IB Biology · Theme C Interaction and interdependence · Molecules
C1.2 Cell respiration
Cell respiration releases energy from carbon compounds and uses it to make ATP. ATP is the cell's short-term energy currency, recycled continuously between ATP and ADP. Aerobic respiration needs oxygen and mitochondria; anaerobic runs in the cytoplasm with a small yield.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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C1.2.1 ATP carries energy from where it is released to where it is used
ATP (adenosine triphosphate) is a nucleotide: adenine, ribose and three phosphates.
Energy released from carbon compounds in respiration is used to make ATP.
ATP then supplies that energy to the processes that need it.
It is soluble, stable until an enzyme acts, and cannot leave the cell.
Students often think ATP is a long-term energy store. In fact cells hold little ATP and recycle it constantly; glycogen and fat are the stores.
Students often think ATP is a special protein. In fact it is a nucleotide, like the monomers of DNA and RNA.
C1.2.2 What cells spend ATP on
Active transport moves substances against a gradient through pumps, using ATP.
Anabolism builds macromolecules from monomers, and needs ATP.
Motor proteins hydrolyse ATP to move whole cells, chromosomes and vesicles.
Diffusion, facilitated diffusion and osmosis are passive and use no ATP.
Students often think all transport across membranes uses ATP. In fact only active transport does; osmosis and diffusion are passive.
Students often think chromosomes are pulled apart passively. In fact motor proteins hydrolyse ATP to move them.
C1.2.3 Hydrolysing ATP releases energy; making it takes energy in
Hydrolysis of ATP gives ADP and phosphate and releases energy.
That energy is coupled to a process in the cell that needs it.
Joining phosphate back to ADP is a condensation that requires energy.
Respiration supplies that energy, so ATP and ADP are interconverted continuously.
Students often think used ATP is lost and rebuilt from glucose. In fact ADP and phosphate are simply rejoined; ATP is recycled.
Students often think making ATP releases energy. In fact hydrolysis releases energy; synthesis takes it in.
C1.2.4 Respiration makes ATP inside cells; gas exchange is something else
Cell respiration releases energy from carbon compounds inside cells to produce ATP.
Glucose and fatty acids are the main substrates, but many carbon compounds can be used.
Gas exchange moves oxygen and carbon dioxide across a surface such as the alveoli.
Gas exchange serves respiration; it is not respiration.
Students often think respiration means breathing. In fact breathing is gas exchange; respiration is chemistry inside cells.
Students often think respiration creates energy. In fact it releases energy already stored in carbon compounds.
C1.2.5 Aerobic and anaerobic respiration in humans compared
Aerobic: glucose + oxygen → carbon dioxide + water; large ATP yield; needs mitochondria.
Anaerobic: glucose → lactate; small ATP yield; no carbon dioxide; all in the cytoplasm.
Anaerobic respiration uses no oxygen and only carbohydrate as substrate.
Fatty acids are respired only aerobically.
Students often think human anaerobic respiration makes ethanol and carbon dioxide. In fact it makes lactate only.
Students often think anaerobic respiration makes no ATP. In fact it yields a net two ATP per glucose from glycolysis.
C1.2.6 Measuring the rate of respiration
Rate is oxygen used or carbon dioxide made per unit time, per unit mass.
In a respirometer, potassium hydroxide absorbs carbon dioxide.
So the gas volume falls as oxygen is taken up, and the liquid drop moves.
Divide volume change by time and by mass: cm³ g⁻¹ min⁻¹.
Temperature raises rate up to an optimum; beyond it the enzymes of respiration are denatured.
Students often think the drop is pushed by carbon dioxide. In fact the alkali absorbs it; the drop moves because oxygen is taken up.
Students often think a fall in oxygen uptake means a switch to anaerobic respiration. In fact heat denatures enzymes; anaerobic respiration answers a lack of oxygen.
C1.2.7 Oxidation is losing hydrogen; NAD carries it HL
2028 guide: scope reduced — Reported (unverified secondary source): FAD / reduced FAD not required from 2028. The 2025 guidance held here names only NAD, so no FAD content was authored; every item on this statement is valid for both guides. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
Oxidation is loss of electrons; reduction is gain. They always happen together.
Removing hydrogen from a substrate (dehydrogenation) removes an electron, so the substrate is oxidised.
NAD accepts that hydrogen and becomes reduced NAD.
Reduced NAD later gives up its electrons, so a small NAD pool is reused.
Students often think oxidation needs oxygen. In fact losing hydrogen with its electron is oxidation, oxygen or not.
Students often think NAD is oxidised when it takes hydrogen. In fact gaining hydrogen is reduction, giving reduced NAD.
C1.2.8 Glycolysis splits glucose into two pyruvate, step by step HL
Glycolysis runs in the cytoplasm and needs no oxygen.
Phosphorylation adds phosphate from two ATP to glucose; lysis splits it into two 3C molecules.
Oxidation by dehydrogenation forms reduced NAD; later steps form ATP.
Net yield per glucose: 2 ATP, 2 reduced NAD, 2 pyruvate.
Students often think glycolysis yields four ATP. In fact four are made but two are spent at the start, so net two.
Students often think one enzyme splits glucose in one step. In fact about ten reactions each use a different enzyme.
C1.2.9 Making lactate keeps glycolysis running without oxygen HL
Glycolysis stops if reduced NAD is not converted back to NAD.
In humans, pyruvate accepts hydrogen from reduced NAD and becomes lactate.
This step makes no ATP; it regenerates NAD.
Glycolysis can then continue, yielding net two ATP per glucose.
Students often think lactate formation releases extra ATP. In fact it makes none; its job is to regenerate NAD.
Students often think pyruvate is oxidised to lactate. In fact pyruvate is reduced, and reduced NAD is oxidised back to NAD.
C1.2.10 Yeast makes ethanol and carbon dioxide instead HL
Glycolysis in yeast is the same pathway as in humans.
Only the NAD-regenerating step differs: pyruvate is decarboxylated, then reduced to ethanol.
Glucose → ethanol + carbon dioxide.
The carbon dioxide raises bread dough; the ethanol is the alcohol in beer and wine.
Students often think yeast makes lactate like muscle. In fact yeast makes ethanol and carbon dioxide.
Students often think anaerobic respiration never makes carbon dioxide. In fact yeast releases it; human anaerobic respiration does not.
C1.2.11 The link reaction turns pyruvate into a 2C acetyl group HL
In the mitochondrial matrix, pyruvate (3C) is oxidised and decarboxylated.
Products: a 2C acetyl group, carbon dioxide and reduced NAD.
Coenzyme A carries the acetyl group into the Krebs cycle as acetyl-CoA.
Acetyl groups from fatty acid breakdown enter the same way.
Students often think pyruvate joins oxaloacetate directly. In fact the link reaction first converts it to an acetyl group.
Students often think coenzyme A is an enzyme. In fact it is a carrier, not a catalyst.
C1.2.12 The Krebs cycle oxidises acetyl groups and regenerates oxaloacetate HL
2028 guide: scope reduced — Reported (unverified secondary source): FAD / reduced FAD not required from 2028. The 2025 guidance held here specifies four oxidations and a yield of ATP and reduced NAD without naming FAD, so items count oxidations without naming FAD and are valid for both guides. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
The acetyl group (2C) joins oxaloacetate (4C) to form citrate (6C).
Citrate returns to oxaloacetate through two decarboxylations and four oxidations.
The oxidations are dehydrogenations, mostly forming reduced NAD.
One ATP is formed directly per turn.
Students often think the Krebs cycle uses oxygen directly. In fact hydrogen goes to NAD; oxygen is used only at the chain's end.
Students often think the Krebs cycle makes most ATP. In fact it makes one per turn; chemiosmosis makes most, using its reduced NAD.
C1.2.13 Reduced NAD hands its electrons to the chain HL
2028 guide: scope reduced — Reported (unverified secondary source): FAD / reduced FAD not required from 2028. The 2025 guidance held here refers only to reduced NAD as the carrier delivering electrons to the chain; no FAD content was authored, so all items are valid for both guides. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
The electron transport chain sits in the inner mitochondrial membrane.
Reduced NAD passes a pair of electrons to the first carrier and becomes NAD.
Electrons pass carrier to carrier, releasing energy at each step.
Reduced NAD comes from glycolysis, the link reaction and the Krebs cycle.
Students often think reduced NAD phosphorylates ADP directly. In fact its electrons drive a proton gradient, which drives ATP synthase.
Students often think all reduced NAD comes from the Krebs cycle. In fact glycolysis and the link reaction make it too.
C1.2.14 Electron flow pumps protons across the inner membrane HL
Energy from electron flow pumps protons from the matrix into the intermembrane space.
The intermembrane space becomes more concentrated in protons than the matrix.
This difference is the proton gradient.
Students often think protons are pumped into the matrix. In fact they go out to the intermembrane space.
Students often think the pumps are powered by ATP. In fact energy released by electron flow along the chain drives them.
C1.2.15 Protons flow back through ATP synthase, making ATP HL
Chemiosmosis uses the proton gradient to drive ATP synthesis.
Protons diffuse down their gradient into the matrix through ATP synthase.
That flow releases energy the enzyme uses to phosphorylate ADP.
ATP forms on the matrix side of the membrane.
Students often think ATP synthase pumps protons using ATP. In fact protons flow through it passively, and that flow makes ATP.
Students often think chemiosmosis is water movement. In fact it is proton diffusion across a membrane; water is not involved.
C1.2.16 Oxygen takes the electrons at the end of the chain HL
Oxygen is the terminal electron acceptor.
It takes electrons from the last carrier and protons from the matrix, forming metabolic water.
Without it the carriers stay reduced and electron flow stops.
Carbon dioxide comes from decarboxylation, not from oxygen.
Students often think the chain runs without oxygen. In fact the final carrier stays reduced and everything halts.
Students often think oxygen becomes carbon dioxide. In fact oxygen becomes water.
C1.2.17 Lipids give more energy per gram but need oxygen HL
Lipids yield about twice the energy per gram of carbohydrates.
They have less oxygen and more oxidisable hydrogen and carbon.
Fatty acids break into 2C acetyl groups that enter as acetyl-CoA.
They skip glycolysis, so only carbohydrate can be respired anaerobically.
Students often think carbohydrate gives more energy per gram. In fact lipid gives roughly twice as much.
Students often think fatty acids are first turned into glucose. In fact they enter the Krebs cycle directly as acetyl-CoA.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement about ATP (adenosine triphosphate) is correct?
Answer and reasoning
It is a nucleotide: adenine and ribose joined to a chain of three phosphate groups. — ATP is a nucleotide made of the base adenine, the pentose sugar ribose and three phosphate groups. It is the molecule that distributes energy within cells.
It is a small protein that binds energy released from glucose in respiration. — A student who files nucleotides under DNA only and thinks of ATP as a special protein picks this. In fact ATP is a nucleotide, not a protein, and energy is not 'bound' but released when ATP is hydrolysed.
It is the cell's main long-term reserve of energy, built up like glycogen. — A student who reads 'energy store' as a reserve for the future picks this. In fact a cell holds only a small pool of ATP and recycles it within seconds; long-term reserves are glycogen and lipid.
It is destroyed once used and is rebuilt from glucose rather than from ADP. — A student who pictures each ATP being made afresh from glucose picks this. In fact hydrolysis gives ADP and phosphate, and these are rejoined to re-form ATP; glucose supplies the energy, not the ADP.
2 Which statement correctly describes a life process that ATP supplies with energy?
Answer and reasoning
Water entering a root hair cell by osmosis uses ATP to cross the membrane against its gradient. — A student who thinks every movement across a membrane needs ATP picks this. In fact osmosis is passive: water moves down its own gradient with no ATP hydrolysed.
Joining amino acids into a polypeptide releases energy, which the cell holds as ATP. — A student who thinks bond-forming synthesis releases energy picks this. In fact synthesis of macromolecules (anabolism) requires energy, which ATP supplies; it does not produce ATP.
Pumping sodium ions out of a neuron against their concentration gradient uses ATP directly. — Active transport across membranes is one of the principal uses of ATP: the sodium-potassium pump hydrolyses ATP to move ions against their concentration gradients.
Chromosomes move to the poles in mitosis without ATP, pulled by spindle fibres as they get shorter. — A student who reads mitosis diagrams as purely mechanical picks this. In fact movement of cell components such as chromosomes is driven by motor proteins that use energy from ATP.
3 Which statement correctly describes the substrates of cell respiration?
Answer and reasoning
Only glucose can be respired, because it is the reactant in the equation for respiration. — A student who takes the glucose word equation as the definition of respiration picks this. In fact glucose is the standard example; a wide range of carbon compounds can be respired.
Glucose and fatty acids are the main substrates, but many other carbon compounds can be used. — Glucose and fatty acids are the principal substrates for cell respiration, but a wide range of carbon (organic) compounds, including other sugars, glycerol and amino acids, can be respired.
Glucose and fatty acids are the substrates, and either can be used with or without oxygen. — A student who keeps substrates and oxygen requirement as separate lists picks this. In fact fatty acids can be respired only aerobically; only carbohydrate can be used anaerobically.
Oxygen and carbon dioxide are the substrates, since they are the substances exchanged at the lungs. — A student who confuses respiration with gas exchange picks the gases as substrates. In fact the substrate is a carbon compound; oxygen is a reactant in aerobic respiration but is not the source of the energy.
4 In a respirometer, carbon dioxide is absorbed by potassium hydroxide. With 5.0 g of germinating seeds, the gas volume in the apparatus fell by 1.2 cm³ in 10 minutes. What is the rate of respiration per gram?
Answer and reasoning
0.024 cm³ of carbon dioxide produced per gram per minute — A student who thinks the liquid is moved by carbon dioxide picks this. In fact the potassium hydroxide absorbs the carbon dioxide, so the volume change measures oxygen uptake.
0.024 cm³ of oxygen taken up per gram per minute — The fall in volume equals the oxygen consumed, because carbon dioxide is removed by the alkali. Rate = 1.2 cm³ ÷ 10 min ÷ 5.0 g = 0.024 cm³ g⁻¹ min⁻¹.
0.12 cm³ of oxygen taken up per gram per minute — A student who divides by time but forgets to divide by the mass of seeds picks this. 1.2 ÷ 10 = 0.12 cm³ min⁻¹ for the whole 5.0 g sample; per gram this is 0.024.
0.024 cm³ of oxygen given out per gram per minute — A student who thinks plant material releases oxygen picks this. In fact germinating seeds respire and do not photosynthesize; the volume fell because oxygen was taken up, not released.
5 In one reaction of the Krebs cycle, two hydrogen atoms (each with an electron) are removed from a substrate and accepted by NAD. Which statement describes this reaction? HL
Answer and reasoning
The substrate is reduced and NAD is oxidized by the transfer. — A student who attaches the labels the wrong way round picks this. In fact the substrate loses hydrogen and electrons, so it is oxidized; NAD gains them, so it is reduced.
Neither substance is oxidized, because no oxygen takes part in it. — A student who defines oxidation as gain of oxygen picks this. In fact oxidation is loss of electrons, so dehydrogenation is an oxidation whether or not oxygen is present.
The substrate is oxidized, but NAD only holds hydrogen and is unchanged. — A student who thinks a carrier is a passive container picks this. In fact a redox reaction always involves both changes: NAD accepts the hydrogen and is reduced to reduced NAD.
The substrate has been oxidized and NAD has been reduced. — Oxidation is loss of electrons. Removing hydrogen with its electron (dehydrogenation) oxidizes the substrate, and NAD, which accepts the hydrogen, is reduced; the two occur together as a redox reaction.
6 During intense exercise, muscle cells convert pyruvate to lactate. What is the purpose of this conversion? HL
Answer and reasoning
To release extra ATP from pyruvate once the mitochondria have stopped working. — A student who thinks lactate formation is where the anaerobic ATP comes from picks this. In fact no ATP is formed in this step; all the ATP comes from glycolysis.
To oxidize pyruvate so that more reduced NAD is available to the cell. — A student who assumes every step of respiration is an oxidation picks this. In fact pyruvate is reduced, accepting hydrogen from reduced NAD, which is thereby oxidized back to NAD.
To regenerate NAD so that glycolysis can continue producing ATP. — Reducing pyruvate to lactate converts reduced NAD back to NAD. This allows glycolysis, which needs NAD for its oxidation step, to continue with a net yield of two ATP per glucose.
To convert pyruvate into a form able to enter the mitochondria. — A student who thinks all respiration passes through mitochondria picks this. In fact lactate formation occurs when oxygen is insufficient and the mitochondrial pathway cannot be used.
7 What happens to pyruvate in the link reaction of aerobic cell respiration? HL
Answer and reasoning
It enters the Krebs cycle directly, where it combines with oxaloacetate to form citrate. — A student who leaves out the link reaction picks this. In fact pyruvate (3C) is first converted to a 2C acetyl group; it is the acetyl group, not pyruvate, that joins oxaloacetate.
It is oxidized and decarboxylated to a 2C acetyl group, which coenzyme A carries to the Krebs cycle. — In the link reaction pyruvate loses carbon dioxide (decarboxylation) and hydrogen (oxidation, reducing NAD) to form an acetyl group, which is bound to coenzyme A and transferred to the Krebs cycle.
It reacts with oxygen in the matrix, forming carbon dioxide and a two-carbon acetyl group that enters the cycle. — A student who thinks carbon dioxide is made by adding oxygen picks this. In fact the carbon dioxide comes from decarboxylation; oxygen is used only at the end of the electron transport chain.
It is bound by coenzyme A, the enzyme that catalyses its conversion into citrate in the matrix. — A student who reads 'coenzyme' as 'enzyme' picks this. In fact coenzyme A is a carrier that accepts the acetyl group; the reactions are catalysed by separate enzymes.
8 Which statement about the Krebs cycle is correct? HL
Answer and reasoning
Oxaloacetate is used up in each turn of the cycle, so it must be supplied continually from pyruvate. — A student who expects every reactant to be consumed, as in a linear pathway, picks this. In fact oxaloacetate is regenerated by the reactions of the cycle.
Most of the ATP from aerobic respiration is formed directly by the reactions of the cycle itself. — A student who sees the cycle as the main energy-releasing stage picks this. In fact the cycle forms only one ATP per turn; most ATP is made by chemiosmosis using the reduced NAD.
Citrate (6C) forms when an acetyl group (2C) joins oxaloacetate (4C), which is then regenerated. — Citrate is produced by transfer of the acetyl group from acetyl-CoA to oxaloacetate. The cycle's oxidations and decarboxylations then convert citrate back to oxaloacetate, ready for the next acetyl group.
Carbon dioxide is released when oxygen combines with the carbon atoms of citrate. — A student who thinks oxygen is a reactant of the cycle picks this. In fact carbon dioxide is released by decarboxylation; oxygen is used only as the terminal electron acceptor.
9 How is the proton gradient across the inner mitochondrial membrane generated? HL
Answer and reasoning
Protons are pumped from the intermembrane space into the matrix as electrons flow along the chain of carriers. — A student who places the proton build-up where ATP is made picks this. In fact protons are pumped out of the matrix into the intermembrane space, and flow back into the matrix through ATP synthase.
Hydrolysis of ATP supplies the energy needed to pump protons across the inner membrane into the intermembrane space. — A student who thinks all active transport uses ATP picks this. In fact the pumping is powered by electron flow along the chain; this is the process that makes ATP, not one that uses it.
Energy released as electrons flow along the chain pumps protons from the matrix into the intermembrane space. — As electrons flow from carrier to carrier along the electron transport chain, energy released at each transfer is used to move protons from the matrix across the inner membrane into the intermembrane space.
Electrons themselves are pumped across the inner membrane, leaving the protons behind in the matrix. — A student who mixes up which particle crosses the membrane picks this. In fact electrons stay within the carriers; the energy they release is used to pump protons across.
10 What is the role of oxygen in aerobic cell respiration? HL
Answer and reasoning
It combines with carbon atoms from the acetyl group in the Krebs cycle to form the carbon dioxide released. — A student who thinks the carbon dioxide is made from the oxygen picks this. In fact carbon dioxide comes from decarboxylation; the oxygen consumed becomes water.
It accepts electrons from the final carrier of the chain and protons from the matrix, forming water. — Oxygen is the terminal electron acceptor. By accepting electrons from the last carrier and protons from the matrix it forms metabolic water and keeps the carriers oxidized so that electron flow continues.
It is not needed for the chain to run; it is used only in the last step to form water. — A student who sees water formation as a minor finishing step picks this. In fact without oxygen to accept electrons the final carrier stays reduced and the whole chain stops.
It oxidizes glucose directly in the cytoplasm, converting it to pyruvate in glycolysis. — A student who thinks oxidation means adding oxygen picks this. In fact glycolysis oxidizes the sugar by removing hydrogen to NAD; oxygen is used only at the end of the chain in the mitochondrion.
Read the ones marked not yet in Learn, then Verify.
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19 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A cell needs an energy currency that can supply many different reactions in the cytoplasm. Which property makes ATP suitable for this role?
Answer and reasoning
It holds a very large amount of energy, so that a cell can store it for many weeks between meals. — A student who thinks ATP is a long-term store picks this. In fact ATP is used within seconds of being formed; energy for weeks is stored as glycogen and lipid, not as ATP.
It releases energy in an amount suited to a single reaction and is quickly re-formed from ADP. — ATP hydrolysis releases a modest packet of energy, enough for many individual tasks, and ADP is phosphorylated back to ATP within seconds, so a small pool of ATP can supply energy continuously.
It is consumed after one use, so the cell then builds fresh ATP directly from glucose. — A student who thinks ATP is used up and replaced from scratch picks this. In fact ATP is recycled: the ADP and phosphate formed by hydrolysis are rejoined using energy from respiration.
It acts as an enzyme, catalysing each of the reactions in the cytoplasm that need energy. — A student who thinks ATP is a protein or enzyme picks this. In fact ATP is a nucleotide; it supplies energy by being hydrolysed, and the reactions are catalysed by separate enzymes.
2 During a sprint, ATP in muscle fibres is hydrolysed and then re-formed thousands of times. Which statement about the energy transfers is correct?
Answer and reasoning
Hydrolysis of ATP to ADP and phosphate needs an input of energy, and re-forming ATP releases it. — A student who merges 'respiration releases energy' with 'respiration makes ATP' reverses the direction and picks this. In fact energy is released by hydrolysis and required for synthesis.
Hydrolysed ATP cannot be re-formed, so each contraction needs new ATP built from a glucose molecule. — A student who thinks ATP is consumed and replaced from scratch picks this. In fact the ADP and phosphate are rejoined; the same ADP molecules are recycled thousands of times during the sprint.
The muscle takes energy directly from glucose during the sprint; hydrolysis of ATP is a minor side reaction. — A student who remembers 'glucose releases energy for the cell' without ATP picks this. In fact contraction is powered by ATP hydrolysis; glucose supplies energy only through the ATP it is used to make.
Hydrolysis of ATP releases energy; energy from respiration is needed to re-form it from ADP. — Energy is released by hydrolysis of ATP to ADP and phosphate, and energy released from carbon compounds in respiration is required to synthesize ATP again from ADP and phosphate.
3 A student writes: 'Cell respiration is when oxygen enters the blood and carbon dioxide leaves it in the lungs.' Which statement correctly evaluates this?
Answer and reasoning
It is wrong: this describes gas exchange; cell respiration releases energy from carbon compounds to make ATP inside cells. — Gas exchange is the movement of gases across a surface. Cell respiration is a chemical process in every cell that releases energy from carbon compounds to produce ATP; the two must be distinguished.
It is right: respiration is the exchange of gases at a respiratory surface, which the cells then make use of. — A student who uses the everyday meaning of 'respiration' as breathing picks this. In fact gas exchange supplies oxygen to and removes carbon dioxide from cells; respiration is what happens inside them.
It is wrong: cell respiration is glucose reacting with oxygen inside cells, and glucose is the only substrate used. — A student who defines respiration by the glucose equation picks this. In fact glucose and fatty acids are the principal substrates, but a wide range of carbon compounds can be respired; the statement is wrong because it describes gas exchange, not because it omits glucose.
It is wrong: cell respiration creates the energy a cell needs, using the oxygen that gas exchange brings into the body. — A student who thinks respiration creates energy picks this. In fact energy cannot be created; respiration releases energy already held in carbon compounds and transfers it to ATP, and the statement is wrong because it describes gas exchange.
4 Which statement about anaerobic cell respiration in humans is correct?
Answer and reasoning
Glucose is converted to ethanol and carbon dioxide, as it is in yeast cells. — A student who confuses the human and yeast equations picks this. In fact humans convert glucose to lactate; ethanol and carbon dioxide are the products in yeast.
Glucose is converted to lactate and carbon dioxide in the cytoplasm of the cell. — A student who thinks all respiration releases carbon dioxide picks this. In fact no carbon dioxide is released in human anaerobic respiration; the only product is lactate.
Glucose is converted to lactate, and no carbon dioxide is released at all. — The word equation is glucose → lactate. Anaerobic respiration in humans uses no oxygen, occurs in the cytoplasm, yields a little ATP and releases no carbon dioxide.
Glucose is converted to lactate using a little oxygen rather than none. — A student who reads 'anaerobic' as 'low oxygen' picks this. In fact anaerobic respiration uses no oxygen at all; it consists only of glycolysis and lactate formation in the cytoplasm, so the oxygen-using reactions in the mitochondria play no part.
5 Mature human red blood cells have no mitochondria. Which statement about cell respiration in these cells is correct?
Answer and reasoning
They cannot respire at all, because all cell respiration takes place in mitochondria. — A student who thinks every form of respiration is confined to mitochondria picks this. In fact glycolysis and lactate formation occur in the cytoplasm, so respiration continues without mitochondria.
They respire anaerobically, and so they gain no ATP from the glucose they take up. — A student who rounds the small anaerobic yield down to nothing picks this. In fact anaerobic respiration yields a net two ATP per glucose, which is enough for the needs of a red blood cell.
They respire anaerobically, releasing ethanol and carbon dioxide into the blood plasma. — A student who applies the yeast equation to human cells picks this. In fact human cells convert glucose to lactate; they do not produce ethanol or carbon dioxide anaerobically.
They respire anaerobically in the cytoplasm, converting glucose to lactate. — Mitochondria are required for aerobic but not anaerobic respiration. Red blood cells respire anaerobically in the cytoplasm, with a small yield of ATP and lactate as the waste product.
6 Aerobic respiration of one glucose molecule yields many times more ATP (roughly 30) than anaerobic respiration (2). Which statement explains the difference?
Answer and reasoning
Without oxygen only glycolysis in the cytoplasm yields ATP; with oxygen the mitochondria release far more of the energy in glucose. — Anaerobic respiration ends with glycolysis and lactate formation in the cytoplasm, so most of the energy remains in the lactate. Aerobic respiration continues in the mitochondria, oxidizing the substrate completely and yielding far more ATP.
Both pathways run in the mitochondria, but without oxygen the mitochondria work more slowly and so make less ATP. — A student who places all respiration in mitochondria picks this. In fact anaerobic respiration does not use mitochondria at all; it is completed in the cytoplasm, where only glycolysis yields ATP.
Without oxygen the cell converts glucose to ethanol, which keeps most of the energy, so much less ATP is made. — A student who applies the yeast pathway to humans picks this. In fact human anaerobic respiration produces lactate, not ethanol, though it is true that most of the energy stays in the product.
Without oxygen the cell uses the little oxygen dissolved in its cytoplasm, so only a small amount of ATP forms. — A student who thinks anaerobic respiration means 'a little oxygen' picks this. In fact anaerobic respiration uses no oxygen; the low yield is because glycolysis alone releases only a small fraction of the energy.
7 A student measured oxygen uptake by yeast (cm³ g⁻¹ min⁻¹) at five temperatures: 20 °C 0.10; 30 °C 0.19; 40 °C 0.31; 50 °C 0.26; 60 °C 0.04. Which conclusion is best supported?
Answer and reasoning
Rate rises with temperature over the whole range, so the low reading at 60 °C must be an experimental error. — A student who applies only the kinetic-energy explanation picks this. In fact the sharp fall from 50 °C to 60 °C is the expected effect of enzyme denaturation, not an error.
Above 40 °C the yeast switches to anaerobic respiration, which uses no oxygen, so oxygen uptake falls. — A student who uses 'switched to anaerobic' as a general explanation picks this. In fact oxygen was freely available; the fall is due to denaturation of the enzymes catalysing respiration.
Rate rises with temperature up to an optimum near 40 °C, then falls as the enzymes of respiration denature. — Every stage of respiration is enzyme-catalysed. Rate increases with temperature until an optimum, then falls rapidly as enzymes denature, which matches the sharp drop between 50 °C and 60 °C.
At 60 °C the extra carbon dioxide produced expands and pushes the liquid back, hiding the true rate. — A student who thinks the respirometer responds to carbon dioxide picks this. In fact carbon dioxide is absorbed by the alkali, so the reading reflects oxygen uptake, which genuinely fell.
8 Which statement correctly summarises the stages of glycolysis? HL
Answer and reasoning
Glucose is phosphorylated using ATP, split into two three-carbon molecules that are oxidized, and ATP is then formed. — Glycolysis involves phosphorylation of glucose (using ATP), lysis into two three-carbon molecules, oxidation of these by removal of hydrogen to reduce NAD, and formation of ATP; each step has its own enzyme.
Glucose is phosphorylated, forming ATP, and then split into two pyruvate molecules that are reduced. — A student who assumes any phosphorylation makes ATP picks this. In fact phosphorylation of glucose uses ATP, and the three-carbon molecules are oxidized, not reduced.
A single enzyme splits glucose directly into two pyruvate molecules, releasing energy in the form of ATP. — A student who mistakes the summary equation for the mechanism picks this. In fact glycolysis is a stepwise pathway in which each reaction is catalysed by a different enzyme.
Glucose is split into two pyruvate molecules without any oxidation, as no oxygen is present in the cytoplasm. — A student who thinks oxidation needs oxygen picks this. In fact the three-carbon molecules are oxidized by dehydrogenation, reducing NAD, even though no oxygen is involved.
9 During glycolysis of one glucose molecule, two ATP are used in phosphorylation and four ATP are formed later. What is the net yield per glucose? HL
Answer and reasoning
A net gain of 4 ATP as well as 2 reduced NAD — A student who counts only the ATP-forming steps picks this. In fact two ATP were used to phosphorylate glucose, so the net gain is 4 − 2 = 2 ATP.
A net gain of 2 ATP together with 2 reduced NAD — Four ATP formed minus two ATP used gives a net yield of two ATP. The oxidation steps also produce two reduced NAD, and two pyruvate are formed.
A net gain of 6 ATP plus another 2 reduced NAD — A student who thinks phosphorylation of glucose produces ATP adds 2 + 4 and picks this. In fact the phosphorylation step consumes ATP, so the net gain is two.
A net gain of 2 ATP together with 2 oxidized NAD — A student who reverses the redox labels picks this. In fact NAD accepts hydrogen during the oxidation steps and so becomes reduced NAD.
10 How does anaerobic cell respiration in yeast differ from anaerobic cell respiration in human muscle? HL
Answer and reasoning
Yeast uses a different pathway from glucose to pyruvate, with its own distinct set of enzymes. — A student who infers different pathways from different products picks this. In fact glycolysis is identical in yeast and humans; only the NAD-regenerating step differs.
Yeast converts pyruvate to ethanol only, since carbon dioxide cannot form without oxygen. — A student who thinks carbon dioxide needs oxygen picks this. In fact yeast decarboxylates pyruvate, releasing carbon dioxide, before reducing the product to ethanol.
There is no difference: both convert pyruvate to ethanol and carbon dioxide when oxygen is absent. — A student who applies the yeast equation to humans picks this. In fact human muscle reduces pyruvate directly to lactate, with no carbon dioxide released.
Yeast regenerates NAD by turning pyruvate into ethanol and carbon dioxide, not lactate. — The pathways are the same as far as pyruvate. Yeast regenerates NAD by decarboxylating pyruvate and reducing the product to ethanol, so the final products are ethanol and carbon dioxide rather than lactate.
11 Yeast is used in baking to make dough rise and in brewing to make beer alcoholic. Which statement explains both uses? HL
Answer and reasoning
Anaerobic respiration of sugars produces carbon dioxide, which raises the dough, and ethanol, which remains in the beer. — In anaerobic respiration yeast converts glucose to ethanol and carbon dioxide. Bubbles of carbon dioxide make dough rise (the ethanol evaporates in baking), while the ethanol is the alcohol in beer.
The carbon dioxide that raises dough comes from aerobic respiration, since anaerobic respiration releases none. — A student who generalises from human anaerobic respiration picks this. In fact yeast releases carbon dioxide anaerobically, by decarboxylating pyruvate, which is what makes dough rise.
Yeast produces lactate, as human muscle does, which sours the dough and makes the beer alcoholic. — A student who thinks all anaerobic respiration gives lactate picks this. In fact yeast produces ethanol and carbon dioxide; lactate is not alcoholic and is not a yeast product.
Yeast uses the small amount of oxygen dissolved in the dough or beer to produce carbon dioxide and ethanol. — A student who thinks anaerobic respiration uses a little oxygen picks this. In fact ethanol is produced only when oxygen is absent; the pathway needs no oxygen at all.
12 One acetyl group enters the Krebs cycle and oxaloacetate is regenerated. Which account of one turn of the cycle is correct? HL
Answer and reasoning
Two carbon dioxide molecules form when oxygen oxidizes the carbon atoms of the acetyl group directly. — A student who thinks oxygen reacts in the cycle picks this. In fact the oxidations are dehydrogenations and the carbon dioxide comes from decarboxylation, without any oxygen.
Each of the four oxidations forms one ATP directly, so the cycle yields most of the cell's ATP supply. — A student who attributes most ATP to the cycle picks this. In fact the oxidations reduce NAD; only one ATP per turn is formed directly, and most ATP comes from chemiosmosis.
Only decarboxylation occurs, since no oxygen enters the matrix to oxidize any of the cycle's intermediates. — A student who thinks oxidation needs oxygen picks this. In fact four oxidations occur in each turn by removal of hydrogen, which reduces NAD, with no oxygen involved.
Two decarboxylations give off carbon dioxide, four dehydrogenations reduce carriers, one ATP forms. — Per turn, two carbons leave as carbon dioxide by decarboxylation, four oxidations by removal of hydrogen produce reduced carriers (mainly reduced NAD), and one ATP is formed directly.
13 What is the role of reduced NAD in aerobic cell respiration? HL
Answer and reasoning
It passes a pair of electrons to the first carrier of the electron transport chain, becoming NAD again. — Energy is transferred when reduced NAD passes a pair of electrons to the first carrier in the chain. This regenerates NAD, which returns to glycolysis, the link reaction and the Krebs cycle.
It phosphorylates ADP directly in the matrix, so each reduced NAD becomes a fixed number of ATP. — A student who reads 'ATP equivalents' tables as a direct exchange picks this. In fact reduced NAD transfers energy to the electron transport chain, and ATP is made by chemiosmosis.
It carries hydrogen from the Krebs cycle only, because glycolysis in the cytoplasm forms none. — A student who forgets the earlier stages picks this. In fact reduced NAD is formed in glycolysis, the link reaction and the Krebs cycle, and all of it feeds the chain.
It carries electrons across the inner membrane into the intermembrane space to build the gradient. — A student who thinks electrons are what cross the membrane picks this. In fact electrons pass along carriers within the membrane; it is protons that are pumped into the intermembrane space.
14 Isolated mitochondria are supplied with pyruvate and oxygen. A poison then blocks the first carrier of the electron transport chain, and the Krebs cycle also stops. Which statement explains why the cycle stops? HL
Answer and reasoning
The poison prevents oxygen from reaching the matrix, so the cycle can no longer oxidize citrate to oxaloacetate. — A student who thinks the cycle uses oxygen directly picks this. In fact oxygen is not a reactant in the cycle; the cycle stops because NAD, not oxygen, is no longer available.
Reduced NAD can no longer pass its electrons to the chain, so no NAD is regenerated for the cycle's oxidations. — The Krebs cycle oxidizes substrates by reducing NAD. NAD is regenerated only when reduced NAD gives its electrons to the first carrier; with that blocked, all NAD stays reduced and the dehydrogenations halt.
Reduced NAD can no longer phosphorylate ADP directly, and the cycle runs on the ATP that this provides. — A student who thinks reduced NAD makes ATP directly picks this. In fact reduced NAD donates electrons to the chain; the cycle halts because NAD is not regenerated, not for lack of ATP.
Oxaloacetate is no longer supplied by the chain, so citrate cannot form from the acetyl groups. — A student who thinks oxaloacetate is consumed and must be resupplied picks this. In fact oxaloacetate is regenerated within the cycle; the chain does not supply it.
15 How does ATP synthase couple the proton gradient to the synthesis of ATP? HL
Answer and reasoning
It hydrolyses ATP to pump protons across the inner membrane into the intermembrane space, building the gradient. — A student who takes ATP synthase for another pump picks this. In fact ATP synthase does not pump; protons flow through it passively and it makes ATP.
Water moves through it by osmosis into the matrix, and the resulting pressure change forces phosphate onto ADP. — A student who reads 'chemiosmosis' as osmosis of water picks this. In fact it is protons, not water, that move through ATP synthase, down their concentration gradient.
Protons collect in the intermembrane space and phosphorylate ADP there, on the outer surface of the enzyme. — A student who places ATP synthesis where the protons accumulate picks this. In fact protons pass through the enzyme into the matrix, and ADP is phosphorylated on the matrix side.
Protons flow through it down their gradient into the matrix, releasing energy that phosphorylates ADP. — ATP synthase is the route by which protons diffuse back into the matrix. The energy released as they move down the gradient is coupled to joining phosphate to ADP: this is chemiosmosis.
16 A chemical makes the inner mitochondrial membrane freely permeable to protons. Electron flow and oxygen consumption continue, but ATP synthesis stops. Which statement explains these observations? HL
Answer and reasoning
Protons leak back into the matrix without passing through ATP synthase, so no gradient remains to drive ADP phosphorylation. — Chemiosmosis needs a proton gradient and a single route back through ATP synthase. If protons can cross the membrane anywhere, the chain still pumps them (so electron flow and oxygen use continue) but no gradient builds up and ATP synthase has nothing to drive it.
Electrons leak through the membrane instead of reaching oxygen, so no energy arrives at ATP synthase. — A student who thinks electrons cross the membrane picks this, but it contradicts the data: oxygen is still consumed, so electrons are still reaching it. Electrons stay in the carriers; it is protons that leak.
Water enters the matrix by osmosis, diluting the ADP and phosphate so that they can no longer be joined together. — A student who reads chemiosmosis as osmosis picks this. In fact water plays no part; ATP synthesis fails because the proton gradient has been dissipated.
Reduced NAD cannot transfer energy directly to ADP, because the leak removes the protons that it needs. — A student who thinks reduced NAD phosphorylates ADP directly picks this. In fact reduced NAD gives electrons to the chain, and the electron flow is unaffected; only the gradient is lost.
17 A student claims: 'Without oxygen the electron transport chain keeps running; only the formation of water stops.' Which statement correctly evaluates this claim? HL
Answer and reasoning
Correct: the chain needs only reduced NAD to keep running, and oxygen is required just to make water. — A student who treats oxygen's role as a trivial last step picks this. In fact allowing continued flow of electrons is precisely oxygen's role; without it the chain, proton pumping and chemiosmosis all stop.
Incorrect: the chain stops because the Krebs cycle needs oxygen to keep supplying it with reduced NAD. — A student who thinks the Krebs cycle uses oxygen directly picks this. In fact the cycle stops for lack of NAD, which is a consequence of the chain stopping, not the cause of it.
Incorrect: with no terminal acceptor the final carrier remains reduced and electron flow stops. — Each carrier can pass electrons on only if the next one is oxidized. Oxygen keeps the final carrier oxidized by accepting its electrons; without oxygen the carriers back up in the reduced state and flow ceases.
Incorrect: the chain stops because without oxygen there is no ATP to power the proton pumps. — A student who thinks the pumps are driven by ATP picks this. In fact proton pumping is driven by electron flow itself; the chain stops because there is nothing to accept the electrons.
18 Which statement correctly compares lipids and carbohydrates as respiratory substrates? HL
Answer and reasoning
Carbohydrates yield more energy per gram, because they are the body's main and most readily used fuel. — A student who confuses how readily a fuel is used with how much energy it holds picks this. In fact lipids yield roughly twice as much energy per gram as carbohydrates.
Lipids yield more energy per gram, because they contain more oxygen atoms that can be oxidized. — A student who links 'oxidation' with oxygen content picks this. In fact lipids contain less oxygen; their higher yield comes from more oxidizable hydrogen and carbon.
Lipids can be respired without oxygen once their fatty acids have been converted into glucose. — A student who thinks every substrate is turned into glucose picks this. In fact fatty acids are broken down to acetyl groups that enter via acetyl-CoA, so they can be respired only aerobically.
Lipids yield more energy per gram, as they have less oxygen and more oxidizable hydrogen. — Fatty acids are less oxidized than sugars: they contain little oxygen and more hydrogen and carbon per gram that can be oxidized, so more reduced NAD is formed and more ATP is made per gram.
19 A sprinter's leg muscles rely on anaerobic respiration during a 100 m race. Which statement explains why the muscles cannot use fatty acids to supply this energy? HL
Answer and reasoning
Fatty acids enter aerobic respiration as acetyl-CoA and cannot enter glycolysis, the only stage that yields ATP without oxygen. — Glycolysis and anaerobic respiration occur only if carbohydrate is the substrate. Fatty acids are converted to acetyl-CoA and oxidized in the Krebs cycle, which depends on the electron transport chain and therefore on oxygen.
Fatty acids must first be converted into glucose, and this conversion is far too slow to keep pace with a sprint. — A student who thinks all substrates are turned into glucose picks this. In fact fatty acids are not converted to glucose in muscle; they enter respiration as acetyl-CoA, which requires the aerobic pathway.
Fatty acids can be respired anaerobically, but the muscle prefers glucose because it is much faster to break down. — A student who thinks any substrate can be used without oxygen picks this. In fact fatty acids cannot be respired anaerobically at all, because they bypass glycolysis and enter only the aerobic pathway.
Fatty acids contain too little energy per gram to supply the very large amount of ATP that a sprint needs. — A student who thinks carbohydrate is the richer fuel picks this. In fact lipids hold roughly twice the energy per gram; the problem is that releasing it requires oxygen, which the sprinting muscle lacks.
That was your twenty minutes. Real practice on C1.2 is past-paper questions marked against the mark scheme.
What the exam asks of C1.2
Paper 1A asks you to recognise ATP's structure and uses, and to tell aerobic from anaerobic respiration. Paper 1B gives respirometer data and asks you to calculate a rate, or a graph of rate against temperature to describe and explain. Paper 2 uses *outline* and *distinguish* at SL: word equations, products and locations. At HL, expect *explain* and *describe* on glycolysis, the link reaction, the Krebs cycle and chemiosmosis: give the stage, the location, what is oxidised and what carries the electrons.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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