Cellular Respiration

The big idea

Your cells need usable energy all the time. They get it by breaking one glucose molecule into smaller pieces in a controlled way and capturing the energy in ATP, the cell’s spendable energy coin. That whole process is cellular respiration, the series of reactions that turns food energy into ATP.

Think of glucose as a full $20 bill and ATP as smaller bills you can spend right away. The cell does not tear the bill apart all at once; it breaks it down in stages so it can keep more of the energy and waste less as heat.

What you'll learn

• Where glycolysis, the Krebs cycle, and the electron transport chain happen
• What each stage makes, and why oxygen matters
• How many ATP a glucose molecule can yield in eukaryotic cells
• What fermentation does when oxygen is missing

Where it happens: the mitochondrion

Most cellular respiration happens in the mitochondrion, a cell organelle with two membranes. The location matters because each stage uses a different part of the cell.

1. Outer membrane is the smooth outside boundary. It is not the main site of ATP-making, but it surrounds the organelle.
2. Inner membrane is the highly selective membrane inside. It holds the protein machines of the electron transport chain.
3. Cristae are the folds of the inner membrane. Folding gives the membrane more surface area, so more energy-making proteins can fit.
4. Matrix is the fluid inside the inner membrane. It contains the enzymes for the Krebs cycle.

This diagram shows where each stage happens.

Keep that map in your head: glycolysis happens before the mitochondrion, the Krebs cycle runs in the matrix, and the electron transport chain runs on the inner membrane.

Stage 1: Glycolysis

Glycolysis is the first stage of cellular respiration, and it happens in the cytosol, the watery fluid outside the organelles. It does not need oxygen, so it can run in both aerobic and anaerobic conditions.

1. Split the sugar. One 6-carbon glucose molecule is rearranged and split into two 3-carbon molecules called pyruvate.
2. Spend a little ATP first. The cell invests 2 ATP to start the process, like paying a small fee to open a locked box.
3. Make ATP and NADH. Later steps make 4 ATP total, so the net gain is 2 ATP. The process also makes 2 NADH, electron carriers that hold high-energy electrons.

Glycolysis is small, fast, and does not need oxygen. It is the same starting point for aerobic respiration and fermentation.

Stage 2: Pyruvate oxidation and the Krebs cycle

After glycolysis, pyruvate moves into the mitochondrion. Before the cycle starts, each pyruvate is changed into acetyl-CoA, a 2-carbon molecule that can enter the cycle. This step is called pyruvate oxidation, and it releases CO₂ and makes NADH.

The Krebs cycle — also called the citric acid cycle — happens in the matrix.

1. Join the cycle. Acetyl-CoA combines with a 4-carbon molecule already in the cycle.
2. Release carbon dioxide. The carbon atoms from glucose leave the cell as CO₂. That is why you breathe out carbon dioxide.
3. Capture energy. The cycle does not make much ATP directly, but it makes lots of electron carriers: NADH and FADH₂.
4. Reset the loop. The 4-carbon starting molecule is remade so the cycle can keep going.

One glucose makes two acetyl-CoA, so the Krebs cycle runs twice per glucose. Across those two turns, it makes 2 ATP, 6 NADH, and 2 FADH₂. This stage loads the “batteries” that power the next stage.

Stage 3: Electron transport chain

The electron transport chain (ETC) happens on the inner membrane of the mitochondrion, especially the cristae. This is where most ATP is made in aerobic respiration.

1. Drop off electrons. NADH and FADH₂ give their electrons to proteins in the inner membrane.
2. Pump protons. As electrons move down the chain, energy is used to pump H⁺ ions out of the matrix into the intermembrane space (the thin gap between the two membranes). This builds a gradient, like water held behind a dam.
3. Use oxygen last. Oxygen is the final electron acceptor. It takes the electrons and H⁺ to form water. Without oxygen, the chain backs up and stops.
4. Make ATP. H⁺ ions flow back through ATP synthase, a protein enzyme that uses that flow to make ATP. This is called chemiosmosis, the use of a proton gradient to drive ATP production.

This stage makes about 26–28 ATP per glucose in eukaryotic cells. The exact number can vary because the electrons from glycolysis enter mitochondria by different shuttle systems.

When oxygen is missing: fermentation

If oxygen is not available, the ETC cannot keep running because electrons have nowhere to go. The cell still needs NAD⁺ for glycolysis, so it uses fermentation, an anaerobic process that regenerates NAD⁺ from NADH.

1. Rebuild NAD⁺. Fermentation turns NADH back into NAD⁺, so glycolysis can continue.
2. Make lactic acid or alcohol. In animal muscle cells and some bacteria, pyruvate becomes lactate in lactic acid fermentation. In yeast and some plants, pyruvate becomes ethanol and CO₂ in alcoholic fermentation.
3. Keep only the small payoff. Fermentation does not add extra ATP. The cell gets only the 2 ATP from glycolysis.

Fermentation is useful because it keeps glycolysis alive, but it is far less efficient than aerobic respiration.

Putting it together

The overall equation for aerobic cellular respiration is:

$\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O} + \text{energy (about 30–32 ATP)}$

Read it left to right: one glucose plus six oxygen molecules make six carbon dioxide molecules, six water molecules, and usable energy in ATP.

Example with the ATP count shown step by step:

• Glycolysis: net 2 ATP
• Pyruvate oxidation: 0 ATP directly
• Krebs cycle: 2 ATP
• Electron transport chain: about 26–28 ATP

So the total is:

$2 + 0 + 2 + 26 = 30\ \text{ATP}$ $2 + 0 + 2 + 28 = 32\ \text{ATP}$

That is why AP Biology usually gives the total yield as about 30–32 ATP per glucose in eukaryotic cells.

Check yourself

1. Where does glycolysis happen? Answer In the cytosol, outside the mitochondrion.
2. Why is oxygen needed for the biggest ATP payoff? Answer It is the final electron acceptor in the ETC, which lets electron flow continue and powers ATP synthase.
3. Why does fermentation help the cell survive without oxygen? Answer It regenerates NAD⁺, so glycolysis can keep making a small amount of ATP.

Common mistakes

• "Glycolysis happens in the mitochondrion." No — it happens in the cytosol.
• "The Krebs cycle makes most of the ATP." No — it makes only 2 ATP directly; the ETC makes most of the ATP.
• "Fermentation is just another way to make lots of ATP." No — fermentation makes no extra ATP. It only regenerates NAD⁺.

Remember this

• Glycolysis: cytosol, 2 ATP net, 2 NADH, 2 pyruvate.
• Krebs cycle: matrix, 2 ATP total, lots of NADH and FADH₂, CO₂ released.
• ETC: inner membrane, oxygen is the final electron acceptor, about 26–28 ATP.
• Fermentation: anaerobic backup that keeps glycolysis going by regenerating NAD⁺.