Page 65 - Biology_F5

Biology for Advanced Level Secondary Schools

(d) Classification of proteins according carry lipids from the site of absorption
to their functions or synthesis to the site of utilisation or
Proteins can be classified into different storage and membrane transport proteins,
categories depending on their physical and which transport substances across the cell
chemical structures as well as their location membranes.
in the cell. They can also be grouped based
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on the metabolic functions they perform in Nutrient and storage proteins: These
the body as follows: are proteins which provide reservoirs of
essential nutrients to a growing embryo.
Enzyme proteins: These are biological A good example of nutrient and storage
catalysts, which are mostly protein in proteins is the albumin of an egg white, used
nature. They are the most varied and as an amino acid source for the developing
highly specialised proteins with catalytic embryo. Casein, the protein of milk, is the
activity. Virtually, all the chemical reactions major source of amino acids for mammalian
of organic biomolecules in the cells are babies. Ferritin is an iron-binding protein;
catalysed by enzymes, resulting into which stores iron in the liver. Moreover,
increased reaction rate. Enzymes, such plants store proteins in seeds, where seeds
as urease, catalase, and lactase catalyse a of many plants store nutrient protein for
variety of body reactions. growth of embryonic plants. Examples of
such plants include wheat, rice, maize, and
Structural proteins: These are proteins, bean.
which aid in strengthening or protecting
biological structures. These proteins act Contractile or motile proteins: These
as supporting filaments, cables or sheets proteins found in the contractile tissues.
to give biological structure, strength, and They include actin, myosin, and tubulin
protection. Collagen is an example of the proteins. They are important for movement
structural protein of the bone and connective of body parts. Contractile proteins are
tissue. Collagen and elastin provide a fibrous responsible for undulation of the cilia and
framework in animal connective tissues, flagella, which propel many cells.
such as tendons and ligaments. Keratin is
the structural protein of hairs, horns, finger Defence proteins: These are proteins which
nails, feathers and other skin appendages defend the body against invaders, such as
of animals. antibodies, fibrinogen and thrombin. These
are highly specialised proteins that recognise
Transport or carrier proteins: These are and combine with foreign substances,
proteins involved in transportation of ions like viruses, bacteria and destroy them.
and other substances. Examples of such Fibrinogen and thrombin are blood clotting
proteins include haemoglobin, the iron- proteins, which protect the body against
containing protein of blood, which transports excessive bleeding.
oxygen from the lungs to other parts of
the body; serum albumin which transports Regulatory proteins: These proteins
fatty acids in the blood; lipoproteins, which regulate gene expression and cellular or

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