TL;DR:
Cells break down glucose in stages to make ATP, their usable energy currency:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.Glycolysis (cytoplasm), the Krebs cycle (mitochondrial matrix) and the electron transport chain (inner mitochondrial membrane) do most of the work.
Without oxygen only glycolysis can run, giving far less ATP, and fermentation recycles what it needs.
You eat a slice of bread and an hour later your muscles are using it to move. Somewhere in between, the sugar's stored energy has to be turned into a form that cells can spend. That form is a small molecule called ATP, and making it from glucose is cellular respiration.
ATP: the energy currency
ATP (adenosine triphosphate) has three phosphate groups. Removing the last one, producing ADP and a free phosphate, releases energy the cell can use to power things like muscle contraction, pumping ions and building molecules. Cells then re-attach a phosphate to ADP to remake ATP, using energy from food.
ATP is not stored in large amounts. It is made and spent continuously, so a cell keeps cycling the same molecules between ATP and ADP.
The overall equation
The summary is C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy. It is the reverse of the equation in how photosynthesis turns light into sugar. The carbon in glucose ends up in carbon dioxide, which you breathe out, and the oxygen you breathe in ends up in water.
Worked example: following one glucose molecule
Glycolysis (cytoplasm). One glucose (6 carbons) is split into two pyruvate molecules (3 carbons each). The process uses 2 ATP and makes 4, so the net gain is 2 ATP, plus 2 NADH. No oxygen is needed.
Pyruvate oxidation (mitochondria). Each pyruvate loses one carbon as CO₂ and becomes a 2-carbon acetyl group. For one glucose: 2 CO₂ released and 2 NADH made.
Krebs cycle (mitochondrial matrix). Each acetyl group goes around once. For one glucose, two turns give 4 CO₂, 6 NADH, 2 FADH₂ and 2 ATP.
Electron transport chain (inner mitochondrial membrane). NADH and FADH₂ hand their electrons to a chain of carriers. The energy released pumps hydrogen ions across the membrane, and they flow back through an enzyme that makes ATP. At the end, oxygen accepts the electrons and, with hydrogen ions, forms water.
Do the carbon bookkeeping: glucose has 6 carbons. They leave as 2 CO₂ (pyruvate oxidation) plus 4 CO₂ (Krebs), which is 6 CO₂. All six carbons are accounted for, matching the equation.
Count the carriers: NADH is 2 + 2 + 6 = 10, and FADH₂ is 2. These feed the electron transport chain, which produces most of the ATP. Many current textbooks give a total of roughly 30 to 32 ATP per glucose, while older ones say 36 to 38. The exact figure depends on how efficiently the cell runs the chain, so teachers may ask for different numbers. Use the one your course uses.
No oxygen: anaerobic respiration and fermentation
If oxygen is missing, the electron transport chain stops, because it has no final electron acceptor. The Krebs cycle backs up as well. Glycolysis can still run, giving the net 2 ATP, as long as the cell regenerates the NAD⁺ it needs.
Fermentation does this. In your muscles, pyruvate is turned into lactate. In yeast, pyruvate becomes ethanol and CO₂, which is how bread rises and drinks ferment. Fermentation makes only the 2 ATP from glycolysis, so it is quick but far less efficient. Compare 2 with roughly 30, and you see why oxygen matters.
A useful comparison is energy accounting. Glucose is a compact store, and cells release its energy in many small steps rather than all at once. If glucose were burned in a single step, the energy would come out as heat and little would be captured. By stepping down through carriers like NADH and an electron transport chain, cells capture a good share of it as ATP and release the rest as heat, which is part of why your body stays warm.
Other foods feed in too. Fats and proteins can be broken down into pieces that enter the pathway at the pyruvate or Krebs stage, so the same machinery runs on whatever fuel is available.
How to check your answer
Carbon. 6 carbons in, 6 CO₂ out.
Places. Glycolysis in the cytoplasm, the rest in mitochondria.
Oxygen's role. It is the final electron acceptor at the end of the chain, making water.
Net ATP. Glycolysis alone nets 2 ATP, while the whole aerobic process gives many more.
Mistakes that cost marks
Saying CO₂ is released in the electron transport chain. It is released earlier, in pyruvate oxidation and the Krebs cycle.
Saying plants do photosynthesis and animals do respiration. Both do respiration.
Believing energy is stored in ATP as a bulk fuel instead of used right away.
Mixing up the places, such as putting glycolysis in the mitochondria.
Forgetting that oxygen is used at the end, not at the start.
Where you will use it next
The movement of electrons here is the same oxidation and reduction idea as in oxidation and reduction. The energy that cells make is spent on growth and division, as in mitosis and meiosis compared.
Frequently asked questions
Why do muscles burn during hard exercise?
When oxygen supply cannot keep up, muscles make more ATP by fermentation, producing lactate. The soreness that follows later has other causes.
Where do you lose the carbon from food?
Mostly as carbon dioxide, which leaves your body when you breathe out.
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