BIOLOGY TEST, MONDAY OCTOBER 21ST: REVISION SHEET AND PRACTICE
QUESTIONS
This test will be based on Chapters 3 and 4 of your IGCSE book (Pages 20 – 38).
Note that you will also be asked on the additional material that we covered in the
class presentations.
You must know the key definitions, which relate to:
 Diffusion (p. 21) and Facilitated diffusion (you should also know the differences
and smiliarities between diffusion and facilitated diffusion)
 Osmosis (p. 23)
 Active transport (protein pumps and endocytosis/exocytosis) (p. 29)
You must know the key ideas featured on Page 30 and Page 89
You should revise for this test using the Revision Questions on Page 30 and Page
38.
You should be familiar with the principles and definitions used in your potato
osmosis lab.
BASIC BIOCHEMISTRY
1 State that the most frequently occurring chemical elements in living things are
carbon, hydrogen, oxygen and nitrogen
The approximate proportions of the four main elements in living things are:
• Carbon (19%)
• Hydrogen (10%)
• Oxygen (65%)
• Nitrogen (3%)
2. Be able to draw and label a diagram showing the structure of water molecules to
show their polarity and hydrogen bond formation
Structure of a Water Molecule:
▪ Water (H2O) is made up of two hydrogen atoms polar-covalently bound to an
oxygen atom
▪ While this bonding involves the sharing of electrons, they are not shared equally
▪ The oxygen atom, having more protons (+ve), attract the electrons (-ve) more
strongly (i.e. the oxygen has a higher electronegativity)
▪ Thus the oxygen atom becomes slightly negative and the hydrogen atoms
become slightly positive
Hydrogen Bonding between Water Molecules
▪ Covalently bonded molecules that have a slight potential charge are said to be
polar
▪ The slightly charged regions of the water molecule can attract other polar or
charged compounds
▪ Water molecules can associate via weak hydrogen bonds (F/O/N bonding with H)
Structure and Bonding of Water Molecules
3. Outline the thermal, cohesive and solvent properties of water
Thermal Properties
▪ Water has a high specific heat capacity (the measure of energy required to raise
the temperature of 1 g of substance by 1°C)
▪ Water has a high heat of vaporisation (amount of energy absorbed per gram as it
changes from a liquid to a gas / vapour)
▪ Water has a high heat of fusion (amount of energy required to be lost to change 1
g of liquid to 1 g of solid at 0°C)
▪ These properties occur as a result of the extensive hydrogen bonding between
water molecules - this allows water to absorb considerable amounts of
energy with little change in form (H-bonds need to be broken first)
Cohesive Properties
▪ Water molecules are strongly cohesive (they tend to stick to one another)
▪ Water molecules will also tend to stick to other molecules that are charged or
polar (adhesion)
▪ These properties occur as a result of the polarity of a water molecule and its
ability to form hydrogen bonds with appropriate molecules
Solvent Properties
▪ Water can dissolve many organic and inorganic substances
▪ This occurs because the polar attraction of large quantities of water molecules
can sufficiently weaken intra-molecular forces (such as ionic bonds) and
result in the dissociation of the atoms
Other Properties
▪ Water expands when frozen, becoming less dense / lighter (important for life on
earth - oceans don't freeze)
3.1.6 Explain the relationship between the properties of water and its use in living
organisms as a coolant, medium for metabolic reactions and transport medium
Coolant
▪ Both plants and animals use the evaporation of water from the surfaces of their
bodies to facilitate cooling (sweating and panting in animals, transpiration
from leaves in plants)
Medium for Metabolic Reactions
▪ Water can dissolve many organic and inorganic substances to facilitate chemical
reactions
▪ (Water can also absorb thermal energy released as a by-product of many chemical
reactions)
Transport Medium
▪ The forces of attraction between water molecules help facilitate the transport of
water up the xylem of plants
▪ Water is an effective transport medium for dissolved substances (in plants,
minerals from the soil and sugars from the leaves can be transported in
water through the xylem and phloem respectively; while in animals, water in
the blood is used to transport oxygen, glucose and urea)
Surface Tension
▪ The force of attraction between water molecules makes water sufficiently dense
for some smaller organisms to move along its surface
CARBOHYDRATES
Carbohydrates are organic compounds consisting of one or more simple sugars that
as monomers follow the general basic formula of (CH2O)x
Note: Exceptions to this basic formula and the inclusion of other atoms (e.g. N) can
occur
1. Identify glucose from diagrams showing ITS structure
Glucose (C6H12O6)
2. List examples of monosaccharides, disaccharides and polysaccharides
Monosaccharides: Glucose, galactose, fructose
Disaccharides: Lactose, maltose, sucrose
Polysaccharides: Cellulose, glycogen, starch
3. State one function of glucose, lactose and glycogen in animals and of fructose,
sucrose and cellulose in plants
Animals
Glucose: A source of energy which can be broken down to form ATP via cellular
respiration
Lactose: A sugar found in the milk of mammals, providing energy for suckling
infants
Glycogen: Used by animals for short term energy storage (between meals) in the
liver
Plants
Fructose: Found in honey and onions, it is very sweet and a good source of energy
Sucrose: Used primarily as a transportable energy form (e.g. sugar beets and sugar
cane)
Cellulose: Used by plant cells as a strengthening component of the cell wall
4. Outline the role of condensation and hydrolysis in the relationship between
monosaccharides, disaccharides and polysaccharides
▪ Condensation reactions occur when molecules are covalently joined together and
water is formed as a by-product
▪ The opposite of a condensation reaction is a hydrolysis reaction, which requires a
water molecule to break a covalent bond between two subunits
▪ Monosaccharides are single monomers that are joined to form disaccharides,
while sugars containing multiple subunits (more than 10) are called
polysaccharides
A Condensation Reaction between Two Monosaccharides
LIPIDS
Lipids are a group of organic molecules that are insoluble in water but soluble in
non-polar organic solvents
Common lipids include triglycerides (fats and oils), phospholipids and steroids
1. Identify fatty acids from diagrams showing their structure
General Structure
(double bonds)
Saturated (no double bonds)
Unsaturated
2. State three functions of lipids
Structure: Phospholipids are a main component of cell membranes
Hormonal signalling: Steroids are involved in hormonal signalling (e.g. estrogen,
progesterone, testosterone)
Insulation: Fats in animals can serve as heat insulators while sphingolipids in the
myelin sheath (of neurons) can serve as electrical insulators
Protection: Triglycerides may form a tissue layer around many key internal organs
and provide protection against physical injury
Storage of energy: Triglycerides can be used as a long-term energy storage source
3. Compare the use of carbohydrates and lipids in energy storage
Similarities:
▪ Complex carbohydrates (e.g. polysaccharides) and lipids both contain a lot of
chemical energy and can be used for energy storage
▪ Complex carbohydrates and lipids are both insoluble in water - they are not easily
transported
Differences:
▪ Lipid molecules contain more energy per gram than carbohydrates (about twice
as much)
▪ Carbohydrates are more readily digested than lipids and release their energy
more rapidly
▪ Monosaccharides and disaccharides are water soluble and easier to transport to
and from storage sites than lipids
▪ Animals tend to use carbohydrates primarily for short-term energy storage, while
lipids are used more for long-term energy storage
▪ Carbohydrates are stored as glycogen in animals while lipids are stored as fats (in
plants carbohydrates are stored as cellulose and lipids as oils)
PROTEINS
Proteins are large organic compounds made of amino acids arranged in a linear
chain
The sequence of amino acids in a protein is defined by a gene and encoded in the
genetic code
1. Identify amino acids from diagrams showing their structure
Generalised Structure of an Amino Acid
2. Outline the role of condensation and hydrolysis in the relationship between amino
acids and polypeptides
▪ A condensation reaction occurs between the amino group (NH2) of one amino
acid and the carboxylic acid group (COOH) of another amino acid
▪ This reaction forms a dipeptide (plus a molecule of water) that is held together by
a peptide bond
▪ Multiple amino acids can be joined together to form a polypeptide chain
▪ In the presence of water, polypeptides can be broken down into individual amino
acids via hydrolysis reactions
Formation of a Dipeptide
Functions of proteins
Structure: Support for body tissue (e.g. collagen, elastin, keratin)
Hormones: Regulation of blood glucose (e.g. insulin, glucagon)
Immunity: Bind antigens (e.g. antibodies / immunoglobulins)
Transport: Oxygen transport (e.g. haemoglobin, myoglobin)
Movement: Muscle contraction (e.g. actin / myosin, troponin / tropomyosin)
Enzymes: Speeding up metabolic reactions (e.g. catalase, lipase, pepsin)
FOOD TESTS
You must know how to test for the presence of simple (reducing) sugars, starch,
protein and fat in different food stuffs.
To revise, you can use this link:
http://www.bbc.co.uk/bitesize/ks3/science/organisms_behaviour_health/food_d
etective/activity/
A Food Test is a chemical test which determines the presence of biological
molecules commonly found in food. These biological molecules include sugars reducing and non-reducing, starch, protein and lipids.
Food tests use reagents which react predictably with the biological molecules to
give observable color changes. More than one test - involving the use of different
reagents, or with an extended procedure - exists for the same biological.
Common Biological Molecules and their Tsts
Biochemical
Reducing Sugar
Food Test
Benedict's Test for Reducing Sugar
Non-Reducing Sugar
Benedict's Test for Non-reducing Sugar
Starch
Iodine Test for Starch
Protein
Biuret Test
Lipid
Ethanol Emulsion Test
Practice Questions from past IGCSE papers
Practice Questions on Cell membrane and transport mechanisms