Cellular respiration: glycolysis, the Krebs cycle, the electron transport chain, and the ATP count

Cellular respiration takes one glucose and oxygen and returns carbon dioxide, water and about 30 to 32 ATP, in three stages: glycolysis in the cytosol (glucose to two pyruvate, net 2 ATP, 2 NADH), the Krebs cycle in the mitochondrial matrix (two turns, 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂, plus 2 NADH and 2 CO₂ from the step before it), and the electron transport chain on the inner membrane, where those carriers' electrons pump protons and the proton flow drives ATP synthase. Below, one glucose traced with every count, the two things students swap, and why your textbook may say 36 or 38.

The one equation, and the three stages it hides

The whole of cellular respiration fits on one line:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (about 30 to 32 ATP per glucose)

The line hides where everything happens. Glucose is taken apart in three stages, each in its own place in the cell, and the energy comes out in two forms. Some is captured directly as ATP. Most is captured first as loaded electron carriers, NADH and FADH₂, which are ATP not yet made: they carry electrons to the last stage, where the ATP is finally produced.

Stage	Where	In (per glucose)	Out (per glucose)
Glycolysis	Cytosol	1 glucose, 2 ATP, 2 NAD⁺	2 pyruvate, 4 ATP (net 2), 2 NADH
Krebs cycle (with pyruvate oxidation)	Mitochondrial matrix	2 pyruvate, 8 NAD⁺, 2 FAD, 2 ADP	6 CO₂, 8 NADH, 2 FADH₂, 2 ATP
Electron transport chain and chemiosmosis	Inner mitochondrial membrane	10 NADH, 2 FADH₂, 6 O₂	about 26 to 28 ATP, 10 NAD⁺, 2 FAD, water

Three rows. If you are here for the table, you have it. If you are here because there is a test, keep reading: every number in it comes from a trace you can redo in your head, and the trace is what a marker is really asking for.

Stage 1, glycolysis: glucose to two pyruvate in the cytosol

Glycolysis happens in the cytosol, outside the mitochondrion, and needs no oxygen. It takes one six-carbon glucose and cuts it into two three-carbon pyruvate molecules. The course asks for the inputs, the outputs and the location, not the ten enzyme steps, so that is what to carry.

It runs in two halves. In the energy-investment half, the cell spends 2 ATP to phosphorylate the sugar, which makes it unstable enough to split. In the energy-payoff half, the two three-carbon pieces are oxidised and 4 ATP are made by substrate-level phosphorylation, with 2 NAD⁺ picking up electrons to become 2 NADH.

The ledger, per glucose:

• In: 1 glucose (6 C), 2 ATP, 2 NAD⁺, 4 ADP.
• Out: 2 pyruvate (3 C each), 4 ATP made, so a net gain of 4 − 2 = 2 ATP, and 2 NADH.
• Carbon count: 6 in, 3 + 3 = 6 out. No carbon has left yet.

Every arrow in this half of the page is an enzyme, and the enzyme page explains what that word is doing: holding one reaction's substrate in an active site so that step runs faster than it otherwise would.

The step between: pyruvate oxidation

Each pyruvate is transported into the mitochondrial matrix. There it loses one carbon as CO₂, hands two electrons to NAD⁺, and the remaining two carbons are attached to a carrier called coenzyme A, giving acetyl-CoA.

Per pyruvate: 1 CO₂, 1 NADH, 1 acetyl-CoA (2 C). Per glucose, with two pyruvate: 2 CO₂, 2 NADH, 2 acetyl-CoA.

Textbooks differ in where they file this step. Some fold it into "the Krebs cycle"; some list it on its own as the link reaction or pyruvate oxidation. That filing is one reason two books' tables look different while describing the same chemistry. If your course lists it apart, keep it apart in your answer and the counts still add up.

Stage 2, the Krebs cycle: two turns per glucose

The Krebs cycle (also called the citric acid cycle) runs in the mitochondrial matrix. One acetyl-CoA enters per turn, so one glucose drives two turns.

Per turn: acetyl-CoA (2 C) joins oxaloacetate (4 C) to make citrate (6 C). Over the turn, two carbons leave as 2 CO₂, three NAD⁺ become 3 NADH, one FAD becomes 1 FADH₂, one ATP is made directly (as GTP in many texts, which the cell converts to ATP), and oxaloacetate is rebuilt to start again.

Per glucose, two turns: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP.

Now close the carbon ledger. Pyruvate oxidation released 2 CO₂ and the Krebs cycle released 4 CO₂, so 2 + 4 = 6 CO₂, which is all six of glucose's carbons. The 6 CO₂ in the equation are now accounted for, and the Krebs cycle is where the last of the carbon leaves.

Keep a running total as you go:

• ATP made directly so far: 2 (glycolysis) + 2 (Krebs) = 4.
• NADH to hand on: 2 (glycolysis) + 2 (pyruvate oxidation) + 6 (Krebs) = 10.
• FADH₂ to hand on: 2.

Four ATP from two whole stages. The rest of the thirty-odd is still sitting in those twelve carriers.

Stage 3, the electron transport chain and chemiosmosis: where the ATP is made

The electron transport chain is a series of protein complexes in the inner mitochondrial membrane. NADH and FADH₂ hand their electrons to it, and the electrons pass from complex to complex, releasing energy at each step. That energy is used to pump H⁺ out of the matrix into the intermembrane space, building up a concentration difference.

The protons flow back into the matrix through one channel, ATP synthase, and the flow turns the enzyme, which phosphorylates ADP to ATP. This is chemiosmosis, Peter Mitchell's idea: the proton gradient is the link between the electrons and the ATP. The same mechanism, run with light as the energy source, is how photosynthesis makes its ATP.

At the end of the chain the electrons have to go somewhere. Oxygen is the final electron acceptor: each O₂ takes four electrons and four H⁺ and becomes two water molecules. This is the only place in the whole process that O₂ is used. Check it against the equation: 10 NADH and 2 FADH₂ carry 12 pairs of electrons, 24 electrons in all, and each O₂ takes 4, so 24 ÷ 4 = 6 O₂, which is the 6 O₂ on the left of the equation.

Now the count, with the convention named. Current biochemistry texts use about 2.5 ATP per NADH and about 1.5 per FADH₂ (these are measured averages, the P/O ratios, not whole numbers):

• 10 NADH × 2.5 = 25 ATP.
• 2 FADH₂ × 1.5 = 3 ATP.
• 25 + 3 = 28 ATP by chemiosmosis.
• Plus the 4 made directly: 28 + 4 = 32 ATP per glucose.

The lower figure, 30, comes from the two glycolytic NADH. They are made in the cytosol and cannot cross the inner membrane. In many cells their electrons are carried in by the glycerol-phosphate shuttle, which hands them to FAD rather than NAD⁺ inside, so those two count at 1.5 each rather than 2.5: a loss of 2 × 1 = 2 ATP, and 32 − 2 = 30. Hence "about 30 to 32".

Why your textbook may say 36 or 38

Older textbooks used whole numbers: 3 ATP per NADH and 2 per FADH₂. Redo the same sum under that convention:

• 10 NADH × 3 = 30 ATP.
• 2 FADH₂ × 2 = 4 ATP.
• 30 + 4 = 34 by chemiosmosis.
• Plus the 4 made directly: 34 + 4 = 38 ATP per glucose.
• With the shuttle cost for the two glycolytic NADH (2 × 1 = 2): 38 − 2 = 36.

So 36 to 38 and 30 to 32 are not two different pathways and not a disagreement about the chemistry. They are two conventions for how many ATP one NADH is worth, applied to the same ten NADH and two FADH₂. The newer figures are lower because the protons needed per ATP, including the cost of moving ATP out of the mitochondrion, were measured rather than rounded to whole numbers.

What a test marks is the convention your course uses. The safe sentence writes the convention beside the number: "about 32 ATP per glucose at 2.5 ATP per NADH and 1.5 per FADH₂" or "about 38 ATP at 3 and 2". A marker who reads that cannot mark it wrong, because you have shown you know where the number comes from. A bare "36 ATP" in a course that teaches 32 is a lost mark for a reason you could have avoided.

The two things students swap

NADH and NAD⁺. NADH is the loaded carrier (reduced, carrying two electrons and a proton); NAD⁺ is the empty one (oxidised). Glycolysis, pyruvate oxidation and the Krebs cycle load NAD⁺ into NADH; the electron transport chain unloads NADH back to NAD⁺. If you write that glycolysis "uses NADH", you have the arrow backwards. The same holds for FAD and FADH₂.

Where the ATP is made. The Krebs cycle makes 2 of the roughly 30 to 32 ATP per glucose. The electron transport chain, through chemiosmosis, makes the other 26 to 28. A sentence that says "the Krebs cycle produces most of the cell's ATP" loses the mark; what the Krebs cycle mostly produces is loaded carriers, and the ATP is made at the inner membrane.

A third, close behind: oxygen's job. Oxygen is not an input of glycolysis or the Krebs cycle. It is the final electron acceptor at the end of the chain, and that is the only place it appears.

Without oxygen: aerobic vs anaerobic respiration

Take oxygen away and the chain has nowhere to put its electrons, so it stops. NADH piles up with no way to unload, NAD⁺ runs out, and glycolysis would stop too, because its payoff half needs NAD⁺. Fermentation is the fix: pyruvate (or a molecule made from it) takes the electrons from NADH, regenerating NAD⁺ so glycolysis can keep going. In muscle the product is lactate; in yeast it is ethanol and CO₂. The yield is glycolysis alone, a net 2 ATP per glucose, against about 30 to 32 with oxygen. That is the whole content of "aerobic vs anaerobic": the chain and the Krebs cycle need oxygen to keep running; glycolysis does not. The next idea in the course, anaerobic respiration and fermentation, traces the two fermentations themselves.

Check: trace it cold, then one sibling

Draw three boxes on a blank page and label them cytosol, matrix, inner membrane. In each, write what goes in and what comes out, with the counts per glucose. Under them write the carbon ledger (6 → 3 + 3 → 2 CO₂ + 4 CO₂ → 0 left) and the ATP total with its convention. Then open the page and mark it. Drawing it from memory and checking is the retrieval practice that Karpicke and Blunt (2011) found beats re-studying the diagram, even when the re-studying is drawing a careful concept map from the book; the honest-marking rules are what make the check worth doing, and how to study biology has the routine for a whole unit of this kind.

Then a sibling, cold: a cell respires two glucose molecules fully, with oxygen, under the 2.5 and 1.5 convention and no shuttle cost. How many ATP?

Answer. Everything per glucose doubles. Carriers: 20 NADH and 4 FADH₂. Chemiosmosis: 20 × 2.5 = 50 and 4 × 1.5 = 6, so 56. Direct: 2 × 4 = 8. Total 56 + 8 = 64 ATP, which is 2 × 32, as it should be. With the shuttle cost it would be 2 × 30 = 60.

If you got the 64 but could not say why the direct count is 8, redo the Krebs section; if you got the chain's share but swapped NADH and NAD⁺ in the boxes, the swap section is tonight's card.

Where this sits in the course

These are ideas 8, 9 and 10 of the 27 in the SBI4U course map: Glycolysis, The Krebs cycle, and The electron transport chain and chemiosmosis, the middle of strand C. Before them comes idea 7, ATP and the energy currency of the cell, which is why ATP is the unit this page counts in. After them come idea 11, Anaerobic respiration and fermentation, which traces the two fermentations named above, and then idea 12, The light-dependent reactions, where photosynthesis runs the same chain idea the other way, with light as the energy source and water as the electron donor. The other biology and chemistry explainers are in the science guides.

Cellular respiration on one screen

Stage	Where	In (per glucose)	Out (per glucose)
Glycolysis	Cytosol	1 glucose, 2 ATP, 2 NAD⁺	2 pyruvate, net 2 ATP, 2 NADH
Krebs cycle (with pyruvate oxidation)	Matrix	2 pyruvate	6 CO₂, 8 NADH, 2 FADH₂, 2 ATP
Electron transport chain	Inner membrane	10 NADH, 2 FADH₂, 6 O₂	about 26 to 28 ATP, water

• The ATP arithmetic (modern convention): 10 × 2.5 + 2 × 1.5 = 28 by chemiosmosis, + 4 direct = 32; 30 with the shuttle cost. Older convention: 10 × 3 + 2 × 2 + 4 = 38; 36 with the shuttle.
• Write the convention beside the number.
• Carbon ledger: 6 in; 2 CO₂ at pyruvate oxidation, 4 CO₂ in the Krebs cycle; 0 left.
• The two swaps: NADH is loaded, NAD⁺ is empty; the chain makes most of the ATP, the Krebs cycle makes 2.
• Oxygen: the final electron acceptor at the end of the chain, and nowhere else.