Page 74 - Biology_F5

Cytology

Enzymes denature at
high temperatures thus
the rate falls rapidly
Rate of reaction The optimum temperature for

FOR ONLINE READING ONLY
the organism depicted in this
curve is 40 °C

0 10 20 30 40 50 60
Temperature (°C)

Figure 1.47: Effect of temperature on the rate of an enzyme-controlled reaction
The effect of temperature on the rate of (alkalis) have pH values above 7 while
reaction can be expressed as temperature the pH of 7 is termed as ‘neutral’.
coefficient, Q . According to the Q rule, Hydrogen (H ) and hydroxyl (OH ) ions
+
-
10
10
the rate of a physiological process or are charged, therefore, they interfere
reaction doubles for every 10 °C rise in with hydrogen and ionic bonds that
temperature, provided that the temperature hold together an enzyme, since they will
is within the range that can be tolerated be attracted or repelled by the charges
by a living system. A mathematical created by the bonds. This interference
expression is presented below: causes a change in shape of the enzyme

Q = rate of reaction at (X+10) °C and, consequently, its active site. Most
enzymes are sensitive to pH and have
10 rate of reaction at X°C

specific ranges of activity. Different
Where: X is the initial temperature enzymes have different optimum pH
values; hence, the bonds within them are
Thus, within the range of 0 - 40°C, Q of influenced by H and OH ions at different

-
+
10
an enzyme controlled reaction is 2. pH values. Thus, the shape of their active
This means that the rate of the enzyme’s site is most complementary to the shape
action doubles for every 10 °C rise in of their substrate.
temperature.
At optimum pH, the rate of reaction is
(d) pH maximum. Any change in pH, above or
The pH scale measures the acidity and below the optimum, will quickly cause a
basicity of a solution. pH is a measure decrease in the rate of reaction, since more
of hydrogen ion (H ) concentration. It is enzyme molecules will have active sites
+
therefore a good indicator of the hydroxy whose shapes are less complementary to
ion (OH ) concentration. Its values range the shape of their substrates (Figure 1.48).
-
from 1 to 14. Lower pH values implie Slight changes in pH above or below
higher H concentration and lower OH - the optimum do not cause a permanent
+
concentration. Acidic solutions have change to the enzyme, since the bonds
pH values below 7, and basic solutions can be reformed. However, extreme

Form Five Student’s Book
67