• 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
See also  THE FRUIT

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

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