Page 67 - Biology_F5

Biology for Advanced Level Secondary Schools

Heat and radiations, such as ultra violet Mechanical forces: Physical movement
rays and infra-red: These supply kinetic of proteins may break hydrogen bonds.
energy which causes strong vibrations For example, stretching of hair breaks
of protein molecules. These result into the hydrogen bonds in the keratin helix.
breaking down of ionic and hydrogen The latter is extended and the hair is
bonds that are weaker than peptide bonds, stretched. If released, the hair returns to
FOR ONLINE READING ONLY
resulting into coagulation of such protein its normal length. However, if it is wetted
molecules. and then dried under tension, it keeps
its new length. This is the basis for hair
Organic solvents and detergents: These styling. Many liquid proteins denature
reagents disrupt hydrophobic interactions and precipitate when they are vigorously
and form bonds with hydrophobic (non- agitated because of incorporation of
polar) groups. Consequently, this disrupts air bubbles and adsorption of protein
hydrogen bonds resulting into denaturation. molecules to the air-liquid interface.
An example of such chemical substances
is methyl alcohol, which is used as a General functions of proteins
disinfectant to clean the skin before (a) They are essential building materials
injection. The alcohol denatures proteins of the body. For that reason, they
in bacterial cell walls. are needed for proper growth and
development. This explains why
Highly concentrated salts, strong acids,
and alkalis: These compounds disrupt the kwashiorkor victims experience
ionic bonds, resulting into coagulation of stunted growth.
proteins. If such proteins remain mixed (b) They are used as alternative respiratory
with these reagents for a long time, peptide substrates when both carbohydrates
bonds may also break down.
and lipids are completely exhausted.
Heavy metals: Cations of heavy metals (c) They absorb excess fluids in the body.
such as mercury (Hg), silver (Ag), and For example, kwashiorkor victims
lead (Pb) tend to form strong bonds with have swollen lower parts of their
the negatively charged carboxyl groups legs and some parts of hands due to
on the proteins, leading to the disruption
of ionic bonds. The protein’s polarity is accumulation of excess fluids. This
reduced and its insolubility becomes high, condition is called oedema.
hence precipitated as an insoluble metal (d) They form structural components
protein. of body parts. For instance, keratin
is a structural protein of hair, horns,
Urea solution: Urea tends to disrupt
hydrogen bonds. Being amide-like, it hooves, and nails.
forms hydrogen bonds of its own via (e) They are structural components of
distorting the unique configuration of the membranes. For example, globular
protein molecule.

Form Five Student’s Book
60