Notes (Module: How Life Begins)
IJSO Training: How Life Begins
Notes
SECTION 1: CELL
1.1
CELL THEORY
1.1.1
DISCOVERY OF CELLS
1665: Robert Hooke discovered cells when he studied cork with a compound microscope.
1702: Van Leeuwenhoek made descriptions of many protists.
1838: Schleiden reported that all plants are made of cells.
1839: Schwann reported that all animals are also made of cells.
1855: Virchow proposed that all cells come from cells.
1.1.2
CELL THEORY
All organisms are composed of cells.
Cells are the structural and functional units of life.
Cells are only formed from the division of pre-existing cells.
Cells contain information for growth, development and functioning.
Cells can exist independently under suitable conditions.
1.2
CHEMICAL CONSTITUENTS OF CELL
1.2.1
CARBOHYDRATES
Carbohydrates are organic compounds made from elements carbon (C), hydrogen (H) and oxygen
(O).
The ratio of H : O in carbohydrates is 2 : 1.
The general formula of carbohydrates is Cx(H2O)y.
A. MONOSACCHARIDES / SIMPLE SUGARS
These are the simplest form of carbohydrates.
All simple sugars are soluble in water.
Use: fuels for respiration
e.g. glucose, fructose
B. DISACCHARIDES
Disaccharides are formed when 2 molecules of monosaccharides joined together.
All disaccharides are soluble in water.
Use: carbohydrate storage of plants
e.g. sucrose, maltose
Notes (Module: How Life Begins)
C. POLYSACCHARIDES
Polysaccharides are formed when numerous units of monosaccharides are linked together.
There 3 most common types of polysaccharides, cellulose, starch and glycogen.
1. Cellulose
It is formed from glucose.
It is insoluble in water.
Use: component of plant cell wall
2. Starch
It is formed from glucose.
It is slightly soluble in water, but soluble in hot water.
Use: carbohydrate storage of plants
3. Glycogen
It is formed from glucose.
It is insoluble in water.
Use: carbohydrate storage of animals, usually stored in muscles and liver.
1.2.2
LIPIDS
Lipids are organic compounds made from elements carbon, hydrogen and oxygen.
The ratio of H : O is not fixed.
They are insoluble in water but soluble in organic solvents e.g. ethanol, chloroform, ether, acetone.
Lipids can be divided into triglycerides (fats), phospholipids and steroids.
A. TRIGLYCERIDES / FATS
Fats are formed when glycerol and fatty acids are linked together.
G
L
Y
C
E
R
O
L
FATTY ACID
FATTY ACID
FATTY ACID
G
L
Y
C
E
R
O
L
FATTY ACID
FATTY ACID
+ water
FATTY ACID
triglyceride / fat
B. PHOSPHOLIPIDS
When one of the fatty acids in a triglyceride is replaced by a phosphate group, it will become a
phospholipid.
The phosphate group ionized in aqueous medium forming the polar head / hydrophilic head of the
phospholipid molecule.
The 2 hydrocarbon chains of the fatty acids forming the non-polar tail / hydrophobic tail.
Notes (Module: How Life Begins)
Uses of fats and phospholipids:
 as respiratory substrate to release energy in respiration
 as an energy reserve in animals, fruits and seeds in plants
 as heat insulator to keep body warm - subcutaneous fat layer
 as an electrical insulator - myelin sheath of neurones
 as a structural component of cell membrane
Steroid
C. STEROIDS
backbone
Steroids consist of 4 rings of carbon atoms.
They form the component of cell membrane (in animal cells only).
They can be used to form hormones and vitamin D.
1.2.3
PROTEINS
Proteins are organic compounds formed from elements carbon, hydrogen, oxygen and nitrogen.
Some proteins may also consist of sulphur.
The basic unit of protein is amino acid.
A. AMINO ACIDS
Each amino acid molecule consist of an amino group (-NH2) and a carboxyl group (-COOH).
The R group is different for different amino acids.
R

H2N-CH-COOH
The structure of an amino acid.
B. PROTEIN (POLYPEPTIDE)
When numerous amino acids are linked together they will form a polypeptide / protein.
Uses of proteins:
 as structural materials in cell membrane and cytoplasm
 as catalyst e.g. enzymes
 for keeping the shape of cells and allow the cell to move – microtubules, microfilaments
 as fuel for respiration
 as hormones to regulate activities of organisms
Notes (Module: How Life Begins)
1.2.4
NUCLEIC ACIDS
Nucleic acids are formed from pentose (a sugar), phosphate and organic nitrogenous bases linked
together.
There are 2 types of nucleic acids: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).
DNA is found inside the nucleus of the cell and acts as the genetic materials of the cell.
RNA is concerned for the synthesis of proteins in the cell.
1.2.5. WATER
Water is necessary for all living organisms. It makes up 65 – 95% by mass of organisms.
Uses:
 as a component of cytoplasm
 as a medium for chemical reactions to take place inside a cell
 as a reactant of certain chemical reactions taken place inside a cell, e.g. in photosynthesis of plant
cells
1.2.6. INORGANIC IONS
Examples of Inorganic Ions Found in Cells
- sodium ion (Na+), potassium ion (K+), calcium ion (Ca2+), magnesium ion (Mg2+), chloride ion
(Cl-), phosphate ion (PO43-), hydrogen carbonate ion (HCO3-)
Uses of inorganic ions in cells:
 Some ions are necessary in enzyme activity, e.g. zinc, magnesium, iron.
 To maintain anion-cation equilibrium, e.g. in neurones
 Form compounds essential for body structural components or metabolism
e.g. PO43- in phospholipids in lipids
magnesium in chlorophyll (found in plant cells)
iron in haemoglobin (found in red blood cells of vertebrates)
 To act as buffers to stabilize pH, e.g. HCO3-.
Notes (Module: How Life Begins)
1.3
STRUCTURE OF CELL
A. ORGANELLES OF ANIMAL AND PLANT CELLS
Organelles are the functional structures inside a cell.
1. Cell Membrane / Plasma membrane
a) Characteristics of cell membrane
It is composed of lipid and protein.
Cell membrane is a lipid bilayer interspersed with protein molecules.
Proteins (integral proteins) may be attached on the inner or outer surface of the cell membrane,
embedded in the cell membrane or extends across the membrane.
Cell membrane encloses the protoplasm to form a barrier between the cell and the external
environment.
It is semi-permeable so it can control the passage of materials in or out of the cell.
Small non-polar molecules e.g. water may pass through the cell membrane freely.
b) Fluid Mosaic Model of Cell Membranes
The phospholipid molecules of the membrane can move sideway.
The proteins of the cell membrane do not form a continuous layer and are not fixed in position.
Positions of the proteins can shift in the plane of the membrane, like icebergs floating on a sea of
phospholipids.
( cholesterol: found in animal cell only; making the cell membrane less fluid)
c) Functions of membrane-bounded proteins
 channels for ions to pass through the membrane
 pumps/carriers for transporting ions across the membrane
 enzymes catalysing chemical reactions at the membrane surface
 attached with carbohydrates forming glycoproteins to serve as cell marker for recognition of cell
types
 receptors for binding hormones or neurotransmitters so as to regulate cellular metabolism
 Protoplasm: fluid inside the cell, provides medium for metabolic reactions
 Protoplasm = Cytoplasm + Nucleoplasm
cell membrane
cytoplasm: fluid surrounded by the cell membrane but
outside the nucleus; site of most metabolic reactions
nuclear
membrane
nucleoplasm: fluid inside the nucleus
Notes (Module: How Life Begins)
2. Mitochondrion (pl. mitochondria)
It is round, oval, sausage shape and
surrounded by a double membrane:
~ outer membrane – smooth
~ inner membrane – folded into numerous
finger-like structure (cristae) to increase
surface area for the processes of
respiration
It has its own DNA which is different
from those inside the nucleus.
It carries out aerobic respiration to release
energy for cellular activities.
inner membrane
outer membrane
folding of inner
membrane (crista)
DNA
3. Endoplasmic Reticulum (ER)
It is a network of folded membranes forming sheets, tubes or flattened sacs in the cytoplasm.
The function of ER is for transport of large molecules inside the cell and other metabolism. It can
provide a structural skeleton to maintain cellular shape.
a) Rough ER
ER attached with ribosomes on its surfaces
It is the site of protein synthesis.
Cells with active protein synthesis contain a lot of RER e.g. liver, enzyme-secreting in the digestive
tract.
Ribosome
- minute structure, not bound with membrane
- occurs freely in cytoplasm or attached with ER
- site of protein synthesis
b) Smooth ER
It is not coated with ribosomes.
It usually appears as tubular structure.
It is concerned with lipid metabolism and abundantly found in cells of steroid synthesizing tissues
e.g. testes, ovaries, adrenal glands
4. Golgi Apparatus / Golgi Body
Golgi apparatus consists of flattened membranous sacs with vesicles bud off from its margin.
Proteins and lipids made in ER are collected, modified and packages them into vesicles by Golgi
apparatus.
The vesicles then moved to other parts of the cell for being secreted or deposited.
5. Lysosome
Lysosomes are vesicles of various sizes containing hydrolytic enzymes to break down cellular
debris, worn-out organelles.
The hydrolytic enzymes break the down the organelle and recover useful biochemicals.
Notes (Module: How Life Begins)
6. Centrioles
It consists of a pair of rod-like structures arranged at right angles to one another.
Spindle fibres (microtubules / filaments of protein) during cell division are formed from the
centrioles.
Centrioles are found in animal cells only.
7. Nucleus
It contains genetic materials to control activities of the cell, the genetic materials are diffused as
deeply stained chromatin.
a) Nucleolus
It is heavily stained round body inside the nucleus, the number may vary from one to several.
It is composed of RNA and protein and forms the site of protein synthesis inside nucleus.
b) Nuclear envelope
It is a double membrane surrounding the nucleus.
There are pores on the nuclear envelope for the passage of large molecules, e.g. RNA.
8. Chloroplast
Chloroplast is a type of plastid. (Plastid is a type of organelle in plant cells to produce and store
food materials.)
It is oval in shape and bounded by a double membrane.
It contains chlorophylls for photosynthesis.
It has its own DNA which is different from those inside the nucleus.
It is found in plant cells.
9. Vacuole
Vacuole is fluid-filled space surrounded by a semi-permeable membrane called tonoplast.
The fluid inside the vacuole is known as cell sap which is a solution of sugar, salts, wastes and
pigments, etc.
It can increase cell surface area and for storage.
It is present in plant cells while animal cells usually do not have vacuoles or the vacuoles are very
small if present.
10. Cell Wall
It is present in plant cells for support and protection of the cell.
Cell wall of plant cells is made up of cellulose.
It is freely permeable.
11. Microtubules and Microfilaments
They are protein fibres inside cells to provide support to the cell, allow transport of materials inside
the cell and movement of the cell.
Notes (Module: How Life Begins)
1.4
COMPARISON BETWEEN PLANT CELLS AND ANIMAL CELLS
Cell wall
Chloroplast
Vacuole
Centrioles
Shape
1.5
Animal Cell
absent
absent
usually absent, very small if present
present
able to change e.g. amoeba, white
blood cells
Plant Cell
present
present
present, usually a large central vacuole
absent
fixed because the cell wall is rigid
PROKARYOTIC CELLS AND EUKARYOTIC CELLS
1.5.1. PROKARYOTIC CELLS
Cells without nucleus are known as prokaryotic cells. The organisms are called prokaryotes, e.g.
bacteria.
No membranous organelles can be found in prokaryotic cells.
Basic structure of a bacterium
1. Nucleoid
The genetic material of bacterium is not surrounded by a membrane. It is in the form of circular
DNA (ring form DNA). The DNA is dispersed in cytoplasm and does not associated with protein.
2. Ribosomes
Small granular structure in the cytoplasm for protein synthesis.
3. Storage granules
These are the nutrients and reserves of the cell, e.g. glycogen, lipids.
4. Capsule
It is a layer of polysaccharide outside the cell wall of bacterium. It protects the bacterial cell from
being phagocytosed by white blood cells or some protozoans.
Notes (Module: How Life Begins)
5. Cell wall
It is made of peptidoglucan (combination of polysaccharide and peptide chains).
6. Cell membrane / Plasma membrane
It is similar to the cell membrane of other cells. It is a lipid bilayer.
7. Mesosomes
It is formed from the infolding (invagination) of the cell membrane into the cytoplasm.
It provides a large surface area for certain metabolic reactions inside the bacterial cells.
8. Pili (si. pilus)
These are hairlike structures made of protein for attachment with other bacterial cells.
They can be used for DNA transfer between cells.
9. Flagella (si. flagellum)
These are threadlike structures for the movement of the bacterial cells.
1.5.2. EUKARYOTIC CELLS
Cells with nucleus (i.e. genetic materials are surrounded by nuclear membrane) are known as
eukaryotic cells. The organisms are called eukaryotes.
Eukaryotic cells have different forms of membranous organelles.
e.g. fungi, algae, protozoans, plants, animals
1.5.3. COMPARISON BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS
Features
Prokaryotic Cells
Eukaryotic Cells
Nucleus
no true nucleus, no nuclear
enclosed in nuclear envelope
envelope
Genetic Material
DNA no associated with protein,
DNA associated with protein
in the form of ring
called histones forming
chromosomes
Cell Wall
present in all except the smallest
variable
prokaryotes; made with different
chemical as eukaryotes
Cell Membrane
present
present
ER
absent
universally present
Organelles
membrane-bounded organelles
present
absent
Photosynthetic
contains chlorophyll, not
present as chloroplast
Apparatus
surrounded by membrane
Ribosomes
free in cytoplasm
free in cytoplasm or associated
with ER
Centrioles
absent
present in animal cells only
Size Range
100 – 2,000 ηm
10,000 – 100,000 ηm
Notes (Module: How Life Begins)
1.6. LEVELS OF ORGANIZATION
1. Cells
Organisms are made up of cells.
Cells are the structural and functional units of living organisms
e.g. blood cells, bone cells, muscle cells
2. Tissues
Cells of similar shape and size grouped together having a particular function is known tissue.
Examples of tissues in animals: bones, muscles, connective tissues, epithelial tissues
Examples of tissues in plants: epidermis, mesophyll, xylem, phloem
3. Organs
Different tissues grouped together to form a functional unit is called an organ
Examples of animal organs: heart, eye, lung, kidney
Examples of plant organs: leaf, flower
4. Organ systems
Organs have related function coordinated to form a system
Organ systems in the human body:
Organ system
Digestive system
Respiratory system
Excretory system
Circulatory system
Nervous system
Endocrine system
Reproductive system
Immune system
Characteristics
- consists of digestive tract and digestive glands
- to digest and absorb food
- consists of lungs, trachea, bronchi
- for gaseous exchange
- consists of kidneys, ureters, urinary bladder, urethra
- removes metabolic wastes and regulates the amount of water and
mineral salts in the body
- consists of heart, blood vessels
- transports food and oxygen to various parts of the body and carries
away wastes and carbon dioxide
- consists of brain, spinal cord and nerves
- to control body activities
- consists of endocrine glands which produce hormone for
coordination
- consists of gametes producing organs (testes in male and ovaries in
female) and external sex organs
- for reproduction
- consists of white blood cells and lymphatic tissues
- to defend the body against diseases
Notes (Module: How Life Begins)
Skeletal system
Muscular system
Integumentary system
- consists of bones and cartilage
- to support and protect the body
- consists of skeletal muscles
- for body movement
- consists of skin, nails and hairs
- to protect the body, regulate body temperature, sensation
5. Organisms
Organism is an individual that can carry out all basic life process. It is usually made up of organ
systems but may be unicellular, e.g. protists, bacteria.
Level of organization in a living organism:
cells  tissues  organs  systems  organism