Question

A researcher uses a radioactively-labeled carbon dioxide molecule to evaluate photosynthetic rate. In her experiments, the researcher finds adequate consumption of water, generation of oxygen and NADPH production, but no change in plant mass. What is most likely the dysfunctional system in the plant (PI, PII, Calvin Cycle, ATP generation)? Explain.

Answer 1

The plant's lack of mass increase despite active photosynthetic processes suggests that the Calvin Cycle is dysfunctional, as this is where CO2 is fixed into sugars contributing to biomass.

Step-by-step explanation:

The core issue appears to be the dysfunction in the Calvin Cycle within the plant's photosynthesis process. While the radioactively-labeled carbon dioxide is adequately being used in the light-dependent reactions — water is consumed, oxygen and NADPH are produced — there is no increase in the plant's mass.

This suggests that although the light reactions (photosystem I and II) and ATP generation seem to be functioning properly, the carbon is not being fixed into sugars due to an issue within the Calvin Cycle.

This cycle is responsible for carbon fixation, converting CO₂ and energy from ATP and NADPH into carbohydrates, which contribute to plant biomass. A failure in this cycle means the plant is not synthesizing new organic material from CO₂, despite having the necessary energy and reducing power available from the light reactions.