Biology

Cellular respiration

Cellular respiration is how cells break down glucose to make ATP, the energy molecule cells spend. With oxygen it runs in three stages: glycolysis in the cytoplasm, the citric acid (Krebs) cycle in the mitochondrial matrix, and the electron transport chain on the inner mitochondrial membrane, which makes most of the ATP. Oxygen is the final electron acceptor. The overall products are carbon dioxide, water and ATP.

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Key ideas

C6H12O6+6 O2→6 CO2+6 H2O+energy (ATP and heat)\mathrm{C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O} + \text{energy (ATP and heat)}
  • C6H12O6\mathrm{C_6H_{12}O_6} is glucose, the fuel. O2\mathrm{O_2} is oxygen, which you breathe in.
  • CO2\mathrm{CO_2} is carbon dioxide, which you breathe out. H2O\mathrm{H_2O} is water.
  • ATP (adenosine triphosphate) stores energy in a form the cell can use right away. NADH and FADH2\mathrm{FADH_2} are carriers that move high-energy electrons to the electron transport chain.
The stages of aerobic respiration, per glucose (OpenStax Biology 2e, 7.2 to 7.4)
StageWhereGoes inComes out
GlycolysisCytoplasm1 glucose2 pyruvate, net 2 ATP, 2 NADH
Pyruvate oxidationMitochondrial matrix2 pyruvate2 acetyl CoA, 2 CO2\mathrm{CO_2}, 2 NADH
Citric acid cycle (2 turns)Mitochondrial matrix2 acetyl CoA4 CO2\mathrm{CO_2}, 2 ATP (or GTP), 6 NADH, 2 FADH2\mathrm{FADH_2}
Electron transport chain and chemiosmosisInner mitochondrial membraneNADH, FADH2\mathrm{FADH_2}, O2\mathrm{O_2}Most of the ATP, and water

Without oxygen, the electron transport chain stops. Many cells then use fermentation, which recycles NADH back to NAD+\mathrm{NAD^+} so glycolysis can keep making its 2 ATP. Animal muscle cells and yogurt bacteria make lactic acid. Yeast makes ethanol and CO2\mathrm{CO_2}.

Worked examples

Example 1: follow the carbons

Problem Glucose has 6 carbon atoms. Show where all 6 end up as CO2\mathrm{CO_2} during aerobic respiration.

  1. Glycolysis splits glucose (6 C) into 2 pyruvate (3 C each). No CO2\mathrm{CO_2} yet.
  2. Pyruvate oxidation removes 1 carbon from each pyruvate as CO2\mathrm{CO_2}: 2 CO2\mathrm{CO_2}, leaving 2 acetyl groups (2 C each).
  3. Each turn of the citric acid cycle releases 2 CO2\mathrm{CO_2}. Two turns: 4 CO2\mathrm{CO_2}.
    2+4=6 CO22 + 4 = 6\ \mathrm{CO_2}

Answer 2 CO2 from pyruvate oxidation + 4 CO2 from the citric acid cycle = 6 CO2

Example 2: grams of carbon dioxide from glucose

Problem A runner's cells completely break down 90.0 g of glucose. How many grams of CO2\mathrm{CO_2} are produced?

  1. Grams of glucose to moles (180.16 g/mol).
    90.0 g180.16 g/mol=0.4996 mol\frac{90.0\ \text{g}}{180.16\ \text{g/mol}} = 0.4996\ \text{mol}
  2. Mole ratio from the equation: 6 CO2\mathrm{CO_2} per glucose.
    0.4996×6=2.997 mol CO20.4996 \times 6 = 2.997\ \text{mol CO}_2
  3. Moles to grams (44.01 g/mol).
    2.997×44.01=131.9 g2.997 \times 44.01 = 131.9\ \text{g}

Answer 132 g of CO2\mathrm{CO_2}

Example 3: why fermentation is a backup

Problem A muscle cell runs short of oxygen during a sprint. Compare the ATP it gets from one glucose by fermentation with what it gets from aerobic respiration, using the textbook counts on this page.

  1. Fermentation only runs glycolysis, so it nets 2 ATP per glucose.
  2. Aerobic respiration adds the citric acid cycle and the electron transport chain. Even the low end of textbook counts is many times more.
  3. That is why sprinting muscles tire fast and why your body needs oxygen for long activity.

Answer Fermentation nets only 2 ATP per glucose, far less than aerobic respiration

Common mistakes and how to fix them

  • Saying respiration means breathing. Breathing brings in O2\mathrm{O_2} and removes CO2\mathrm{CO_2}. Cellular respiration is the chemistry inside each cell that uses them.
  • Saying glycolysis happens in the mitochondria. Glycolysis is in the cytoplasm. Only the later stages are in the mitochondria.
  • Thinking oxygen becomes CO2\mathrm{CO_2}. The carbon in CO2\mathrm{CO_2} comes from glucose. Oxygen accepts electrons at the end of the chain and becomes water.
  • Saying plants don't do respiration. Plants make glucose by photosynthesis and break it down by respiration in their mitochondria.

Practice questions

  1. Where does glycolysis take place?

    1. Cytoplasm
    2. Mitochondrial matrix
    3. Inner mitochondrial membrane
    4. Chloroplast
    Show answer

    Answer: Cytoplasm

    Glycolysis runs in the cytoplasm of every cell, including bacteria, which have no mitochondria.

  2. What is the final electron acceptor in the electron transport chain?

    1. NAD+
    2. Oxygen
    3. Carbon dioxide
    4. Glucose
    Show answer

    Answer: Oxygen

    Oxygen takes the electrons at the end of the chain and combines with hydrogen ions to form water.

  3. How many moles of O2\mathrm{O_2} are needed to completely break down 2.0 mol of glucose?

    Show answer

    Answer: 12 mol

    The equation uses 6 O2\mathrm{O_2} per glucose: 2.0×6=122.0 \times 6 = 12 mol.

  4. What is the net ATP gain from glycolysis for one glucose?

    1. 0
    2. 2
    3. 4
    4. 36
    Show answer

    Answer: 2

    Glycolysis spends 2 ATP and makes 4, for a net gain of 2.

  5. Which organism makes ethanol and CO2\mathrm{CO_2} by fermentation?

    1. Yeast
    2. Human muscle cells
    3. Yogurt bacteria
    4. Green plants in light
    Show answer

    Answer: Yeast

    Yeast uses alcohol fermentation, which is why bread rises (CO2\mathrm{CO_2}) and why it is used to make alcoholic drinks. Muscle cells and yogurt bacteria make lactic acid.

Frequently asked questions

How is cellular respiration related to photosynthesis?

They run in roughly opposite directions. Photosynthesis stores light energy in glucose and releases O2\mathrm{O_2}. Respiration releases that energy as ATP and gives back CO2\mathrm{CO_2} and water. Plants do both, while animals only do respiration.

What is chemiosmosis?

The electron transport chain uses energy from electrons to pump hydrogen ions across the inner mitochondrial membrane, building up a gradient. The ions flow back through ATP synthase, an enzyme that works like a turbine, and that flow powers ATP production. OpenStax says chemiosmosis makes about 90% of the ATP from aerobic glucose breakdown.

Why do my muscles burn during hard exercise?

When muscle cells cannot get oxygen fast enough, they rely more on lactic acid fermentation to keep glycolysis going. The burning feeling is linked to the chemical changes of this fast, low-oxygen work. Your body clears the lactate once oxygen supply catches up.

Sources

  1. OpenStax Biology 2e, 7.2 Glycolysis, accessed October 1, 2026
  2. OpenStax Biology 2e, 7.3 Oxidation of Pyruvate and the Citric Acid Cycle, accessed October 1, 2026
  3. OpenStax Biology 2e, 7.4 Oxidative Phosphorylation, accessed October 1, 2026
  4. OpenStax Biology 2e, 7.5 Metabolism without Oxygen, accessed October 1, 2026
  5. OpenStax Anatomy and Physiology 2e, 24.2 Carbohydrate Metabolism (36 ATP count), accessed October 1, 2026
  6. OpenStax Microbiology, 8.3 Cellular Respiration (theoretical maximum of 38 ATP), accessed October 1, 2026

Try asking Ducky

  • “Check my table of inputs and outputs for each stage of respiration.”
  • “Quiz me on where each stage happens until I stop mixing up the matrix and the membrane.”
  • “Draw the electron transport chain with me and explain chemiosmosis simply.”

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