Page 66 - Biology_F5

Cytology

metabolic activities of an organism. They such as albumin. Soluble protein
include hormones, such as, insulin hormone can be precipitated from solution by
which is secreted by the pancreas, and addition of certain salts, such as NaCl
helps in the regulation of the blood sugar and (NH ) SO . Decrease in solubility
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concentration that regulates the metabolism occurs due to competition between salt
of the cells. Transcription factors are and protein molecules for water, as well
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proteins that regulate gene expression. Some as a decrease in charge on the protein
repress gene expression by binding to target molecule.
genes and activating their transcription. (e) Proteins are also coagulated by heat
The examples of such protein include lac and agents such as strong acids, alkali,
repressor and catabolite activator protein alcohol, acetone, urea and salts of
(CAP). Lac repressor is a DNA-binding heavy metal.
protein which inhibits the expression of
genes coding for proteins involved in the (f) Proteins are specific in their reactions
metabolism of lactose in bacteria, whereas and substrates they act upon. This
CAP promotes transcription at several property is clearly illustrated by
sites, as it affects the metabolism of sugars enzymes.
and amino acids, protein folding, toxin (g) Proteins are colloidal in nature, that is,
production and pilus synthesis. they can be hydrolysed into their amino
acid constituents.
Receptor proteins: Receptor proteins are
built into the membrane of a nerve cell (h) They can be denatured or changed from
and they detect chemical signals released their natural state by heat or chemicals.
by other nerve cells. Other proteins act
as chemical messengers within the brain Denaturation of proteins
throughout the body. They are involved A protein is said to be denatured, if
in the cell’s response to chemical stimuli. it loses its natural three-dimensional

Properties of proteins conformation or shape. This change may
Proteins have the following properties: be temporary or permanent, but the amino
acid sequence of the protein remains intact
(a) They are colourless, tasteless, and
amphoteric in nature, as they have both because the peptide bonds are not cleaved.
acidic and basic properties derived from Denaturation results in the alteration of
–COOH and –NH groups respectively. physical properties of a protein in terms of
2 solubility and other criteria. However, the
(b) They exhibit characteristic isoelectric protein in this situation can no longer carry
points and have buffering properties.
out its normal biological functions. The
(c) They are large molecules; hence, causes of this condition include heat and
they have high molecular weight. radiation, organic solvents and detergents,
For instance, haemoglobin has the highly concentrated salts and strong acids
molecular weight of about 68,000 g/mol.
and alkalis. Denaturation may also be
(d) Proteins differ in their solubility in caused by heavy metals, urea solution and
water, some are insoluble, example mechanical forces.
keratin while others are highly soluble

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