AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/3/3-5)
Unit 3 · Topic 3.5
3.5 Cellular Respiration
Cellular respiration releases the energy stored in food molecules and uses it to make ATP. Glycolysis splits glucose into pyruvate in the cytosol, the Krebs cycle in the mitochondrial matrix releases CO₂ and loads electrons onto NADH and FADH₂, and the electron transport chain uses those electrons to build a proton gradient that powers ATP synthase. When oxygen is missing, fermentation keeps glycolysis running.
Key terms
- glycolysis
- Krebs cycle
- electron transport chain
- chemiosmosis
- oxidative phosphorylation
- fermentation
The big picture
Overall, aerobic respiration (respiration that uses oxygen): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (captured as ATP, with the rest lost as heat).
Cellular respiration and fermentation happen in all forms of life, not just animals. Plants, fungi, bacteria and archaea all break down organic molecules to make ATP. In eukaryotes, aerobic respiration is a chain of linked stages, each step run by its own enzyme, so energy is released in small, usable steps (3.3).
Stage by stage
Glycolysis doesn't need oxygen. It releases some of glucose's energy and produces pyruvate, a little ATP and NADH. Pyruvate then moves from the cytosol into the mitochondrion, where it's oxidized. During the Krebs cycle, the carbon atoms from glucose are released as CO₂, a little ATP is made, and electrons are transferred to the carriers NAD⁺ and FAD, forming NADH and FADH₂.
In total, a cell makes roughly 30–32 ATP per glucose. Know the pattern, not the exact numbers: glycolysis and the Krebs cycle make a little ATP, and the electron transport chain powers most of it. You don't need to memorize the steps, intermediates or enzyme names in glycolysis or the Krebs cycle.
| Stage | Where (eukaryotes) | Goes in | Comes out (per glucose) |
|---|---|---|---|
| Glycolysis | Cytosol | Glucose, ADP, NAD⁺ | 2 pyruvate, a net 2 ATP, 2 NADH |
| Pyruvate oxidation | Mitochondrion (matrix) | 2 pyruvate, NAD⁺ | 2 CO₂, 2 NADH, and the carbon fragments that enter the Krebs cycle |
| Krebs (citric acid) cycle | Mitochondrial matrix | Carbon fragments from pyruvate, ADP, NAD⁺, FAD | 4 CO₂, 2 ATP, 6 NADH, 2 FADH₂ |
| Electron transport + chemiosmosis | Inner mitochondrial membrane | NADH, FADH₂, O₂, ADP | Most of the ATP (roughly 26–28), H₂O, NAD⁺ and FAD |
Electron transport and chemiosmosis
NADH and FADH₂ carry their electrons to the electron transport chain (ETC) in the inner mitochondrial membrane. The electrons are passed from one carrier to the next in a series of redox (oxidation-reduction) reactions, where each carrier gains electrons and then passes them on, falling to lower energy each time, until they reach the final (terminal) electron acceptor: oxygen. Oxygen combines with the electrons and H⁺ to form water.
The energy released as electrons move down the chain is used to pump H⁺ from the matrix into the intermembrane space (the space between the two mitochondrial membranes). So H⁺ is concentrated in the intermembrane space, and the pH inside the matrix is higher (less acidic).
H⁺ flows back into the matrix through ATP synthase, which uses that flow to make ATP from ADP and inorganic phosphate (Pi). This is chemiosmosis, and making ATP this way in respiration is called oxidative phosphorylation. The inner membrane's folds (cristae) increase its surface area, giving room for more ETCs and ATP synthases, so more ATP can be made (2.1).
Prokaryotes don't have mitochondria, so their ETCs sit in the plasma membrane, and they pump H⁺ across it. In prokaryotes that live with oxygen, O₂ is the last stop for the electrons, just as in mitochondria. Prokaryotes that live without oxygen hand their electrons to something else instead, such as sulfate or nitrate.
Heat from uncoupling
Normally, electron transport and ATP synthesis are linked (coupled) by the proton gradient. Some cells have proteins that let H⁺ leak back into the matrix without passing through ATP synthase. This uncoupling means the energy in the gradient is released as heat instead of being used to make ATP. Endotherms use this to keep warm. Brown fat in newborn babies and hibernating mammals is full of mitochondria that do exactly this.
Fermentation: when there's no oxygen
Without oxygen, the ETC stops, because there's no final electron acceptor. NADH can't drop off its electrons, so NAD⁺ runs out, and glycolysis needs NAD⁺ to keep going.
Fermentation solves this. It transfers NADH's electrons to an organic molecule made from pyruvate, which regenerates NAD⁺ so glycolysis can continue. In lactic acid fermentation (in your muscle cells during intense exercise, and in bacteria used to make yogurt), pyruvate becomes lactic acid. In alcohol fermentation (in yeast, used to make bread and beer), pyruvate becomes ethanol and CO₂. Fermentation makes only the 2 ATP per glucose from glycolysis, far less than aerobic respiration.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Respirometer data
In a respirometer, potassium hydroxide (KOH) absorbs CO₂, so the volume change shows O₂ used. Readings over 20 minutes at 5-minute intervals: germinating peas: 0, 0.12, 0.25, 0.36, 0.49 mL. Dry peas: 0, 0.02, 0.03, 0.05, 0.06 mL. Glass beads (control): 0, 0.01, 0.01, 0.02, 0.02 mL. Calculate the corrected rate of O₂ consumption for each type of pea, and explain the difference.
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- Step 1: The glass beads don't respire, so their change shows effects of temperature or pressure. Subtract the bead reading from each pea reading.
- Step 2: Germinating peas at 20 min: 0.49 − 0.02 = 0.47 mL. Rate = 0.47 mL ÷ 20 min ≈ 0.024 mL O₂/min.
- Step 3: Dry peas at 20 min: 0.06 − 0.02 = 0.04 mL. Rate = 0.04 mL ÷ 20 min = 0.002 mL O₂/min.
- Step 4: Explain: germinating seeds are breaking down stored food to fuel growth, so they respire much faster and use more O₂. Dry (dormant) seeds are alive but have a very low metabolic rate.
Answer: Germinating peas ≈ 0.024 mL O₂/min (0.0235); dry peas = 0.002 mL O₂/min. Germinating peas respire about 12 times faster because they're actively growing.
- Example 2
Predicting the effect of cyanide
Cyanide blocks the last protein of the electron transport chain, so electrons can't be passed to oxygen. Predict the effects on O₂ use, the proton gradient, ATP production, and the cell's NADH and NAD⁺ levels. Explain why the cell may start making lactic acid.
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- Step 1: Electrons can't reach O₂, so O₂ consumption stops.
- Step 2: Electrons back up in the chain, so pumping of H⁺ stops and the proton gradient runs down.
- Step 3: Without the gradient, ATP synthase can't make ATP. Oxidative phosphorylation stops, and ATP falls sharply.
- Step 4: NADH can't drop off its electrons, so NADH builds up and NAD⁺ runs low. The Krebs cycle, which needs NAD⁺, slows to a stop.
- Step 5: Cells that can ferment turn to lactic acid fermentation to regenerate NAD⁺ so glycolysis can make its small amount of ATP, which is why lactic acid rises.
Answer: O₂ use stops, the proton gradient collapses, ATP production drops sharply, NADH builds up and NAD⁺ runs out. Cells rely on glycolysis plus fermentation, producing lactic acid.
- Example 3
Where do the atoms go? (classic trap)
A mouse breathes O₂ labeled with the isotope ¹⁸O. A student predicts that the label will show up in the CO₂ the mouse breathes out. Where does it actually appear, and why?
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- Step 1: The trap: since O₂ goes in and CO₂ comes out, it's tempting to assume the O₂ becomes CO₂.
- Step 2: Recall oxygen's job: O₂ is the final electron acceptor at the end of the ETC. It combines with electrons and H⁺ to form water.
- Step 3: Recall where CO₂ comes from: it's released from the carbon compounds made from glucose during pyruvate oxidation and the Krebs cycle.
- Step 4: So the labeled oxygen ends up in the water made at the end of the ETC, not in the CO₂ released by the Krebs cycle.
Answer: The ¹⁸O first shows up in water made by the mouse's cells, because O₂ is the final electron acceptor in the ETC. The CO₂ comes from the food molecules.
Common mistakes
- Saying glycolysis needs oxygen or happens in the mitochondrion. It's in the cytosol and doesn't use O₂.
- Saying the ETC makes ATP directly. The ETC builds a proton gradient; ATP synthase uses the H⁺ flow to make ATP.
- Putting the high H⁺ concentration in the matrix. Protons are pumped into the intermembrane space; the matrix has the higher pH.
- Saying fermentation's purpose is to make ATP. Its job is to regenerate NAD⁺ so glycolysis can keep making its 2 ATP.
On the exam
- Expect 'predict and justify' questions: an ETC poison, an uncoupler, a missing O₂ supply, or a leaky inner membrane. Trace effects through electron flow, the proton gradient, ATP synthase and NAD⁺.
- Respiration lab questions often ask you to calculate rates from respirometer or CO₂ data and explain the role of controls.
Connected topics
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Check yourself
4 questions on 3.5 Cellular Respiration. Pick an answer to see if you got it, and why.
Cyanide prevents electrons from being passed to oxygen at the end of the electron transport chain in mitochondria. Which of the following is the most likely immediate effect?
Brown fat cells in hibernating mammals contain a protein that lets H⁺ ions leak back across the inner mitochondrial membrane without passing through ATP synthase. Which of the following is the most likely result?
Fermentation produces no additional ATP beyond the ATP made in glycolysis. Which of the following best explains why fermentation is still important to cells without oxygen?
In an actively respiring mitochondrion, where is the H⁺ concentration highest?
0 of 4 answered