Proteins are the most abundant organic compounds in cells and constitute 50% of total dry weight Proteins are compounds which are made up of carbon, hydrogen, nitrogen, oxygen and sometimes sulphur and phosphorus.
The structural units of proteins are amino acids The nature of a protein is determined by the types of amino acids it is made of There are about 20 common amino acids that make up proteins
Essential and Non-Essential Amino Acids
- Essential amino acids are those which cannot be synthesised in the body of an organism and must therefore be provided in the diet
- There are ten amino acids which are essential for humans
- These are valine, leucine, phenylalanine, lysine, tryptophan, isoleucine, methionine, threonine, histidine and arginine
- Non-essential amino acids are those which the body can synthesise and therefore need not be available in the diet
- There are ten of them
- These are glycine, alanine, glutamic acid, aspartic acid, serine, tyrosine, proline, glutamine, arginine and cysteine
- Proteins are essential in the diet because they are not stored in the body
- Excess amino acids are deaminated
Formation of Proteins
- Proteins are made up of many amino acid units joined together through peptide bonds
- When two amino acids are joined together a dipeptide is formed
- The chemical process involved is called condensation and a molecule of water is eliminated
- When many amino acids are joined together a polypeptide chain is formed
- The nature of a particular protein depends on the types, number and sequence of amino acids from which it is made
Functions of Proteins As structural materials proteins
- Are the basic building structures of protoplasms
- Proteins in conjunction with lipid form the cell membrane
Examples of structural proteins include:
- Keratin (in hair, nails, hoofs, feathers and wool)
- Silk in spider’s web
- Elastin forms ligaments that join bones to each other
Protective proteins
- Antibodies that protect the body against foreign antigens
- Fribrogen and thrombin are involved in clot formation, preventing entry of micro-organisms when blood vessel is cut
As functional chemical compounds
- Examples are hormones and enzymes that act as regulators in the body
- Respiratory pigments
- Examples are haemoglobin that transports oxygen in the blood and myoglobin that stores up oxygen in muscles
- Contractile proteins – make up muscles, i.e myosin and actin
- Proteins combine with other chemical groups to form important substances e.g. mucin in saliva
Source of energy
- Proteins are a source of energy in extreme conditions when carbohydrates and fats are not available e.g. in starvation
See also:
FACTORS AFFECTING TRANSPIRATION