How do cells release energy from glucose in the four stages of aerobic respiration, and what happens without oxygen?
5.2.2 Respiration: the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation); the role of decarboxylation, dehydrogenation, reduced NAD and FAD, the electron transport chain, chemiosmosis and ATP synthase; the synthesis of ATP and the role of oxygen as the final electron acceptor; and anaerobic respiration in animals (lactate) and in yeast (ethanol).
A focused answer to the OCR H420 5.2.2 dot point on respiration. Covers the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), chemiosmosis and ATP synthase, the role of oxygen, and anaerobic respiration producing lactate or ethanol.
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What this dot point is asking
OCR wants you to describe the four stages of aerobic respiration, explain the roles of decarboxylation, dehydrogenation, reduced NAD and FAD, the electron transport chain, chemiosmosis and ATP synthase, explain the role of oxygen as the final electron acceptor, and describe anaerobic respiration in animals and yeast.
The answer
Overview
Respiration releases energy from glucose to make ATP, the universal energy currency. Aerobic respiration has four stages. ATP is made in two ways: substrate-level phosphorylation (directly, in glycolysis and the Krebs cycle) and oxidative phosphorylation (the bulk, using the electron transport chain).
Stage 1: glycolysis (in the cytoplasm)
Glucose (6C) is phosphorylated using 2 ATP, then split into two molecules of pyruvate (3C). This produces a net gain of 2 ATP (substrate-level) and 2 reduced NAD. Glycolysis does not need oxygen.
Stage 2: the link reaction (in the mitochondrial matrix)
Each pyruvate is decarboxylated (losing carbon dioxide) and dehydrogenated, forming a 2-carbon acetyl group that joins coenzyme A to make acetyl coenzyme A, and reduced NAD. This happens twice per glucose.
Stage 3: the Krebs cycle (in the matrix)
Acetyl CoA (2C) combines with a 4-carbon molecule to form a 6-carbon citrate. Through a series of decarboxylation and dehydrogenation steps the cycle regenerates the 4-carbon molecule, releasing carbon dioxide and producing, per turn, 3 reduced NAD, 1 reduced FAD and 1 ATP. The cycle turns twice per glucose.
Stage 4: oxidative phosphorylation (on the inner mitochondrial membrane)
This makes most of the ATP:
- Reduced NAD and FAD are oxidised, releasing electrons that pass along the electron transport chain.
- The energy released pumps protons from the matrix into the intermembrane space, creating a proton gradient.
- The protons flow back through ATP synthase, driving the synthesis of ATP (chemiosmosis).
- At the end of the chain, electrons and protons combine with oxygen, the final electron acceptor, to form water.
Without oxygen, the chain cannot pass on its electrons, so it backs up and stops, and reduced NAD cannot be reoxidised.
Anaerobic respiration
Without oxygen, only glycolysis runs, so NAD must be regenerated some other way:
- In animals (lactate fermentation): pyruvate is reduced to lactate by reduced NAD (regenerating NAD), allowing glycolysis to continue. Lactate accumulation causes fatigue and an oxygen debt; it is later oxidised back or converted to glucose in the liver.
- In yeast and plants (alcoholic fermentation): pyruvate is decarboxylated and reduced to ethanol and carbon dioxide, regenerating NAD.
Anaerobic respiration yields far less ATP (only the 2 net ATP from glycolysis) than aerobic respiration.
Examples in context
Example 1. Muscle fatigue in a sprint. During intense exercise, oxygen cannot reach the muscles fast enough, so they respire anaerobically to lactate, which builds up and causes fatigue; afterwards extra oxygen (the oxygen debt) is needed to oxidise it.
Example 2. Brewing and baking. Yeast respiring anaerobically produces ethanol (used in brewing) and carbon dioxide (which makes bread rise), a direct industrial use of alcoholic fermentation.
Try this
Q1. State the products of glycolysis from one glucose molecule. [2 marks]
- Cue. Two pyruvate, a net 2 ATP and 2 reduced NAD.
Q2. Explain the role of oxygen in oxidative phosphorylation. [2 marks]
- Cue. Oxygen is the final electron acceptor at the end of the electron transport chain, combining with electrons and protons to form water; this keeps the chain running so ATP can be made.
Q3. Name the molecule produced when pyruvate is reduced during anaerobic respiration in a muscle cell. [1 mark]
- Cue. Lactate.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20186 marksDescribe the process of oxidative phosphorylation and explain how it produces most of the ATP in aerobic respiration.Show worked answer →
Connect the electron transport chain, the proton gradient and ATP synthase.
Reduced NAD and reduced FAD (made in the earlier stages) are oxidised, releasing electrons that pass along the electron transport chain on the inner mitochondrial membrane. The energy released is used to pump protons (H+) from the matrix into the intermembrane space, building a proton gradient (proton motive force).
The protons flow back into the matrix through ATP synthase, and this flow drives the synthesis of ATP from ADP and phosphate (chemiosmosis).
At the end of the chain, the electrons and protons combine with oxygen (the final electron acceptor) to form water; without oxygen the chain backs up and stops. Markers reward the chain, proton pumping, the gradient, ATP synthase and chemiosmosis, and oxygen as the final electron acceptor forming water.
OCR H420/01 20214 marksExplain why anaerobic respiration in muscle cells allows glycolysis to continue, and state the disadvantage of producing lactate.Show worked answer →
Link lactate fermentation to regenerating NAD, then the cost.
In anaerobic conditions there is no oxygen to accept electrons, so the electron transport chain stops and reduced NAD cannot be reoxidised there. Glycolysis needs a supply of NAD.
So pyruvate is reduced to lactate by reduced NAD (catalysed by lactate dehydrogenase), which regenerates NAD. This lets glycolysis continue and keep producing a small amount of ATP (substrate-level phosphorylation) without oxygen.
The disadvantage is that lactate builds up, lowering pH and causing fatigue; it must later be oxidised back (the oxygen debt) or converted to glucose in the liver. Markers reward no oxygen so NAD is regenerated by reducing pyruvate to lactate, allowing glycolysis and ATP to continue, with lactate accumulation as the cost.
Related dot points
- 5.2.1 Photosynthesis: the structure of the chloroplast; the light-dependent stage (photolysis of water, photophosphorylation and the reduction of NADP); the light-independent stage (the Calvin cycle, fixing carbon dioxide using RuBP, forming GP and TP and regenerating RuBP); and the effect of limiting factors (light intensity, carbon dioxide concentration and temperature).
A focused answer to the OCR H420 5.2.1 dot point on photosynthesis. Covers chloroplast structure, the light-dependent stage (photolysis, photophosphorylation and reduced NADP), the light-independent stage (the Calvin cycle with RuBP, GP and TP), and the effect of limiting factors.
- 2.1.4 Enzymes: the role of enzymes as biological catalysts in metabolic reactions; the mechanism of enzyme action including the lock-and-key and induced-fit models; the effects of temperature, pH, enzyme and substrate concentration on the rate of reaction; the action of competitive and non-competitive inhibitors; the roles of cofactors, coenzymes and prosthetic groups.
A focused answer to the OCR H420 2.1.4 dot point on enzymes. Covers enzymes as catalysts, the lock-and-key and induced-fit models, activation energy, the effects of temperature, pH and concentration, competitive and non-competitive inhibition, and cofactors.
- 5.1.4 Hormonal communication: the principles of hormonal coordination and the contrast with nervous coordination; the structure and function of the adrenal glands and pancreas; the control of blood glucose concentration by insulin and glucagon (glycogenesis, glycogenolysis and gluconeogenesis); the second messenger model of adrenaline and glucagon; and the causes of type 1 and type 2 diabetes.
A focused answer to the OCR H420 5.1.4 dot point on hormonal communication. Covers hormonal versus nervous coordination, the adrenal glands and pancreas, the control of blood glucose by insulin and glucagon, the second messenger model, and the causes of type 1 and type 2 diabetes.
- 5.1.3 Neuronal communication: the structure of a neurone; the establishment of the resting potential by the sodium-potassium pump; the generation of an action potential by voltage-gated channels (depolarisation and repolarisation); the all-or-nothing principle and the refractory period; saltatory conduction in myelinated neurones; and synaptic transmission by acetylcholine at a cholinergic synapse.
A focused answer to the OCR H420 5.1.3 dot point on neuronal communication. Covers neurone structure, the resting potential, the action potential and its ionic basis, the all-or-nothing principle and refractory period, saltatory conduction, and synaptic transmission by acetylcholine.
- 2.1.2 Biological molecules: the structure and function of triglycerides and phospholipids; the structure of amino acids, the formation of peptide bonds and the four levels of protein structure; the structure of nucleotides, DNA and RNA; the biochemical tests for lipids (emulsion test) and proteins (biuret test).
A focused answer to the OCR H420 2.1.2 dot point on lipids, proteins and nucleic acids. Covers triglycerides and phospholipids, amino acids and the four levels of protein structure, nucleotide and DNA and RNA structure, and the emulsion and biuret tests.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)