Which cycle is described as light-independent reactions of photosynthesis?

The correct choice, the Calvin Cycle, is known as the set of light-independent reactions of photosynthesis. This cycle takes place in the chloroplasts of plant cells and is where carbon dioxide is fixed into organic molecules.

During the Calvin Cycle, ATP and NADPH, produced during the light-dependent reactions, are used to convert carbon dioxide and water into glucose. This process does not require light directly; instead, it utilizes the chemical energy captured from light reactions.

In this cycle, three main steps occur: carbon fixation, reduction, and regeneration of ribulose bisphosphate (RuBP). The significance of the Calvin Cycle lies in its role in synthesizing carbohydrates, which serve as an energy source for the plant and, ultimately, for other organisms within the ecosystem.

The other processes mentioned do not fit as light-independent reactions. The Krebs Cycle is part of cellular respiration, glycolysis is an anaerobic pathway that breaks down glucose, and the Electron Transport Chain involves the transfer of electrons in the presence of oxygen to produce ATP. Thus, the Calvin Cycle is uniquely positioned as the light-independent phase of photosynthesis.