Question

The Calvin Cycle requires ATP and NADPH, products of light reactions. If a classmate asserted that the light reactions don’t depend on the Calvin cycle and, with continual light, could just keep on producing ATP and NADPH, how would you respond?

Short answer

It is necessary to understand that light reactions and the Calvin cycle are inter-dependent because the intermediates produced during the Calvin cycle are required by light reactions to synthesize ATP and NADPH.

Step-by-step solution

Light reaction of photosynthesis 

There are two stages to photosynthesis. The light reaction is the first stage of photosynthesis, followed by the Calvin cycle.

In light reactions, the pigment chlorophyll collects light energy and converts it into chemical energy in the form of electron charge carrier molecules like NADPH and ATP.

Water splits at this point to produce oxygen as well as protons (H+) and electrons. These are transported to an NADP⁺ (nicotinamide adenine dinucleotide phosphate) acceptor and reduced to NADPH. The photophosphorylation process also produces ATP (adenosine triphosphate).

Calvin cycle of photosynthesis

The Calvin cycle is also known as dark reactions or light-independent photosynthesis processes. It is a set of chemical reactions carried out by plants to convert carbon dioxide into organic molecules.

Carbon fixation is the first step in this process, in which carbon from carbon dioxide is absorbed into an organic substance. The cycle subsequently adds electrons to the fixed carbon to convert it to carbohydrate.

The reducing power created by the light reaction is NADPH, which provides the electrons and protons required for carbohydrate synthesis. The chemical energy given by ATP drives these processes.

Inter-dependence of two stages of photosynthesis

The products of the light reaction, ATP, and NADPH are required to convert CO₂ into glucose during the Calvin cycle.

ADP, iP, and NADP are regenerated during the Calvin cycle, and they are required for the production of ATP and NADPH during the light reaction.

Calvin cycle is required for light reactions; without it, ATP and NADPH cannot be produced. As a result, the two processes of photosynthesis are linked.