Page 280 - Biology_F5

Nutrition

Exercise 5.3 Inhibitors
An inhibitor is a substance or factor which
1. Describe how C plants are adapted may slow down the rate of reaction.
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to synthesize sugars in the tropical For example, many herbicides such as
and subtropical climates. Dichlorophenyl Dimethyl Urea (DCMU)
interfere with the electron flow in the
2. Using your knowledge on C and C chloroplast. Thus, they inhibit the light
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plants, explain which of these are reaction, and hence no photophosphorylation
more vulnerable to climate changes. can occur. In addition, a relatively higher
concentration of oxygen (above 21%) in
Factors affecting the rate of the atmosphere tends to inhibit the rate of
photosynthesis photosynthesis, because it lowers the amount
For photosynthesis to take place efficiently, of carbondioxide available to the plant.
certain conditions must be met, and raw Leaf structure and position
materials must be present. The process Leaf is a photosynthetic organ of a plant. It
of photosynthesis is influenced by both contains chloroplasts, which are organelles
internal and external factors. for food synthesis. Leaves have special
adaptation to enhance carbon dioxide
(a) Internal factors and light absorption for photosynthesis.
The internal factors which are also called For example, they have a broad surface
plant factors are chlorophyll concentration, area exposed to light illumination. Thin
enzymes, inhibitors, and leaf structure and leaves with large surface area absorb more
position. light than small and thick leaves. Carbon
dioxide penetrates more quickly in thin
Chlorophyll concentration leaves than in thick leaves. Additionally,
Chlorophyll is a green photosynthetic leaves with thin and transparent epidermis
pigment located in the chloroplast, allow more light to reach the chloroplasts.
especially in the grana. Chlorophyll is Furthermore, leaves, particularly of most
tropical tree species, are positioned at
responsible for trapping light energy a certain angle which is normally more
during light reaction. When chlorophyll or less vertical. This reduces excessive
concentration in the leaves is very low, light interception and temperature on the
the rate of light reaction will be reduced, leaf. Reducing this natural angle of leaf
because only a little amount of light will may be detrimental to the chloroplast
be trapped for the reaction, resulting in because it increases light interception and
reduction in the rate of photosynthesis. excessive temperature. Thus, maintaining
high leaf angle orientation in tropical
Low concentrations of chlorophyll in plant trees is necessary in reducing excessive
leaves can be caused by several factors, light interception and leaf temperature to
such as mineral deficiency, ageing, lack of protect chlorophyll from photodamage
light and diseases such as fungal diseases due to excessive light. This in turn
or infections. enhances photosynthetic activity.

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