9th Class Biology Chapter 6 Notes English Medium
Important Notes of 9th Class Biology Chapter 6 Notes English Medium written by Honorable Sir Adnan Haider Suib. These notes are very helpful in the preparation of Enzymes 9th Class Notes for students of the 9th Class Biology Chapter 6 Notes English Medium and these are according to the paper patterns of all Punjab boards.
Summary and Contents:
Topics which are discussed in the notes are given below:
- Very Important Multiple Choice Questions (MCQs) of Chapter No.6 Enzymes English Medium.
- Factors Affecting The Rate Of Enzyme Action: Enzymes are very sensitive to the environment in which they work. Any factor that can change the
chemistry or shape of enzyme molecule, can affect its activity. Some of the factors that can affect
the rate of enzyme action are being discussed next.
- Temperature: Increase in temperature speeds up the rate of enzyme catalyzed reactions, but only to a point. Every enzyme works at its maximum rate at a specific temperature called as the optimum
temperature for that enzyme.
- When temperature rises to a certain limit, heat adds in the activation energy and also provides
kinetic energy for the reaction. So reactions are accelerated. But when temperature is raised well
above the optimum temperature, heat energy increases the vibrations of atoms of enzyme and the
globular structure of enzyme is lost. This is known as the denaturation of enzyme. It results in a
rapid decrease in rate of enzyme action and it may be blocked completely.
The optimum temperature for the maximum working speed of human enzymes is 37°C.
- Substrate concentration: If enzyme molecules are available in a reaction,
increase in substrate concentration increases the
rate of reaction. If enzyme concentration is kept
constant and amount of substrate is increased,
a point is reached where any further increase in
substrate does not increase the rate of reaction
any more. When the active sites of all enzymes are
occupied (at high substrate concentration), any more
substrate molecules do not find free active sites. This
state is called saturation of active sites and reaction
rate does not increase.
- pH: All enzymes work at their maximum rate at a narrow range of pH, called as the optimum pH. A slight change in this pH causes retardation in enzyme activity or blocks it completely.
Every enzyme has its specific optimum pH value. For example pepsin (working in stomach) is active
in acidic medium (low pH) while trypsin (working in small intestine) shows its activity in alkaline
medium (high pH). Change in pH can affect the ionization of the amino acids at the active site.
- Mechanism Of Enzyme Action: When enzyme attaches with substrate, a temporary enzyme-substrate (ES) complex is formed.
Enzyme catalyzes the reaction and substrate is transformed into product. After it, the ES complex
breaks and enzyme and product are released. E + S ----> E S comlex ----> E + P
- In order to explain the mechanism of enzyme action a German chemist Emil Fischer, in 1894,
proposed lock and key model. According to this model, both enzyme and substrate possess
specific shapes that fit exactly into one another. This model explains enzyme specificity. In 1958, an American biologist Daniel Koshland suggested a modification to lock and key model
and proposed induced-fit model. According to this model, active site is not a rigid structure rather
it is molded into the required shape to perform its function. Induced fit model is more acceptable
than “lock and key” model of enzyme action.
- Specificity Of Enzymes: There are over 2000 known enzymes, each of which is involved in one specific chemical reaction.
Enzymes are also substrate specific. The enzyme protease (which breaks peptide bonds in proteins)
will not work on starch (which is broken down by an enzyme amylase . Similarly lipase enzyme acts
only on lipids and digests them into fatty acids and glycerol. Specificity of different enzymes is
determined by the shapes of their active sites. Active sites possess specific geometric shapes that
fit with specific substrates.