How cells turn glucose into ATP
HESI A2 topic: Cellular respiration
- Respiration turns glucose and oxygen into CO₂, water, and ATP.
- Glycolysis runs in the cytoplasm, needs no oxygen, and nets 2 ATP.
- The Krebs cycle runs in the mitochondrial matrix and turns twice per glucose.
- The electron transport chain makes most ATP; oxygen becomes water there.
- Without oxygen, fermentation keeps glycolysis going; it makes no extra ATP.
Key facts
- Overall equation: C6H12O6 + 6O2 → 6CO2 + 6H2O, with part of the released energy captured as ATP; it is the reverse of the photosynthesis equation.
- Glycolysis happens in the cytoplasm, does not use oxygen, and turns one glucose into two pyruvate for a net gain of 2 ATP and 2 NADH.
- Pyruvate oxidation and the citric acid (Krebs) cycle happen in the mitochondrial matrix; the cycle turns twice per glucose and releases CO2.
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- The electron transport chain sits in the inner mitochondrial membrane; oxygen is the final electron acceptor and becomes water.
- Chemiosmosis through ATP synthase makes about 90 percent of the ATP from aerobic glucose breakdown.
- OpenStax Biology 2e says the total ATP per glucose varies; OpenStax Anatomy & Physiology 2e counts a net 36 ATP.
- Without oxygen, fermentation regenerates NAD+ so glycolysis can keep making its 2 ATP; muscle cells make lactate and yeast make ethanol and CO2.
Quick check1 question · 2 more at the end
Where in a eukaryotic cell does glycolysis take place?
Correct answer: B.
Glycolysis, the splitting of glucose into two pyruvate molecules, happens in the cytoplasm of both prokaryotic and eukaryotic cells. The later stages move into the mitochondrion: pyruvate oxidation and the citric acid cycle in the matrix, and the electron transport chain in the inner membrane.
A The mitochondrial matrix is where pyruvate oxidation and the citric acid cycle happen, after glycolysis.
B Correct: glycolysis takes place in the cytoplasm.
C The nucleus stores DNA; no stage of cellular respiration takes place there.
D The inner mitochondrial membrane holds the electron transport chain, the final stage, not glycolysis.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Cellular respiration breaks glucose down with oxygen to make ATP, releasing carbon dioxide and water. Glycolysis in the cytoplasm splits glucose into two pyruvate for a net 2 ATP. In the mitochondria, pyruvate oxidation and the citric acid (Krebs) cycle release CO2 and load NADH and FADH2, and the electron transport chain uses them to make most of the ATP.
1 What is cellular respiration for?Cellular respiration is the catabolic pathway that turns the energy in glucose into ATP the cell can spend. 1 min
Cellular respiration is the catabolic pathway that turns the energy in glucose into ATP the cell can spend. With oxygen available, the overall reaction is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy, partly captured as ATP)
Glucose and oxygen go in; carbon dioxide and water come out. This is the reverse of the photosynthesis equation. It happens in a series of stages, each in its own part of the cell.
2 Where does each stage happen?Each stage has a fixed address in the cell, and that location is worth memorizing. 1 min
Each stage has a fixed address in the cell, and that location is worth memorizing. Numbers below are per glucose.
| Stage | Where | Uses O₂? | Main products |
|---|---|---|---|
| Glycolysis | Cytoplasm | No | 2 pyruvate, net 2 ATP, 2 NADH |
| Pyruvate oxidation | Mitochondrial matrix | Not directly | 2 acetyl CoA, 2 CO₂, 2 NADH |
| Citric acid (Krebs) cycle | Mitochondrial matrix | Not directly | 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP |
| Electron transport chain + chemiosmosis | Inner mitochondrial membrane | Yes | Most of the ATP, plus water |
The Krebs cycle never touches oxygen itself. It still counts as aerobic because the NADH and FADH₂ it makes can only unload at the electron transport chain, which needs oxygen.
3 What happens in glycolysis?Glycolysis splits one six-carbon glucose into two three-carbon pyruvate molecules in the cytoplasm. 1 min
Glycolysis splits one six-carbon glucose into two three-carbon pyruvate molecules in the cytoplasm. The cell first invests 2 ATP to get glucose ready to split, then makes 4 ATP, for a net gain of 2 ATP. It also loads 2 NADH, an electron carrier.
Glycolysis needs no oxygen, and nearly all living things run it. Mature red blood cells have no mitochondria, so glycolysis is their only source of ATP.
4 What happens inside the mitochondrion?Inside the mitochondrion, the rest of glucose's carbon is stripped off as CO₂ while electron carriers fill up. 1 min
Inside the mitochondrion, the rest of glucose's carbon is stripped off as CO₂ while electron carriers fill up.
- Pyruvate oxidation (matrix): each pyruvate loses one carbon as CO₂, becomes acetyl CoA, and loads one NADH.
- Citric acid (Krebs) cycle (matrix): each acetyl CoA enters one turn, so the cycle turns twice per glucose. Each turn releases 2 CO₂ and makes 3 NADH, 1 FADH₂, and 1 ATP (or GTP).
- Electron transport chain (inner membrane): NADH and FADH₂ hand their electrons to a chain of proteins. The chain uses that energy to pump hydrogen ions (H⁺) across the membrane. Oxygen is the final electron acceptor: it picks up the spent electrons and H⁺ and becomes water.
- Chemiosmosis: the H⁺ flow back through ATP synthase, which uses the flow to attach phosphate to ADP. This step makes about 90 percent of the ATP.
5 How much ATP does one glucose make?Most of the ATP comes from the electron transport chain, not from the earlier stages. 1 min
Most of the ATP comes from the electron transport chain, not from the earlier stages. Glycolysis nets 2 ATP and the Krebs cycle makes 2; the rest comes from chemiosmosis.
OpenStax Biology 2e does not give a single total, because the number varies between species and with how electrons are carried into the mitochondrion. OpenStax Anatomy & Physiology 2e counts a net total of 36 ATP per glucose. Know the pattern: a small amount from glycolysis, the large majority from the electron transport chain.
6 What happens without oxygen?Without oxygen, the electron transport chain stops, and cells fall back on glycolysis alone. 1 min
Without oxygen, the electron transport chain stops, and cells fall back on glycolysis alone. Glycolysis needs a steady supply of NAD⁺, so cells use fermentation to turn NADH back into NAD⁺. The cell then harvests only the 2 ATP from glycolysis per glucose.
| Type | Who does it | Products from pyruvate |
|---|---|---|
| Lactic acid fermentation | Red blood cells; skeletal muscle short on oxygen | Lactate (lactic acid) |
| Alcohol fermentation | Yeast | Ethanol and CO₂ |
The liver can later convert lactate back to pyruvate. Some prokaryotes use anaerobic respiration instead, running an electron transport chain with a non-oxygen final acceptor such as sulfate.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Glycolysis happens in the cytoplasm, not in the mitochondria.
Watch out: Oxygen is used at the end of the electron transport chain, where it becomes water; the CO2 you release comes from pyruvate oxidation and the Krebs cycle, not from oxygen.
Watch out: Most ATP comes from the electron transport chain and chemiosmosis, not from glycolysis or the Krebs cycle.
Watch out: Fermentation does not make extra ATP; it only regenerates NAD+ so glycolysis can continue.
Watch out: Lactic acid fermentation happens in human muscle and red blood cells; alcohol fermentation (ethanol and CO2) happens in yeast, not humans.
How to remember it
Memory hook: "Get Kids Energy": Glycolysis, Krebs, Electron transport chain, in order. Then pin each to its address: cytoplasm, matrix, inner membrane.
Self-check:
- Name the stage that runs in the cytoplasm and its net ATP per glucose.
- Explain what oxygen does in the electron transport chain and what it becomes.
- A muscle runs short of oxygen. Say what pyruvate becomes and why the cell bothers.