Energy in the cell

Cells obtain usable energy through a network of chemical reactions. In aerobic cellular respiration, glucose is broken down and oxygen is consumed, producing carbon dioxide, water and ATP. Glycolysis begins in the cytosol; mitochondria carry out the later aerobic stages. Cells can also use other fuels, and some cells use pathways that do not require oxygen. Mature red blood cells have no mitochondria and obtain ATP through glycolysis.

These linked processes explain why digestion, breathing and circulation cooperate: nutrients supply fuel, lungs exchange gases, and blood transports nutrients, oxygen and wastes. Many tissues, especially the brain and heart, depend strongly on a continuous oxygen supply; requirements and tolerated interruptions differ between cell types.