When oxygen is scarce, human muscle cells switch from aerobic respiration to lactic acid fermentation. Lactic acid accumulates, but it is gradually carried to the liver where it is converted back to pyruvic acid when oxygen becomes available. There are three major catabolic processes for harvesting food's chemical energy aerobic respiration anaerobic respiration fermentation. Cellular respiration includes all types of catabolism that use electron transport chains to make ATP, regardless of the substance used as the final electron acceptor. It also includes the Krebs cycle or some modification of the Krebs cycle.
Aerobic respiration occurs only in the presence of oxygen.
Uses oxygen as the final electron acceptor. Produces most ATP by oxidative phosphorylation and some ATP by substrate-level phosphorylation. Is used by plants and animals. Anaerobic respiration is a catabolic process that does not require the presence of oxygen. Produces most ATP by oxidative phosphorylation and some by substrate-level phosphorylation.