Question

carbon fixation occurs in the second stage of photosynthesis, during the light-independent reactions of the calvin cycle. in the first step of this cycle, the enzyme rubisco adds co2 to the energy-rich compound ribulose 1,5-bisphosphate, ultimately producing two molecules of 3-phosphoglycerate. in a culture of green alga that is carrying out photosynthesis in the presence of co2 in the laboratory, what would happen to the levels of ribulose 1,5-bisphosphate and 3-phosphoglycerate in the minutes after the lights were turned off and the cultures were plunged into darkness? ribulose 1,5-bisphosphate would be depleted, but 3-phosphoglycerate would accumulate. ribulose 1,5-bisphosphate would accumulate, but 3-phosphoglycerate would be depleted. nothing would happen because the calvin cycle is not light-dependent. both would be depleted. both would accumulate.

Answer 1

Upon the lights being turned off, the Calvin cycle is disrupted due to the lack of ATP and NADPH from the light-dependent reactions. Initially, RuBP would accumulate as the input of CO₂ is halted, while 3-PGA levels would decrease given that its production requires continuous energy. If darkness continues, both RuBP and 3-PGA levels would decline as the reactions of the Calvin cycle slow and eventually stop.

Step-by-step explanation:

When the lights are turned off in a culture of green alga carrying out photosynthesis, the levels of ribulose 1,5-bisphosphate (RuBP) and 3-phosphoglycerate (3-PGA) will be affected due to the cessation of the light-dependent reactions. The light-independent reactions of the Calvin cycle are powered by ATP and NADPH which are produced during the light-dependent reactions. Once the light is removed, the production of these energy carriers stops, consequently inhibiting the Calvin cycle as the energy inputs become depleted.

Carbon fixation during the Calvin cycle begins when the enzyme RuBisCO catalyzes the reaction between CO₂ and RuBP, leading to the formation of 3-PGA. In the absence of light, the supply of ATP and NADPH will be reduced, slowing down the photosynthetic process. As RuBisCO requires the products of the light-dependent reactions to continue catalyzing reactions between CO₂ and RuBP, the immediate aftermath would see accumulating levels of RuBP, as it is less likely to be used without available CO₂, and decreased levels of 3-PGA, since its production is slowed down without the continuous input of CO₂ and the necessary energy molecules.

Eventually, if the darkness persists, RuBP levels may also begin to decrease as the remaining ATP and NADPH are used up and the cycle slows to a stop. The Calvin cycle is the stage in photosynthesis where carbon is fixed into organic molecules like glucose and relies on the regeneration of RuBP to continue the process.