NAT 5 Biology
Multicellular Organisms
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National 5 Biology
Use the following table as a checklist for your revision.
Remember to ask your teacher for help with anything you don’t understand.
Learning Outcomes/ Mandatory Course Key Content
Learned
notes
Completed
questions
1. Specialised Cells
Specialisation of cells, in animals and plants, leads to the formation of a
variety of tissues and organs. Groups of organs which work together
form systems.
Stem cells in animals can divide and have the potential to become
different types of cell. Stem cells are involved in growth and repair.
Meristems are the sites of production of non-specialised cells in plants
and are the sites for mitosis in a plant. These cells have the potential to
become other types of plant cell and they contribute to plant growth.
2. Communication and Control
Nervous control. Nervous system consists of central nervous system
(CNS) and nerves. CNS consists of brain and spinal cord. Structure and
function of brain to include cerebrum, cerebellum and medulla. Neurons
are of three types, sensory, relay and motor. Receptors detect sensory
input/stimuli. Electrical impulses carry messages along neurons. A
synapse occurs between neurons. Chemicals transfer these messages
across synapses.
Structure and function of reflex arc.
Hormonal control. Endocrine glands release hormones into the blood
stream. Hormones are chemical messengers. Target tissues have cells
with receptor proteins for hormones, so only some tissues are affected
by specific hormones.
Blood glucose regulation including the role of insulin, glucagon,
glycogen, pancreas and liver.
3. Reproduction and Inheritance
The structure of gametes and the sites of their production in plants and
animals. Cells are diploid, except gametes, which are haploid.
Fertilisation is the fusion of the nuclei of the two haploid gametes to
produce a diploid zygote
Comparison of discrete and continuous variation.
Most features of an individual phenotype are polygenic and show
continuous variation.
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Got help
from my
teacher
Genetic terms, including gene, allele, phenotype, genotype, dominant,
recessive, homozygous, heterozygous and P, F1 and F2. Carry out
monohybrid crosses from parents through to F2. Reasons why predicted
ratios are not always achieved.
4. World of Plants
Leaf structure to include upper epidermis, palisade mesophyll, spongy
mesophyll, vein, lower epidermis, guard cells and stomata.
Other parts of the plant involved in water transport including root hairs
and xylem vessels. Water minerals are transported in xylem vessels.
Xylem vessels are dead and contain lignin for support. Water is required
for transporting materials and for photosynthesis.
The process of transpiration.
Sugar is transported up and down the plant in living phloem. Structure of
phloem tissue.
5. Health and Physiology
Animal transport and exchange systems In mammals, nutrients, oxygen
and carbon dioxide are transported in the blood.
Pathway of oxygenated and deoxygenated blood through heart, lungs
and body. Heart structure to include right and left atria and ventricles
and location and function of valves. Blood vessels to include aorta, vena
cava, pulmonary arteries and veins, coronary arteries and their function.
Arteries have thick, muscular walls, a narrow central channel and carry
blood under high pressure away from the heart. Veins carry blood under
low pressure; have thinner walls and a wide channel. Veins contain
valves to prevent backflow of blood and carry blood towards the heart.
Capillaries form networks at organs and tissues, and are thin walled and
have a large surface area, allowing exchange of materials.
Red blood cells are specialised by being biconcave in shape, having no
nucleus and containing haemoglobin. This allows them to transport
oxygen efficiently in the form of oxyhaemoglobin.
Rings of cartilage keep main airways open. Oxygen and carbon dioxide
are exchanged through the alveolar walls. Alveoli have a large surface
area, thin walls and a good blood supply for more efficient diffusion of
gases. Mucus traps dirt and microorganisms and cilia moves this up and
out of the lungs.
Food is moved through the digestive system by peristalsis. Villi in the
small intestine have a large surface area, thin walls and a good blood
supply to aid absorption of glucose and amino acids. The lacteals
absorb fatty acids and glycerol (the products of fat digestion).
Effects of lifestyle choices on human transport and exchange systems
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1. Specialised Cells
Cells, Tissues and Organs
Unicellular organisms are living things which consist of only one cell. To ensure the survival
of the organism this one cell must carry out all the necessary functions and processes in
order to sustain life.
Multicellular organisms are highly developed organisms which consist of many (possibly
millions) of cells. These cells are responsible for the continuation of life and will have a
specific cell structure suited to their function. Cells like this are known as specialised cells.
A group of specialised cells which carry out the same function are known as tissue (e.g.
smooth muscle is composed of smooth muscle
cells). Different tissue joined together to form a
structural and functional unit is known as an
organ (e.g. the stomach which is composed of
different types of tissue including smooth
muscle tissue and nerve tissue).
Section of smooth muscle tissue.
Animal
Tissue
Cell Type
Specialised Structural Features
Function
Nerve
Motor neurone
Axon (long insulated extension of cytoplasm)
Transmission of nerve
impulses
Blood
Red blood cell
Small, biconcave shape presents a large surface
area. Rich supply of haemoglobin present
Uptake and transport of
oxygen to living cells.
White blood cell
Able to change shape; sacs of microbe-digesting
enzymes present in some types
Destruction of invading
pathogens.
Plant
Tissue
Phloem
Root
Cell Type
Specialised Structural Features
Function
Sieve Tube
Sieve plates and continuous system of cytoplasmic
strands
Transport of glucose and
soluble carbohydrates
Companion Cell
Large nucleus in relation to cell size
Controls sieve tube
Epidermal cell
Box-like shape allowing cells to fit together like a
brick wall
Protection
Root Hair
Long extension presenting large surface area in
contact with soil solution
Absorption of water and
mineral salts.
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Animal Cells
Motor neurone
Red blood cells and white blood cells
Plant Cells
Phloem containing companion cells and sieve tubes
Epidermal cell
Root section composed of epidermal cells and root hairs
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Stem Cells
Stem cells are undifferentiated cells (general cells) that have the potential to become any
type of cell in animals. In humans there are two types of stem cell; embryonic stem cells
and adult stem cells.
Embryonic Stem Cells
Embryonic stem cells account for all of the cells present in an embryo before development
begins. They have the ability to differentiate into many of the cell types which make up the
fully developed organism.
Adult Stem Cells
Adult stem cells occur in most of the organs in the body. They can be used in growth and
repair of tissue that may have dead or damaged cells. These stem cells are slightly more
restricted in development compared to embryonic stem cells as they are limited to the types
of tissue in which they are found.
Stem Cell Research
In medicine there is potential use for stem cells including repairing damaged organs. This is
a highly controversial topic as it can require the extraction of stem cells from embryos that
are not implanted during IVF treatment or are still in the early stages of development. In this
case the embryo can still develop normally after having cells removed.
It is hoped that in the future stem cell research will be able to replace or repair organs of
people who suffer from various illnesses including diabetes and Parkinson’s disease.
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Meristems
Unlike in animals where stem cells are found throughout the body, undifferentiated cells in
plants are restricted to areas called meristems. These are the only sites of cell division in
plants. There are two types of meristems present in plants; apical meristems and lateral
meristems.
Apical Meristems
Primary growth occurs at the apical meristems. These meristems are found at the root tips
and shoot tips of plants. Cells produced here allow the plants to grow in height and increase
their root length. Once the cells have multiplied by the process of mitosis, they then
undergo differentiation. Some cells, for example, develop into xylem vessels which
transport water in the plant or phloem tissue which transports glucose.
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2. Communication and Control
The Nervous System
The nervous system consists of the brain, spinal cord and nerves.
The brain and spinal cord together make the central nervous system (CNS).
The nerves (neurons) make up the peripheral nervous system. There is one set of nerves
known as the sensory nerves that carry information from receptors at the body’s sense
organs to the central nervous system. These receptors have received information from a
stimulus (e.g. seeing a football coming towards you). Another set of nerves known as
motor nerves carry impulses from the central nervous system to other parts of the body
e.g. muscles. These parts of the body are known as effectors as they carry out a response
(e.g. kicking the ball).
Component
Function
System
Brain


Controls the body
Coordinates nerve impulses
Central Nervous System
Spinal Cord

Relays impulses between the nerves
and the brain
Controls reflexes
Central Nervous System

Nerves
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
Carries impulses to and from the
CNS
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Peripheral Nervous System
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Nerves
Nerves (or neurones) make up the peripheral nervous system. They consist of a cell body
attached to extending nerve fibres. The cell body contains the nucleus. A nerve impulse is
carried towards the cell body along a dendrite and away from it on an axon fibre. Axon
fibres can differ in length depending on the type of neurone.
A tiny space called a synapse occurs between the axon of one neurone and the dendrites
of another. When a nerve impulse reaches the nerve endings of an axon, a chemical is
released which diffuses across the synapse and triggers an impulse at the dendrite of the
next neurone.
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The Brain
The brain is a complex organ consisting of several different regions. The cerebrum is the
largest part of the brain and can be split into hemispheres. The cerebellum is found at the
back of the brain and the medulla is found at the top of the brainstem.
Structure
Cerebrum
Cerebellum
Medulla
Hypothalamus
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Function
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Conscious thought
Limb movement
Personality etc.
Controls balance
Controls muscular coordination
Breathing rate
Heart rate
Regulates water balance
Regulates temperature
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Reflex Action
Internal communication is required for survival of a multicellular organism. Cells in
multicellular organisms do not work independently. An example of this internal
communication is the one which is involved in the reflex action.
A reflex action is what results from the transmission of nerve impulses through a reflex arc.
The reflex arc is an arrangement of nerve cells that allow the body to respond rapidly and
automatically to harmful situation. This protects the body from damage.
Since many reflex actions need to be performed quickly they do not involve the brain.
Instead a sensory nerve sends the information to a relay nerve in the spinal cord. This then
relays the information to a motor nerve which can bring about a fast muscle contraction or a
slower response from a gland.
Reflex Action
limb withdrawal
blinking
release of adrenaline
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Stimulus
Response
Protective Function
heat from a naked
flame
contraction of flexor
muscle
removal of limb to
safety
harmful object
approaching eye
contraction of eyelid
muscle
prevention of damage
to eye
stress e.g. physical
threat or excitement
increases heart rate
and blood sugar level.
Diverts blood to brain
and muscles
prepares body for rapid
activity e.g. running
from danger
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Ouch!
Maintaining Homeostasis
For the human body to function efficiently it must maintain its internal environment. The
body has various actions it can carry out in response to internal and external changes to
maintain stable body conditions. This maintenance of the internal environment is controlled
by homeostasis. Despite extreme changes in the external environment, homeostasis
works to ensure that the internal environment stays within certain tolerable limits.
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Hormonal Control
Hormones are chemical messengers that are transported in the blood. They are secreted
by endocrine glands in the body and affect different target organs. The body’s response to
hormones is usually slower and longer lasting than its response to nerve impulses.
Below are an example of some hormones.
Blood Glucose Regulation
The body requires glucose for energy. Cells use up glucose during the process of
respiration. The body only obtains glucose from the digestion of food, meaning that it is
only present in the blood when food is being eaten. It is important that the blood glucose
concentration is controlled so that there is a constant supply of glucose for respiration.
The pancreas monitors and controls the concentration of glucose in the blood. If blood
sugar levels get too high (above a set point) then special receptor cells in the pancreas
respond by producing the hormone insulin. Insulin is a hormone that is transported in the
bloodstream and allows glucose to be taken up by body tissues. It also travels to the liver
where it activates an enzyme. This enzyme catalyses a reaction that converts glucose to
glycogen.
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Insulin
Glucose
Glycogen
About one hundred grams of glucose are stored as glycogen in the liver. Glucose can be
added to or removed from the liver depending on the blood sugar levels.
When the blood sugar level is low (i.e. between meals or at night) different cells in the
pancreas respond by releasing glucagon into the bloodstream. When this hormone
reaches the liver it activates a different enzyme which catalyses the reaction that converts
glycogen to glucose.
Glucagon
Glycogen
Glucose
This returns the blood sugar level back to normal.
Some people suffer from the disease diabetes which is when their blood sugar levels are
not monitored properly. Type 1 diabetes is when cells in the pancreas do not produce
enough insulin. This means that the blood sugar level is extremely high and the cells cannot
use glucose efficiently. Most of the glucose is expelled from the body in urine. Type 1
diabetes used to be fatal but can now be treated with insulin injections and a controlled diet.
Type 2 diabetes is when a person becomes resistant to insulin. People who are obese or
have a poor diet are at more risk of developing type 2 diabetes. This form of diabetes
usually develops later in life and can be treated with regular exercise and a controlled diet.
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3. Reproduction and Inheritance
All animals and plants reproduce. If a species did not reproduce they would become extinct.
Some plants and unicellular organisms (e.g. amoeba) reproduce by asexual
reproduction. This is when an organism can reproduce on its own. In
unicellular organisms this is done by cell division. Most animals reproduce by
sexual reproduction.
Amoeba
Sex Cells (gametes)
For sexual reproduction to occur an organism must produce gametes. Gametes are
another name for sex cells. These gametes are produced in specialised organs.
The male gamete is called sperm. Sperm are very small and do not contain a food store.
They have a tail which they use to propel themselves towards the female gamete. The
female gamete is known as an ovum (egg). The ovum is large as it contains a food store.
The ovum cannot move by itself. For a new individual to be formed, sperm must first fuse
with the ovum allowing the nuclei of the two cells to fuse. The single cell resulting from this
fusion of the parental gametes is termed the zygote. Once this cell divides the newly
formed cells are now known as the embryo.
It is the random combination of the sex cells from each parent that produces variety in
offspring. This explains why children born to the same parents do not look identical to each
other.
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Fertilisation in Animals
The number of chromosomes in a cell of a living organism are known as the chromosome
complement. The chromosome complement can vary depending on the species. Human
body cells contain 46 chromosomes. These chromosomes can be arranged into 23 pairs.
When a cell has a double set of chromosomes which can be arranged into pairs they are
known as diploid cells. Human body cells are diploid cells.
Gametes only have a single set of chromosomes that cannot be arranged into pairs. These
are known as haploid cells. The fusing of two gametes ensures that the offspring produced
contains the correct number of chromosomes (46 chromosomes).
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Gamete Production
Sperm are produced in the testes of the male. Once the sperm has been produced,
muscles around the testes force the sperm into the sperm duct and then into the urethra
where they and then are expelled from the penis.
Bladder
Bladder
Ova are produced in the ovaries of a female. The ovum enters the oviduct which transports
the ovum towards the uterus. If sperm is expelled from the penis into the vagina then the
sperm swim through the uterus into the oviduct where fertilisation occurs. Only one sperm
can fertilise an ovum. The fertilised egg (zygote) is then transported through the oviduct into
the uterus where the embryo develops.
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Fertilisation in Plants
Structure of a Flower
Sexual reproduction (requiring the fusion of two gametes) occurs in plants as well as
animals. The flower contains the reproductive organs of a plant.
The flowers of many different plants are built to the same basic plan although they may not
all be exactly alike.
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Structure
Function
Sepals
To protect the flower when it is a bud
Nectary
Makes sugary nectar which insects eat
Carpel
Female part containing female gametes
Ovary
Produces ovules (female gametes)
Stamen
Male part containing male gametes
Anther
Produces pollen grains (male gamete)
Stigma
Catches pollen
Petals
Bright colours to attract insects.
Pollination
Pollen is made inside the anther. When they ripen the pollen is released. Pollen grains are
like specks of dust. They contain the plant’s male sex cell. This has to reach the egg cell
(ovule) in the ovary.
Pollination is the transfer of pollen from the anther to the stigma.
Variation
The differences between members of the same species are called variation. A species is a
group of organisms that can interbreed to produce fertile offspring. There are two types of
variation, continuous and discrete.
Discrete Variation
If the variation in a population allows individuals to be divided into two or more distinct
groups, the characteristic shows discrete variation.
Examples of discrete variation are:

Ear Lobe Attachment

Tongue Rolling Ability

Eye Colour

Flower Colour
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Continuous Variation
If the variation in a characteristic can be measured (e.g. height) and it shows a wide range
occurring between two extremes (e.g. the shortest and the tallest), it is an example of
continuous variation. In this case individuals cannot be easily divided into distinct groups.
Examples of continuous variation are:

Height

Pulse Rate

Hand Span

Seed Mass
Inherited Characteristics
Inherited characteristics are determined by genetic information received from the parents.
Some examples of inherited characteristics and their possible phenotypes are shown in the
table below:
Organism
Inherited Characteristics
Possible Phenotypes
Human
hair colour
black, brown, red, blonde
tongue-rolling ability
roller, non-roller
height
tall, dwarf
seed shape
smooth, wrinkled
wing shape
straight, curved
eye colour
red, white
Pea Plant
Fruit Fly
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The development of these inherited characteristics are controlled by genes. Genes are the
small units that make up chromosomes. Genes are arranged along the chromosomes like a
string of beads.
Gene
At least two forms of a particular gene exist, e.g. tongue roller or non-tongue roller, attached
or unattached earlobe. The different forms of a gene are called alleles.
Some characteristics are controlled by the allele of a single gene. An example of this is the
ability to roll the tongue. Other characteristics are controlled by alleles of many genes. An
example of this is height which varies from one extreme (very tall) to another extreme (very
small). This is known as polygenic inheritance. Many features of a person’s appearance
are polygenic and show continuous variation.
Genotype and Phenotype
The outward appearance of a characteristic in an organism is its phenotype.
The complete set of genes an organism possesses is its genotype.
We have all started life as a zygote with 46 chromosomes – 23 from the father, 23 from the
mother. This explains why we have some characteristics from both parents.
At fertilisation each chromosome from the sperm pairs up with the matching chromosome
from the egg. This brings together the two sets of genes in the chromosomes together. The
genes for hair colour pair up, the genes for nose shape pair up etc.
The genes in the pair may be identical. For example, both genes might produce a tongue
roller, but if one gene is for tongue rolling and the other gene is for non-tongue rolling the
genes are now in competition.
In this example the person will be a tongue roller because the gene for tongue rolling is
more powerful than the gene for non-tongue rolling.
Genes which dominate other genes are called dominant genes.
Genes which are dominated by other genes are called recessive genes.
When a dominant gene pairs up with a recessive gene the dominant one produces the final
phenotype.
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An organism with two identical genes for a characteristic is said to be homozygous (you
may also see the terms pure-breeding and true-breeding. These both mean
homozygous).
An organism with two different types of genes for a characteristic is said to be
heterozygous.
Monohybrid Crosses
In diagrams genes are usually represented by letters. Capital letters are used for
dominant genes. Lower case letters are used for recessive genes.
Example
T = gene for tongue rolling
t = gene for non-tongue rolling
Genotype = TT
Genotype = tt
Phenotype = tongue roller
Phenotype = non-tongue roller
Remember that when the gametes are produced they will have half the number of
chromosomes and therefore half the number of genes.
The sex cells will contain one allele from each gene pair, not two. For example the gamete
produced from the tongue roller will contain the allele T and the gamete produced from the
non-tongue roller would contain allele t. If these two gametes were to fuse then the
offspring produced would have the genotype Tt and would be able to roll their tongue (as
they contain at least one dominant gene).
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A cross such as this one can be described as a monohybrid cross because only one
difference is being investigated (tongue rolling ability). In a monohybrid cross each original
parent is true-breeding for the form of the inherited characteristic it possesses.
Example
P Generation
X
Phenotype – Blue Eyes
Phenotype – Brown Eyes
Genotype - bb
Genotype - BB
F₁ Generation
Phenotype – Brown Eyes
Genotype - Bb
F₂ Generation
B
b
B
BB
Bb
b
Bb
bb
3:1 Ratio
3 Brown : 1 Blue
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Gregor Mendel (1822-84) was a monk who performed monohybrid crosses using pea
plants. These pea plants demonstrated discrete variation. Mendel made important
discoveries regarding inherited characteristics.
Monohybrid crosses like this always produce a 3:1 ratio in the F₂ generation. This however
rarely happens in nature because fertilisation is a random process.
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4. The World of Plants
The Need for a Transport System in Plants
Plants need water for photosynthesis. Water enters the roots of the plant and must be
transported upwards to the leaves where photosynthesis takes place. All living cells need a
supply of glucose as well as water. The glucose made in the leaves by photosynthesis
must be transported downwards to the parts of the plant that have no chlorophyll and
cannot make their own food e.g. root cells.
Water and minerals are transported upwards from the roots to the leaves in tubes called
xylem vessels. Water is absorbed by root hairs from the soil by the process of osmosis.
Xylem vessels are hollow tubes that carry water and minerals to all parts of the plant.
Xylem vessels are strengthened by rings or spirals of tough, woody lignin. This lignin helps
to support the plant. Xylem vessels are dead as they have no nucleus or cytoplasm.
The food made by plants during photosynthesis is called glucose. Glucose is carried from
the leaves to every part of the plant in tubes called phloem sieve tubes. Phloem contain
two types of cells; sieve tubes and companion cells. All phloem cells are living. The end
walls have pores and together these are called sieve plates. These allow glucose to be
transported in the strands of cytoplasm from one cell to another.
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Transport tissues in Plants
xylem

Water is carried to
the leaves from the
roots in transport
tissue called xylem
phloem

Food (glucose) is
carried from the leaves
to the roots and growing
part of the plant in
transport tissue called
phloem
Water can however be lost from a plant by the process of transpiration.
Transpiration is when water inside the leaves of a plant evaporates and escapes through
tiny holes on the underside of leaves called stomata. Factors such as light intensity,
temperature, wind and humidity can affect the rate of transpiration.
Factor
Description
Explanation
Light Intensity
In bright light transpiration increases
Temperature
Transpiration is faster in higher
temperatures
Transpiration is slower in humid
conditions
The stomata open wider to allow more
carbon dioxide into the leaf for
photosynthesis
Evaporation and diffusion are faster at
higher temperatures
Diffusion of water vapour out of the leaf
slows down if the leaf is already
surrounded by moist air
Water vapour is removed quickly by air
movement, speeding up diffusion of
more water vapour out of the leaf
Humidity
Wind
Transpiration is faster in windy
conditions
Stomata are pores which allow for gas exchange by plants. They are open during daylight
but closed in darkness. Guard cells are pairs of cells which surround each stoma and
control when the stomata open and close.
The sections of leaf closest to the stomata constantly have a low concentration of water as
it is continually being lost to the environment. This creates a water concentration
gradient. As water is lost through the stomata, more water is drawn out of the xylem
vessels to replace what is lost. Water moves from the xylem into the leaf by osmosis. Some
of this water is used by the spongy mesophyll, palisade mesophyll and guard cells for
photosynthesis whilst some water is used to keep the plant cells turgid.
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Structure of a leaf

Leaves generally have a large surface area to trap the light energy from the sun.

They are also very thin to allow carbon dioxide to diffuse quickly into the leaf cells
and allow the excess oxygen to diffuse out.

The leaf is made of layers of cells, each with a different function to make
photosynthesis an efficient process
Structure
Function
Waxy cuticle
Prevents evaporation of water vapour from the upper surface of the
leaf
Thin outer layer - has no chloroplasts so allows light to pass through
to mesophyll cells
Main site of photosynthesis. Cells contain many chloroplasts. Cells
are arranged to allow maximum absorption of light energy
Cells are loosely packed, with moist air spaces between them to
allow gases to diffuse quickly into the cells
Lower layer of cells containing many pores called stomata (singular,
stoma)
Cells that surround the stomata and control the opening and closing
of the stomata. Stomata allow entry of carbon dioxide and the exit of
excess oxygen and water vapour.
Upper epidermis
Palisade mesophyll
Spongy mesophyll
Lower epidermis
Guard cells
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5. Health and Physiology
The Heart
The heart is a muscular pump which pumps blood to all body cells and to the lungs.
The heart has four chambers. The two upper chambers are called atria (singular: atrium)
and the two lower changes are called ventricles.
Note that the wall of the left ventricle is thicker than the wall of the right ventricle. This is
because the left ventricle pumps blood all around the blood whereas the right ventricle only
pumps blood to the lungs.
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Blood from all parts of the body enters the heart through two main veins called vena cava.
This blood is deoxygenated and enters the right atrium of the heart. It then passes into the
right ventricle. From the right ventricle the deoxygenated blood is pumped through the
pulmonary arteries to the lungs. In the lungs, the blood loses carbon dioxide and gains
oxygen by diffusion. The pulmonary veins return the oxygenated blood to the heart
through the left atrium. From there the oxygenated blood is pushed into the left ventricle.
The left ventricle pumps the oxygenated blood into the aorta and then onto all parts of the
body.
pulmonary artery
pulmonary vein
aorta
vena cava
Blood Vessels
There are three types of blood vessel: arteries, veins and capillaries.
Arteries
Arteries carry blood away from the heart. Each time the heart beats blood is
forced into the arteries at high pressure. Arteries have thick muscular walls to
withstand this high pressure. Every time the heart beats, a spurt of blood
passes along the arteries. This can be felt as a pulse. Apart from the
pulmonary artery, all arteries carry oxygenated blood.
Veins
Veins carry blood towards the heart. By the time the blood starts moving back
to the heart it is at low pressure. The inside of a vein is relatively wider than an
artery allowing the blood to flow through it more easily. Veins also contain
valves which prevent blood flowing backwards.
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Blood Capillaries
Capillaries connect arteries to veins and allow exchange of materials between
blood cells and cells of the body. They are microscopic and highly branched.
The walls of the capillaries are only one cell thick. This allows exchange of
material (e.g. glucose) to take place between the blood and surrounding cells.
The aorta branches into other arteries which deliver oxygenated blood to the body tissues.
As the heart is a working muscle it requires a constant supply of glucose and oxygen to
keep it functioning. These substances are
delivered to the cells of the heart tissue
through the coronary artery.
When the coronary artery gets blocked
with plaque, it deprives the heart of
oxygen. This can lead to health issues
such as angina or more seriously cardiac
arrest. These conditions can arise from a
poor diet, lack of exercise or underlying
health issues such as diabetes.
Fatty deposits and blockages in blood
vessels can lead to other health issues.
Blood clots can occur in the blood
vessels. Blood clots are vital when
someone is injured as it prevents them
from bleeding out. Blood clots in blood vessels however can deprive body tissues from
oxygen. In the brain this can lead to a stroke. A stroke can lead to brain damage and can
be fatal. A stroke can also be caused when a blood vessel ruptures at the brain. Some
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National 5 Biology
factors that increase the risk of blood clots are high fat or high salt diet, trauma and
inherited disorders. Stress can also lead to health issues including blood clots.
Blood
In mammals nutrients, oxygen and waste products such as carbon dioxide are carried in the
blood.
The blood is composed of red blood cells, white blood cells, platelets and plasma.
White blood cells are a part of the immune
system and platelets are what help blood
clot.
Plasma is the watery fluid in which blood
cells are carried. It also contains dissolved
substances such as glucose and amino
acids. A small volume of carbon dioxide is
carried in the plasma but it is limited as it
can make the blood acidic. The majority of
the carbon dioxide enters the red blood cells
to be transported.
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National 5 Biology
Red blood cells carry oxygen and some carbon dioxide. Red blood cells
contain a pigment called haemoglobin which readily combines with
oxygen when it is in high concentration. This occurs at the lungs. Oxygen
is released in the capillaries around body tissues where the oxygen
concentration is low. A lack of iron means that haemoglobin cannot be
made. This can lead to anaemia which is when body tissues do not
receive enough oxygen.
Red blood cells are extremely numerous and have a biconcave shape which offers
maximum surface area for oxygen uptake. Red blood cells are specialised to carry oxygen.
Lungs
Every person has two lungs located inside the chest. Inside each lung there is a series of
branching tubes through which air passes.
When you breathe in, air passes down the trachea (windpipe). The trachea is held open by
rings of cartilage. The trachea then splits into two bronchi (single – bronchus), one leading
to each lung. Each bronchus then splits to form many smaller branching tubes called
bronchioles.
The bronchioles end in tiny air sacs
called alveoli.
There are millions of alveoli in each lung.
It is in the alveoli that gas exchange
takes place. When we breathe in, oxygen
from the air is absorbed into the blood
and carbon dioxide from the blood is
passed into the air.
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There are four main features of the alveoli that make them efficient at gas exchange:
1. There are many alveoli which creates a large surface area.
2. They have thin walls which allows for quick diffusion of gases.
3. They receive a good blood supply so that large volume of gases can be exchanged.
4. They have a moist inner lining which allows gases to dissolve before diffusing.
The inside of the alveoli are lined with mucus. Mucus also lines the trachea. The function of
mucus in the trachea is to catch dirt and microorganisms that may enter the body when a
person inhales. Hair-like extensions called cilia line the trachea. They beat creating a
wavelike motion that moves the mucus up out of the trachea and
into the stomach where the microorganisms are destroyed by the
stomach acid.
Tobacco smoke contains harmful substances like tar and smoke
which can damage the lungs.
Tar coats the inside of the lungs and causes coughs. This can
damage the alveoli which makes it difficult for gas exchange. Tar
can cause cancer of the lungs, mouth and throat.
Hot smoke can damaged the ciliated cells. This means that people have to cough to
remove the mucus from their lungs. This can result in bronchitis.
The Small Intestines
The small intestines are part of the digestive system.
The small intestine is designed to complete the
digestive process and then absorb the soluble food
products into the bloodstream. These include the
products of fat, carbohydrate and protein digestion.
Digestion is necessary to breakdown the large insoluble
molecules into small soluble molecules so they can
diffuse into the bloodstream.
Food moves from the stomach into the small intestine.
In the intestines food is moved by the process of
peristalsis.
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Peristalsis is a wave of muscular
contractions behind the ball of food
(bolus) which pushes it along the length
of the gut. This also happens in the
oesophagus.
The small intestine has the following features which make it efficient at absorbing soluble
food products:
1. It is very long.
2. The inner surface is folded to increase the surface area for absorption.
3. It has very thin walls to allow for easy diffusion of the digested products.
4. It has a rich blood supply to absorb the digested food products and transport them
elsewhere in the body.
The inner surface of the small intestine is lined with finger-like extensions called villi. Inside
each villus is a dense network of blood capillaries and a single lacteal. It is into these two
kinds of vessel that the digested food products
pass.
The blood capillaries absorb:

Simple sugars (products of carbohydrate
digestion)

Amino acids
digestion)
(products
of
protein
The lacteal absorbs:

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Fatty acids and glycerol (products of
fat digestion)
National 5 Biology