AP Biology · Lesson 3

Cellular respiration and fermentation

Know Know it

Cellular respiration releases the energy in food in small steps and stores part of it in ATP. Overall, with oxygen: C6H12O6 + 6 O2 gives 6 CO2 + 6 H2O, with part of the energy captured as ATP. Breaking glucose down is strongly exergonic, and the cell couples it to the endergonic job of making ATP from ADP and phosphate.

Glycolysis (cytosol): one glucose is split into two pyruvate. It spends 2 ATP and makes 4, a net gain of 2 ATP, and it reduces 2 NAD+ to NADH. It needs no oxygen.

Pyruvate oxidation and the Krebs cycle (mitochondrial matrix): each pyruvate becomes acetyl CoA, and the cycle removes its carbons as CO2. The energy goes mostly into NADH and FADH2, with a little ATP.

Electron transport chain (inner mitochondrial membrane): NADH and FADH2 hand electrons to a chain of proteins. As electrons move down the chain, the proteins pump H+ from the matrix into the space between the two membranes. Oxygen takes the electrons at the end and, with H+, forms water.

Chemiosmosis: H+ flows back down its gradient through ATP synthase, which uses that flow to make most of the cell's ATP. In all, one glucose yields around 30 ATP when oxygen is present. If a drug makes the inner membrane leaky to H+, electrons still flow to oxygen, but the gradient leaks away, so ATP output falls and the energy is lost as heat.

Fermentation: without oxygen the chain stops and NADH piles up. Fermentation passes the electrons from NADH to pyruvate (making lactate) or to a product of pyruvate (making ethanol and CO2). That regenerates NAD+, so glycolysis can keep making 2 ATP per glucose.

Trap Breathed-in oxygen does not become CO2

The oxygen you breathe in takes electrons at the end of the chain and becomes water. The carbon dioxide you breathe out comes from the carbons of food, removed in pyruvate oxidation and the Krebs cycle.

Worked example Easy

Where in a eukaryotic cell does glycolysis take place?

  1. Mitochondrial matrix
  2. Inner mitochondrial membrane
  3. CytosolAnswer
  4. Thylakoid membrane

How to solve it

  1. Glycolysis is the first stage and needs no oxygen or organelle.
  2. Its enzymes are in the fluid part of the cell.
  3. So it happens in the cytosol.

Why each choice is right or wrong

  • A. Incorrect. The mitochondrial matrix holds pyruvate oxidation and the Krebs cycle, which come after glycolysis.
  • B. Incorrect. The inner mitochondrial membrane holds the electron transport chain and ATP synthase, not glycolysis.
  • C. Correct. The enzymes of glycolysis are dissolved in the cytosol, so glucose is split there before pyruvate enters the mitochondria.
  • D. Incorrect. The thylakoid membrane is inside chloroplasts and runs the light-dependent reactions of photosynthesis.

Explain Explain it

Task verbs: explain, predict

To explain a respiration result, follow the electrons: glucose gives electrons to NAD+ and FAD, NADH and FADH2 give them to the chain, the chain pumps H+, and oxygen takes them at the end. Then follow the H+: the gradient drives ATP synthase. A block at any link stops everything after it.

Worked example Medium

Which process makes most of the ATP from one glucose in aerobic respiration?

  1. Glycolysis in the cytosol
  2. Chemiosmosis through ATP synthaseAnswer
  3. The Krebs cycle in the mitochondrial matrix
  4. Fermentation in the cytosol

How to solve it

  1. Count ATP by stage: glycolysis nets 2, the Krebs cycle makes 2.
  2. That leaves most of the roughly 30 ATP for the last stage.
  3. The last stage makes ATP by chemiosmosis through ATP synthase.

Why each choice is right or wrong

  • A. Incorrect. Glycolysis nets only 2 ATP per glucose, a small share of the total of around 30.
  • B. Correct. The H+ gradient built by the electron transport chain drives ATP synthase, which makes most of the roughly 30 ATP from one glucose.
  • C. Incorrect. The Krebs cycle makes only 2 ATP per glucose directly; its main output is NADH and FADH2.
  • D. Incorrect. Fermentation makes no ATP itself; it only regenerates NAD+ so glycolysis can keep making its 2 ATP.

Predict Your turn

Yeast cells growing with plenty of oxygen are moved to a sealed flask with no oxygen. Predict how the amount of ATP the cells get from each glucose molecule changes. Then explain why the electron transport chain stops, and explain how fermentation lets glycolysis keep making ATP.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Predicts that the ATP made per glucose falls
  • Explains that the chain stops because no oxygen is there to accept electrons at its end
  • Explains that fermentation turns NADH back into NAD+, which glycolysis needs to keep running

Model answer

Each glucose now yields much less ATP: only the 2 ATP made by glycolysis, instead of around 30 with oxygen. The electron transport chain stops because oxygen is its final electron acceptor; with no oxygen at the end, electrons back up and cannot keep moving down the chain, so NADH cannot unload its electrons there. Glycolysis needs NAD+ to accept electrons from the sugar being broken down, so it would soon stop too. In fermentation, the yeast pass the electrons from NADH to a product of pyruvate, making ethanol and CO2, and NADH becomes NAD+ again. That supply of NAD+ lets glycolysis keep running and making 2 ATP per glucose.

Apply Apply it

Practice these topics. AP Practice questions on exactly what this lesson covers, with an explanation for every answer.

Practice these topics