Biology 9700 · AS & A Level · Photosynthesis as an energy transfer process

Photosynthesis as an energy transfer process — practice question

Most plants are C3 plants, and they are called that because the first photosynthetic product they make is a three-carbon compound. The enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyses the fixation of carbon dioxide in the Calvin cycle and is used by both C3 and C4 plants. Each molecule consists of eight large polypeptides and eight small polypeptides. Fig. 2.1 shows the molecule from the side.
(a(i))[2]

Outline the differences between the biochemistry of C4 plants and that of C3 plants.

(a(ii))[1]

State why rubisco is described as having quaternary structure.

(a(iii))[2]

Explain what makes a molecule such as rubisco soluble.

(b)[2]

The active sites of rubisco accept ribulose bisphosphate (RuBP) together with either carbon dioxide or oxygen, and the two reactions below can be catalysed: either $\text{RuBP} + \text{CO}_2 \rightarrow \text{unstable intermediate compound} \rightarrow 2\text{GP (PGA)}$ or $\text{RuBP} + 2\text{O}_2 \rightarrow \text{unstable intermediate compound} \rightarrow \text{GP (PGA)} + 2\text{CO}_2$. Explain the effect on the plant of the oxygen-based reaction.

(c)[3]

In the absence of light, rubisco changes from an active form to an inactive one. Explain why rubisco does not have to stay active when light is absent.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “In $C_4$ plants, PEP carboxylase fixes $CO_2$.” …

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