recap

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10.4 recap

Rubisco catalyzes the carboxylation of RuBP to form two 3PG, and the oxygenation of RuBP to form one 3PG and one phosphoglycolate. The diversion of rubisco to its oxygenase function decreases net CO2 fixation. C4 photosynthesis and CAM allow plants to fix CO2 under warm, dry conditions when stomata are closed and CO2 entry into the leaf is limited.

learning outcomes

You should be able to:

  • Explain how and why C4 plants maintain a high concentration of CO2 around rubisco.

  • Explain how CAM plants carry out carbon fixation separately from Calvin cycle reactions.

Question 1

How do C4 plants keep the concentration of CO2 around rubisco high, and why?

In C4 plants, CO2 is initially fixed in the leaf mesophyll cells but is then transferred (as a four-carbon molecule) to the bundle sheath cells, where decarboxylation reactions release CO2 for use in the Calvin cycle. The bundle sheath cells are located in the interior of the leaf where less atmospheric O2 can reach them than reaches cells near the surface of the leaf.

Question 2

Describe CAM plants and explain how they can temporally segregate CO2 fixation from the Calvin cycle.

In CAM plants, CO2 is initially fixed into a four-carbon compound (malate) at night when it is cooler and water loss is minimized, and the stomata open. During the day, when the stomata close to reduce water loss, the accumulated malate is transferred from the vacuole to the chloroplasts, where its decarboxylation supplies the CO2 for the Calvin cycle and the light reactions supply the necessary ATP and NADPH.

Table 10.1 compares photosynthesis in C3, C4, and CAM plants.

Now that you understand how photosynthesis produces carbohydrates, let’s see how the pathways of photosynthesis are connected to other metabolic pathways.