recap

10.3 recap

ATP and NADPH produced in the light reactions power the synthesis of carbohydrates by the Calvin cycle. This cycle fixes CO2, reduces it, and regenerates the acceptor, RuBP, for further fixation.

learning outcomes

You should be able to:

  • Describe the experiment that led to identification of the steps in the Calvin cycle.

  • Explain how light reactions stimulate the Calvin cycle.

Question 1

What experiments led to the identification of 3PG as the initial product of carbon fixation?

Researchers exposed algae to CO2 labeled with 14C for varying times and looked to see where the 14C label ended up. After a very short exposure time of 3 seconds, all of the label was found in 3-phosphoglycerate (3PG). After a longer exposure time of 30 seconds, the label could be found in more compounds, suggesting that 3PG is the initial product that is then used for synthesis of additional molecules. See Figure 10.10.

Question 2

In what ways does light stimulate the Calvin cycle?

  1. The light reactions produce ATP and NADPH, which are used in CO2 fixation reactions (Calvin cycle).
  2. Light-induced pH changes in the stroma activate some Calvin cycle enzymes.
  3. Light-induced electron transport reduces disulfide bridges in four of the Calvin cycle enzymes, thereby activating them.

Question 3

Both photosynthetic electron transport and the Calvin cycle stop in the dark. Which specific reaction stops first? Which stops next? Continue answering the question “Which stops next?” until you have explained why both pathways have stopped.

In the dark, photosynthetic electron transport stops at photosystem II → reduced PQ (plastoquinone). Initially the chlorophylls in light-harvesting complexes remain reduced, so reaction center chlorophylls remain reduced and thus photosystem II remains reduced. In the dark, the Calvin cycle stops at the reduction phase, which requires NADH. No RuBP is regenerated, so there is no rubisco activity. The initial reactions are no oxidation of photosystem I and no reduction of NADP to NADPH.

Although all green plants carry out the Calvin cycle, some plants have evolved variations on, or additional steps in, the light-independent reactions. These variations and additions have permitted plants to adapt to and thrive in certain environmental conditions. Let’s look at these environmental limitations and the metabolic bypasses that have evolved to circumvent them.