Practise AP Biology 3.4.B questions by tracing electron flow, proton gradients, chemiosmosis and ATP or NADPH production in photosynthetic light reactions.
Noncyclic electron flow and cyclic electron flow are two major pathways of the light-dependent reactions of photosynthesis. In noncyclic electron flow, electrons pass through photosystem II, then components of a chloroplast electron transport chain, and then photosystem I before finally reducing NADP to NADPH. In cyclic electron flow, electrons cycle through photosystem I and some components of the electron transport chain (Figure 1).
Figure 1. The pathways of noncyclic and cyclic (heavy arrows) electron flow. The cytochrome complex is a component of the electron transport chain between the two photosystems.
Describe the role of chlorophyll in the photosystems of plant cells.
Based on Figure 1, explain why an increase in the ratio of NADPH to NADP will cause an increase in the flow of electrons through the cyclic pathway.
Using rice plants, scientists examined the effect of a mutation that results in the loss of the protein CRR6. CRR6 is a part of the photosystem I complex, and its absence reduces the activity of photosystem I. Predict the effect of the mutation on the rate of biomass (dry weight) accumulation.
Justify your prediction in part (c).
Write your responses to this question only on the designated pages in the separate Free Response booklet.