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.
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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