2.3 Cell Respiration, ATP and Anaerobic Pathways

Cell respiration is the controlled release of energy from organic compounds to produce ATP. Much of the energy originally stored in a respiratory substrate ultimately disperses as heat; a usable fraction is transferred to ATP. Cells spend ATP on active transport, biosynthesis, movement and other work. ATP is therefore an immediate energy-transfer molecule, not a long-term energy store comparable to glycogen or lipid.

ATP contains adenine, ribose and three phosphate groups. Hydrolysis of the terminal phosphate produces ADP and inorganic phosphate. The overall reaction releases free energy because the products are more stable and become hydrated; it is misleading to imagine that simply breaking a bond releases energy in isolation. ATP synthesis from ADP and phosphate is endergonic and must be coupled to respiration, photosynthesis or another energy source.

Aerobic respiration transfers much more of glucose's energy to ATP because pyruvate is oxidized further and its electrons pass to oxygen through an electron transport chain. Textbook totals such as 36 or 38 ATP are useful idealized accounts, but measured eukaryotic yields are often closer to about 30 because transport and membrane leakage have costs. The defensible comparison is that aerobic yield is far greater, not that every cell always produces one exact total.

During intense exercise, ATP demand can exceed the rate at which oxygen delivery supports aerobic respiration. Anaerobic glycolysis supplies ATP rapidly but briefly. Lactate and H⁺ handling, depletion of fuel and other changes contribute to fatigue; lactate is not simply a poisonous waste that directly causes next-day soreness. Once oxygen supply is adequate, lactate can be oxidized or transported to the liver for further metabolism.

A respirometer estimates respiration by measuring gas-volume or pressure change. If carbon dioxide is absorbed by alkali, a fall in gas volume reflects oxygen uptake. Temperature, organism mass, activity and apparatus volume must be controlled, and a control apparatus accounts for pressure changes unrelated to respiration. The movement of a fluid marker is evidence for gas change; converting it into a rate requires distance or volume per unit time and appropriate normalization.