Four primary regions are the brainstem, the cerebellum, the cerebrum, and the cerebral cortex. Each brain region is responsible for a variety of functions, and injury to any of these regions impairs specific functions.
Cellular respiration is a pathway that organisms use to generate energy. Oxygen is required and is used to break down nutrients, like glucose sugar. The nutrients are broken down to generate ATP (energy) and carbon dioxide and water (waste). Organisms from all kingdoms of life can use cellular respiration including bacteria, archaea, plants, protists, animals, and fungi.
However, many organisms have developed strategies to carry out metabolism without oxygen, or can switch from aerobic to anaerobic cell respiration when oxygen is scarce. During cellular respiration, some living systems use an organic molecule as the final electron acceptor. Processes that use an organic molecule to regenerate NAD + from NADH are collectively referred to as fermentation.