Page 279 - Biology_F5

Biology for Advanced Level Secondary Schools

of it is produced during the light reaction. photosynthesise, even if the stomata are
The fact that the C pathway produces closed due to the presence of high levels
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high yields under high concentration of of carbon dioxide in the bundle sheath
carbon dioxide makes photosynthesis in cells, about 20 to 120 times higher than
C plants more efficient in carboxylation normal.
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compared to C plants, which under certain Differences between C and C plants
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conditions such as hot or dry conditions The major differences between these
undergo photorespiration. The PEP two types of plants are evident in the
carboxylase in the mesophyll of C plants photosynthesis process. Generally, the
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has a high affinity to carbon dioxide, and basic metabolism of the C and C plants
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it does not get competitively inhibited is similar, but the latter is more complex.
by oxygen to cause photorespiration. The C plants are therefore evolutionarily
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Although carbon dioxide is a limiting more advanced than C plants (Table 5.2).
factor in photosynthesis, C plants can 3
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Table 5.2: Differences between C and C plants
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C plants C plants
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The first product in carbon dioxide fixation is The first product in carbon dioxide
a 3-carbon compound called phosphoglyceric fixation is a 4-carbon compound called
acid oxaloacetic acid
Photosynthesis is limited by low levels Photosynthesis is not limited by low levels
of atmospheric carbon dioxide, hence of atmospheric carbon dioxide, hence
photorespiration may occur under limited photorespiration does not occur
supply of carbon dioxide

Carboxylation reaction is accomplished by Carboxylation reaction is accomplished by
the enzyme RuBP carboxylase to fix carbon the enzymes RuBP carboxylase and PEP
dioxide carboxylase to fix carbon dioxide
The carbondioxide acceptor is a 5-carbon The carbondioxide acceptors are the
compound called RuBP 3-carbon compound (PEP) and the 5-carbon
compound (RuBP)
Carbondioxide fixation occurs in bundle Carbondioxide fixation occurs in both,
sheath cells only mesophyll cells and bundle sheath cells
They are less efficient in fixing carbon They are more efficient in fixing carbon
dioxide dioxide
They have only one type of chloroplast in They have two types of cells, each with its
their leaf bundle sheath cell own type of chloroplast
They do not tolerate hot and dry They tolerate hot and dry conditions
conditions. They are adapted to cool and
wet environment
Example of C plants are soybeans spinach, Example of C plants are millet, sorghum and
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rice and grasses sugarcane

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