What you need to know
- 7.2glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD. in aerobic and anaerobic respirationRespiration without oxygen: glycolysis followed by conversion of pyruvate to lactate (animals) or ethanol and CO₂ (yeast and plants), which regenerates NAD.: phosphorylation of hexoses, substrate-level ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi., reduced coenzymeA non-protein molecule that helps an enzyme work, often by carrying hydrogen or chemical groups, such as NAD., pyruvateThe three-carbon product of glycolysis. and lactateThe product of anaerobic respiration in animals, formed when pyruvate is reduced by reduced NAD.. Intermediate names are not required.
- 7.5What happens to lactate after anaerobic respiration in animals.
Do not name glycolytic intermediates. Yeast ethanol is mentioned only so you do not write it for human muscle. The Krebs cycle is 7A 3. A blood lactate test is not this pathway.
By the end, you should be able to…
- Describe phosphorylation of glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. and the net substrate-level ATP.
- Explain why lactate formation lets glycolysis continue.
- State the fate of lactate in the liver.
Build your understanding
Glycolysis in the cytoplasm
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. is the cytoplasmic pathway from glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. to pyruvateThe three-carbon product of glycolysis.. It happens whether or not oxygen is available. The hexose is first phosphorylated: phosphate from ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. is added. That makes the sugar more reactive and keeps it in the cell, because the phosphorylated molecule does not diffuse out easily. Two ATP are used in this investment.
The six-carbon sugar is then split into two three-carbon molecules, which are converted to pyruvate. You do not need the names of those intermediates. Along the way, dehydrogenationRemoval of hydrogen from a molecule, usually accepted by a coenzyme such as NAD. produces reduced NADNAD that has accepted hydrogen in glycolysis, the link reaction or the Krebs cycle. It passes electrons to the electron transport chain., and ATP is made directly by substrate-level phosphorylationMaking ATP by transferring a phosphate group directly from a substrate molecule to ADP, as in glycolysis and the Krebs cycle.. Four ATP are formed and two were spent, so the net yield is two ATP per glucose. That net two is the glycolysis yield, not the yield of the whole aerobic pathway.
Anaerobic means regenerating the coenzyme
Glycolysis needs a supply of NAD in the oxidised form. In aerobic conditions the reduced NAD is reoxidised by the mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place.. If oxygen is not available, that route stops. In animal cells, pyruvate is reduced to lactateThe product of anaerobic respiration in animals, formed when pyruvate is reduced by reduced NAD.. The hydrogen comes from reduced NAD, so NAD is regenerated and glycolysis can continue. The ATP yield stays at the net two. No carbon dioxide is made in this animal pathway.
Yeast uses a different anaerobic ending: pyruvate is converted to ethanol, and carbon dioxide is released. That matters for Core Practical 15. It is the wrong product if the question is about human muscle. Do not write “lactic acid and ethanol” as if they were the same thing.
Lactate does not stay in the muscle
After a period of anaerobic respirationRespiration without oxygen: glycolysis followed by conversion of pyruvate to lactate (animals) or ethanol and CO₂ (yeast and plants), which regenerates NAD., lactate leaves the muscle in the blood and is taken up by the liver. In the liver it is converted back to pyruvate. When oxygen is available, that pyruvate can enter 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 be respired aerobically. Some can also be converted to glucose.
The extra oxygen used afterwards is sometimes called an oxygen debtThe extra oxygen taken in after exercise, used partly to oxidise lactate back to pyruvate.. The specification point is the fate of the lactate, not a required definition of debt. Lactate is a product of anaerobic glycolysis. It is not, by itself, an explanation of next-day muscle soreness, and a clinical lactate measurement is not this liver pathway.
Follow the process
With or without oxygen, the cytoplasmic order is the same: glucose is phosphorylated using two ATP → the hexose splits into two three-carbon molecules → those are dehydrogenated, so NAD is reduced, and substrate-level phosphorylation makes ATP → two pyruvate remain. Four ATP are made and two were spent, so the net is two ATP per glucose. Glycolysis still ends at pyruvate when oxygen is absent. It does not make lactate instead of pyruvate.
Follow the process (continued)
Animal cells, no oxygen: pyruvate is reduced to lactate → the hydrogen comes from reduced NAD → NAD is free for another glycolysis. That step releases no carbon dioxide. Afterwards: lactate in the blood → liver → pyruvate, which can be respired when oxygen returns, or converted to glucose. Yeast: pyruvate loses carbon dioxide first, then the remaining molecule is reduced to ethanol, which also regenerates NAD. A yeast answer therefore includes carbon dioxide. An animal-muscle answer does not. enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. names are not required.
NOT: human muscle does not make ethanol, and glycolysis does not stop at the lack of oxygen.
- Net two ATP per glucose, in the cytoplasm
- Lactate regenerates NAD
- No carbon dioxide in the animal anaerobic step
- Liver converts lactate back to pyruvate
Two traps this diagram blocks
- Do not write ethanol and carbon dioxide as the products in human muscle.
- Do not claim anaerobic respiration in muscle makes a large ATP yield.
From the diagram alone
Question
Explain how lactate formation allows glycolysis to continue, and state what then happens to the lactate. [4]
Show a model answer
Pyruvate is reduced to lactate, and the hydrogen comes from reduced NAD, so NAD is available for glycolysis. Lactate is carried in the blood to the liver and converted back to pyruvate.
| Condition | Fate of pyruvate | What is regenerated or released |
|---|---|---|
| Aerobic, animal | Enters the mitochondrion | Reduced NAD used by the later stages |
| Anaerobic, animal | Reduced to lactate | NAD regenerated; no CO₂ in glycolysis |
| Anaerobic, yeast | Converted to ethanol | CO₂ released; not the human answer |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: 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. continues without oxygen. Y: pyruvateThe three-carbon product of glycolysis. accepts hydrogen from reduced NADNAD that has accepted hydrogen in glycolysis, the link reaction or the Krebs cycle. It passes electrons to the electron transport chain., so NAD is regenerated. Z: a net of two ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. can still be made in the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles.. Incomplete: “No oxygen, so respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. stops.” Improved: “Without oxygen the mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place. cannot reoxidise NAD, but reducing pyruvate to lactateThe product of anaerobic respiration in animals, formed when pyruvate is reduced by reduced NAD. regenerates NAD, so glycolysis and its net two ATP continue.”
Say net two, and where
Two ATP used, four made, net two per glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate., in the cytoplasm, by substrate-level phosphorylationMaking ATP by transferring a phosphate group directly from a substrate molecule to ADP, as in glycolysis and the Krebs cycle..
Lactate has a destination
Blood, liver, pyruvate. “It is broken down in the muscle” misses 7.5.
Avoid this misconception
The claim: anaerobic respirationRespiration without oxygen: glycolysis followed by conversion of pyruvate to lactate (animals) or ethanol and CO₂ (yeast and plants), which regenerates NAD. in muscle produces carbon dioxide and a large ATP yield.
The correction: Animal anaerobic glycolysis produces lactate and a net of two ATP per glucose. Carbon dioxide comes from 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 Krebs cycle, which need the aerobic route.
How can a cell keep making ATP without oxygen?
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: set Oxygen to Absent and press Glycolysis until it stops. Then see what Make lactate does to the NAD pool.
Respiration without oxygen: keeping NAD free
Run glycolysis without oxygen until the NAD runs out, then use lactate or ethanol to regenerate it. The endings add no ATP. Counts are per model cell, and the carrier pools are model numbers.
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 9
- In Stages, set Oxygen to Absent.
- Keep Cell on Animal muscle (options: Animal muscle, Yeast).
Without oxygen the electron transport chain stops, so the matrix carriers stay reduced and the link reaction and Krebs cycle stop too.
Stages
Questions
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Why is glucose phosphorylated at the start of glycolysis?
Show suggested answer
Phosphorylation uses ATP, makes the hexose more reactive, and helps keep the phosphorylated sugar inside the cell.
Practice question
Retrieval and explanation · suggested answer
State the net ATP yield of glycolysis and how that ATP is made.
Show suggested answer
Net two ATP per glucose, made by substrate-level phosphorylation. Four are produced and two were used.
Practice question
Retrieval and explanation · suggested answer
How does lactate formation allow glycolysis to continue?
Show suggested answer
Pyruvate is reduced to lactate and reduced NAD is reoxidised to NAD, which glycolysis needs.
Practice question
Retrieval and explanation · suggested answer
Describe the fate of lactate after exercise.
Show suggested answer
Lactate is carried in the blood to the liver and converted back to pyruvate, which can be respired aerobically or converted to glucose.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two athletes produce the same amount of lactate in a one-minute sprint. One then sits still; the other walks gently. Assume blood flow to the liver is higher in the walker, and oxygen is available. Who is likely to clear that lactate sooner, and which 7.5 step is doing the work?
Show a hint
Lactate has to reach the liver, then become pyruvate.
Explore the reasoning
The walker is likely to clear it sooner if more blood delivers lactate to the liver. The specification step is conversion of lactate back to pyruvate in the liver, not a special property of walking. The assumption about blood flow is given; you should not invent a fitness rule beyond it.
Recall this lesson
Lesson recall questions
Question 1 of 5: List four named products or events of glycolysis that IAL requires.
Glycolysis
Practice question
Retrieval and explanation · suggested answer
List four named products or events of glycolysis that IAL requires.
Lesson: 7A 2: Glycolysis and Anaerobic Respiration
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A lactate number is a blood test
7A · Glycolysis and Anaerobic RespirationMedlinePlus describes a lactate test as a clinical measurement. Lactate can rise during hard exercise, when oxygen demand is high, and also in illness. The page is about…Specification 7.2Read & use the scienceClose article
Why this matters
MedlinePlus describes a lactate test as a clinical measurement. Lactate can rise during hard exercise, when oxygen demand is high, and also in illness. The page is about detecting a high level. It is not a description of the liver converting lactate back to pyruvate, and it does not say that lactate causes muscle soreness. 7.2 and 7.5: lactate is the anaerobic product in animals, then a substrate the liver can turn back into pyruvate.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.2 Glycolysis in aerobic and anaerobic respiration.
Use the science
Why is “my lactate test was high, so my liver enzymes have the wrong names” a muddle of this lesson?
Explore a possible response
A high reading means a lot of lactate was in the sample at that time. It does not name glycolytic enzymes, which are not required anyway. The fate of lactate is still blood to liver to pyruvate.
Explore the source
- MedlinePlus: lactate testUS National Library of Medicine
Cancer cells rewrite the glycolysis story
7A · Glycolysis and Anaerobic RespirationMany proliferating cancer cells take up much glucose and make lactate even when oxygen is available. Researchers call this aerobic glycolysis or the Warburg effect. The NCI account…Specification 7.2Read & use the scienceClose article
Why this matters
Many proliferating cancer cells take up much glucose and make lactate even when oxygen is available. Researchers call this aerobic glycolysis or the Warburg effect. The NCI account makes clear that the reason remains an active research question; it should not be reduced to “cancer cells have broken mitochondria”. Your 7.2 route still applies: glycolysis makes pyruvate, reduced coenzyme and a little ATP. The extension is that cells can route carbon and regenerate oxidised coenzyme in different ways while building new material. A high lactate reading is therefore not, by itself, proof that oxygen was absent.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.2 Glycolysis in aerobic and anaerobic respiration.
Use the science
Why does lactate production by an oxygenated tumour cell not prove that all its mitochondria have stopped working?
Explore a possible response
Lactate can regenerate oxidised NAD after glycolysis even while oxygen is present. The cell may still use mitochondrial reactions; lactate alone does not measure those reactions.
Explore the source
- New clarity on the Warburg effectUS National Cancer Institute