Aerobic glucose respiration is easier to understand by tracking three ledgers separately:

1. carbon atoms;
2. reduced electron carriers;
3. ATP formed by different mechanisms.

The pathways then fit together without treating “energy” as an untracked substance.

Carbon ledger

Start with one glucose molecule:

$$6\ \mathrm{C}.$$

Glycolysis converts it to two pyruvates:

$$2\times3\ \mathrm{C}=6\ \mathrm{C},$$

with no $\mathrm{CO_2}$ released.

Pyruvate oxidation releases one $\mathrm{CO_2}$ from each pyruvate:

$$2\ \mathrm{CO_2},$$

leaving two acetyl groups containing four carbons total.

Two turns of the citric-acid cycle release four more $\mathrm{CO_2}$:

$$4\ \mathrm{CO_2}.$$

Thus the six glucose carbons are accounted for as

$$\boxed{6\ \mathrm{CO_2}}.$$

Reduced-carrier ledger

Per glucose, a common bookkeeping total is

These carriers temporarily hold reducing equivalents removed during carbon oxidation.

During oxidative phosphorylation, they are reoxidized and their electrons ultimately reduce molecular oxygen to water.

Direct ATP ledger

Substrate-level phosphorylation gives approximately

• $2$ net ATP from glycolysis;
• $2$ ATP-equivalent nucleotide triphosphates from two turns of the citric-acid cycle.

So direct phosphorylation contributes about

$$4\ \mathrm{ATP\ equivalents}.$$

Oxidative-phosphorylation estimate

Using illustrative coupling values of about

$$2.5\ \mathrm{ATP/NADH}$$

and

$$1.5\ \mathrm{ATP/FADH_2},$$

the reduced-carrier total could support approximately

$$10(2.5)+2(1.5)=28\ \mathrm{ATP}.$$

Adding substrate-level ATP gives an upper-style bookkeeping estimate near

$$28+4=32\ \mathrm{ATP/glucose}.$$

In many eukaryotic cells, commonly quoted totals are roughly 30–32 ATP per glucose because reducing equivalents generated by cytosolic glycolysis must be transferred indirectly into the mitochondrial respiratory system, and membrane leak and metabolite transport also affect effective yield.

The exact ATP number is therefore not a fixed stoichiometric law.

Overall redox equation

The overall chemical oxidation is commonly summarized as

$$\boxed{\mathrm{C_6H_{12}O_6+6O_2\rightarrow6CO_2+6H_2O}}.$$

But this net equation hides the mechanism. Cells do not oxidize glucose in one step. They partition the free-energy decrease across substrate-level phosphorylation, reduced carriers, electron transport, proton pumping and ATP synthase.

The full conceptual chain is

glucose carbon oxidation
      ↓
NADH/FADH2 + a little direct ATP
      ↓
electron transport to O2
      ↓
proton-motive force
      ↓
ATP synthase
      ↓
most respiratory ATP

Separate carbon, electron and ATP ledgers make cellular respiration a conservation-and-coupling problem rather than a list of pathway names.