What you need to know
- 7.1(i)aerobic respirationRespiration using oxygen: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. It releases much more ATP than anaerobic respiration. splits the respiratory substrateAn organic molecule that is broken down in respiration to release energy, such as glucose, fatty acids or amino acids., releases carbon dioxide, and reunites hydrogen with oxygen, transferring a large amount of energy.
- 7.1(ii)respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. is a many-step pathway. Each step is catalysed by a specific intracellular enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction.. Enzyme names are not required.
Do not memorise a single ATP total per glucose. Glycolysis detail is 7A 2, the Krebs cycle is 7A 3, and chemiosmosis is 7A 4. Plant respiration in the NPP equation is 5C, not this pathway.
By the end, you should be able to…
- Write the overall aerobic equation for glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate..
- Explain why the pathway is stepped rather than one reaction.
- State that enzyme names are not required.
Build your understanding
One equation hides a pathway
A respiratory substrateAn organic molecule that is broken down in respiration to release energy, such as glucose, fatty acids or amino acids. is an organic molecule that can be oxidised to transfer energy to ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi.. glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. is the usual example. The overall aerobic reaction is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Carbon dioxide is a waste product. The hydrogen from the substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. is eventually reunited with atmospheric oxygen, and water is formed.
That equation does not happen as a single collision. If it did, the energy would be transferred too quickly to be useful and too much would leave as heat in one step. respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. is a sequence of small oxidations. Each step is controlled and catalysed by its own intracellular enzyme. You need that idea. You do not need the names of those enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction..
Where the hydrogen and the carbon go
dehydrogenationRemoval of hydrogen from a molecule, usually accepted by a coenzyme such as NAD. removes hydrogen from the substrate and passes it to a coenzyme, so the coenzyme is reduced. Those reduced coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD. carry the hydrogen to the inner mitochondrial membrane. Oxygen is the final acceptor. That is the “reuniting hydrogen with atmospheric oxygen” in 7.1. The carbon leaves as carbon dioxide in the link reactionThe stage in the mitochondrial matrix in which pyruvate is decarboxylated and dehydrogenated and joined to coenzyme A to form acetyl coenzyme A. and the Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP., not in glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD..
Most of the ATP is made later, by oxidative phosphorylationMaking ATP using energy from electrons passed along the electron transport chain, with oxygen as the final electron acceptor.. A smaller amount is made earlier by substrate-level phosphorylationMaking ATP by transferring a phosphate group directly from a substrate molecule to ADP, as in glycolysis and the Krebs cycle.. Saying “respiration makes 38 ATP” is a textbook model that changes with the shuttle used, and it is not a specification number. Say where the ATP is made instead.
What this page is not
Breathing moves oxygen into the blood and carbon dioxide out. Cellular respiration is the pathway inside cells. A person can breathe and still have a blocked electron transport chainA series of carriers in the inner mitochondrial membrane. Energy from electrons passing along it pumps H⁺ ions into the intermembrane space.. Do not use the two words as synonyms.
anaerobic respirationRespiration without oxygen: glycolysis followed by conversion of pyruvate to lactate (animals) or ethanol and CO₂ (yeast and plants), which regenerates NAD. regenerates oxidised coenzyme without using oxygen, and it yields much less ATP. It is the next lesson. photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. stores energy from light; it is not the reverse spelling of this equation, even though the overall gases look opposite.
NOT: the overall equation is not one reaction, and the lung does not run these stages.
- Glycolysis is cytoplasmic
- Link and Krebs are in the matrix
- Electron transport is on the inner membrane
- Most ATP is not made in glycolysis
Two traps this diagram blocks
- Do not put the Krebs cycle in the cytoplasm.
- Do not treat breathing as the same process as these mitochondrial stages.
From the diagram alone
Question
State the location of glycolysis and of the Krebs cycle. [2]
Show a model answer
Glycolysis occurs in the cytoplasm. The Krebs cycle occurs in the matrix of the mitochondrion.
Make your reasoning visible
Quote the fate of both elements
Carbon leaves as CO₂. Hydrogen is carried by coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD. and ends in water when it meets oxygen.
Refuse the enzyme list
If a question says “name the enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction.”, the specification still does not require those names. Describe the type of reaction instead.
Key explanations
- glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD., from glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. to pyruvate, happens in the cytoplasm.
Avoid this misconception
The claim: aerobic respirationRespiration using oxygen: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. It releases much more ATP than anaerobic respiration. is one reaction that releases all the energy as ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. in the cytoplasm.
The correction: It is a many-step, enzyme-controlled pathway. Most ATP is formed later by oxidative phosphorylationMaking ATP using energy from electrons passed along the electron transport chain, with oxygen as the final electron acceptor. in the mitochondrion, not as one cytoplasmic event.
Where do the carbon and hydrogen from glucose go?
Respiration takes glucose apart in stages. Carbon atoms leave as carbon dioxide, and hydrogen is carried by NAD and FAD to oxygen, which releases the energy used to make ATP.
What the model shows
One glucose at a time going through glycolysis, the link reaction, the Krebs cycle and the electron transport chain, or through an anaerobic ending in a muscle cell or yeast.
What to look for
Where each carbon leaves as CO₂. How the NAD pool fills up with hydrogen. What happens to glycolysis when there is no oxygen and no ending.
What it tells us
Oxygen is needed to reoxidise the carriers. Without it, cells can only keep glycolysis going by regenerating NAD with lactate or ethanol, which adds no ATP.
Be careful what you claim
The carrier pools are small model numbers, and the model does not give an ATP total for oxidative phosphorylation.
Try it in the model: press Glycolysis once and count the beads: one 6-carbon glucose has become two 3-carbon pyruvate, and two NAD have turned gold.
Respiration: where the carbon and hydrogen go
Follow one glucose through glycolysis, the link reaction, the Krebs cycle and the electron transport chain. Every carbon is tracked to CO₂, and every reduced carrier to oxygen. No total ATP yield is given.
What you see
- The cytoplasm on the left and a mitochondrion on the right, with its matrix and the inner membrane where the electron transport chain sits.
- Each bead is one carbon atom: glucose has 6, pyruvate 3, an acetyl group 2. Pills are carriers: white when oxidised, gold with two H when reduced.
- The carbon ledger at the bottom shows where every carbon from the glucose used is now. The hydrogen row shows reduced carriers and hydrogen passed to oxygen.
How to use it
- Press the stage buttons one at a time, or Next step (or Play) to run the next sensible stage.
- Switch oxygen off and choose an animal muscle cell or yeast to try the anaerobic endings.
What it shows
- Glycolysis runs with or without oxygen and nets 2 ATP; carbon leaves as CO₂ only in the link reaction and Krebs cycle (7.2, 7.3).
- Reduced NAD and FAD are reoxidised by the electron transport chain, with oxygen as the final acceptor (7.4).
- Without oxygen, lactate or ethanol regenerates NAD so glycolysis can continue; lactate is taken to the liver (7.2, 7.5).
Limits of the model
- Carrier pools are small model numbers so you can see them run out; real cells have many more.
- Cytoplasmic reduced NAD is shown going straight to the chain; how its hydrogen enters the mitochondrion is not required.
- Most ATP comes from oxidative phosphorylation, and its yield per glucose is not a specification number, so none is given.
Guided tour · step 1 of 12
- In Stages, just below these steps, keep Oxygen on Available (options: Available, Absent).
- Press Glycolysis once.
- Count the carbon beads and the NAD in the simulator.
Glycolysis splits one 6-carbon glucose into two 3-carbon pyruvate in the cytoplasm. It reduces 2 NAD and nets 2 ATP, with or without oxygen.
Stages
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Write the overall equation for aerobic respiration of glucose.
Show suggested answer
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Energy is transferred, but a single ATP number is not part of the equation.
Practice question
Retrieval and explanation · suggested answer
Why is it useful that respiration happens in many steps?
Show suggested answer
Each small step can be catalysed and controlled, and energy can be transferred to ATP in manageable amounts rather than lost in one uncontrolled release.
Practice question
Retrieval and explanation · suggested answer
A student lists ten named enzymes. What should you tell them about 7.1?
Show suggested answer
Specific enzymes do catalyse each step, but their names are not required. Describing the step is enough.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A cell has plenty of glucose and a normal cytoplasm, but no oxygen reaches the mitochondrion. Use only 7.1 to explain why the overall aerobic equation stops, even though the enzymes are still present.
Show a hint
Look at the oxygen on the left of the equation and the hydrogen story.
Explore the reasoning
The overall aerobic reaction reunites hydrogen with oxygen. Without oxygen that reunion cannot happen, so the aerobic pathway cannot be completed. The enzymes may still be present; the missing reactant is oxygen. What the cell can still do anaerobically is the next lesson, not this equation.
Recall this lesson
Lesson recall questions
Question 1 of 5: State what happens to the respiratory substrate in aerobic respiration, and name the waste gas released.
Aerobic respiration overview
Practice question
Retrieval and explanation · suggested answer
State what happens to the respiratory substrate in aerobic respiration, and name the waste gas released.
Lesson: 7A 1: Respiration in Cells
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The large yield needed a mechanism, not a bigger explosion
7A · Respiration in CellsThe 1978 Nobel Prize in Chemistry recognised Peter Mitchell for the chemiosmotic theory of biological energy transfer. That theory is the membrane-gradient explanation you meet in 7A 4.…Specification 7.1(i)Read & use the scienceClose article
Why this matters
The 1978 Nobel Prize in Chemistry recognised Peter Mitchell for the chemiosmotic theory of biological energy transfer. That theory is the membrane-gradient explanation you meet in 7A 4. It is enrichment here: 7.1 only asks you to know that a large amount of energy is transferred, and that the chemistry is stepped. 7.1: carbon dioxide is released and hydrogen meets oxygen, with a large energy transfer spread across many enzyme-controlled steps.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.1(i) Overall aerobic respiration: substrate, carbon dioxide, oxygen and energy transfer.
Use the science
Why is “Mitchell discovered a new enzyme that burns glucose” a poor summary of why this prize is linked to 7.1?
Explore a possible response
The prize was for a theory of how energy is transferred across membranes, not for naming a glucose-burning enzyme. Enzyme names are not the 7.1 requirement anyway.
Explore the source
- Nobel Prize in Chemistry 1978: chemiosmotic theoryNobel Prize Outreach
When mitochondria cannot meet demand
7A · Respiration in CellsThe US neurological institute describes mitochondrial disorders that can affect muscles, nerves, the heart and several other organs. Those tissues have high energy demands, so disruption of mitochondrial…Specification 7.1(i)Read & use the scienceClose article
Why this matters
The US neurological institute describes mitochondrial disorders that can affect muscles, nerves, the heart and several other organs. Those tissues have high energy demands, so disruption of mitochondrial function may have effects in more than one body system. The connection to 7.1 is the scale of energy transfer made available by aerobic respiration, not a claim that every disorder blocks the same step. A diagnosis needs evidence about the particular defect; the word “mitochondrial” alone does not identify an enzyme or predict one symptom. This is why a biological mechanism and a clinical label are different levels of explanation.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.1(i) Overall aerobic respiration: substrate, carbon dioxide, oxygen and energy transfer.
Use the science
Why might a mitochondrial defect affect both a skeletal muscle and a neurone, yet cause different symptoms?
Explore a possible response
Both tissues depend on ATP from respiration, but they use it for different tasks: contraction in muscle and ion gradients or signalling in neurones. The affected pathway and distribution of defective mitochondria also matter.
Explore the source
- Mitochondrial disordersUS National Institute of Neurological Disorders and Stroke