Textbooks promise 38 ATP per glucose, but real cells manage about 30
Generations of students memorised that burning one glucose molecule yields 38 units of ATP, the cell's energy currency. Real cells fall short. Leaky membranes and the cost of shuttling molecules into mitochondria mean current estimates sit around 29 to 30, still roughly fifteen times more than surviving without oxygen.
Cellular respiration oxidises fuels such as sugars, amino acids and fatty acids, passing electrons to an acceptor, usually oxygen, and capturing the released energy as ATP. Chemically it is a kind of combustion, but an unusually gentle one, releasing energy in a controlled series of small steps rather than a single blaze. The ATP then powers building molecules, movement and pumping substances across membranes.
The first stage, glycolysis, literally sugar splitting, happens in the cytosol of every living thing, with or without oxygen. It spends two ATP to prime a glucose molecule, then earns four back while breaking it into two pyruvates, for a net gain of two. When oxygen is present, pyruvate is converted into acetyl-CoA and fed into the citric acid cycle, also known as the Krebs cycle, which produces carbon dioxide along with energy-rich carriers called NADH and FADH2.
Most of the payoff comes last. In oxidative phosphorylation, the carriers drive an electron transport chain that pumps protons across a membrane, and the resulting pressure spins the enzyme ATP synthase to make ATP. In eukaryotes these later steps happen in mitochondria; in bacteria, in the cytoplasm. Without oxygen, glycolysis alone yields only two ATP per glucose.
Not all respiration needs oxygen. Anaerobic respiration uses other inorganic acceptors, which lets organisms like methanogens squeeze out more energy than simple glycolysis. It differs from fermentation, which involves no external electron acceptor and therefore does not count as respiration. Plants respire too: although photosynthesis makes them net consumers of carbon dioxide, their respiration produces about half of the carbon dioxide released each year by land ecosystems.
Source: Cellular respiration
