Aerobic respiration

Aerobic respiration uses molecular oxygen as the final electron acceptor. For complete oxidation of glucose, the net material balance is

C_{6} H_{12} O_{6} + 6 O_{2} ⟶ 6 CO_{2} + 6 H_{2} O .

(1)

This records the overall conversion of glucose, molecular oxygen, carbon dioxide and water; it is not one enzyme-catalyzed step, and it omits the separate ATP accounting. For an original stoichiometry check, each side contains six carbon, twelve hydrogen and eighteen oxygen chemical atoms. If exactly

1 mmol

of glucose undergoes this complete conversion, the equation requires

6 mmol

of molecular oxygen and yields

6 mmol

each of carbon dioxide and net water. This is an equation-based calculation, not an observed seed measurement.

The molecular oxygen is reduced to water, rather than being the source of the carbon dioxide's carbon. A formal redox electron balance, using acidic half-reaction notation, makes the roles explicit:

C_{6} H_{12} O_{6} + 6 H_{2} O 6 O_{2} + 24 H^{+} + 24 e^{-} ⟶ 6 CO_{2} + 24 H^{+} + 24 e^{-}, ⟶ 12 H_{2} O .

(2)

Adding and cancelling gives the net equation above. Both half-reactions conserve chemical atoms and electric charge. The

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Electrons balance a net transfer; they are carried through cellular intermediates, rather than released as a pool of free Electrons. The final respiratory reduction can be written

O_{2} + 4 H^{+} + 4 e^{-} ⟶ 2 H_{2} O

. These equations are bookkeeping, not a tracing experiment that assigns every product oxygen chemical atom to its original molecule.^[ref]

Table of contents
- Respiration and ATP coupling Aerobic respiration

Respiration and ATP coupling ( $ADP + P_{i} ⟶ ATP$ )

 0  0

Glycolysis converts glucose into two pyruvate molecules, with a net two ATP and reduced NADH; it does not directly consume molecular oxygen. Pyruvate processing and the citric acid cycle, also called the TCA cycle, release carbon dioxide and supply reduced carriers. Thus releasing carbon dioxide and consuming molecular oxygen occur at different stages.^[ref]

Figure 1.
Cellular respiration: separate carbon, Electron and energy accounts
. Original pathway schematic. Blue arrows trace the fuel pathway and carbon dioxide release; purple arrows indicate reduced-carrier delivery and terminal molecular oxygen reduction; orange dashed arrows denote energy coupling. Carrier chemistry and intermediate steps are abbreviated. Arrow widths encode no rates or yields. **Details**.

An electron transport chain reoxidizes carriers such as NADH. Coupling some of its redox steps to proton transport establishes an electrochemical gradient; ATP synthase can use that gradient to form ATP from adenosine diphosphate and inorganic phosphate. This is oxidative phosphorylation. A separate route, substrate-level phosphorylation, transfers a phosphate group from a reacting substrate to adenosine diphosphate. In typical eukaryotic biological cells, the respiratory chain occupies the inner membrane of a mitochondrion.^[ref]

The useful achievement is controlled transfer: fuel oxidation can replenish an ATP supply that supports active transport and synthesis. ATP is a reusable chemical carrier, not newly created energy; ATP hydrolysis and the coupled cellular process have their own net free energy balance. Some fuel energy is dissipated as heat.^[ref]

There is no universal yield of

36

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ATP per glucose. Actual ATP recovery depends on carrier transfer routes, coupling and diversion of intermediates to other metabolism. In particular, an oxygen uptake rate cannot be converted into an ATP production rate by assuming one fixed multiplier.^[ref]