How do cells release the chemical energy in glucose to make ATP?
Topic 3.6 Cellular Respiration: explain how glycolysis, the Krebs cycle and oxidative phosphorylation release energy from glucose to make ATP, and how fermentation allows ATP production without oxygen.
A focused answer to AP Biology Topic 3.6, covering glycolysis, the Krebs cycle, oxidative phosphorylation, chemiosmosis, the role of oxygen, and fermentation, with the link back to photosynthesis.
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What this topic is asking
The College Board (Topic 3.6) wants you to explain how cellular respiration releases the energy stored in glucose to make ATP, through glycolysis, the Krebs cycle and oxidative phosphorylation (electron transport chain and chemiosmosis), and to explain how fermentation lets cells make some ATP without oxygen. You should also connect respiration back to photosynthesis.
The three stages
The first stage is shared by almost all life and happens in the cytoplasm; the later stages occur in the mitochondrion, the link to the endosymbiotic theory of Unit 2.
Oxidative phosphorylation and chemiosmosis
This stage makes the bulk of the ATP.
This is the same chemiosmotic mechanism used in the light reactions of photosynthesis: an electron transport chain builds a proton gradient that ATP synthase uses to make ATP.
Fermentation
When oxygen is absent, the electron transport chain stops, so NADH cannot be reoxidised there. Cells use fermentation to keep glycolysis running.
Fermentation does not make ATP itself; its job is to regenerate NAD+ from NADH so that glycolysis has the oxidized carrier it needs to continue and keep producing its small net yield of ATP. In animals and many bacteria this produces lactic acid; in yeast it produces ethanol and carbon dioxide. The ATP yield is far lower than with full aerobic respiration.
Respiration and photosynthesis are complementary
Photosynthesis stores energy in sugar using carbon dioxide and water, releasing oxygen. Cellular respiration releases that energy from sugar, using oxygen and producing carbon dioxide and water. The products of one are the reactants of the other, and both use an electron transport chain and chemiosmosis to handle energy.
Try this
Q1. Identify where glycolysis occurs and whether it requires oxygen. [2 points]
- Cue. In the cytoplasm; it does not require oxygen.
Q2. Explain the role of oxygen in oxidative phosphorylation. [2 points]
- Cue. Oxygen is the final electron acceptor; it accepts electrons (and hydrogen ions) to form water, keeping the electron transport chain running so the proton gradient and chemiosmotic ATP synthesis continue.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt). A poison blocks the final electron acceptor of the electron transport chain in mitochondria. (a) Predict the effect on ATP production by oxidative phosphorylation. (b) Explain the effect, including what happens to the proton gradient.Show worked answer →
A 4-point predict-and-explain FRQ on chemiosmosis.
(a) Predict (1 point): ATP production by oxidative phosphorylation falls sharply (close to zero).
(b) Explain (3 points): (1 point) oxygen is the final electron acceptor; if it (or its acceptor role) is blocked, electrons cannot leave the chain, so the chain stops; (1 point) with no electron flow, hydrogen ions are no longer pumped across the inner membrane and the proton gradient collapses; (1 point) without the gradient, ATP synthase cannot make ATP by chemiosmosis.
Markers reward linking the blocked acceptor to a stalled chain, a collapsed gradient, and no chemiosmotic ATP synthesis.
AP 2021 (style)1 marksSection I (multiple choice). Why does fermentation allow glycolysis to continue without oxygen? (A) It produces large amounts of ATP. (B) It regenerates NAD+ so glycolysis can keep oxidizing glucose. (C) It makes oxygen. (D) It runs the Krebs cycle in reverse.Show worked answer →
The correct answer is (B).
Fermentation regenerates NAD+ from NADH, so glycolysis has a supply of the oxidized carrier it needs to keep going and make a small amount of ATP without oxygen. (A) is wrong because fermentation yields little ATP; (C) and (D) are incorrect.
Related dot points
- Topic 3.5 Photosynthesis: explain how the light-dependent reactions and the Calvin cycle capture light energy and use it to fix carbon dioxide into sugar.
A focused answer to AP Biology Topic 3.5, covering the light-dependent reactions, the electron transport chain, chemiosmosis, the Calvin cycle, and how light energy is converted to the chemical energy of sugars.
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- Topic 2.11 Origins of Cell Compartmentalization: describe the similarities and differences in compartmentalization between prokaryotic and eukaryotic cells, and the evidence for the endosymbiotic origin of mitochondria and chloroplasts.
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Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)