to incur an ​oxygen debt​. Lactic acid concentrations build
up slowly in the muscles, and may eventually become high enough to cause fatigue and
muscular pains. The body then needs to rest and recover.
Recovery period
- During the period of rest, the breathing rate continues to be fast for some time.
- This is to provide sufficient oxygen to repay the oxygen debt.
- Lactic acid is removed from the muscles and transported to the liver.
- In the liver, some of the lactic acid is oxidised (using the oxygen that is taken in) to release
energy. This energy is used to convert the remaining lactic acid back into glucose.
- When all the lactic acid has been converted to glucose, the oxygen debt is repaid.
- Glucose is then transported back to the muscles and the body is ready.
50
Human Gas Exchange System
Nose
-
Air enters the body through the two external nostrils.
The walls of the nostrils bear a fringe of hairs. The nostrils lead into two nasal passages
which are lined with a moist mucous membrane.
Breathing through the nose has the following advantages:
- Dust and foreign particles, including bacteria in the air, are trapped by the hairs in
the nostrils as well as by the mucus on the mucous membrane.
- As air p[asses through the nasal passages, it is warmed and moistened.
- Harmful chemical may be detected by small sensory cells in the mucous membrane.
Nose to trachea
- Air passes into the pharynx from the nose. From the pharynx, air passes into the larynx and
then into the trachea through the glottis.
51
Trachea
- The trachea (windpipe) is supported by C-shaped rings of cartilage. The cartilage keeps the
lumen of the trachea open. The membrane next to the lumen is the epithelium.
- The epithelium consists of:
- Gland cells
- Secrete mucus to trap dust particles and bacteria
- Ciliated cells
- Contain hair-like structures called cilia on their surfaces. The cilia sweep the
dust-trapped mucus up the trachea.
Bronchi and bronchioles
- The trachea divides into two bronchi. Each bronchi carries air into the lung. The bronchi are
similar in structure to the trachea. Each bronchus branches repeatedly, forming numerous
bronchioles. Bronchioles are very fine tubes that end in a cluster of alveoli.
Alveoli
- Gas exchange takes place through the walls of the alveoli. Numerous alveoli are found in the
lungs, providing a very large surface area for gas exchange.
Adaptations of the Lungs
-
The numerous alveoli in the lungs provide a large surface area.
The wall of the alveolus is only one cell thick. This provides a short diffusion distance for
gases, ensuring a faster rate of diffusion.
A thin film of moisture covers the surface of the alveolus. This allows oxygen to dissolve in it.
The walls of the alveoli are richly supplied with blood capillaries. The flow of blood maintains
the concentration gradient of gases.
Gas Exchange in the Alveoli
-
-
-
Gas exchange in the lungs occurs by diffusion. Blood entering the lungs has a lower
concentration of oxygen and a higher concentration of carbon dioxide than the atmospheric
air entering the alveoli in the lungs.
A concentration gradient for oxygen and carbon dioxide is set up between blood and
alveolar air. Oxygen diffuses from the alveolar air into the blood capillaries. Carbon dioxide
diffuses in the opposite direction.
Oxygen and carbon dioxide concentration gradients between the alveolar air and the blood
are maintained by:
- Continuous flow of blood through the blood capillaries.
- Movement of air in and out of the alveoli, caused by breathing.
52
Absorption of oxygen in the lungs
1) One cell thick alveolar wall that separates the blood capillaries from the alveolar air is
permeable to oxygen and carbon dioxide.
2) Since the alveolar air contains a higher concentration of oxygen than the blood, oxygen
dissolves in the moisture lining the alveolar walls and then diffuses into the blood capillaries.
3) Oxygen combines with the haemoglobin in red blood cells to form oxyhaemoglobin. This
reaction is reversible. The direction in which the reaction takes place depends on the
amount of oxygen in the surroundings.
4) In the lungs where the oxygen concentration is high, oxygen combines with haemoglobin to
form oxyhaemoglobin.
5) When the blood passes through oxygen-poor tissues, the oxyhaemoglobin releases oxygen,
which will then diffuse through the walls of the blood capillaries into the cells of the tissues.
Removal of carbon dioxide from the lungs
1) Tissue cells produce a large amount of carbon dioxide as a result of aerobic respiration.
2) As blood passes through these tissues via blood capillaries, carbon dioxide diffuses into the
blood and enters the red blood cells.
3) The carbon dioxide then react with water in the red blood cells to form carbonic acid. This
reaction is catalysed by the enzyme​ carbonic anhydrase​ which is present in red blood cells.
4) The carbonic acid is then converted into hydrogencarbonate ions which diffuse out of the
red blood cells. Hence, most of the carbon dioxide is carried as hydrogencarbonate ions in
the blood plasma. A small amount of carbon dioxide is also carried and dissolved in the red
blood cells.
5) In the lungs, hydrogencarbonate ions diffuse back into the red blood cells where they are
converted into carbonic acid, and then into water and carbon dioxide.
6) The carbon dioxide then diffuses out of the blood capillaries and into the alveoli, where it is
expelled when the body breathes out.
Breathing Mechanisms in Humans
-
Inspiration​ or ​inhalation ​is the taking in of air.
Expiration ​or ​exhalation​ is the giving out of air.
Thoracic cavity (chest cavity)
- The chest wall is supported by the ribs. The ribs are attached dorsally to the ​vertebral
column​ in such a way that they can move up and down.
- The ribs are attached ventrally to the ​sternum​. Humans have 12 pairs of ribs but only the
first 10 pairs are attached to the sternum. The remaining pairs ar free ribs that are not
attached to the sternum.
- Two sets of muscles, the ​external ​and ​internal intercostal muscles, ​can be found between
the ribs. They are antagonistic muscles.
- The thorax is separated from the ​abdomen​ by a dome-shaped sheet called the ​diaphragm​.
The diaphragm is made of muscle and elastic tissue. When the diaphragm muscles contract,
the diaphragm flattens downwards, and when they relax, the diaphragm arches upwards
again. The intercostal muscles and the diaphragm change the volume of the thoracic cavity.
53
Inspiration
1) Diaphragm muscle contracts and diaphragm flattens.
2) External intercostal muscles contract, while internal intercostal muscles relax.
3) Ribs move upwards and outwards. Sternum moves up and forward.
4) Volume of thoracic cavity increases.
5) Lungs expand and air pressure inside decreases as volume increases.
6) Atmospheric pressure is higher than the pressure within the lungs. This forces atmospheric
air into the lungs.
Path of air into lungs
Nostrils → Nasal Passages → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
Expiration
1) Diaphragm muscle relaxes and diaphragm arches upwards.
2) Internal intercostal muscles contract while external intercostal muscles relax.
3) Ribs move downwards and inwards. Sternum moves down to its original position.
4) Volume of thoracic cavity decreases.
5) Lungs are compressed and air pressure inside increases as volume decreases.
6) Pressure within the lungs is higher than atmospheric pressure. Air is forced out of the lungs
to the exterior environment.
Inspired vs expired air
Component
Inspired air
Expired air
Oxygen
About 21%
About 16.4%
Carbon dioxide
About 0.03%
About 4.0%
Nitrogen
About 78.0%
About 78.0%
Water vapour
Variable (rarely saturated)
Saturated
Temperature
Variable
About body temperature (37℃)
Dust particles
Variable but usually present
Little
Breathing stimulus
- High concentration of carbon dioxide in the blood.
- No lack of oxygen.
54
Effects of Tobacco Smoke on Human Health
Chemical
Properties of chemical
Effects on body
Nicotine
Addictive drug that causes the
release of the hormone adrenaline.
Increases heartbeat rate and blood
pressure.
Makes blood clot easily.
Increases risk of blood clots in the
arteries, which leads to increased
risk of coronary heart disease.
Combines with haemoglobin to
form carboxyhemoglobin.
Reduces ability of blood to carry
oxygen.
Increases the rate of fatty deposits
on the inner arterial wall, which
leads to increased risk of coronary
heart disease.
Narrows the lumen of arteries and
leads to increase in blood pressure.
Causes uncontrolled cell division.
Increases risk of cancer in lungs.
Paralyses cilia lining the air
passages.
Dust particles trapped in the
mucus ;ining the air passages
cannot be removed, increasing
risks of chronic bronchitis and
emphysema.
Paralyse cilia lining the air
passages.
Dust particles trapped in the
mucus lining the air passages
cannot be removed, increasing
risks of chronic bronchitis and
emphysema.
Carbon Monoxide
Tar
Irritants
55
Chronic bronchitis
- Prolonged exposure to irritant particles that are found in tobacco smoke may cause chronic
bronchitis.
- The epithelium lining of the air passages become inflated.
- Excessive mucus is secreted by the epithelium.
- The cilia on the epithelium are paralysed. Mucus and dust particles cannot be removed.
- The air passages become blocked, making breathing difficult.
- Persistent coughing to clear air passages, in order to breathe. This increases the risk of
getting lung infections.
Emphysema
- Persistent and violent coughing due to bronchitis may lead to emphysema.
- Partition walls between the alveoli break down due to persistent and violent coughing.
- Decreased surface area for gaseous exchange.
- Lungs lose their elasticity and become inflated with air.
- Breathing becomes difficult. Wheezing and severe breathlessness result.
Lung cancer
- Risk of lung cancer increases when a person smokes tobacco.
- Cancer is the uncontrolled division of cells producing outgrowths or lumps of tissues. Apart
from lung cancer, smoking also increases the risk of cancers of the mouth, throat, pancreas,
kidneys and urinary bladder.
56
Chapter 10: Excretion in Humans
Excretion
-
Excretion is the removal of toxic materials and the waste products of metabolism from
organisms.
NOTE: Excretion ≠ Egestion
- Egestion is the elimination of undigested material from the alimentary canal.
Need for excretion
- Chemical reactions take place in living cells to keep the organism alive. The sum of all
chemical reactions in the body is called metabolism. Metabolism consists of catabolic
reactions and anabolic reactions.
- Catabolic reactions break up complex molecules into simpler molecules.
- Hydrolysis
- Digestion in the alimentary canal
- Respiration
- Glucose broken down to release energy, producing waste and carbon
dioxide.
- Deamination
- Excess amino acids are deaminated in the liver to produce urea.
- Anabolic reactions consist of chemical reactions which build up simpler molecules into
complex molecules.
- Condensation reaction
- Synthesis of proteins from amino acids.
- Photosynthesis
- Carbon dioxide reduced to glucose in the presence of water and sunlight.
- Metabolic reactions produce waste products, which can be harmful and prevent the
maintenance of a steady state in the body if allowed to accumulate.
Excretory waste products in the human body
Excretory products
Carbon dioxide
Excess minerals and salts
Nitrogenous waste products:
- Urea
- Uric acid
- Creatinine
Excretory organs
Excreted as
Lungs
Gas in expired air
Kidneys
Constituent of urine
Skin
Constituent of sweat, but only
in small quantities for
nitrogenous waste products.
Kidneys
Main constituent of urine
Lungs
Water vapour in air
Liver
Constituent of faeces
Excess water
Bile pigments
57
Human Urinary System
Kidneys
- Contain kidney tubules which remove urea, excess water and mineral salts from blood to
form urine.
- Responsible for osmoregulation - the process of keeping the water potential of the body
fluids constant.
Ureter
- Tube that connects the kidney to the urinary bladder. Urine flows from the kidneys to the
bladder through the ureter.
Bladder
- A muscular bag which stores urine.
Urethra
- A muscular tube through which urine flows from the bladder to the exterior.
58
Kidney
Cortex
- The outer dark red region.
Nephron
- Responsible for urine formation.
Medulla
- Inner pale red regions consisting of renal pyramids.
Renal pelvis
- Enlarged portion of the ureter inside the kidneys.
Renal pyramid
- Conical structures consisting of nephrons.
59
Nephrons
Bowman’s capsule
- Each nephron begins in the cortex as a cup-like structure.
Proximal convoluted tubule
- A short convoluted tubule which straightens out as it passes into the medulla.
Loop of Henlé
- In the medulla, the tubule extends into the renal pyramid and makes a u-turn back into the
cortex, called the loop of Henlé.
Distal convoluted tubule.
- When the tubule enters the cortex again, it begins convoluted again, known as the distal
convoluted tubule.
Collecting duct
- Duct that runs straight through the medulla and opens into the renal pelvis.
Path of blood in nephron
1) Blood enters the kidney by the renal artery, which branches out into arterioles.
2) Each arteriole further branches into a mass of blood capillaries in the Bowman’s capsule.
This mass of blood capillaries is called the glomerulus. The bowman’s capsule with its
glomerulus is called the renal corpuscle of Malpighian corpuscle.
3) Blood leaving the glomerulus enters blood capillaries surrounding the nephron.
4) These blood capillaries then unite to form venules, which join to form a branch of the renal
vein.
60
Urine Formation
Ultrafiltration
- Ultrafiltration is the removal of small molecules from the blood in the glomerulus and the
formation of the glomerular filtrate in the Bowman’s capsule.
- As blood flows through the glomerulus, ultrafiltration of plasma occurs through the
glomerular capillaries into the Bowman’s capsule.
- It is a non-selective process. Both useful substances and waste products are filtered into the
Bowman’s capsule.
1) Blood enters the kidney by the renal artery. The renal artery splits into numerous arterioles,
each leading into a nephron. The arteriole splits into a network of capillaries known as
glomerulus.
2) Most of the blood plasma is forced out of the glomerular blood capillaries into the bowman’s
capsule. The lumen of the afferent arteriole that brings blood into the glomerulus is wider
than the efferent arteriole which brings blood away. This creates high blood pressure in the
glomerulus.
a) Blood can enter the glomerulus more readily through the wider afferent arteriole
than it can leave through the narrower efferent arteriole. As a result, blood dams up
in the glomerulus, creating a high blood pressure.
b) This pressure forces blood plasma out of the glomerular blood capillaries into the
Bowman’s capsule along the entire length of the glomerulus.
3) The basement membrane that wraps around the glomerular blood capillaries has very small
pores that only allow water and very small molecules to pass through.
4) Blood plasma that is forced out contains water and small molecules and it forms the filtrate
in the Bowman’s capsule.
5) Blood cells, platelets and large molecules such as blood proteins and fats remain in the
glomerulus.
Selective reabsorption
- Selective reabsorption is the transport of useful substances (glucose, vitamins, amino acids
and water) from the glomerular filtrate to the bloodstream. These useful solutes are
reabsorbed back by active transport and diffusion. Water is reabsorbed back by osmosis.
- Selective reabsorption occurs at the proximal convoluted tubule, loop of Henlé, distal
convoluted tubule and collecting duct.
1) Selective reabsorption of most of the useful substances is completed at proximal convoluted
tubule. In a healthy person, ​all ​glucose, amino acids and vitamins are absorbed through the
walls of the nephron into the surrounding capillaries. Most of the sodium ions, chloride ions
and water are reabsorbed. The active transport of sodium and other ions into the blood
increases the water potential in the nephron, causing water to leave the nephron into the
capillaries by osmosis.
2) Some water is reabsorbed at the ​loop of Henlé
3) Smaller concentration of sodium ions, chloride ions are reabsorbed at the ​distal convoluted
tubule.
4) Remaining water as required by the body is reabsorbed at the ​collecting duct​.
61
Composition of urine
-
-
Water
Salts (mainly
sodium chloride)
Urea
Nitrogenous
substances
Total
96%
1.8%
2%
0.2%
100%
Composition of urine will depend on:
- A protein rich diet will result in more urea being present in the urine. Urea is formed
when excess amino acids are deaminated in liver.
- Taking in more liquids or water-rich food increases the water potential of the blood.
Larger volume of urine is excreted.
- Taking in more salty food will result in the excess salts being excreted in the urine.
- Glucose can appear in the urine after taking in large concentration of sugar-rich
food.
A patient with ​diabetes mellitus​ excretes large amount of glucose in the urine.
- The diabetic patient is unable to convert excess glucose into glycogen in the body.
Hence, there is high concentration of glucose in the blood. The glucose is filtered off
in the glomerular filtrate. Since the nephrons are unable to reabsorb all the glucose
fast enough, a lot of glucose passes out in the urine.
62
Osmoregulation
-
Osmoregulation ​is the maintenance of a correct balance between water and dissolved
solute in the blood to maintain ​constant water potential​ in the body.
The volume of water in the blood is controlled by the ​anti-diuretic hormone (ADH)​. ADH is
produced by the hypothalamus and released by the ​pituitary gland​. The target organ of ADH
is the ​kidney​.
Negative feedback
mechanism
Osmoregulation
Stimulus
Increase in water potential in
blood.
- Large volume of water
being ingested
- Little sweating
- Low salt intake
Receptor
Osmoreceptors in the hypothalamus detects the
increase/decrease in water potential and sends signal to the
hypothalamus.
Control centre
Effector
Norm
Decrease in water potential in
blood.
- Small volume of water
being ingested
- Increase in sweat secretion
- Large concentration of
salts ingested
Hypothalamus in the brain sends signals to the pituitary glands.
Hypothalamus produces less
ADH.
Hypothalamus produces more
ADH.
Pituitary gland releases less
ADH.
Pituitary gland releases more
ADH.
Walls of collecting duct
become less permeable, less
water is reabsorbed.
Walls of collecting duct
become more permeable,
more water is reabsorbed.
More urine produced, urine
becomes less concentrated.
Less urine produced, urine
becomes more concentrated.
Water potential decreases
back to normal.
Water potential increases back
to normal.
A negative feedback is sent to
the hypothalamus via the
osmoreceptors to prevent
further corrective actions.
A negative feedback is sent to
hypothalamus via the
osmoreceptors to prevent
further corrective actions.
63
Kidney Failure
-
Kidneys are important organs as they are the major excretory organs and osmoregulators in
the body. Common causes of kidney failure include:
- High blood pressure
- Diabetes
- Alcohol abuse
- Accidents or infections to the kidney.
Dialysis
1) Blood is drawn from the ​vein ​in patient’s arm and is allowed to be pumped through in
dialysis machine.
2) The tubing is ​bathed ​in specially controlled dialysis fluid with no waste products. The walls of
tubing are​ partially permeable​.
3) Small molecules such as urea and other metabolic waste products ​diffuse out​ of the tubing
into the dialysis fluid. Blood cells, platelets and other large molecules remain in the tubing.
4) Blood is then returned to the vein in the patient’s arm.
Features of a dialysis machine
- Tubing is narrow, long and coiled
- To ​increase surface area to volume ratio ​and to speed up the rate of diffusion.
- Direction of blood flow is opposite to the flow of dialysis machine
- To maintain the concentration gradient for the removal of waste products.
- Dialysis fluid contains the same concentration of essential substances as blood
- To ensure essential substances will not diffuse out of the blood to the dialysis fluid.
- Dialysis fluid does not contain metabolic waste products
- Sets up suitable concentration gradient for waste products to diffuse out of the
tubing into the dialysis fluid.
- Maintains the correct solute concentration and water potential of blood.
64
Chapter 11: Homeostasis
-
Homeostasis is the maintenance of a constant internal environment.
- Internal Environment refers to conditions within the body of the organism.
Homeostasis ensures that the composition of the body fluids is kept within narrow limits.
Homeostasis allows an organism to be independent from changes in the external
environment.
Importance of constant body temperature
- Enzymes in our bodies can only work within a certain range of temperature.
- Changes in body temperature may result in enzyme inactivation or denaturation.
Importance of constant pH and water potential in blood
- Drastic change in pH of tissue fluid will affect the enzyme reactions in cells. Drastic changes
in water potential will also affect our cells.
Homeostasis involves negative feedback
- In homeostatic control, the body reacts to bring about an opposite effect to the changes
detected. This is the negative feedback process.
- In a negative feedback control loop, there must be:
- a norm or set-point to be maintained
- a stimulus, which is a change in the internal environment
- a receptor that can detect the stimulus and sends signal to the control center
- a corrective mechanism, which brings about the reverse effect of the stimulus
- a feedback to the receptor when the set-point is reached. This causes the corrective
mechanism to stop.
Examples of Homeostasis in Humans
Regulation of blood glucose concentration
- Cellular respiration provides cells with energy to perform their vital activities. A drastic
change in the blood glucose concentration can thus be dangerous.
65
-
When blood glucose rises above the normal level,
1) Stimulus:
Blood glucose concentration​ ​rises above normal
2) Receptor:
Islets of Langerhans in the pancreas stimulated
3) Corrective mechanism:
Islets of Langerhans secrete insulin into the bloodstream
Blood transports insulin to the liver and muscles
- Insulin increases the permeability of the cell surface membrane to glucose. Glucose
is absorbed more quickly by the cells.
- Insulin causes the liver and muscles to convert excess glucose to glycogen. Glycogen
is stored in the liver and muscles.
4) Feedback
Blood glucose concentration decreases. This provides a feedback to the receptor to
reduce insulin production.
-
When blood glucose falls below normal level,
1) Stimulus:
Blood glucose concentration falls below normal
2) Receptor:
Islets of Langerhans in the pancreas are stimulated
3) Corrective mechanism:
Islets of Langerhans secrete glucagon into the bloodstream.
Blood transports the glucagon to the liver and muscles.
- Glucagon causes the conversion of stored glycogen back to glucose.
From the liver, glucose enters the bloodstream.
4) Feedback
Blood glucose concentration increases. This provides a feedback to the receptor to
decrease glucagon production.
Regulation of water potential in blood
Stimulus
Water Potential of blood decreases
below the norm
Receptor
Corrective
Mechanism
Hypothalamus stimulated
-
Feedback
Water Potential of blood decreases
above the norm
More ADH released by pituitary
gland into the bloodstream
More ADH transported to the
kidneys
Cells in the walls of the
collecting ducts become more
permeable to water
More water reabsorbed into
the bloodstream
Less water excreted
Urine is more concentrated
Less urine produced
Water potential of blood increases
66
-
-
Less ADH released by pituitary
gland into the bloodstream
Less ADH transported to the
kidneys
Cells in the walls of the
collecting ducts become less
permeable to water
Less water reabsorbed into
the bloodstream
More water excreted
Urine is more diluted
More urine produced
Water potential of blood decreases
Structure of the Human Skin
-
Skin is composed of two parts, an outer part called the epidermis, and an inner thicker part,
the dermis. The upper part of the dermis is thrown into ridges or papillae. The dermis has a
rich supply of nerves and blood capillaries
Structures Involved In Temperature Regulation
Blood vessels in the dermis layer
- Body temperature can be regulated through contraction and dilation or the arterioles
- Dilation of the arterioles is called vasodilation.
- Constriction of arterioles is called vasoconstriction
Hair
-
The Malpighian layer of the epidermis sinks into the dermis, forming a hollow tube called the
hair follicle. Each hair grows inside the hair follicle.
At the base of the hair follicle is a mass of tissue called the hair papilla, which contains blood
capillaries and nerves.
Hair erector muscles are attached to the hair follicles. When these muscles contract, the
hairs “stand on their ends” and the skin around the hair is raised, producing “goose pimples”
in humans.
Sweat glands
- Each sweat gland is a coiled tube formed by a downgrowth of the epidermis. It forms a tight
knot in the dermis and is surrounded by many blood capillaries.
- Secreted sweat flows through a sweat duct to a sweat pore that opens at the skin surface.
- Secreted sweat is mainly made up of water, dissolved salts and small amounts of urea.
- Sweat is secreted continuously. The amount of sweat secreted varies on the external and
internal environmental conditions.
Sensory receptors
- Sensory receptors are structure in the body that detect changes in the environment.
- They enable us to sense pain, pressure and temperature changes.
- Receptors that detect temperature changes are called thermoreceptors.
Subcutaneous fat
- Beneath the dermis are several layers of adipose cells where fat is stored. The fat in these
cells also serves as an insulating layer, reducing heat loss.
67
Temperature Regulation
-
Heat is produced within our bodies as a result of metabolic activities such as cellular
respiration. Heat is distributed to the rest of the body via the bloodstream.
Methods of gaining heat
- vigorous muscular exercise
- consumption of hot food
- being in warm environment
Methods of losing heat
- radiation, convection and conduction
- evaporation of water in sweat
- excretion of faeces or urine
- exhalation of air
- dilation and constriction of shunt vessels
Regulation of body temperature
- Hypothalamus in our brains monitors and regulates body temperature
- Hypothalamus receives information about temperature changes from two sources:
- thermoreceptors in our skin which detects temperature from the
environment
- Thermoreceptors in the hypothalamus which detect temperature of the
blood
When temperature rises,
- Dilation of arterioles in our skin and constriction of shunt vessels allow more blood to flow
through blood capillaries in your skin. This allows more heat to be lost through our skin by
radiation, convection and conduction.
- Sweat glands become more active, resulting in increased production of sweat. As more
water in sweat evaporates from the surface of our skin, more latent heat of vaporisation is
lost from our bodies.
- Decreased metabolic rate, to reduce the amount of heat released within our bodies.
When temperature falls,
- Constriction of our skin arterioles and dilation of shunt vessels so that less blood flows
through blood capillaries in our skins. Tis reduces the heat lost by radiation, convection and
conduction.
- Sweat glands become less active, resulting in decreased production of sweat. As less water
in the sweat evaporates from the surface of our skin, less latent heat of vaporisation is lost
from our body.
- Increased metabolic rate, to increase the amount of heat released within our bodies.
- When the above reactions ar4e not sufficient to prevent a drop in body temperature,
shivering, a reflex contraction of our body muscles, occurs. This spasmodic contraction of the
skeletal muscles increases the amount of heat released and helps to raise our body
temperatures to normal.
68
Chapter 12: The Nervous System
Sensitivity
-
A living organism is able to react to changes in its surroundings. Any change in the
environment that causes an organism to react is called a stimulus. An organism’s reaction to
a stimulus is called a response.
An organism usually responds to a stimulus by moving in a way that benefits itself.
- Plants need light for photosynthesis, so they grow towards the light. This response occurs
over time.
- Euglena, a single-celled organism, makes food by photosynthesis. Euglena swims towards
light.
- Some organisms, such as cockroaches, move away from the light. They hide in dark areas
during the day.
Role of nervous system in humans
- The nervous system consists of a well-developed brain, spinal cord, spinal nerves and highly
specialised sense organs.
- Sense organs:
- help the body to adjust rapidly to any changes in the environment
- enable the various parts of the body to coordinate with one another quickly
- Activities that not controlled consciously are called involuntary actions
- Activities that are controlled consciously are voluntary actions.
Human Nervous System
Components of the human nervous system
- Nervous system is made up of:
- the central nervous system consisting of the brain and the spinal cord
- the peripheral nervous system consisting of the cranial nerves, the spinal nerves and
the sense organs.
- Sense organs receive stimuli and are also called receptors. They inform the
central nervous system of any changes in the surroundings by producing
electrical messages called nerve impulses. These nerve impulses are carried
or transmitted to the central nervous system by nerves.
- Nerve impulses are transmitted within a fraction of a second.
- Muscles are called effectors.
69
Nervous tissue
- The nervous system is made up of nervous tissue. Nervous tissue consists of nerve cells
called neurons.
1) Sensory neuron (receptor neuron)
Transmits nerve impulses from the sense organs or receptors to the central nervous system.
2) Relay neuron (intermediate neuron)
Transmits nerve impulses from the sensory neuron to the motor neuron. They are found
within the central nervous system.
3) Motor neuron (effector neuron)
Transmits nerve impulses from the central nervous system to the effectors.
Structure of a Motor Neuron
Cell body
- The cell body contains a nucleus, cytoplasm, cell surface membrane and organelles. The cell
body of a motor neuron is irregular in shape.
Dendron
- The nerve fibres that transmit nerve impulses towards the cell body are called dendrons. A
motor neuron has many dendrites. The dendrites of a dendron of a motor neuron receive
nerve impulses from other neurons.
Axon
-
The nerve fibres that transmit nerve impulses away from the cell body are called axons.
Axons in motor neurons are usually long.
Myelin sheath
- The layer of fatty substances enclosing many nerve fibres is the myelin sheath. It insulates
the axon, just as a rubber sheath would insulate an electricity-conducting wire.
Node of Ranvier
- The regions where the myelin sheath is absent are called nodes of Ranvier. Nerve impulses
cannot be transmitted through the myelin sheath, so they “jump” from one node to the
next. In this way, they help to speed up the transmission of impulses along the nerve fibre.
70
Axon terminals
- The axon terminals of a motor neuron transmit nerve impulses to the effector.
Motor end plate
- The junction between the axon terminal and the muscle fibre is the motor end plate. Nerve
impulses are transmitted across the motor end plate by chemicals which stimulate the
muscles.
Structure of a Sensory Neuron
- The sensory neuron has a circular cell body. It has only one long dendron and a short axon.
Synapses
- A synapse is a junction between two neurons, or a junction between a neuron and an
effector such as a muscle or a gland.
- At a synapse, impulses are transmitted from the axon of one neuron to the dendron of
another neuron across a tiny space.
- Nerve impulses are transmitted across the synapse by chemicals called neurotransmitters
released by the neuron.
Nerves
- A nerve is a bundle of nerve fibres enclosed in a sheath of connective tissue.
- Nerves may emerge from the brain (cranial nerve) or spinal cord (spinal nerve). They may
contain:
- Sensory nerve fibres only. Such nerve impulses from sense organs.
- Motor nerve fibres only. Such nerves conduct nerve impulses to effectors.
- Both sensory and motor nerve fibres. Spinal nerves contain mixed fibres.
71
Brain, spinal cord and spinal nerves
- The spinal cord passes through the vertebral column (backbone) which protects it. The brain
and the spinal cord consist of two distinct regions, grey matter and white matter. A central
canal is present in both the brain (inside the white matter) and the spinal cord (inside the
grey matter)
Grey matter
- Grey matter consists mainly of the cell bodies of neurons. It forms the outer layers of the
brain and the central parts of the spinal cord.
White matter
- The white matter consists mainly of nerve fibres. It forms the central parts of the brain, and
the outer layers of the spinal cord.
Central canal
- The central canal contains a fluid called cerebrospinal fluid that brings nutrients to the spinal
cord.
Dorsal root
- The dorsal root joins the dorsal part (towards the back) of the spinal cord. It contains only
the nerve fibres of sensory neurons that transmit impulses to the spinal cord.
Dorsal root ganglion
- The dorsal root ganglion is a small swelling in the dorsal root that contains cell bodies of
sensory neurons.
72
Ventral root
- Ventral root joins the ventral part (towards the front) of the spinal cord. It contains only the
nerve fibres of motor neurons that transmit impulses from the spinal cord.
Spinal nerve
- The dorsal root and ventral root join to form the spinal nerve. The spinal nerve contains both
sensory and motor neurons.
Processing of Information by the Nervous System
Sensation
- When touching a piece of ice, a temperature receptor in our skin is stimulated. Impulses are
produced. Impulses are transmitted to the forebrain. The brain interprets the impulses, and
a sensation of coldness is felt.
- Receptor in skin → sensory neuron →relay neuron in spinal cord → forebrain
Voluntary action
- A voluntary action is a deliberate action.
- Impulses are produced by a forebrain. Impulses are transmitted by a relay neuron from the
forebrain, down the white matter of the spinal cord, and then into the grey matter. In the
grey matter, impulses are transmitted to the motor neuron which transmits the impulses to
the effector muscles. The muscle contracts, and an action is carried out.
- Forebrain → relay neuron in spinal cord → motor neuron → effector
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Reflex Action
-
-
A reflex action is an immediate response to a specific stimulus without conscious control..
- It is an involuntary action.
The spinal cord and the brain are reflex centres. Reflex actions may be classified as:
- Cranial reflexes --- controlled by the brain and usually occur in the head region.
- Spinal reflexes ---- controlled by the spinal cord.
When your hand touches a hot object,
1) Receptors in skin
The heat on the object stimulates the nerve endings (receptors) in your skin.
Impulses are produced.
2) Sensory neuron
The nerve impulses travel along the sensory neuron to your spinal cord.
3) Spinal cord
In your spinal cord, the nerve impulses are transmitted first across a synapse to the
relay neuron, and then across another synapse to the motor neuron.
4) Motor neuron
The motor neuron transmits the impulses from the spinal cord to the effector.
5) Effector muscle contracts
Your biceps muscle (effector) then contracts and causes your hand to withdraw
suddenly.
Reflex Arc
-
A reflex arc is the shortest path way by which nerve impulses travel from the receptor to the
effector in a reflex action.
A reflex arc consists of:
1) A receptor or sense organ
2) A sensory neuron
3) Central nervous system (brain or spinal cord)
4) A motor neuron
5) An effector (muscle or gland)
74
Chapter 13: The Human Eye
-
Each eyeball lies in a hollow in the skull called the orbit.
Each eyeball is attached to the skull by rectus muscles. The rectus muscles control eye
movement.
The front part of the eyeball is covered by eyelids.
External Structure of the Eye
Iris
-
A circular sheet of muscles. It contains a pigment which gives the eye its colour. The amount
of light entering the eye is controlled by the two sets of involuntary muscles in the iris, the
circular muscles and the radial muscles.
Conjunctiva
- A thin transparent membrane covering the sclera in front. It is a mucous membrane as it
secretes mucus. This helps to keep the front of the eyeball moist. It is continuous with the
skin of the eyelids.
Sclera
- A tough, white outer covering of the eyeball. It is continuous with the cornea. It protects the
eyeball from mechanical damage.
Eyelids
- They protect the cornea from mechanical damage.
- The eyelids can be partly closed. This is known as squinting. Squinting prevents excessive
lighting from entering the eye and damaging the light-sensitive tissues inside.
- Blinking spreads tears over the cornea and conjunctiva and wipes dust particles off the
cornea.
Tear gland
- The tear gland lies at the corner of the upper eyelid.
- It secretes tears which:
- Wash away dust particles and my pain
- Keep the cornea moist for atmospheric oxygen to dissolve. The dissolved oxygen
diffuses into the cornea.
- Lubricate the conjunctiva, helping to reduce friction when the eyelids move.
Eyelashes
- Eyelashes help to shield the eye from dust particles.
Pupil
-
A hole in the centre of the iris. The pupil allows light to enter the eye.
75
Internal Structure of the Eye
Ciliary body
- A thickened region at the front end of the choroid.
- Contains ciliary muscles which control the curvature or thickness of the lens.
Suspensory ligament
- A connective tissue that attaches the edge of the lens to the ciliary body
Cornea
- A dome-shaped transparent layer continuous with the sclera or white of the eye. It refracts
or bends light rays into the eye.
- The cornea causes most of the refraction of light that occurs in the eye.
Aqueous chamber
- The space between the lens and the cornea. It is filled with aqueous humour, a transparent,
watery fluid. Aqueous humour keeps the front of the eyeball firm and helps to refract light
into the pupil.
Lens
-
A transparent, circular and biconvex structure. It is elastic and changes its shape or thickness
in order to focus light into the retina.
76
Retina
- The innermost layer of the eyeball.
- Contains light-sensitive cells or photoreceptors.
- Photoreceptors enable us to see colours in bright light while rods enable us to see in black
and white in dim light.
- The photoreceptors are connected to the nerve endings from the optic nerve.
Blind spot
- The region where the optic nerve leaves the eye, It does not contain any rods or cones,
therefore it is not sensitive to light.
Optic nerve
- A nerve that transmits nerve impulses to the brain when the photoreceptors in the retina
are stimulated.
Fovea (Yellow spot)
- A small yellow depression in the retina.
- Situated directly behind the lens.
- This is where images are normally focused.
The fovea contains the greatest concentration of cones, but no rods.
- The fovea enables a person to have detailed colour vision in bright light.
Vitreous chamber
- The space behind the lens. It is filled with vitreous humour, a transparent, jelly-like
substance. Vitreous humour keeps the eyeball firm and helps to refract light onto the retina.
Choroid
- The middle layer of the eyeball (between the sclera and the retina). It has two functions:
- It is pigmented back to prevent internal reflection of light.
- It contains blood vessels that bring oxygen and nutrients to the eyeball and remove
metabolic waste products.
77
Photoreceptors in the Retina
-
Photoreceptors in the retina consist of rods and cones. The photoreceptors are connected to
the nerve endings from the optic nerve.
Functions of cones
- Cones enable us to see colours in bright light.
- There are three types of cones, red, green, and blue that allow us to see a wide variety of
colours by containing a different pigment which absorbs light of different wavelengths.
- Cones do not work well in dim light.
Functions of rods
- Rods are more sensitive to light than cones. They enable us to see in dim light, but only in
black and white.
- Rods are sensitive to light of low intensity as they contain pigment called visual purple.
When the eye is exposed to bright light, all the visual purple is bleached. Visual purple must
be reformed for a person to see in the dark.
- Formation of visual purple requires vitamin A.
Function of the Iris
-
-
-
The size of the pupil determines how much light enters the eye.
Size of the pupil is controlled by two sets of involuntary muscles in the iris:
- Circular muscles are arranged in a circle around the pupil.
- Radial muscles are arranged radially.
They are called antagonistic muscles as when one set contracts, the other set relaxes.
In bright light
1) Circular muscles of the iris contract
2) Radial muscles of the iris relax
3) The pupil becomes smaller or constricts. This reduces the amount of light entering
the eye.
In dim light
1) Radial muscles of the iris contract
2) Circular muscles of the iris relax
3) The pupil enlarges or dilates. This increases the amount of light entering the eye.
Pupil reflex
- Reflex action where the pupil changes size as a result of changes in light intensity.
- The pupil becomes larger when the surrounding light intensity is low, and smaller when the
light intensity is high.
- Stimulus (change in light intensity) → receptor (retina) → sensory neuron in optic nerve →
brain → motor neuron → effector (iris)
78
How our eyes see
-
-
When light falls on an object, rays of light are reflected from the object. Some of these
reflected rays fall on the eye.
1) Light rays are refracted through the cornea and the aqueous humour onto the lens
2) The lens causes further refraction and the rays are brought to a focus on the retina.
3) The image on the retina stimulates either the rods or the cones, depending on the
intensity of the light. The image formed on the retina is:
- Upside down
- Laterally inverted
- Diminished (smaller in size than the actual object)
Nerve impulses are produced when light falls on the rod and cones. These impulses are
transmitted via the optic nerve to the brain. The brain interprets these impulses so that we
can see the object the right way up, front to back, and the right size.
Focusing
- Focusing or accomodation is the adjustment of the lens of the eye so that clear images of
objects at different distances are formed on the retina.
Focusing on a distant object
1) Ciliary muscles relax, pulling on the suspensory ligaments.
2) Suspensory ligaments become taut, pulling on the edge of the lens.
3) Lens becomes thinner and less convex, increasing its focal length.
4) Light rays from the distant object are sharply focused on the retina.
5) Photoreceptors are stimulated.
6) Nerve impulses produced are transmitted by the optic nerve to the brain. The brain
interprets the impulses and the person sees the distant object.
Focusing on a close object
1) Ciliary muscles contract, relaxing their pull on the suspensory ligaments.
2) Suspensory ligaments slacken, relaxing their pull on the lens.
3) The lens, being elastic, becomes thicker and more convex, decreasing its focal length.
4) Light rays from the near object are sharply focused on the retina.
5) Photoreceptors are stimulated.
6) Nerve impulses produced are transmitted by the optic nerve to the brain. The brain
interprets the impulses and the person sees the near object.
79
Chapter 14: Hormones
-
-
A hormone is a chemical substance produced in minute quantities by an endocrine gland. It
is transported in the bloodstream to target organ(s) where it exerts its effect(s). After
hormones have performed their functions, they are eventually destroyed by the liver.
Hormones can influence growth, development and activity of an organism. They are
chemical messengers that help the various parts of the body to respond, develop and work
together smoothly.
Hormone production
- Hormones are produced by glands.
- Glands that have a duct or tube for carrying away their secretions are called exocrine
glands.
- Hormones are produced by ductless glands. They do not have a duct to carry away
its secretion, and the hormone is instead secreted directly into the bloodstream.
They are also called endocrine glands.
- Glands such as the adrenal glands are purely endocrine glands, producing only hormones.
Other glands such as the pancreas produce both hormones and other secretions.
- The pancreas produces pancreatic juice, which is carried by the pancreatic duct to
duodenum.
- The pancreas also contains special groups of cells known as the islets of Langerhans
which secrete the hormones insulin and glucagon into the bloodstream.
- Hormone production of some endocrine glands is controlled by the nervous system. Other
endocrine glands are regulated by certain chemical substances. These chemical substances
may be hormones from other endocrine glands.
Endocrine Glands and Their Hormonal Secretions
Pituitary gland
- Plays an important role as a “controller”. It secretes a number of hormones, which control
the secretion of hormones of several other endocrine glands. The pituitary gland is often
referred to as the “master gland”. The pituitary gland also secretes antidiuretic hormone.
Hypothalamus
- An endocrine gland that regulates the secretion of some pituitary hormones.
Adrenal gland (medulla)
- The adrenal gland secretes adrenaline. Adrenaline had wide-ranging effects on the body.
Pancreas
- Islets of Langerhans in the pancreas secrete insulin and glucagon.
Testis (in males)
- The testis secretes testosterone.
Ovary (in females)
- The ovary secretes oestrogen and progesterone.
80
Effects of Hormones
Effects of insulin
- Islets of Langerhans in the pancreas increase the secretion of the hormone insulin when the
concentration of blood glucose increases above normal levels. Insulin promotes the
utilisation of glucose by the cells.
Amount of insulin secreted
Effect
Normal
Decreases blood glucose concentration by:
- Increasing the permeability of cell membranes to glucose, which increases
the rate of glucose uptake by the cells.
- Stimulating the liver and muscle cells to convert glucose into glycogen for
storage.
- Increasing oxidation of glucose during tissue respiration.
Lack of secretion
-
-
Over-secretion
-
Glucose cannot be stored or utilised by tissue cells, so blood glucose
concentration rises. Some glucose is subsequently lost in the urine. This
gives rise to a disease called diabetes mellitus.
Since muscle cells have non reserves of glycogen, the body grows weak
and continuously loses weight.
The body oxidises fats instead of glucose to produce energy. This results
in production of poisonous substances called ketones which are excreted
in urine.
Abnormal decrease in blood glucose concentration.
Low blood glucose concentration results in a condition called shock.
Coma and death may follow.
Diabetes Mellitus
- Diabetes mellitus is a diseases in which the body is unable to control its blood glucose
concentration so that it remains within normal limits.
- Type 1 diabetes develops early in a person’s life. It is known as juvenile or
early-onset diabetes. The islets of Langerhans are unable to produce or secrete
sufficient insulin
- Type 2 diabetes occurs later in a person’s life and hence is called late-onset
diabetes. Overweight people are more likely to develop type 2 diabetes. Type 2
diabetes develops when the target cells, such as the muscle cells, do not respond
well to insulin.
Signs of diabetes mellitus
- a persistently high blood glucose level.
- the presence of glucose in the urine after a meal.
- slow or difficult healing of wounds.
Treatment of diabetes mellitus
- Type 1 diabetes patients have to inject insulin directly into their bloodstream. They have to
ensure that they have a supply of sugary food such a glucose sweets, as their blood glucose
can drop too low if they use too much insulin, exercise too much or eat too little.
81
-
Type 2 diabetes patients can control their blood sugar level by carefully regulating the
carbohydrate content in their diet and by exercising. If lifestyle changes fail, type 2 diabetics
may have to take medication and insulin injections.
Effects of glucagon
- Islets of Langerhans increase the secretion of the hormone glucagon when the concentration
of blood glucose decreases below normal levels.
- Glucagon increases blood glucose concentration by stimulating:
- The conversion of glycogen into glucose
- The conversion of fats and amino acids into glucose
Effects of adrenaline
- Stimulates the liver to convert glycogen to glucose so that more glucose is available for
muscle contraction.
- Increases blood glucose level.
- Increases metabolic rate. This means morte energy is released in tissue respiration.
- Increases the rate of heartbeat and causes a rise in blood pressure so that oxygen and
glucose are carried faster to the muscles.
- Increases the rate of blood clotting.
- Constricts arterioles to the gut, thus decreasing digestive activities.
- Constricts arterioles in skin, causing paleness, thus channelling more blood to the muscles.
- Dilate pupils to enhance vision.
- Contracts hair muscles, producing “goosebumps” and causing hair to stand on end.
- Stimuli activate the ​hypothalamus​ in the brain → transmission of impulses down the ​spinal
cord​ → motor neuron transmits impulses to ​adrenal gland​ → adrenal gland secretes
adrenaline​ into the bloodstream → blood transports ​adrenaline to target organs
Comparing Endocrine and Nervous Controls
-
-
The endocrine (hormonal) system, like the nervous system, serves as a means of
coordination within our bodies. In both cases, a stimulus causes the transmission of a
message to a target organ (effector) which carries out the response.
The nervous control differs from the endocrine control in several ways. For example,
nervous control may affect only a particular part of our bodies, and is localised. As hormones
are usually transported around your body by blood, several target organs may be affected by
the same hormone.
Endocrine control
Nervous control
Involves hormones (chemical substances)
Involves nerve impulses (electrical signals)
Hormones are transported by the blood
Impulses are transmitted by neuron
Usually slow responses
Usually quick responses
Responses may be short-lived or long-lived
Responses are short-lives
Always involuntary
May be voluntary or involuntary
May affect more than one target organ
Usually localised
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Chapter 15: Cell Division
Growth
-
Growth is the characteristic of all living things. It is a permanent increase in size. It is
accompanied by cell division and differentiation to form tissues and organs.
Growth in plants is restricted to the tips of shoots and roots, also known as growing points.
In animals, growth occurs throughout the body.
For a multicellular organism to grow normally, its cells must first divide. The cell division that
takes place during growth is called mitosis.
Mitosis
- Mitosis is a form of nuclear division that produces 2 daughter nuclei containing the same
number of chromosomes as the parent nucleus. The daughter nuclei are genetically
identical.
Importance of genetically identical daughter cells
- Dna replication copies all the information stored within the chromosomes. This ensures that
daughter cells produced by mitosis contain all the sections of DNA needed for subsequent
cell division and differentiation.
- A zygote divides to form an embryo. The cells formed as the zygote divides must be
genetically identical for the embryo to develop normally. Errors occuring in dna replication
or mitosis can lead to harmful changes for the genes and affect how the cells functions.
- Changes in gene during dna replication may also cause abnormal proteins to be produced,
which may be rejected or destroyed by the body’s immune system.
- Mistakes made in DNA replication may cause the uncontrolled division of cells, known as
cancer.
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The Cell Cycle
-
The cell cycle is divided into three stages:
- Interphase, or “resting stage”
- Mitosis, which is nuclear division.
- Cell division, which is division of the cytoplasm or cytokinesis.
Interphase
- Most of the cells’ lives are spent here
- Cell gets ready for mitosis by:
- Absorbing nutrients.
- Building up protoplasm.
- Replicating chromatin threads, producing two identical chromatin threads joined at
the centromere, known as sister chromatids.
Prophase
- Chromatin threads condense, coil and shorten to become chromosomes.
- Chromosomes visible under a microscope as X-shaped structures
- In an animal cells, asters, made of microtubules, form around the centrioles.
- Nucleolus and nuclear envelope disappear.
- A spindle forms with spindle fibers extending from one pole of the cell to the other.
Metaphase
- Chromosomes line up along the equatorial plane of the spindle.
- Centromere of each chromosome is attached on both sides to a spindle fibre.
Anaphase
- Each centromere divides.
- Spindle fibres shorten and pull the chromatids apart to opposite poles of the cell.
- Once the chromatids are separated, they are called daughter chromosomes
Telophase
- Spindle fibres break down.
- Nuclear envelope forms around the chromosomes at each pole of the cell.
- Nucleolus reforms in each nucleus and the chromosomes uncoil and lengthen to become
thin chromatin threads.
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Cytokinesis
- In animal cells, cleavage or furrows appear in the the cytoplasm between the two nuclei. The
furrows deepen and two identical daughter cells are produced.
- In plant cells, a cell plate is formed between the two daughter nuclei, dividing the cell into
two.
- Cell plate is formed by the fusion of small fluid-filled vesicles produced by the Golgi
apparatus.
Importance of Mitosis
- Mitosis enables the growth of an organism
- New cells must be produced by mitosis for a multicellular organism to grow.
- Mitosis is needed for the repair of worn-out parts of the body
- Mitosis constantly replaces dead cells and helps to heal from wounds.
- Mitosis allows asexual reproduction to occur
- Mitosis causes shoots and roots to develop in storage organs, which grow into new
daughter plants that are identical to their parent plants.
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Meiosis
-
Meiosis is a form of nuclear division that produces daughter nuclei containing half the
number of chromosomes as the parent nucleus.
Gametes
- Gametes are reproductive cells that contain half the number of chromosomes as the normal
body cell.
- A cell with 2n chromosomes divides to form 4 gametes with n chromosomes.
Prophase I
- Chromatin threads condense, coil and shorten to become chromosomes.
- Homologous chromosomes pair along their whole length (synapsis)
- Each chromosome consists of two chromatids attached at the centromere.
- Chromatids of homologous chromosomes may cross each other at the chiasma.
- Asters form around the centrioles which move apart to opposite poles of the cell. Nuclear
envelope and nucleolus disappear. Spindle fibres form.
Metaphase I
- Pairs of homologous chromosomes arrange themselves along the equatorial plane of the
spindle.
- Two chromosomes of each pair face opposite poles of the cell.
- Each chromosome is attached to a spindle fibre.
Anaphase I
- Homologous chromosomes separate and are pulled to opposite poles of the cell as the
spindle fibres shorten.
Telophase I
- A nuclear envelope forms around the chromosomes at each pole.
- Division of cytoplasm occurs
- Cytoplasm cleaves into two, producing two daughter cells, each with haploid
number of chromosomes.
- Centrioles divide.
Prophase II
- Centrioles move to opposite poles of the cell.
- Nuclear envelope disappears.
- Spindle fibres appear.
Metaphase II
- Chromosomes arrange themselves along the equatorial plane of the spindle.
Anaphase II
- Centromeres divide.
- Sister chromatids separate to become daughter chromosomes, which are pulled towards
opposite poles of the cell.
Telophase II
- Spindle fibres disappear.
- Nuclear envelopes form around the two daughter chromosomes at each pole.
- Nucleolus reforms.
- Cleavage of cytoplasm results in four daughter cells being produced, each with half the
number of chromosomes as the parent cell.
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Importance of Meiosis
- Meiosis produces haploid gametes
- Maintains the normal diploid number of chromosomes in the species when a male
gamete fuses with a female gamete.
- Meiosis results in variations in the gametes produced
- When chromatids of homologous chromosomes cross over, genes are transferred
and results in variations of gametes.
- Variation increase the chances of survival of the species during changes in the
environment.
- Those with favourable genes will survive and pass on their favourable genes to their
offspring via natural selection.
Mitosis vs Meiosis
Mitosis
Meiosis
Daughter cells contain same number of
chromosomes as the parent cell.
Daughter cells contain half the number of
chromosomes as the parent cell.
Pairing of homologous chromosomes does not
occur.
Pairing of homologous chromosomes pair at
Prophase I.
No crossing over of homologous chromosomes.
Crossing over of homologous chromosomes
may occur.
Daughter cells are genetically identical to the
parent cell.
Daughter cells are genetically dissimilar
compared to the parent cell.
Two daughter cells are produced from one
parent cell.
Four daughter cells are produced from one
parent cell.
Involves only one nuclear division.
Involves two nuclear divisions.
Occurs in normal body cells during growth or
repair of body parts.
Occurs in the gonads during gamete formation.
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Chapter 16: Reproduction in Plants
Asexual Reproduction
-
Asexual reproduction is the process resulting in the production of genetically identical
offspring from one parent, without the fusion of gametes.
Advantages of asexual reproduction
- Only one parent required.
- Fusion of gametes is not required.
- All beneficial qualities are passed onto the offspring.
- Faster method of producing offspring as compared with sexual reproduction.
- Since organisms are already in a suitable habitat, they can colonise the area rapidly.
Disadvantages of asexual reproduction
- No genetic variation in the offspring. Hence, species are not well adapted to changes in the
environment.
Sexual Reproduction
-
Sexual reproduction is a process involving the fusion of two gametes to form a zygote. It
produces genetically dissimilar offspring.
Advantages of sexual reproduction
- Offspring may inherit beneficial qualities from both parents.
- There is greater genetic variation in the offspring, leading to species that are better adapted
to changes in the environment.
Disadvantages of sexual reproduction
- Two parents are required (except in plants with bisexual flowers)
- Fusion of gametes is required
- Slower method of producing offspring as compared to asexual reproduction
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Parts of a Flower
-
Petal
-
Sepal
-
Flowers contain the reproductive organs of flowering plants.
An inflorescence is a cluster of flowers borne on the same stalk.
A complete flower consists of:
- Pedicel
- Receptacle
- Sepals
- Petals
- Stamens
- Carpels (pistil)
Petals are modified leaves forming a large part of the flower.
The petals combine to form the corolla.
In insect-pollinated flowers, petals:
- are brightly coloured to attract insects for pollination.
- provide a platform for insects to land.
Sepals are modified leaves which enclose and protect the other parts of the flower in the
bud stage.
The sepals combine to form the calyx.
Some flowers have another layer of floral leaves outside the sepal which make up the
epicalyx of the flower.
Pedicel
- The pedicel is the flower stalk, connecting the flower to the plant.
Receptacle
- The receptacle is the enlarged end of the flower stalk which bears the other parts of the
flower.
Stamen
- The stamen is the male part of the flower.
- The stamen consists of the anther and filament.
Anther
-
The anther consists of two lobes and a vascular bundle
Each lobe contains two pollen sacs, which contain pollen grains.
The anther produces pollen grains via meiosis.
When the anther matures, it splits open to release the pollen grains.
Filament
- The filament is the stalk that holds the anther in a suitable position to disperse the pollen.
Carpel (pistil)
- The carpel is the female part of the flower.
- The carpel consists of an ovary, a style above the ovary and one or more stigmas.
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Stigma
- The stigma is a swollen structure at the end of the style.
- It receives the pollen grains.
- A mature stigma secretes a sugary fluid that stimulates the pollen grains to germinate.
Style
-
The style is a stalk that connects the stigma to the ovary.
Ovary
-
The ovary develops into a fruit after fertilisation.
It produces and protects one or more ovules.
The ovule develops into a seed after fertilisation.
The ovule produces an ovum by meiosis.
The ovule is attached to the placenta by the funicle.
Self-pollination
-
Self-pollination is the transfer of pollen grains from the anther to the stigma of the same
flower or of a different flower on the same plant.
NOTE: self-pollination ≠ asexual reproduction
Features favouring self-pollination
- Flowers are bisexual with anthers and stigmas maturing at the same time.
- The stigma is situated directly above the anthers.
Advantages of self-pollination
- Only one parent plant is required.
- The offspring inherits its genes from the parent plant. Beneficial qualities are more likely to
be passed down to the offspring.
- It does not depend on external factors such as insects or wind for pollination.
- Higher chance of pollination to occur as the anthers are close to the stigmas of the same
flower.
- Less pollen and energy is wasted in self-pollination as compared to cross-pollination.
Disadvantages of self-pollination
- Less genetic variation in the offspring as compared to cross-pollination. As a result, the
species is less well-adapted to changes in the environment.
- Possibility of harmful recessive alleles being expressed in the offspring is higher as compared
with cross-pollination.
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Cross-pollination
-
Cross-pollination is the transfer of pollen grains from one plant to the stigma of a flower in
another plant of the same species.
Features favouring cross-pollination
- Dioecious plants that have either male or female flowers.
- Bisexual flowers that have anthers and stigmas mature at different times.
- Stigmas of bisexual flowers may have been situated away from the anthers so that
self-pollination is unlikely.
Advantages of cross-pollination
- Offspring produced may have inherited beneficial qualities from both parents.
- More varieties of offspring can be produces as there is greater genetic variation. This
increases the chance of the species surviving changes in the environment.
- Increased probability of offspring being heterozygous.
Disadvantages of cross-pollination
- Two parent plants are required.
- Depends on external factors such as insects or wind for pollination.
- Lower probability that pollination will occur.
- More energy and pollen is wasted.
Insect-pollinated Flowers vs Wind-pollinated Flowers
Characteristics of insect-pollinated flowers
Characteristics of wind-pollinated flowers
Flowers are usually large with brightly-coloured
petals to attract insects.
Flowers are usually small and dull-coloured,
without petals.
Nectar is present.
Nectar is absent.
Flowers are fragrant or sweet-smelling.
Flowers are odourless.
Stigmas are usually small, compact and do not
protrude out of the flower.
Stigmas are large, feathery and usually
protrude out of the flower to provide a large
surface area to trap pollen.
Stamens are not pendulous and usually do not
protrude out of the flower.
Stamens have long pendulous filaments and
protruding anthers. Pollen grains are hence
easily shaken out from the anthers.
Pollen is fairly abundant. Pollen grains are
usually larger with rough surfaces so that they
can readily cling onto the body insects.
Pollen is more abundant. Pollen grains have
smooth surfaces and are tiny and light so that
they are easily blown about by the wind.
Nectar guide may be present on the petals to
guide insects towards the nectar.
Nectar guide absent.
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Fertilisation in Plants
1) After pollination, the pollen grains germinate after it comes into contact with the stigma, in
response to the sugary fluid secreted by the mature stigma.
2) A pollen tube grows out from each pollen grain. The male gamete enters the pollen tube.
3) As the pollen tube grows, it secretes enzymes to digest the surrounding tissue of the stigma
and style. Thus, the pollen tube grows down the style into the ovary.
4) The pollen tube enters the ovule through the micropyle.
5) Within the ovule, the tip of the two pollen tubes absorbs sap and bursts, releasing the two
male gametes.
6) The nucleus of one male gamete fuses with the nucleus of the ovum to form the zygote.
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Chapter 17: Reproduction in Humans
Male Reproductive System
Testis
- There are two testes. Each testis produces sperm.
- Each testis also produces male sex hormones.
- The male sex hormones are responsible for the development and maintenance of the
secondary sexual characteristics in males.
- Each testis receives blood from blood vessels in a spermatic cord.
- Each testis has a narrow, coiled tube called the epididymis which stores inactive sperms
from the testis before they enter the sperm duct.
Scrotum
- The testes lie between the thighs, in a pair of pouch-like sacs called the scrotums or scrotal
sacs.
- The scrotums are outside the main body cavity and thus are at a slightly lower temperature
than body temperature. The lower temperature is essential for the sperm to develop
properly.
Sperm duct / Vas deferens
- Each sperm duct or vas deferens loops over a ureter and then opens into the urethra.
- After sperms are released from a testis, they travel through a sperm duct.
Glands
- The prostate gland is at the base of the urinary bladder, where the two sperm ducts join the
urethra.
- Beneath the prostate gland is the Cowper’s gland.
- The seminal vesicle is a gland that opens into each sperm duct. It stores sperm temporarily
before they are released through the urethra.
- The prostate gland, the seminal vesicles and the Cowper’s gland secrete a fluid which mixes
with the sperms. This fluid contains nutrients and enzymes which nourish the sperms and
stimulate them to swim actively.
- The mixture of fluid and sperms is called semen.
Urethra
- The urethra is a tube which passes from the bladder through the center of the penis to the
outside of the body.
- Both semen and urine pass out of the body through the urethra.
Penis
-
The penis is an erectile organ.
The penis enters the vagina of a woman during sexual intercourse to deposit semen
containing sperm.
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Sperm
- Numerous sperm are produced throughout the life of a physically mature male human. Each
sperm is about 60 ​µm long.
- Head
- The head is about ​2.5 µm wide.
- It contains a large nucleus with small amounts of cytoplasm. The nucleus carries a
haploid number of chromosomes.
- An acrosome is also present. The acrosome is a vesicle containing enzymes. The
enzymes break down part of the egg membranes so that the sperm can penetrate
the egg during fertilisation.
- Middle piece
- The middle piece contains numerous mitochondria.
- The mitochondria provides energy for the sperm to swim towards the egg.
- Tail
- The beating movement of the tail or flagellum enables the sperm to swim towards
the egg.
- The sperm is motile (can move on its own).
Female Reproductive System
Ovary
- The two ovaries produce eggs.
- Each ovary also produces hormones such as oestrogen and progesterone.
- The female sex hormones are responsible for the development and maintenance of the
secondary sexual characteristics in females.
- When the eggs become mature, they are released from the ovaries.
Oviduct
- Each ovary releases mature eggs into an oviduct or fallopian tube.
- Each oviduct is a narrow muscular tube leading from the ovary to the uterus. It has a
funnel-like opening lying close to the ovary. This makes it easier for the egg to enter the
oviduct.
- The egg is usually fertilised in the oviduct.
Uterus
- The uterus or womb is where the fetus or unborn baby develops during pregnancy.
- The uterus is shaped like an upside-down pear. It had elastic muscular walls. The smooth
muscle tissue in the walls of the uterus contract to push the fetus out during birth.
- The soft, smooth inner lining of the uterus is called the uterine lining or endometrium. It is
where the embryo implants.
Cervix
- The cervix is the lower portion of the uterus where it joins the vagina.
- The opening of the cervix allows menstrual blood to flow out into the vagina during
menstruation.
Vagina
- Leading from the cervix to the outside is the vagina or birth canal.
- The opening of the vagina is the vulva.
- Semen is deposited in the vagina during mating or sexual intercourse.
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Ovum
- The ovum or egg is the female gamete. A female is born with all the potential eggs she will
have.
- The egg is spherical and about 120 ​µm - 150 µm wide.
- The egg:
- has a large nucleus containing one haploid set of chromosomes.
- has abundant cytoplasm which may contain a small amount of yolk.
- is surrounded by a cell surface membrane which in turn is surrounded by an outer
membrane.
Male vs Female Gamete
Male Gamete
Female Gamete
Has a head, a middle-piece and a tail. Nucleus
contains either X or Y chromosome. About 60
µm long with a diameter of 2.5 µm for the
head.
Spherical in shape. Nucleus has one X
chromosome. Diameter of 120 ​µm to 150 µm.
Motile, has a tail that enables it to swim
towards the oviduct.
Non-motile, passive movement of egg along
oviduct is due to the action of cilia in oviduct
and peristalsis of oviduct wall.
Numerous sperm are produced through life
from puberty onwards. Large number of sperm
is released per ejaculation.
Number of eggs is determined at birth. Only
one egg is released per month. Both ovaries
together produce about 500 mature eggs.
Puberty
-
Puberty is the stage of human growth and development in which a person becomes
physically mature.
During puberty, the reproductive system of a young person begins to function. The sex
organs mature and begin to produce gametes.
Secondary sexual characteristics develop and are brought about by sex hormones.
The female sex hormones, oestrogen and progesterone, are produced by the ovaries.
The male sex hormones, testosterone, is produced by the testes.
Secondary sexual characteristics
In Males
In Females
Facial hair starts to grow and hair appears in
the pubic region and under armpits.
Hair appears in the pubic region and under
armpits.
The penis and testicles increase in size.
The breasts and uterus enlarge.
Larynx enlarges and voice deepens.
Hips broaden
Production of sperms starts.
Menstruation and ovulation start.
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The Menstrual Cycle
-
The first sign of puberty for a female is the monthly discharge of blood or menses from the
uterus via the vagina.
The menstrual period usually lasts for five days.
Natural variation in the menstrual cycle
- Menstruation is part of a cycle of events that takes place in the female reproductive organs
every month. This cycle of events is called the menstrual cycle.
- The average menstrual cycle is 28 days.
- There is natural variation in the length of the menstrual cycles.
- As a female ages, the ovaries will stop releasing eggs and the menstrual cycle stops.
Menopause usually takes place between 45 and 55 years of age.
Changes in a follicle during the menstrual cycle
1) Primary follicles
- Young follicles are called primary follicles.
- Each primary follicle consists of a potential egg cell surrounded by a layer of smaller cells
called follicle cells.
2) Graafian follicle
- A primary follicle may develop into a Graafian follicle.
- The graafian follicle contains an egg surrounded by follicle cells and a fluid-filled space. The
egg has a haploid number of chromosomes.
- Usually, only one egg is released every month. The ovaries take turns to release an egg.
3) Ovulation
- On day 14 of a 28-day cycle, the Graafian follicle ruptures and releases the egg into the
oviduct through the funnel-like opening.
- The release of the egg from the ovary is called ovulation.
4) Corpus luteum
- After ovulation, the Graafian follicle develops into a corpus luteum.
- The corpus luteum produces hormones that prepare the body for pregnancy.
5) Corpus luteum breaks down
- If no fertilisation occurs, the corpus luteum will persist for some time and then eventually
breaks down.
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Stages in the menstrual cycle
Days
Stage
Menstrual flow stage
1-5
Low levels of oestrogen and progesterone causes uterine lining to break
down and to be shed in the form of menstrual blood out of the body
through the vagina.
Follicle stage
6-13
Oestrogen stimulates the repair and growth of the uterine lining. The
uterine lining becomes thick and spongy with blood vessels.
Ovulation
14
Egg cell is released from a ovary.
Corpus Luteum stage
15-28
Progesterone maintains the uterine lining by stimulating it to thicken
further and be supplied with blood capillaries, preparing it for the
fertilised egg.
Progesterone inhibits ovulation.
Early Development of Zygote
Fertilisation
- The egg released from the ovary is usually surrounded by a few layers of follicle cells.
- To penetrate the egg, the acrosome of the sperm releases an enzyme to disperse the follicle
cells and break down part of the egg membranes for the sperm to enter.
- Only one sperm nucleus enters the egg. The haploid sperm nucleus fuses with the haploid
egg nucleus, and a fertilised egg or diploid zygote is formed.
- As soon as the sperm has entered the egg, the membrane of the egg changes so that no
other sperms can enter. The remaining sperms, which do not fertilise the egg, eventually die.
Implantation
- Cilia lining the oviduct sweep the fertilised egg or zygote along the oviduct.
- Peristaltic movement of the oviduct also help the zygote move towards the uterus.
- The zygote divides by mitosis to form a hollow ball of cells called the embryo.
- It takes about five days for the embryo to reach the uterus.
- The developing embryo moves down the uterus and eventually embeds itself in the uterine
lining.
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Development of placenta and amniotic sac
- After implantation, villi, which contain blood capillaries of the embryo, grow from the
embryo into the uterine lining. The villi and the uterine lining in which the villi are embedded
in make up the placenta. The umbilical cord attaches the embryo to the placenta.
- A membrane known as the amniotic sac develops at the same time as the placenta. The
membrane encloses the embryo in a fluid-filled space known as the amniotic cavity. The fluid
is known as the amniotic fluid.
Structure
Function
Amniotic fluid
Acts as a cushion to absorb shock and protect the fetus against mechanical
injury.
Allows the fetus to move freely.
Prevents the fetus from dehydration.
Maintains a constant temperature for optimum development of the fetus.
Acts as lubricating fluid for the passage of the baby at birth.
Amniotic sac
Membrane which surrounds embryo.
Secretes amniotic fluid.
Function of umbilical cord
- Attaches foetus to the placenta.
- Contains two umbilical arteries that transports deoxygenated blood and metabolic waste
products from foetus to placenta.
- Umbilical vein transports oxygenated blood and food substances from the placenta
to the foetus.
- Umbilical arteries transport deoxygenated blood and metabolic waste products from
the foetus to the placenta.
- Contains one umbilical vein that transports oxygenated blood and food substances to foetus.
Function of placenta
- Performs the functions of nutrition, gaseous exchange and excretion.
- Oxygen and nutrients diffuse through the placenta from the maternal blood to the
embryonic blood.
- Carbon dioxide and urea diffuse through the placenta from the maternal blood to
the embryonic blood.
- Attaches embryo to the uterus.
- Separates embryonic blood from maternal blood.
- Prevents the high blood pressure of the mother from destroying the delicate embryo
- Prevents pathogens in the maternal blood from entering the embryo.
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Structure
Function
Foetal part of placenta consists of chorion
which produces numerous finger-like
projections called chorionic villi.
Increase total surface area to volume ratio for
diffusion of substances.
Chorionic villi are surrounded by maternal
blood spaces supplied with blood from the
arterioles at low pressure.
Low pressure ensures that there is sufficient
time for efficient transfer of nutrients and
waste materials between maternal and foetal
blood.
Blood capillaries of the foetus are separated
from the mother’s blood system by only a thin
layer of tissue.
Thin barrier ensures that the exchange of
substances by diffusion occurs efficiently.
Maternal and foetal bloods are only a short
distance away but they do not mix.
The two blood systems do not mix to prevent
damage to foetus’ blood vessels due to
mother’s higher blood pressure. Also to prevent
agglutination if mother and child do not have
compatible blood types.
Sexually Transmitted Infections
-
Sexually transmitted infections are spread through sexual intercourse.
Acquired Immunodeficiency Syndrome (AIDS)
- AIDS is caused by ​Human Immunodeficiency Virus​, which destroys the body’s immune
system.
- AIDS is the most advanced stages of HIV.
Modes of transmission
- Sexual intercourse with an infected person.
- Sharine hypodermic needles with an infected person.
- Blood transfusion with blood from an infected person.
- Pregnancy from mother to fetus.
- Exchange of body fluids.
Prevention and control
- Keep to one sex partner or abstain from sex.
- Males should wear a condom when having sex.
- Do not abuse drugs.
- Do not share instruments that are likely to break the skin.
- Sterilise all needles before using.
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Chapter 18: Heredity
-
Hereditary traits are passed on from parents to offspring.
Monohybrid inheritance ​is an inheritance involving only one pair of contrasting traits.
Basic Terms
Term
Gametes
Definition
Male or female sex cells
Dominant allele
An allele that can always be expressed in the phenotype, when it occurs in
the homozygous or heterozygous condition.
Recessive allele
An allele that is only expressed in the phenotype, when it occurs in the
homozygous recessive condition.
Homozygous
dominant
Organisms having two identical dominant alleles of a particular gene.
Homozygous
recessive
Organisms having two identical recessive alleles of a particular gene.
Heterozygous
Organisms having two different alleles of a particular gene.
Codominance
When both alleles have an equal effect on the phenotype of the offspring.
Both alleles are expressed in the phenotype.
Chromosome
A chromosome is a rod-like structure visible in the nucleus during cell
division. It is made up of DNA.
DNA in chromosomes carries the hereditary information for making new
organisms. Each chromosome may carry many genes along its length.
Gene
A gene is a unit of inheritance with specific sequence of nucleotides, as part
of a DNA molecule that contains the information to make a polypeptide.
It is a unit of inheritance, passed on from parent to offspring via
chromosomes of the parent’s gametes.
Allele
Alleles are different forms of a gene.
Alleles of a gene occupy the same locus on a pair of homologous
chromosomes.
Alleles of a gene impart a particular characteristic.
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Homologous
chromosomes
A pair of chromosomes, one from the male parent and one from the
female.
They are similar in shape and size (with the exception of sex
chromosomes).
Exactly the same order of sequence of gene loci. The alleles may not be the
same.
Phenotype
Phenotype refers to the expressed trait in an organism.
The phenotype of an organism is the result of its genes and the effects of its
environment.
Genotype
A genotype is the genetic makeup of an organism.
An organism is homozygous for a trait if the two alleles controlling the trait
are identical. An organism is heterozygous for a trait if the alleles
controlling the trait are different.
Codominance
-
Codominance results when the two alleles controlling a trait express themselves in the
organism.
ABO Blood group
- A person’s blood group may be one of four types, A, AB, B and O. They are determined by 3
different alleles of a single gene.
- I​A
- Allele for the production of Type A antigen (Blood Group A)
- I​B
- Allele for the production of type B antigen (Blood Group B)
- I​O
- Allele that produces neither antigen (Blood Group O)
Phenotypes
Genotypes
Blood Group A
I​A​I​A​ / I​A​I​O
Blood Group B
I​B​I​B​ / I​B​I​O
Blood Group AB
I​A​I​B
Blood Group O
I​O​I​O
Sex determination
- XX - Female
- XY - Male
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Variation
-
Variations are differences in traits between individuals of the same species.
Discontinuous variation
- Traits that show clear-cut phenotypes ​with no​ intermediate forms between traits.
- Traits that are easily indistinguishable and are not affected by environmental conditions.
Continuous variation
- Traits that ​do not ​show clear-cut phenotypes ​with​ intermediate forms between traits.
- Traits that are ​not ​easily indistinguishable and ​are ​affected by environmental conditions.
Continuous vs Discontinuous variation
Factor
Phenotype
Continuous variation
Discontinuous variation
Range of phenotypes.
Few clear-cut phenotypes.
Affected by
environment
Greatly affected by environmental
conditions.
Relatively unaffected by the
environmental conditions.
Controlled by
Controlled by many genes.
Controlled by one or few genes.
Differences between individuals are
quantitative.
The differences between the
individuals are qualitative.
Continuous range of intermediate
values.
Discrete groups, no intermediate
forms.
Data
Every possible form between the
two extremes will exist.
Examples
Height
Tongue rolling ability
Body mass
Blood group
Skin colour
Single or double eyelid
Intelligence
Albinism.
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Mutation
-
Mutation is a ​sudden random change​ in the structure of a ​gene​ or in the ​chromosome
number​.
Gene mutation produces variation between individuals as it results in new alleles of genes.
Mutations that take place in body cells other than gametes are called ​somatic mutations​.
Gene mutation
- Albinism
- Albinism is a recessive gene mutation. Individuals who are homozygous for the
albinism allele are albinos.
- Albinism is characterised by the absence of pigments in the skin, hair and eyes.
- An albino individual has a reddish-white skin and white hair. Since the iris does not
contain any pigment, it will appear red because of the colour of the blood vessels in
it.
- Albinos are very sensitive to sunlight and their skin is easily sunburnt.
- Sickle-cell anaemia
- Sickle-cell anaemia is a recessive gene mutation. Individuals who are homozygous
for the allele suffer from sickle-cell anaemia.
- The gene controlling haemoglobin production mutates and produces haemoglobin
S(HbS), which differs from normal Haemoglobin A by one amino acid. This causes a
change in the 3D shape of the haemoglobin molecule. HbS molecules clump
together, making the cell sickle-shaped.
- This reduces the oxygen-carrying capabilities of the red blood cell, making the
person fragile.
- Sickle-cell anaemia is fatal.
Chromosome mutation
- Down’s syndrome
- Individuals with Down’s syndrome have an extra chromosome 21.
- The older the mother, the higher the chances that copies of chromosome 21 will not
separate during gamete formation.
Mutagenic Agents
- Rate of spontaneous mutation is usually very low, but is greatly increased with the presence
of certain agents in the external environment. Examples include:
- UV light
- Alpha, beta and gamma radiations
- Tar
- Formaldehyde (in cigarette smoke)
- Lysergic acid diethylamide (LSD)
Mutations and selections
- Individuals with harmful mutations will die off more frequently, thus bad mutations are
usually not passed on for long.
- Individuals with beneficial mutations will leave more offspring than those with harmful
mutations, thus the beneficial mutations are more likely to be passed down to offspring.
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Selection
Natural selection
1) Variation within the population is caused by ​spontaneous mutation.
2) When there is a change in environment / competition among varieties, only the ​fittest will
survive.
3) Those with the genetic variation ​best suited to the environment​ allows them to ​survive till
maturity.
4) They will then have a higher chance of ​reproducing ​and ​passing down​ these​ desirable genes
to the ​offspring​.
5) Their offspring increase in proportion in the population and so the proportion of favourable
genes in the gene pool of the species increases.
6) Such preservation of favourable variations and elimination of unfavorable variations is
known as natural selection. When natural selection is repeated in every generation, more
and more favourable genes are accumulated in the genepool and the species will be better
adapted to the environment. Over a long period of time, it could lead to evolutionary
changes and possibly the formation of a new species.
Environmental factors in natural selection
- Amount of food or water
- Breeding space available for animals
- Availability of mineral salts, light and water for plants.
- Number of predators or pathogens.
Artificial selection
- Also known as selective breeding.
- In the process of artificial selection, humans, not the environment, perform the selecting.
People deliberately choose to breed organisms with particular characteristics.
- Man, rather than nature, selects individuals with desirable alleles to produce offspring and
prevent individuals lacking the desirable alleles from breeding. This increases the frequency
of desirable alleles at the expense of undesirable alleles.
- Many crop plants and farm animals are the results of selective breeding programmes. Some
examples are:
- Increase in milk production in cows.
- Increase in meat production in farm animals.
- Increase in yield from cereals.
- Increase in disease resistance in many crops.
- As a result, greater profits are made from greater quantities of better quality produce.
- Inbreeding will increase the chances of two recessive alleles coming together, which may
give rise to a genetically-controlled deformity.
Natural vs artificial selection
Natural selection
Artificial selection
Selection occurs when natural environmental
condition changes.
Humans select the varieties of organism that
suits their needs.
Varieties are produced by mutations.
Varieties are produced by selective breeding.
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Chapter 19: Molecular Genetics
DNA
-
Deoxyribonucleic acid is a molecule that carries genetic information.
A small segment of DNA carries a gene that stores information used to make a polypeptide.
Each DNA molecule consists of two strands twisted around each other to form a ​double
helix​.
A molecule of DNA is wrapped around proteins to form a single chromatin thread.
During cell division, chromatin threads coil tightly into structures called chromosomes inside
the cell nucleus.
Basic units of DNA
- One DNA molecule is a double helix of two complementary antiparallel polynucleotide
chains.
- One nucleotide consists of:
- Phosphate group
- Deoxyribose sugar
- Nitrogenous base
- Adenine
- Guanine
- Cytosine
- Thymine
- Nucleotides can be joined together to form polynucleotides.
- The phosphate group of one nucleotide joins with the deoxyribose sugar of the next
nucleotide to form the backbone of the DNA molecule.
- The nitrogenous bases pair with each other using complementary base-pairings to form a
double-stranded structure. Complementary bases are joined together by hydrogen bonds.
- Adenine bonds with thymine
- Cytosine bonds with guanine.
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Genes
-
A gene is a ​sequence ​of DNA nucleotides that controls the formation of a single polypeptide.
Structure of a gene
- Each gene consists of two polynucleotide chains. One of the chains determines the type of
protein made, called the ​template​.
- The ​template​ contains a sequence of nucleotides or bases. This polynucleotide sequence
stores information as ​three bases to one amino acid.
Formation of Polypeptides
Transcription
- Transcription is the process by which the DNA template is used to make a single-stranded
molecule called ​messenger RNA (mRNA). ​mRNA has a base sequence complementary to
that of the DNA template. It carries the message from the DNA codons out of the nucleus to
the cytoplasm, where the protein is synthesised.
- mRNA contains Uracil (U) instead of Thymine.
Translation
- Translation is the process by which the sequence of mRNA codons is used to make a
polypeptide.
Sequence of events
1) The region of DNA carrying the gene unwinds and unzips. The two DNA strands are
separated. One of the DNA strands called the template is used to make the mRNA.
2) mRNA leave the nucleus through the nuclear pore.
3) mRNA attaches to a ribosome.
4) Ribosome translates the message in mRNA into a sequence of amino acids joined together to
form a polypeptide. As the ribosome moves along the mRNA, the polypeptide produced gets
longer as more amino acids are joined together by peptide bonds. At the end of the mRNA
strand, the ribosome detaches from the mRNA, and the polypeptide is released.
DNA vs RNA
DNA
RNA
The sugar unit is deoxyribose.
The sugar unit is oxyribose.
The nitrogen-containing bases are adenine,
thymine​, cytosine, and guanine.
The nitrogen-containing bases are adenine,
uracil​, cytosine and guanine.
Double stranded molecule.
Single stranded molecule.
Ratio of A:T and C:G is 1:1
No fixed ratio between A and U, C and G.
Permanent molecule in the nucleus.
Temporary molecule and is made only when
needed. Can move out of the nucleus.
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Genetic Engineering
-
Genetic engineering​ is used to transfer genes from one organism to another. Individual
genes may be cut off from the cells of one organism and inserted into the cells of another
organism of the same or different species. The transferred gene can express itself in the
recipient organism.
Transfer of human insulin gene into bacteria
1) Obtain the fragment of DNA in human chromosome that contains the insulin gene. Cut the
gene using a ​restriction enzyme​. The enzyme cuts the restriction site at the two ends of the
gene to produce sticky ends. Each sticky end is a single-stranded sequence of DNA bases.
These bases can pair with complementary bases to form a double strand.
2) Obtain a plasmid from a bacterium. Cut the plasmid with the ​same​ restriction enzyme. This
produces sticky ends complementary to those of the insulin gene.
3) Mix the plasmid with the DNA fragment containing the human insulin gene. The human
insulin gene will bind to the plasmid by complementary base pairing between their sticky
ends. Add the enzyme ​DNA ligase​ to seal the human insulin gene to the plasmid. This
plasmid containing DNA from two different organisms is called ​recombinant plasmid​.
4) Mix the recombinant plasmid with E.coli bacterium. Apply ​temporary heat or electric shock​.
This opens up pores in the cell surface membrane of the bacterium for plasmid to enter.
5) This ​transgenic bacterium​ will use the new gene to make insulin. Such bacteria can be
isolated and grown for mass production of human insulin. The insulin protein has to be
extracted and purified before it can be used.
-
Advantages
- It does not induce allergic response in the patient as the insulin produced is identical
to human insulin.
- It is easier and cheaper to produce insulin in large quantities.
- There is less risk of contamination by disease-causing microorganisms like bacteria
as compared to insulin obtained from the pancreas of animals.
- The ethical concerns of vegetarians or religious groups can be overcome.
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Transfer of pest-resistant gene from a bacterium to a crop plant
1) Use restriction enzyme to cut out the gene from the bacterial DNA to produce sticky ends
2) Use the same restriction enzyme to cut the plasmid to produce complementary sticky ends.
3) Insert the gene into the plasmid.
4) Insert the recombinant plasmid into the bacterium.
5) Allow this bacterium to infect plant cells.
6) Induce the plant cells to produce recombinant plants. A plant that has acquired a foreign
gene is a ​transgenic plant.
-
-
Advantages
- Food production increased
- Reduce environmental pollution as less pesticides are used.
Disadvantages
- Insect pests may develop resistance to the poison produced by the plant.
- Pest-resistance may be spread to weeds through cross-pollination.
- Useful insects may be killed.
- Producing herbicide-resistant crops may lead to more effective destruction of
weeds. This may break the links in the food web and upset the ecological balance.
Selective breeding vs Genetic engineering
Selective breeding
Genetic engineering
Plants and animals used for breeding must be
closely related or belong to the same species.
Genes from any plant or animal can be inserted
into non-related species or different species.
Defective genes may be transmitted along with
the healthy genes to the offspring.
Genes are carefully selected before transfer
into an organism. This reduces the risk of
genetic defects being passed on to the
offspring.
Selective breeding is a slow process. It involves
breeding over several generations. Selective
breeding requires large amounts of land.
Genetic engineering uses individual cells which
reproduce rapidly in the laboratory in a small
container.
Less efficient. Organisms grow slower and may
require more food.
More efficient. Transgenic organisms may grow
master and require less food than ordinary
organisms.
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Effects of Genetic Engineering on Society
Benefits
Applications of genetic engineering
Low-cost production of medicines.
Benefits to society
Genetic engineering of important drugs such as
human insulin has drastically reduced the cost
of these medicines. With these drugs becoming
more affordable, more patients can get access
to them and be treated.
Production of crops that grow in extreme
conditions, for example:
- Drought-resistant crops.
- Salt-tolerant crops.
- Crops that make more efficient use of
nutrients.
Allows farmers to grow crops even when the
environmental conditions are not suitable for
cultivating most crops.
Development of:
- Crops that produce toxins that kill
insect pests.
- Pesticide-resistant crops.
The use of costly pesticides that may damage
the environment is reduced.
Development of foods designed to meet
specific nutritional goals.
Improved nutritional quality of foods.
Social and ethical issues
- New proteins in genetically modified (GM) food might cause allergies in humans that
consume them.
- GM food may prove to be toxic or cancer-causing to people that consume them. Modifying a
single gene in plants could result in the alteration of some metabolic processes within the
plant. This may result in the production of toxins not usually found within these plants. The
consumption of these plants or products made from these plants by humans can pose
serious health problems.
- The resulting deaths of useful insects, like the honey bees and butterflies that feed on the
nectar of GM crops may result in a loss of biodiversity.
- Some biotechnology companies have engineered crop plants such that these plants produce
seeds that cannot germinate. This means that farmers have to buy special seeds from these
companies every year. This poses a serious problem to poorer societies.
- There are objections to consuming animal genes in plants or vice versa.
- Some people feel that it is morally wrong to exploit animals for medical research, especially
when the animals are designed to suffer.
- Some people may deliberately create new combinations of genes which they may use in
chemical or biological warfare.
- Genes that code for antibiotic resistance may be used in genetic engineering. There is
concern that such genes might accidentally be incorporated into bacteria that cause diseases
to humans, making antibiotics ineffective in treating these diseases.
- Genetic engineering may lead to class distinctions. Only individuals with sufficient financial
means can afford certain gene technologies.
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Chapter 20: Ecology
-
Ecology is the study of how organisms continually interact with one another, as well as with
their surroundings.
Habitat
- Place where an organism lives.
Population
- Group of organisms of the same species that live together in a habitat.
Community
- Community is all the population living and interacting with one another in a habitat.
Ecosystem
- Ecosystem is a community of organisms interacting with one another and with its abiotic
environment.
Abiotic Environment
-
The abiotic environment is made up of the physical features of the surroundings.
Light intensity
- Light intensity affects the rate of photosynthesis and growth in plants. This affects the rate
of growth of animals, as animals depend either directly or indirectly on plants for food.
Temperature
- Temperature affects the rate of enzyme reactions, and therefore it affects the metabolic
rate and growth of organisms.
Water availability
- Water is important for the survival of all living things. It is the main component of
protoplasm.
Oxygen content
- Oxygen is needed for aerobic respiration.
Salinity of soil and water
- Salinity affects the water potential of a solution. Organisms living in freshwater have to
constantly remove excess water that enters their bodies via osmosis. Those living in
seawater have to conserve water.
pH of soil and water
- Organisms are sensitive to the pH of the soil or water in which they live in. pH of the
surroundings affects the rate of enzyme reactions within the bodies of the organisms.
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Biotic Environment
-
The biotic environment comprises all the living organisms that an organism interacts within
its habitat.
Organisms are ​interdependent ​as each organism depends on, and is influenced by, other
organisms around it.
Ecological Community
- An ecological community is made up of different plants and animals living together and
interacting within the same environment.
- Various populations in any community live interdependently. A change in one population
affects the other populations of the community.
- Breaking a single strand in this biological balance or equilibrium will affect the whole system.
Energy and Nutrient Flow
Producers
- Producers such as plants, algae and photosynthetic bacteria are organisms that can make
their own food by photosynthesis.
- They contain chlorophyll which traps and converts light energy into chemical energy.
- They synthesise glucose from carbon dioxide and water, and they produce oxygen as a
by-product during photosynthesis.
- Producers affect the lives of other organisms because they provide them with energy and
oxygen.
Consumers
- Consumers are organisms that are not able to make their own food.
- They obtain energy and nutrients by feeding on other organisms.
- Types of consumers:
- Primary consumers
- Primary consumers are herbivores. They feed directly on plants.
- Secondary consumers
- Secondary consumers are carnivores. They feed on primary consumers.
- Tertiary consumers
- Tertiary consumers are carnivores. They feed on secondary consumers.
Decomposers
- Decomposers such as bacteria and fungi feed on decaying organic matter.
- They break down dead bodies of organisms, faeces and excretory products.
- Their activities return nutrients to the environment.
Food chain
- A food chain is a series of organisms through which energy is transferred in the form of food.
- Each stage in a food chain is a trophic level.
1​st​ trophic level
2​nd​ trophic level
Producer (green plant) Primary consumer
(herbivore)
3​rd​ trophic level
4​th​ trophic level
Secondary consumer
(carnivore)
Tertiary consumer
(carnivore)
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Food web
- A food web consists of interlinked food chains.
Non-cyclic energy flow in an ecosystem
1) In any ecosystem, the ultimate source of energy is the Sun. The Sun provides energy in the
form of light energy.
2) Light energy absorbed by chlorophyll in producers is converted into chemical energy during
photosynthesis.
3) Energy in the producers is passed from one trophic level to another by feeding (holozoic
nutrition).
4) The flow of energy through the ecosystem is non-cyclic. Energy is lost as heat to the
environment, through respiration, as it flows through the ecosystem. Since this heat energy
does not return to the same system or to the organisms that produced it, it cannot be
recycled in the ecosystem.
5) Egested and excreted materials, and dead organisms contain trapped chemical energy. This
energy is released through the activity of decomposers. Decomposers use some of this
trapped chemical energy for their needs. The rest of the energy is lost as heat.
Predator-prey relationship
- An animal that feeds on another animal is called a predator. An animal that is eaten by
another animal is called the prey.
- An increase in the population size of the prey means that more food is available for the
predators. This leads to an increase in the number of the predators. This then causes a
decrease in the population of the prey, which in turn results in an inevitable decrease in the
predator population as less food is available. The decrease in the number of the predators
allows an increase in the number of the prey.
- This cycle of predator-prey relationship repeats over time. The increase and decrease in the
population of the predator follows the corresponding increase and decrease in the
population of its prey. The average size of the population of the prey is larger than that of
the predator.
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Ecological Pyramids
Pyramid of numbers
- A pyramid of numbers shows the relative number of organisms at each stage of a food chain
at a particular time.
Pyramid of biomass
- A pyramid of biomass allows us to compare the mass of organisms present in each trophic
level at a particular time.
- The pyramid of biomass is constructed based on the standing mass - the ​dry mass ​of
organisms in each trophic level at any one time. The dry mass of an organism is the mass of
the organism when all its water has been removed.
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Nutrient Cycling in an Ecosystem
Carbon cycle
- Removal of carbon dioxide from the environment
- Photosynthesis
- During photosynthesis, plants absorb carbon dioxide and use it to synthesise
carbohydrates. Some of the carbohydrates are converted into proteins and
fats.
- Feeding
- Animals obtain carbon compounds by feeding on plants or other animals.
- Return of carbon dioxide to the environment
- Respiration
- Plants and animals respire, releasing carbon dioxide into the atmosphere.
- Decomposition
- Decomposers, such as bacteria, microbes and fungi, break down dead
organic matter and release carbon dioxide.
- Combustion
- Combustion of fossil fuels releases carbon dioxide into the environment.
- Importance of carbon cycle
- Ensures that there is a continuous supply of carbon dioxide for plants to carry out
photosynthesis. Photosynthesis converts light energy from the Sun into chemical
energy in food, which non-photosynthetic organisms can sue to stay alive.
- Enables energy to flow through the ecosystem. Carbon compounds carry the stored
energy from organism to organism in the food chains of an ecosystem.
- Maintains the correct concentration of carbon dioxide in the atmosphere.
Carbon Sinks
-
A carbon sink is an area that stores carbon compounds for an indefinite period. It stores
more carbon than it releases.
Oceans
- One-third of carbon dioxide released by human activities is absorbed by the ocean.
- Carbon dioxide that dissolves in the ocean’s water is absorbed and used by phytoplankton
and algae in photosynthesis.
- Iron compounds increases the photosynthetic activity of phytoplankton.
Forests
- Forests absorb about 20% of all the carbon dioxide released by the burning of fossil fuels.
- Atmospheric carbon dioxide is absorbed by the plants and used in photosynthesis.
- When trees die, their remains may be buried deep in the ground as fossil fuels.
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Chapter 21: Our Impact on the
Ecosystem
-
Natural resources are resources supplied by nature. Air, water, soil, forests and wildlife are
renewable natural resources. They can be replaced after a long period of time.
Natural resources such as fossil fuels take millions of years to form naturally, and are thus
non-renewable resources.
Human activities such as industry and agriculture use natural resources, leading to
deforestation, overfishing and pollution.
Deforestation
-
The clearing of forests is known as deforestation. Reasons for deforestation include:
- Land is needed for urban development.
- Land is needed for growing crops such as rice.
- Land is used for growing grassland for animal grazing.
- Firewood is a source of fuel.
- Wood is used as construction materials and is turned into pulp for making paper.
Soil erosion
- Deforestation causes soil erosion.
- When trees are removed, the soil is directly exposed to the force of the rain. There are also
no roots to bind the soil. The topsoil is eroded during heavy rains.
- Soil erosion can lead to flooding.
Flooding
- Soil erosion due to deforestation can lead to floods. The eroded soil may be deposited in
rivers and streams, blocking the flow of water. The water levels in rivers rise rapidly, causing
floods.
Desertification
- When forests are cleared, sunlight falls directly onto the soil.
- Water evaporates rapidly from the soil, causing it to harden. The land becomes barren and
plants cannot grow in the soil.
- This leads to desert-like conditions called desertification.
Climate changes
- Deforestation causes climate changes. Rainwater that is retained and absorbed by the roots
of trees is lost as water vapour during transpiration.
- The water vapour eventually condenses and falls as rain. When trees are cleared, there are
fewer clouds, less transpiration and less rainfall.
- The area becomes dry and warm, and annual rainfall decreases.
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Uncontrolled Fishing Practices
Overfishing
- As the human population increases, demand for fish increases. Species whose populations
have decreased significantly due to fishing have been overfished.
- Most modern fishing gear used in commercial fishing catches marine life indiscriminately. It
does not distinguish between the targeted and non-targeted catch.
- Drift nets are nets that are left to drift freely in the seas. These nets trap almost
everything in their path.
- Shrimp or prawn trawlers drag large fishing nets along the bottom of the sea,
trapping marine life indiscriminately.
- Scallop dredges scrape the seabed, destroying coral reefs and organisms that live on
the seabed.
- Marine life such as turtles, sharks and dolphins are often unintentionally caught using these
fishing methods. Although these ‘accidental catches’ are dumped back into the sea, they
often do not survive.
Effects of overfishing
- Some species will be caught faster than they can be replaced.
- Young fish that are caught will not have a chance to grow and reproduce while marine
organisms that are unintentionally caught often do not survive.
- Populations of organisms will decrease and eventually some species may become
endangered and even extinct.
Pollution
-
Pollution is the addition of substances to the environment that damage it, making it
undesirable or unfit for life. Substances that cause pollution are called pollutants.
Eutrophication
- Eutrophication is the process where water receives excess nutrients like phosphates and
nitrates, which causes excessive growth of algae and water plants.
1) Untreated sewage or fertilisers that are not absorbed by crops may end up in nearby rivers
or lakes.
2) The nitrates and phosphates in untreated sewage or fertilisers enhance the growth of algae
and water plants. They are used in the synthesis of proteins and nucleic acids. This leads to
the increased growth and multiplication of algae and floating water plants in the rivers or
lakes.
3) Overgrowth of algae and floating water plants prevents sunlight from reaching the
submerged plants.
4) Submerged algae and water plants due due to lack of sunlight.
5) The dead algae and water plants are decomposed by aerobic bacteria and fungi.
6) As the bacteria feed on the decaying organic matter, they grow and multiply rapidly, using
up the oxygen in the water.
7) Other organisms such as fish die due to lack of oxygen.
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Sewage treatment
1) Sewage from households is channeled into the water reclamation plant. The used water is
first passed through bar screens to remove large coarse materials like grit, rags and sticks.
2) The screened liquid is then sent to the large ​primary settlement tank​. Here the liquid flows
through the tank very slowly. This allows the solid suspensions to settle to the bottom of the
tank as primary sludge. This sludge is removed and fed into an aerobic digester. The top
liquid from this tank flows into an ​aeration tank​.
3) Secondary treatment involves the ​activated sludge process​. In the aeration tank, the liquid is
mixed with aerobic microorganisms, mainly bacteria. Bubbles of compressed air are pumped
into the liquid through pipes in the floor of the tank. The dissolved oxygen is used by the
microorganisms for aerobic respiration. The microorganisms absorb and breakdown the
organic pollutants in the water.
4) The treated water, together with the microorganisms, is channeled into the ​final settlement
tank​. Here, the microorganisms settle to the bottom of the tank as sludge. A portion of this
sludge is returned to the aeration tank for reuse. The excess sludge is sent to the anaerobic
digester. The clean water at the top of the tank may be discharged as effluent into nearby
rivers, streams and lakes.
5) The anaerobic digester is a closed tank, hence, no oxygen is supplied. Anaerobic bacteria
decompose the sludge. They break down the organic matter, producing biogas, mainly
methane. The biogas is used as a fuel to generate electricity for the functioning of the
reclamation plant. The remaining solid material is removed from the tank. It may be used as
fertiliser or burnt in an incinerator.
Inorganic wastes
1) Waste water containing poisonous metals is discharged into the water body.
2) Poisonous metals are absorbed by the water plants in the water body.
3) The water plants are eaten by fish.
4) Fish caught from the sea contain high concentrations of poisonous metals.
5) Villages who eat the fish are poisoned.
Bioaccumulation
- Certain chemicals are not excreted, but are accumulated in the bodies of organisms.
Bioamplification.
- These chemicals are passed along the food chain, increasing its concentration in the bodies
of organisms along the trophic levels.
- Top consumers can accumulate large amounts of these chemicals.
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Conservation
-
Conservation is the protection and preservation of natural resources in the environment.
Conservation is necessary for the maintenance of Earth’s biodiversity.
- Biodiversity refers to the range of species that are present in a particular ecosystem.
Need for conservation
- Maintain biodiversity by preventing the extinction of species
- Maintenance of a large gene pool is important as many wild plants and animals
possess favourable genes. By cross-breeding the different varieties of wild plants
and animals, we can improve agricultural produce.
- Many tropical plants are of great importance as they are sources of medicinal drugs.
- For scientific research
- The study of wildlife provides useful information to humans.
- For economic purposes
- Marine life needs to be conserved as they are a major source of human food.
- Tropical rainforests also provide food.
- Tropical rainforests provide raw materials for industries.
- To maintain a stable and balanced ecosystem
- This prevents disruption of natural cycles such as carbon cycles and also prevents
global warming.
- To preserve natural scenery and wildlife for people to appreciate
- Natural resources enable outdoor recreational activities such as fishing, hiking and
skiing.
Conservation measures
- Keeping the environment clean
- Managing the use of Earth’s natural resources
- Protecting wildlife
- Human population control
Conservation of forests
- Prevent tree felling.
- Plant trees to replace trees that have been removed or destroyed via ​reforestation​.
Conservation of fishing grounds
- Banning the use of drift nets which indiscriminately trap all forms of marine life.
- Using nets with a certain mesh size so that young or immature fish are not caught.
- Regulating the entry of ships into fishing grounds.
- Limiting the period of fishing in fishing grounds.
- Banning the harvesting or fishing of endangered species.
- Raising endangered species of fish in hatcheries and releasing them into fishing grounds
where the fish populations are decreasing.
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