Plant cells and cyanobacteria (also photosynthetic) obtain the ATP required to power their usual cellular functions from Cellular respiration.
Plant cells and cyanobacteria are photosynthetic creatures that can produce their nourishment through the process of photosynthesis. They use light energy to transform carbon dioxide and water into glucose and oxygen during this process. However, to fuel their usual cellular functions, including protein synthesis, DNA replication, and cell division, cells require a steady source of energy in the form of ATP.
Cellular respiration produces the ATP necessary for plant cells and cyanobacteria. While photosynthesis generates glucose and oxygen, cellular respiration breaks down glucose molecules to release energy as ATP. This occurs in the mitochondria of plant cells and cyanobacteria. Glycolysis, the Krebs cycle, and oxidative phosphorylation are the three steps of cellular respiration. The first stage, glycolysis, involves the breakdown of glucose molecules into pyruvate, which releases a tiny quantity of ATP.
Pyruvate is transformed into acetyl-CoA in the Krebs cycle, which then enters a loop of reactions that creates additional ATP and carbon dioxide. Finally, electrons from the Krebs cycle are passed via a series of protein complexes in the third stage, oxidative phosphorylation, to form a proton gradient across the mitochondrial membrane. This gradient is then used to drive the production of ATP.
While photosynthesis also produces ATP through light reactions, this ATP is primarily used to power the photosynthesis itself, rather than the regular cellular processes of the plant cell or cyanobacterium. Therefore, the correct answer to the question is Cellular respiration.