Energy Metabolism in Cells
Photosynthesis: The chloroplasts of green plants capture photons to convert ADP to ATP. Water is split to reduce NADP to NADPH, releasing oxygen.
Respiration: The mitochondria use that oxygen and convert ADP to ATP. In the process, NADH is oxidized to NAD and water.
Glycolysis: Lacking illuminated chloroplasts or lacking oxygen, cells metabolize sugars by fermentation to make a little ATP from ADP. Historically, this was probably the first way to make the high-energy pyrophosphate bond of ATP. All cells maintain this pathway. Most cells fall back on glycolysis only when they have no better alternative, but it is common to have no better alternative. The microorganisms of yogurt, sauerkraut, gangrene, and food poisoning, for example, subsist wholly on glycolysis, as do facultative anaerobes such as intertidal bivalves (e.g., oysters) and parasitic helminths (e.g., schistosomes) and diving vertebrates (e.g., green sea turtles) during their prolonged periods of contented abstinence from respiration. Red blood cells have no other energy supply. Poorly vascularized tissue such as the cornea of the eye, compact tumors, and embryos rely heavily on glycolysis for their energy needs.
KeywordsPyruvate Kinase Oxygen Pulse NADH Fluorescence Mutual Synchronization Adenosine Phosphate
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- 1.Before 1970, NADH was called DPNH.Google Scholar
- 2.Today we would say “by F2,6B,” discovered in the 1980s; but Richard et al. (1996b) found that [F2,6B] varies only insignficantly during the oscillation.Google Scholar
- 3.They do in Aplysia neurons, whose electrical rhythms are regulated by PFK activity (Chaplain, 1976, and Chapter 14). Chou et al. (1992) and O’Rourke et al. (1994, 1995) have since found glycolytic ocillations driving action potentials also in cardiac myocytes and in pancreatic beta cells. Besides oxygen, also glycerol, acetaldehyde, and ethanol cross the cell membrane quickly by diffusion. In 1970–2, various prior experiments (that later turned out to mislead) informed us that these do not oscillate measurably, and/or do not affect the oscillation in return, or permanently alter its amplitude and period. So none but oxygen seemed good candidates for testing phase responses.Google Scholar
- 4.Othmer and Aldridge (1978), and Aldridge and Pye (1979b) favor an alternative interpretation, i.e., that each cell quits oscillating when too isolated from its neighbors by dilution. This possibility can be checked by spectrophotometry of single-cell NADH rhythms (Chance et al., 1968) but definitive results have not been reported.Google Scholar