Water is a polar covalent compound.

Due to difference in electro-negativities of oxygen and

hydrogen, one end of the water molecule bears a slight negative charge, while the other end bears
a slight positive charge. This is called the dipolar nature of water.

The positive end of one water molecule is attracted to the negative end of another water
molecule. This force of attraction is called a hydrogen bond.
• The polar nature of water molecule makes it a good solvent.
Almost all ionic compounds (NaCl, KNO3, (NH4)2SO4, etc.) and small organic
compounds (Glucose, amino acids, Glycerol, Fatty acids) are soluble in water. This
enables easy transport of materials. e.g.; sap through xylem and phloem, Glucose, amino
acids, Hormones, etc. in blood. Water also provides a good medium for soluble
substances to collide with each other and react. These collisions would not be possible or
would be too slow in solid state. e.g.; Enzyme-substrate collisions.
• Water has a high latent heat of vaporization - this means that when water evaporates
from the surface of a body, it takes away a lot of heat from the body surface, thus cooling
it. This makes water a good coolant. This is especially useful in cooling of plant tissues
by transpiration and cooling of mammals by sweating or panting.
• Water has a high specific heat capacity - This means that a lot of heat must be added /
removed to change the temperature of water. This property of water prevents sudden
fluctuations in temperature of organisms or aquatic environment. The gradual change in
temperature gives organisms enough time to cope with the change. The high specific heat
capacity also helps to resist temperature changes (maintain constant temperature).
• The density of pure water is 1g/cm3.The density of water changes with temperature.
Thus in aquatic habitats there will be layers of water with different densities. The
differences in density cause the circulation of water and nutrients within the habitat, thus
affecting the vertical distribution of organisms. Water has maximum density at 40 C.
This means that the densest water (at 40 C) will remain at the bottom of an aquatic
habitat. This prevents aquatic habitats from freezing completely, so that aquatic
organisms can survive at the bottom (unfrozen at 40 C).
• Surface tension is the property of a liquid which makes its surface behave like a stretched
membrane, mainly caused due to hydrogen bonding between molecules (water). This is
especially useful to some aquatic invertebrates that can skate or lay eggs on the water
surface. Mosquito larvae also use the surface tension of water to cling to the surface and
breathe air, through siphons. Surface tension decreases the ease with which gases
dissolve into water.
Carbohydrates
Carbohydrates contain only the elements carbon, hydrogen and oxygen. The group includes
Monosaccharides
These all have the formula (CH2O)n, where n can be 3-7. The most common and important
monosaccharide is glucose, which is a six-carbon or hexose sugar, so has the formula C6H12O6.
Its structure Glucose forms a six-sided ring. In animals glucose is the main transport sugar in the
blood, and its concentration in the blood is carefully controlled.

There are many isomers of glucose, with the same chemical formula (C6H12O6), but different
structural formulae. These isomers include galactose and fructose:

Disaccharides
Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond
(C–O–C). The reaction involves the formation of a molecule of water (H2O):

This shows two glucose molecules joining together to form the disaccharide maltose. This kind
of reaction, where two molecules combine into one bigger molecule, is called a condensation
reaction. The reverse process, where a large molecule is broken into smaller ones by reacting
with water, is called a hydrolysis reaction. In general: · polymerisation reactions are
condensations· breakdown reactions are hydrolyses
Polysaccharides
Starch
It is polysaccharides made up of many α glucose residues linked by glycosidic bonds. Starch is a
mixture of amylase and amylopectin.
Functions: Energy storage molecule in plant cells.
Structure related to function;
 Compact: takes up less space in the cell.
 Insoluble: cannot leave the cell easily.
 Insoluble: No osmotic effect.
 Insoluble/unreactive: does not get involved in chemical reactions in cell.
 Can easily be hydrolyzed by enzymes into glucose and used for respiration.

Glycogen
This is also polymer of α glucose residues linked by 1.4 and 1,6 glycosidic bonds. It is highly
branched (branches after every 8 to 10 glucose residues). 1, 4 - glycosidic bonds are in the
unbranched part of glycogen, while 1, 6 - glycosidic bonds are responsible for formation of
branches.
Structure: Similar to amylopectin, but branches more frequently.
Functions: Energy storage molecule in animal cells (liver and muscle cells), and bacterial cells.
Structure related to function;
 Compact: takes up less space in the cell.
 Insoluble: cannot leave the cell easily.
 Insoluble: No osmotic effect.
 Insoluble / unreactive: does not get involved in chemical reactions in cell.
 Can easily be hydrolyzed by enzymes into glucose and used for respiration
Lipids

▪ Lipids are made of the elements Carbon , Hydrogen and Oxygen, although they have a
much lower proportion of water than other molecules such as Carbohydrates. They are
insoluable in water
▪ Lipids perform many functions, such as:
• Energy Storage
• Making Biological Membranes
• Insulation
• Protection - e.g. protecting plant leaves from drying up
• Boyancy
• Acting as hormones
▪ They are made from two molecules: Glycerol and Fatty Acids.
▪ A Glycerol molecule is made up from three Carbon atoms with a Hydroxyl
Group attached to it and Hydrogen atoms occupying the remaining positions.

Fatty Acids

▪ Fatty acids consist of an Acid Group at one end of the molecule and a Hydrocarbon
Chain, which is usually denoted by the letter ‘R’.
Fatty acids may be saturated or unsaturated. A fatty acid is saturated if every possible bond is
made with a Hydrogen atom, such that there exist no C=C bonds. Saturated fatty acids on the
other hand do contain C=C bonds. Obviously monounsaturated fatty acids have one C=C bond,
and polyunsaturated have more than one C=C bond.

▪ If fatty acids are unsaturated, their shape is altered from a saturated molecule so the
molecules in the Lipid push apart, thus making it more fluid and oily.
▪ Animals tend to have more saturated, and consequently solid at room temperature
lipids whereas plants tend to have more unsaturated and so fluid at room temperature
lipids.

Triglycerides

▪ Triglycerides are lipids consisting of one glycerol molecule bonded with three fatty acid
molecules. The bonds between the molecules are covalent and are called Ester bonds.
They are formed during a condensation reaction.

▪ Triglycerides are hydrophobic and so insoluble in water. The charges are evenly
distributed around the molecule so hydrogen bonds to not form with water molecules.

Phospholipids

▪ Phospholipids are similar to triglycerides in they consist of a glycerol ‘backbone’ and

fatty acid ‘tails’, however, the third fatty acid has been replaced by a phosphate group
‘head’.
The structure of a phospholipid

▪ While the fatty acid ‘tails’ are hydrophobic, the phosphate ‘head’ is hydrophilic. This
means the phosphate group will orientate itself towards water and away from the rest
of the molecule, and also gives rise to the special properties that allow phospholipids to
be used to form membranes.
▪ Phospholipids can contain saturated and unsaturated fatty acids. This allows for the
control of the fluidity of membranes, which is useful, for example, in maintaining
membrane fluidity at low temperatures.

Proteins
Amino acids are the building blocks of all proteins.
The general structure of all amino acids involves one ‘central’ Carbon atom with bonds to 4
groups:

1. Hydrogen atom
2. Carboxyl group
3. Amine group
4. R group

The R group is the variant in the amino acid - there are 20 different R groups, meaning there are
20 different types of amino acid. For example, the simplest R group is a single H atom, and this
amino acid is called Glycine.

Dipeptide = two amino acids joined together by a peptide bond

Polypeptide = long chain of two or more amino acids joined together by peptide bonds. A
functional protein may contain one or more polypeptides
Peptide bonds

• Formed in condensation reactions

• Result in a bond between the carboxyl group of one amino acid (aa) and the hydroxyl
group of another
• C-N bond
Primary structure of a protein

➢ The primary structure is the sequence of amino acids in a polypeptide chain. This
sequence is determined by the genetic code on DNA.
➢ The primary structure determines the secondary, tertiary or quaternary structure of
a protein.
Secondary structure of a protein
➢ The folding of the polypeptide chain ( primary structure ) into helices and pleated
sheets, due to the formation of hydrogen bonds between the R-Groups of amino
acids, results in the secondary structure.

Tertiary structure of a protein

Tertiary structure of a protein is the complex three - dimensional shape the polypeptide chain
takes when the polypeptide helix ( secondary structure ) twists and folds around it self .The
tertiary structure is maintained by Hydrogen bonds , disulphide bridges ( covalent bonds) and
ionic bonds between the R groups of amino acids. Hydrophobic interactions also help to
maintain the shape of globular proteins ( Eg: enzymes ).
Quaternary structure of a protein
Quaternary structure is the linking together of two or more
polypeptide chains.
Examples:
• Haemoglobin consist of four polypeptide chains,
• Insulin consists of two polypeptide chains,
• Collagen consists of three polypeptides chains.

Haemoglobin and Collagen

▪ Haemoglobin is a water soluble globular protein which is composed of two α

polypeptide chains, two β polypeptide chains and an inorganic prosthetic haem group.
Its function is to carry oxygen around in the blood, and it is facilitated in doing so by the
presence of the haem group which contains a Fe2+ ion, onto which the oxygen molecules
can bind.
▪ Collagen is a fibrous protein consisting of three polypeptide chains wound around each
other. Each of the three chains is a coil itself. Hydrogen bonds form between these coils,
which are around 1000 amino acids in length, which gives the structure strength. This is
important given collagen’s role, as structural protein. This strength is increased by the
fact that collagen molecules form further chains with other collagen molecules and
form Covalent Cross Linkswith each other, which are staggered along the molecules to
further increase stability. Collagen molecules wrapped around each other form Collagen
Fibrils which themselves form Collagen Fibres.
▪ Collagen has many functions:
▪ Form the structure of bones
▪ Makes up cartilage and connective tissue
▪ Prevents blood that is being pumped at high pressure from bursting the walls of
arteries
▪ Is the main component of tendons, which connect skeletal muscles to bones
▪ Haemoglobin may be compared with Collagen as such:
▪ Basic Shape - Haemoglobin is globular while Collagen is fibrous
▪ Solubility - Haemoglobin is soluble in water while Collagen is insoluble
▪ Amino Acid Constituents - Haemoglobin contains a wide range of amino acids
while Collagen has 35% of it primary structure made up of Glycine
▪ Prosthetic Group - Haemoglobin contains a haem prosthetic group while
Collagen doesn’t possess a prosthetic group
▪ Tertiary Structure - Much of the Haemoglobin molecule is wound into α helices
while much of the Collagen molecule is made up of left handed helix structures