# Cellular respiration

Canonical: https://duckyhelper.com/learn/biology/cellular-respiration/
Updated: 2026-10-01

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.

## Key ideas

$$
\mathrm{C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O} + \text{energy (ATP and heat)}
$$

- \(\mathrm{C_6H_{12}O_6}\) is glucose, the fuel. \(\mathrm{O_2}\) is oxygen, which you breathe in.
- \(\mathrm{CO_2}\) is carbon dioxide, which you breathe out. \(\mathrm{H_2O}\) is water.
- **ATP** (adenosine triphosphate) stores energy in a form the cell can use right away. **NADH** and **\(\mathrm{FADH_2}\)** 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)**

| Stage | Where | Goes in | Comes out |
| --- | --- | --- | --- |
| Glycolysis | Cytoplasm | 1 glucose | 2 pyruvate, net 2 ATP, 2 NADH |
| Pyruvate oxidation | Mitochondrial matrix | 2 pyruvate | 2 acetyl CoA, 2 \(\mathrm{CO_2}\), 2 NADH |
| Citric acid cycle (2 turns) | Mitochondrial matrix | 2 acetyl CoA | 4 \(\mathrm{CO_2}\), 2 ATP (or GTP), 6 NADH, 2 \(\mathrm{FADH_2}\) |
| Electron transport chain and chemiosmosis | Inner mitochondrial membrane | NADH, \(\mathrm{FADH_2}\), \(\mathrm{O_2}\) | Most of the ATP, and water |

> **Note: How many ATP per glucose?**
>
> The exact number is not fixed. OpenStax Biology 2e explains that it changes with the cell type and with how electrons from glycolysis get into the mitochondria. Older textbook counts give a maximum of 36 to 38 ATP, and real cells make fewer. What is certain: glycolysis nets 2 ATP, the citric acid cycle makes 2, and chemiosmosis makes about 90% of the ATP from aerobic glucose breakdown. Use the number your course gives.

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

## Worked examples

**Example 1: follow the carbons**

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

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

   $$
   2 + 4 = 6\ \mathrm{CO_2}
   $$

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 \(\mathrm{CO_2}\) are produced?

1. Grams of glucose to moles (180.16 g/mol).

   $$
   \frac{90.0\ \text{g}}{180.16\ \text{g/mol}} = 0.4996\ \text{mol}
   $$
2. Mole ratio from the equation: 6 \(\mathrm{CO_2}\) per glucose.

   $$
   0.4996 \times 6 = 2.997\ \text{mol CO}_2
   $$
3. Moles to grams (44.01 g/mol).

   $$
   2.997 \times 44.01 = 131.9\ \text{g}
   $$

Answer: 132 g of \(\mathrm{CO_2}\)

**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 \(\mathrm{O_2}\) and removes \(\mathrm{CO_2}\). 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 \(\mathrm{CO_2}\).** The carbon in \(\mathrm{CO_2}\) 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?
   A. Cytoplasm
   B. Mitochondrial matrix
   C. Inner mitochondrial membrane
   D. Chloroplast

   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?
   A. NAD+
   B. Oxygen
   C. Carbon dioxide
   D. Glucose

   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 \(\mathrm{O_2}\) are needed to completely break down 2.0 mol of glucose?

   Answer: 12 mol. The equation uses 6 \(\mathrm{O_2}\) per glucose: \(2.0 \times 6 = 12\) mol.

4. What is the net ATP gain from glycolysis for one glucose?
   A. 0
   B. 2
   C. 4
   D. 36

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

5. Which organism makes ethanol and \(\mathrm{CO_2}\) by fermentation?
   A. Yeast
   B. Human muscle cells
   C. Yogurt bacteria
   D. Green plants in light

   Answer: Yeast. Yeast uses alcohol fermentation, which is why bread rises (\(\mathrm{CO_2}\)) 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](https://duckyhelper.com/learn/biology/photosynthesis/) stores light energy in glucose and releases \(\mathrm{O_2}\). Respiration releases that energy as ATP and gives back \(\mathrm{CO_2}\) 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

- [OpenStax Biology 2e, 7.2 Glycolysis](https://openstax.org/books/biology-2e/pages/7-2-glycolysis), accessed 2026-10-01
- [OpenStax Biology 2e, 7.3 Oxidation of Pyruvate and the Citric Acid Cycle](https://openstax.org/books/biology-2e/pages/7-3-oxidation-of-pyruvate-and-the-citric-acid-cycle), accessed 2026-10-01
- [OpenStax Biology 2e, 7.4 Oxidative Phosphorylation](https://openstax.org/books/biology-2e/pages/7-4-oxidative-phosphorylation), accessed 2026-10-01
- [OpenStax Biology 2e, 7.5 Metabolism without Oxygen](https://openstax.org/books/biology-2e/pages/7-5-metabolism-without-oxygen), accessed 2026-10-01
- [OpenStax Anatomy and Physiology 2e, 24.2 Carbohydrate Metabolism (36 ATP count)](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-2-carbohydrate-metabolism), accessed 2026-10-01
- [OpenStax Microbiology, 8.3 Cellular Respiration (theoretical maximum of 38 ATP)](https://openstax.org/books/microbiology/pages/8-3-cellular-respiration), accessed 2026-10-01

## Related

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- [Enzymes](https://duckyhelper.com/learn/biology/enzymes/)
- [Biology study guides](https://duckyhelper.com/learn/biology/)

## 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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