IB Biology HLCellular RespirationPaper 1 & 2~13 min read
Anaerobic Respiration
Without oxygen the cell has a supply problem, but not the one most people assume. The shortage that stops everything is not glucose — it is NAD. Anaerobic respiration is a trick for getting NAD back.
📚 What you need to know
With little or no oxygen, glucose cannot be completely oxidised, so most ATP-producing reactions stop.
Cells can still oxidise the reduced NAD made in glycolysis, which frees it to carry hydrogen again.
That means glycolysis can continue, giving a net yield of about two ATP per glucose.
Animals and some microorganisms convert pyruvate to lactate; yeast and plants convert it to ethanol.
In the lactate pathway, reduced NAD gives its hydrogens to pyruvate, which is therefore the hydrogen acceptor. The enzyme is lactate dehydrogenase.
Lactate can be oxidised back to pyruvate and fed into the Krebs cycle, or converted to glycogen in the liver. This needs extra oxygen — the oxygen debt.
In the ethanol pathway, pyruvate is first decarboxylated to ethanal, releasing CO2. Ethanal is the hydrogen acceptor and is reduced to ethanol by alcohol dehydrogenase.
Ethanol cannot be metabolised further — it is a waste product. Lactate can.
The real problem with no oxygen
A cell contains only a small amount of NAD. In glycolysis, NAD+ collects hydrogens and becomes reduced NAD. Normally it hands them straight to the electron transport chain and comes back as NAD+, ready to go again.
Take oxygen away and the chain stops, so nothing collects those hydrogens. Reduced NAD builds up, no NAD+ is left, and the oxidation step of glycolysis grinds to a halt. Even the two ATP would stop coming.
So the cell does something slightly desperate: it dumps the hydrogens onto pyruvate (or onto a molecule made from it). That regenerates NAD+, which lets glycolysis keep running. The pyruvate is sacrificed to keep the NAD cycling.
If you remember one sentence from this page, make it this one: the purpose of fermentation is not to make lactate or ethanol. It is to reoxidise NAD so glycolysis can continue. The lactate and ethanol are just where the hydrogens end up.
Notice where the hydrogens go in each branch: onto pyruvate itself in animals, but onto ethanal in yeast — one step further along.
The lactate pathway
Reduced NAD transfers its hydrogens to pyruvate, so pyruvate is the hydrogen acceptor.
Pyruvate is reduced to lactate by the enzyme lactate dehydrogenase.
NAD is reoxidised in the absence of oxygen, so glycolysis and pyruvate formation can continue.
Lactate is not a dead end — it can be further metabolised.
What happens to the lactate
Once oxygen becomes available again, the body has two options:
Oxidise it back to pyruvate, which then enters the Krebs cycle for full ATP production.
Convert it to glycogen for storage in the liver.
The first route needs extra oxygen, and that requirement is the oxygen debt. It explains something you have felt yourself: why you keep breathing hard for several minutes after you stop sprinting. The exercise is over, but the lactate still has to be dealt with.
🧠
Oxygen debt
You borrowed ATP quickly during the sprint without paying the oxygen cost. Afterwards, the bill arrives — and you pay it in deep breaths.
The ethanol pathway
Yeast and plants take an extra step before unloading the hydrogens.
Pyruvate is decarboxylated — a carbon is removed as CO2 — producing ethanal.
Reduced NAD transfers its hydrogens to ethanal, so ethanal is the hydrogen acceptor.
Ethanal is reduced to ethanol by alcohol dehydrogenase, and NAD is reoxidised.
Ethanol cannot be metabolised further. It is a waste product, and in high concentrations it is toxic to the yeast that made it.
Ethanal or ethanol? One letter apart, one step apart. Ethanal is the intermediate that accepts the hydrogens; ethanol is the final product. Getting them the wrong way round in a pathway answer changes the meaning completely, so slow down when you write them.
Bread making, step by step
🧩 Why dough rises
Flour contains starch. Mixed with water and yeast it forms a dough, which is kneaded to combine everything.
The dough is left in a warm place to encourage the yeast to respire.
Yeast hydrolyses the starch into maltose and glucose and respires those sugars aerobically at first, multiplying rapidly while oxygen lasts.
The oxygen in the dough is used up, so the dough becomes anaerobic and alcoholic fermentation takes over.
CO2 bubbles form and are trapped, so the dough rises.
Baking kills the yeast and evaporates the ethanol. The gas pockets left behind give the bread its texture.
Comparing the two pathways
Feature
Lactate fermentation
Alcoholic fermentation
Where
Animals and some microorganisms
Yeast, other fungi and plants
Hydrogen acceptor
Pyruvate
Ethanal
Enzyme named
Lactate dehydrogenase
Alcohol dehydrogenase
Carbon dioxide released?
No
Yes
Final product
Lactate
Ethanol
Can the product be reused?
Yes — oxidised to pyruvate or stored as glycogen
No — it is waste
ATP yield per glucose
2 (from glycolysis)
2 (from glycolysis)
Worked examples
WE 1
Explain why fermentation is necessary
Explain why converting pyruvate to lactate allows a muscle cell to keep producing ATP without oxygen. (4 marks)
Point 1: the blockage
Without oxygen the electron transport chain stops, so reduced NAD cannot be reoxidised there.
Point 2: why that matters
A cell has only a limited supply of NAD, and glycolysis needs NAD+ for its oxidation step.
Point 3: what the pathway does
Reduced NAD transfers its hydrogens to pyruvate, reducing it to lactate and regenerating NAD+.
Point 4: the outcome
Glycolysis can therefore continue, giving a net two ATP per glucose.
Recycling NAD is the point; the lactate is a side effectanswers that only say “lactate is made” without mentioning NAD rarely get past two marks
WE 2
Compare the two fermentation pathways
Give two differences between anaerobic respiration in yeast and in human muscle cells. (2 marks)
Difference 1: the products
Yeast produces ethanol and carbon dioxide; muscle produces lactate only, with no CO2.
Difference 2: what happens next
Lactate can be oxidised back to pyruvate or stored as glycogen, whereas ethanol is a waste product that cannot be metabolised further.
Different products, and only one of them is reusablethe hydrogen acceptor differs too — pyruvate in muscle, ethanal in yeast
WE 3
Apply it to exercise
A sprinter continues to breathe deeply for several minutes after finishing a race. Explain why. (3 marks)
Point 1: during the race
Oxygen supply could not keep up with demand, so muscles respired anaerobically and lactate accumulated.
Point 2: after the race
Oxidising that lactate back to pyruvate, so it can enter the Krebs cycle, requires extra oxygen.
Point 3: the term
This extra requirement is the oxygen debt, so deep, fast breathing continues until it is repaid.
Lactate built up, and clearing it costs oxygenuse the phrase “oxygen debt” explicitly — it is usually a marking point
💡 Exam tips
Always explain fermentation in terms of regenerating NAD, not just making a product.
Name the hydrogen acceptor for each pathway: pyruvate in animals, ethanal in yeast.
Remember only the yeast pathway releases carbon dioxide.
Both pathways yield 2 ATP, and both come from glycolysis, not from the fermentation step itself.
Learn the two fates of lactate: oxidised to pyruvate, or stored as glycogen in the liver.
Spell ethanal and ethanol carefully — they are different molecules.
⚠ Common mistakes
Saying the fermentation step makes ATP. It makes none — the 2 ATP come from glycolysis.
Saying lactate causes muscle fatigue and stopping there. The exam point is the oxygen debt and how lactate is cleared.
Giving CO2 as a product of lactate fermentation. There is none.
Saying ethanol is converted back to pyruvate. Ethanol cannot be metabolised further.
Saying anaerobic respiration happens in the mitochondria. It is entirely cytoplasmic.
Writing that yeast only respires anaerobically. Yeast respires aerobically when oxygen is available.
Up next: The Link Reaction & Krebs Cycle. Now assume oxygen is available. Pyruvate goes into the mitochondrion instead, and the cell starts extracting the energy that fermentation throws away.
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