Page 276 - Biology_F5

Nutrition

carbon dioxideand convert it into organic O + RuBP RuBP carboxylase Phosphoglycolate + PGA

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molecules, such as carbohydrates. Basing 5C 2C 3C
on the first product of photosynthesis just Since the reaction is an oxygenation
after carboxylation, plants are catagorised reaction, the enzyme RuBP carboxylase
into two groups, namely C and C plants. is also known as RuBP oxygenase. There
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is no energy-rich compound produced,
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C plants since 2-phosphoglycolate produced cannot
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In some plants, the first product of enter the Calvin cycle; rather it enters in
photosynthesis immediately after the conversion pathways and use NADPH
carboxylation is a 3-phosphoglyceric acid and ATP to generate PGA, hence decrease
(3-PGA). These plants fix carbon dioxide, in yield. Decreased concentration of carbon
following the Calvin cycle, as explained dioxide occurs during dry or hot conditions
in the preceding section. Plants which because the stomata pores through which
yield a 3-PGA, as the first product after carbon dioxide diffuses into the plant
carboxylation, are described as C plants. normally close to minimise water loss
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Fixation of carbon dioxide occurs in the through transpiration under such conditions.
chloroplasts of the bundle sheath cells. C The global increase in temperature and
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plants account for approximately 85% of drought conditions in some places emanating
all plants including bryophytes, ferns, most from climatic changes is likely to impact
grasses, and trees. They also include crops C plants more than C plants, which are
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such as cotton, tobacco, spinach, soybean, and more efficient photosynthetically.
cereal grains (rice and wheat). This implies
that most food that we eat comes from C Photorespiration involves a complex network
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plants. C plants perform well in temperate of enzyme controlled reactions, which
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conditions, but they suffer photorespiration, exchange the cellular metabolites between
which is also called oxidative photosynthetic three organelles, namely chloroplasts, leaf
carbon cycle or C photosynthesis in hot peroxisomes and mitochondria (Figure
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and dry conditions. 5.6). The formed 2-phosphoglycolate is
dephosphorylated by removing of phosphate
Photorespiration group (Pi) to form glycolate which leaves
This is a condition which occurs when carbon the chloroplast to the peroxisome. In the
dioxide concentration in the chloroplast drops peroxisomes, glycolate is oxygenated to
below 50 ppm while the level of oxygen give glyoxylate and in this process hydrogen
is relatively higher. Under this condition, peroxide gas is evolved. The formed
oxygen and carbon dioxide compete for glyoxylate is converted into glycine, then
the same active site on RuBP carboxylase this product enters the mitochondrion in
enzyme. This in turn triggers the enzyme which it is converted into serine through
to utilise oxygen to oxidise RuBP to form the series of reactions. The produced serine
one molecule of 3-PGA (three carbon) and is transported to the peroxisome and it is
2-phosphoglycolate (two carbon compound) converted into hydroxypyruvate then to
instead of acting on carbon dioxide to produce glycerate (Glyceric acid). The glycerate
two molecules of PGA (three carbon).

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