Covalent bonds - are strong. they are the main bonds holding the atoms together in the organic molecules in living organisms. Because they are strong they don't break or form spontaneously at the temperatures found in living cells. Therefore covalent bonds are always broken or made by the action of enzymes.

Hydrogen bonds - much weaker. Formed between an atom with a slight positive charge and an atom with a slight negative charge. Due to there weak form they can break and form spontaneously at the temperatures found in living cells without the need of enzymes.

Charged or polar molecules such as salts, sugars, amino acids dissolve readily in water and so are called hydrophilic ("water loving").

Uncharged or non-polar molecules such as lipids do not dissolve so well in water and are called hydrophobic ("water hating").

Water molecules "stick together" due to their hydrogen bonds, so water has high cohesion.

Carbohydrates contain only the elements carbon, hydrogen and oxygen. The group includes monomers (monosaccharides), dimers (disacchairdes) and polymers (polysaccharides).

All have the formula (CH2O)n where n can be 3-7.

Most important monosaccharide is glucose, which is a hexose sugar (six carbon) so has the formular C6H12O6. There are many isomers of glucose with the same chemical formula but different structural formulae. These isomers include galactose and fructose. Common pentose sugars (n=5, C5H10O5) include ribose and deoxyribose and ribulose. Triose sugars (where n=3, C3H6O3) are found in respiration and photosynthesis.

Formed when two monosaccharides are joined together by a glycosidic bond (C-O-C). (The reaction incloves the formation of a molecule of water)

The reaction where two molecules combine into one bigger molecule is called condensation reaction. The reverse process is called hydrolysis reaction.

Three common disaccharides:

-Maltose is glucose-glucose. Formed on digestion of starch by amylase.

-Sucrose is glucose-fructose. Common in plants as it is less reactive than glucose and it is their main transport sugar.

-Lactose is galactose-glucose. Found only in mammalian milk and is the main source of energy for infant mammals.

Triglycerides are made of glycerol and fatty acids.

Glycerol is a small, 3-carbon molecule with three alcohol (OH) groups.

Fatty acids are long molecules made of non-polar hydrocarbon chain with a polar carboxyl acid group at one end. Because the length of the hydrocarbon chain can vary it is sometimes called an R group.

One molecule of glycerol joins together with three fatty acid molecules by ester bonds to form triglyceride molecule, in an condensation polymerisation reaction.

Trigluceride are used for storage, insulation and protection in fatty tissue found under the skin or surround organs.

If the fatty acid chains in a triglyceride have no C=C double bonds then they are called saturated fatty acids (saturated with hydrogen). 

Triglycerides with saturated fatty acids:

-Have high melting point (at room temp are solids e.g fats)

-Are found in warm-blooded animals.

If the fatty acid chains in triglyceride do have C=C double bonds they are called unsaturated fatty acids (unsaturated with hydrogen). Fatty acids with more than one double bond are called polu-unsaturated fatty acids. 

Triglycerides with unsaturated fatty acids:

-Have low melting point (at room temp are liquids e.g oils)

-are found in cold-blooded animals and plants.

Phospholipids have a similar structure to triglycerides, but with a phosphate group in place of one fatty acid chain. There may also be other groups attached to the phosphate. 

Phospholipids have a polar hydrophilic head(the negatively-charged phosphate group) and two non-polar hydrophobic tails(the fatty acid chains)

When mixed with water, phospholipids form droplet spheres with a double-layered phospholipid bilayer. The hydrophillic heads facing the water and the hydrophobic tails facing eachother. This traps a comparement of water in the middle separated from the external water by the hydrophobic sphere. This naturally-occuring structure is called a liposome and is similar to a membrane surrounding a cell.

Structure  - e.g collagen (bone, cartilage, tendon), karatin (hair)

Enzymes  - e.g amylase, pepsin, catalase

Transport - e.g haemoglobin (oxygen), transferrin (iron)

Pumps     - e.g Sodium potassium pump in cell membranes

Motors     - e.g myosin (muscle), kinesin (cilia)

Hormones - e.g insulin, glucagon

Receptors - e.g rhodopsin (light receptor in retina) 

Antibodies - e.g immunoglobulins

Storage    - e.g albumins in eggs and blood, caesin in milk 

Proteins are made of amino acids. 

Amino acids are made of the five elements C, H, O, N, S.

The general structure of an amino acide molecule is a central carbon atom (called the 'alpha carbon') with four different chemical groups attached to it:

1. a hydrogen atom

2. a basic amino group (NH2 or NH3+)

3. an acidic carboxyl group (COOH or COO-)

4. a variable "R" group (or side chain)

There are 20 different R groups therefore 20 different amino acids