Respiration is the process through which energy stored in organic molecules is released to do metabolic work. A stepwise process conducted in all living cells, it is controlled by enzymes, and releases carbon dioxide and water.
Breathing, the inspiration and expiration of air by animals, is not the same as respiration. Both animals and plants respire, but plants neither breathe nor have specialized respiratory systems as do animals. In plants, gases diffuse passively into the plant (through the stomata or directly into the epidermal cells) where they come into contact with the moist cellular membranes and then move in water along diffusion gradients between and within cells. No special carriers (such as the hemoglobin of human blood) or organs (such as lungs or gills) aid in the diffusion.
Glucose is the originating molecule for respiration; other reserve foods either follow different utilization pathways or, in the case of complex carbohydrates, are broken down to glucose before undergoing respiratory oxidation.
Respiration can be divided into the following stages (see Figure ):
- Glycolysis is the breakdown of a 6‐carbon glucose molecule into two molecules of 3‐carbon pyruvate; it takes place in the cytoplasm of all living cells.
- If oxygen is present ( aerobic respiration), pyruvate is used in the following reactions that take place in the mitochondria:
- The Krebs cycle (citric acid cycle)occurs in the matrix.
- Electron transport chain and oxidative phosphorylation occur deep in the cristae.
- If oxygen is not present ( anaerobic respiration), pyruvate is used in fermentation.
- Lactate formation occurs in in animal, bacteria, fungi, and protist cells.
- Alcohol fermentation occurs in in yeast and plant cells.
The thermodynamic efficiency—the percentage of the potential energy of the glucose molecule that is recovered to do work in the cells—varies between 22–38 percent in aerobic respiration and is considerably less in anaerobic. (Gasoline engines average less than 25 percent efficiency.) The lost energy is released as heat, some of which is used by plants in interesting ways.
The first steps of energy release (glycolysis) in all organisms follow the same general pattern. This pattern presumably originated early on Earth with the single‐celled prokaryotes before molecular oxygen was plentiful in the atmosphere and before the advent of cellular organelles. Only after photosynthesis altered the gaseous content of the atmosphere could the second chain of respiratory reactions develop in which oxygen is used as an electron acceptor. Some of the small, obligate anaerobes today still respire exclusively with the glycolytic pathway, but most larger organisms resort to glycolysis only for short periods when oxygen is temporarily absent (plant roots in flooded soils, for example) or oxygen can't get to cells fast enough (such as when human muscles are worked hard during exercise).