1. Carbohydrates
1. The carbohydrates are made up of carbon, hydrogen and oxygen. The ratio of hydrogen to oxygen
atoms in the molecules is usually 2 : 1.
2. Many carbohydrates have the general formula Cx (H2O)y, where x is approximately equal to y.
3. The three basic types of carbohydrates are monosaccharides, disacharides and polysaccharides.
Monosaccharides
1. Monosaccharides are also called simple sugars.
2. The common monosaccharides are six-carbon sugars with a molecular formula of C6H12O6.
3. Examples of monosaccharides are glucose, fructose (fruit sugar) and galactose.
4. Glucose is the most common monosaccharides and respiratory substrate.
5. Monosaccharides are sweet-tasting crystalline substances which are soluble in water.
Disacharides
1. Disacharides are formed from two monosaccharides molecules combining together with the
elimination of a molecule of water. The chemical reaction of the formation is known as
condensation.
2. The general formula of a disacharide is C12H22O11.
3. Disacharides are also called double sugars.
4. Disacharides can be broken down to their constituent monosaccharides by a chemical reaction
involving the addition of water. The reaction is known as hydrolysis.
5. Like monosaccharides, they are sweet-tasting crystalline substances that are soluble in water.
6. The most common disacharides are maltose, lactose and sucrose.
Polysaccharides
1. Many monosaccharides molecules can be added by a series of condensation reactions to form a
very large molecule (macromolecule).
2. Polysaccharides are polymers of monosaccharide monomers.
3. Polysaccharides have a general formula of (C6H10O5)n, where n is the number of monomers.
4. The large size of polysaccharides makes them more or less insoluble in water. They are not
sweet and cannot be crystallised.
5. The common polysaccharides are starch, glycogen and cellulose.
6. The addition of iodine solution to a solution containing starch yields a blue-black colour.

2. Proteins
About two-thirds of the total dry mass of a cell is composed of proteins.
Proteins contain carbon, hydrogen, oxygen and nitrogen. Sulphur is often present and sometimes
phosphorus and other elements too.
Amino acids are the monomers of all proteins.
A dipeptide consists of 2 molecules of amino acids that are linked together by a peptide bond through
condensation. Conversely, a dipeptide can be broken down into amino acids by means of hydrolysis.
Amino acid + amino acid —condensation–> dipeptide + water
Dipeptide + water —hydrolysis–> amino acid + amino acid
Long chains of amino acids are called polypeptides. A protein consists of one or more polypeptide
chains twisted and folded in an appropriate way.
Protein Structures
1. Primary Structure
• The linear sequence of amino acids in a protein molecule. The first protein to have its sequence
of amino acids determined was the hormone insulin.
2. Secondary Structure
• Peptide chain becomes folded or twisted forming a helix or pleated sheet. The structure is
maintained by hydrogen bonding.
3. Tertiary Structure
• Bending and twisting of the polypeptide helix into a compact structure. Structure is maintained
by ionic, disulphide and hydrogen bonding. Myoglobin, a protein found in muscles, has the
tertiary structure. Enzymes, antibodies and many hormones have this structure as well.
4. Quaternary Structure
• Different polypeptide chains combine with associated non-protein groups forming a large
complex protein molecule. Haemoglobin, an oxygen-binding protein, has a quaternary structure.
The peptide bond can be broken by hydrolysis with heat, dilute acids or by enzymes.
The loss of the three-dimensional structure of a protein molecule is known as denaturation.
Denaturation can be caused by changes in temperature, pH and salt concentration.
Heating proteins usually denatures the protein irreversibly. For example, the transparent egg white
irreversibly solidifies and becomes opaque on boiling.
Essential and non-essential amino acids
• In the digestive tract, protein is broken down into its amino acid subunits by hydrolysis.
• Humans can synthesise 11 of the 20 different amino acids used in protein synthesis. These 11
amino acids are known as non-essential amino acids.
• The remaining 9 amino acid cannot be synthesised in human. They must be included in the diet.
They are known as essential amino acids.
• Examples of essential amino acids are lysine, valine, leucine and tryptophan.
• Examples of non-essential amino acids are proline, glycine and glutamic acid.

3. PROTEIN STRUCTURES

4. Lipids
Lipids are a diverse group of substances that contain carbon, hydrogen and oxygen. The proportion of
oxygen is lower than that in carbohydrates. For example, the general formula of stearic acid is
C18H36O2.
All lipids are insoluble in water.
Lipids dissolve readily in other lipids and in organic solvents such as ether and ethanol.
The main types of lipids are:
• Fats
• Oils
• Waxes
• Phospholipids
• Steroids
Fats and Oils
Fats are solid at room temperature (20′ C), whereas oil are liquid.
Each molecule of fats or oils is made up of one glycerol combined with three fatty acids which may be
the same or may be different. Three molecules of water are removed in this condensation reaction.
If the process is inversed, the process is called hydrolysis.
These molecules of fats and oils are known as triglycerides.
Fats often contain only saturated fatty acids and oils usually contain unsaturated fatty acids
In a saturated fatty acid, the carbon atoms are bonded to the maximum number of other atoms.
Saturated fatty acid has only single bond and the hydrocarbon chain is relatively straight.
Unsaturated fatty acids has double bonds in the form of -CH=CH- in the hydrocarbon chain. Fatty acids
with one double bond are called monounsaturated fatty acid; those with two or more double bonds are
called polyunsaturated fatty acids.
Fats and oils function efficiently as energy storage material. Fats and oils provide 38kJ per gram, while
carbohydrates can provide only 17kJ per gram.
Waxes
Waxes are similar to triglycerides, but the fatty acids are bonded to long-chain alcohols rather than
glycerol.
Waxes are usually hard solids at room temperature.
Waxes are used to waterproof the external surfaces of plants and animals. The cuticle of a leaf and the
protective covering on an insect’s body are made of waxes.
Wax is also a constituent of the honeycomb of bees.

5. Phospholipids
Phospholipids have a similar structure to triglucerides but one of the fatty acids is replaced by a
phosphate group.
The end of the phospholipid molecule containing the phosphate group is hydrophilic (water-loving).
The other end containing the hydrocarbon chains of the fatty acids is hydrophobic (water-hating).
The hydrophilic end is soluble in water while the hydrophobic end is insoluble in water.
Phospholipid bilayers (double layers) form the basis of all cell membranes.
Steroids
A steroid molecule has a complex ring structure.
Steroids occur in plants and animals.
Examples of steroids are cholesterol, testosterone, oestrogen and progesterone.
• Cholesterol – Strengthens the cell membrane at high temperatures.
• Testosterone – Male reproductive hormone
• Oestrogen, progesterone – Female reproductive hormones
Saturated and Unsaturated Fats
Animal fats such as lard, butter and cream are examples of saturated fats
Vegetable oils such as olive oil, corn oil and sunflower oil are examples of unsaturated fats.

6. Enzymes
Enzymes are proteins which act as biological catalysts. They speed up biochemical reactions in the cell.
The substance whose reactivity is increased by an enzyme is known as substrate.
Example:
substrate —enzyme–> products
sucrose + water —sucrase–> glucose + fructose
Thousands of simultaneous biochemical reactions occur in living cells. Without enzymes, these
biochemical reactions would be too slow to sustain life.
General characteristics of enzymes
1. Enzymes work very rapidly
• One molecule of enzyme can turn thousands or millions of substrate molecules into products per
minute. For example, catalyse can transform approximately six million hydrogen peroxide
molecules into oxygen and water molecules per minute.
2. Enzymes are not destroyed by the reactions which that catalyse
• Since enzymes are not altered by the reactions they catalysed, they can be used again. A smaill
concentration of enzymes can bring about a large amount of biochemical reactions
3. Enzyme-catalysed reactions are reversible
• lactose + water —lactase–> glucose + galactose
• lactose + water <–lactase— glucose + galactose
• The enzyme which catalyses a reaction works in such a way that the reaction can proceed from
left to right or from right to left, depending on circumstances. Note the two way arrows.
4. Enzymes are extremely specific
• Most enzymes are specific to one particular substrate molecule. Thus, a given enzyme will
catalyse only one reaction or one type of reaction. Maltase, for example, acts only on maltose.
5. Enzymes are denatured by high temperature
• An enzyme inactive at very low temperature. As temperature rises, its activity increases until the
optimum temperature is reached. The optimum temperature is around 40′ C. Above the
optimum temperature, the rate of reaction decline rapidly, ceasing altogether at about 60′ C.
This is because enzymes are made of protein, so they are denatured at high temperature. When
an enzyme becomes denatured, the bonds are broken and the polypeptide chains open up. The
enzyme loses its normal shape and becomes inactive.
6. Enzymes are sensitive to pH
• Every enzymes has its own optimum pH in which it functions best. Small changes in the pH of
the medium will denature the enzyme and render its activity. Alterations in the ionic charges of
the acidic and basic groups of the enzyme change the shape of the enzyme.
Naming of enzymes based on the substrate

7. • An enzyme is named by attaching the suffix -ase to the name of the substrate on which it acts.
For example, maltase acts on maltose, sucrase on sucrose and cellulase on cellulose.
• The ‘-ase‘ rule does not apply to enzymes discovered before the ‘-ase‘ idea was introduced. For
example, pepsin rennin, ptyalin and trypsin.
Intracellular and extracellular enzymes
• Enzymes can be divided into two groups: intracellular and extracellular.
• Enzymes formed and retained in the cell are known as intracellular enzymes, and occur in the
cytoplasm, organelles or the nucleus. Examples of intracellular enzyme are DNA polymerase,
RNA polymerase and ATP synthetase.
• Extracellular enzymes are produced in the cell then packed and secreted from the cell,
Extracellular enzymes caralyse their reactions outside the cell. Most digestive enzymes are
extracellular enzymes. For example, amylase, cellulase and zymase.
Site of Enzyme Synthesis
• Since enzymes are made of proteins, they are synthesised by ribosomes.
• Intracellular enzymes are synthesised on ‘free’ ribosomes.
• Extracellular enzymes are synthesised on ribosomes attached to the endoplasmic reticulum.
Formation and secretion of extracellular enzymes:
1. The instruction for making the extracellular enzyme is transcribes from deoxyribonucleic acid
(DNA) to ribonucleic acid (RNA) in the nucleus.
2. The RNA then leaves the nucleus through the nuclear pore and attaches itself to the ribosome
located on the endoplasmic reticulum.
3. When the enzyme synthesis has completed, it is extruded into the interior of the endoplasmic
reticulum.
4. The enzyme is then encapsulated in a transport vesicle.
5. The transport vesicle fuses with the Golgi apparatus, releasing the enzyme into the Golgi
apparatus.
6. In the Golgi apparatus the enzyme is further modified before packing the enzyme in a secretory
vesicle.
7. The secretory vesicle transports the enzyme to the plasma membrane.
8. The secretory vesicle membrane fuses with the plasma membrane and the enzyme is release
outside the cell.
Mechanism of enzyme action

8. • Each enzyme molecule has a region with very precise shape called the active site.
• The substrate molecule fits into the active site of the enzyme like a key into a lock.
• Various types of bonds including hydrogen bonds and ionic bonds hold the substrate(s) in the
active site to form a enzyme-substrate complex.
• The enzyme then changes the substrate(s) either by splitting it apart (for example, hydrolysis) or
linking them together (for example, condensation)
• Once formed, the products no longer fit into the active site and escape into the surrounding
medium, leaving the active site free to receive further substrate molecules.
enzyme+substrate —enzyme-substrate complex–> enzyme+product
• The explanation of enzyme action is known as the ‘lock and key hypothesis’, where the
substrate is like a key whose shape is complementary to the enzyme or lock.
• The ‘lock and key’ hypothesis is able to explain why enzymes are specific and why any change
in enzyme shape alters its effectiveness.
Factors afftecting enzymes
1. pH
• Most enzymes are effective in only a narrow pH range.
• The optimum pH is the particular pH at which the rate of reaction is the highest.
• Deviations from the optimum pH decrease the rate of reaction because bonds maintaining the
tertiary shape of the enzyme are broken.
• The active site loses its shape and the enzyme-substrate complex can no longer be formed. The
enzyme is denatured.
2. Temperature
• Initially an increase in temperature leads to an increase in the rate of reaction because the
kinetic energy of the enzyme and substrate molecules produce more collisions, and therefore
more enzyme-substrate complexes are formed.
• The rate of reaction will increase up to a maximum, known as the optimum temperature.
• After the optimum temperature, the rate of reaction falls quickly because the bonds maintaining
the structure of the enzyme start to break and the active site loses its shape.
• The enzyme-substrate complexes can no longer form and the enzyme is denatured.

9. Substrate Concentration
• Initially an increase in substrate concentration increases the chance of enzyme-substrate
collisions, and the rate of reaction increases.
• Eventually all the active sites are filled at any one time and the rate remains constant The
reaction has reached its maximum rate, Vmax.
• Further addition of substrate will not increase the rate of reaction anymore because the constant
enzyme concentration becomes the limiting factor.

10. 4. Enzyme Concentration
• As the concentration of the enzyme increases there are more chances of enzyme-substrate
collisions. The rate of reaction increases linearly as long as no other factors are limiting.
• As more active sites are available, more substrates can be converted to products.