BOTANY Higher Secondary Second Year - Textbooks Online

8. Ribose 5-phosphate and xylulose 5-phosphate molecules are
transformed to ribulose 5-phosphate (Ru5P).
9. Ru5P molecules are then phosphorylated by ATP to form RuBP
molecules, which again enter into the cycle of CO 2 fixation.
In the above illustration, three CO molecules are fixed and the net
2
gain is a 3C called DHAP. These triose phosphate molecules combine to
form hexose phosphates, which are used to form sucrose. For every
carbon fixation 3ATP and 2 NADPH are consumed.
2
5.2.6. C and C pathways
3 4
It was once thought that all green plants fix CO through Calvin cycle
2
only. Now, we know that certain plants fix CO in a different photosynthetic
2
mechanism called C pathway. In this chapter, we will know more about
4
this. Hatch and Slack observed that 4C compounds such as oxaloaceticacid,
malate and aspartate were the first formed compounds, when the leaves of
sugarcane were exposed to 14CO for one second. So, sugarcane is an
2
example for C plant. When the leaves of rice plant are exposed to 4 14CO2, 3C compound called phosphoglyceric acid is formed. So, rice plant is an
example for C plant. 3
In C plants, photosynthesis occurs only in mesophyll cells. We already
3
learnt that photosynthesis has two types of reactions – light reactions and
dark reactions (Calvin cycle). In light reactions ATP and NADPH are 2
produced and oxygen is released as a byproduct. CO is reduced to
2
carbohydrates by dark reactions. In C plants both light reactions and
3
dark reactions occur in mesophyll cells, whereas in C plants, the mechanism
4
of photosynthesis requires two types of photosynthetic cells – mesophyll
cells and bundle sheath cells. The C plants contain dimorphic chloroplasts
4
i.e. chloroplasts in mesophyll cells are granal (with grana) whereas in
bundle sheath chloroplasts are agranal (without grana). The presence of
two types of cells leads to segregation of photosynthetic work i.e. light
reactions and dark reactions separately.
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