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Educerie · IB Diploma · Biology
Theme C Interaction and interdependence · C1.2 Cell respiration
What you must be able to do
| You must be able to | Level | What it looks like in the exam |
|---|---|---|
| Describe ATP as a nucleotide and explain why it suits its role as the cell's energy currency | SL, HL | "Outline the properties of ATP…" (3 marks) |
| List processes that ATP powers, and state the energy changes in ATP–ADP interconversion | SL, HL | Paper 1A, or a 2-mark "state" |
| Explain respiration as the system that makes ATP from carbon compounds, and distinguish it from gas exchange | SL, HL | "Distinguish between cell respiration and gas exchange" (2 marks) |
| Compare aerobic and anaerobic respiration in humans, with word equations | SL, HL | "Compare and contrast…" (4 marks) |
| Measure and calculate the rate of respiration; explain the variables affecting it | SL, HL | Paper 1B respirometer data: calculation plus evaluation (4 to 6 marks) |
| Explain oxidation and reduction in terms of hydrogen and electrons, and the role of NAD | HL only | Paper 1A, or a 2-mark definition |
| Outline glycolysis, and the regeneration of NAD in anaerobic respiration in humans and yeast | HL only | "Outline glycolysis" (4 marks); brewing and baking context |
| Outline the link reaction and Krebs cycle, naming citrate and oxaloacetate | HL only | Label a diagram, or a 4-mark "outline" |
| Explain the electron transport chain, chemiosmosis and the role of oxygen | HL only | Section B: "Explain how ATP is produced in the mitochondrion" (6 to 8 marks) |
| Compare lipids and carbohydrates as respiratory substrates | HL only | 3-mark "explain" |
Before you start
You need enzymes from C1.1, because every step below is catalysed by one, and the idea of a pathway. You need the structure of a mitochondrion from B2.2: an outer membrane, a folded inner membrane (the cristae), a narrow intermembrane space between them, and a fluid matrix inside. You also need to know that a nucleotide is a sugar, a phosphate and a base.
1The idea in one paragraph
Cells cannot use the energy in glucose directly. They use ATP, a small molecule that releases a convenient packet of energy when its last phosphate is removed and is rebuilt as soon as it is used. Cell respiration is how the cell rebuilds it: carbon compounds such as glucose and fatty acids are broken down in a series of enzyme-controlled steps, and the energy released is used to make ATP from ADP and phosphate. With oxygen, the breakdown goes all the way to carbon dioxide and water in the mitochondria and yields a lot of ATP. Without oxygen, human cells stop at lactate in the cytoplasm and get very little. At HL you learn why: respiration is mostly the stripping of hydrogen from the fuel, and oxygen is where that hydrogen finally goes.
2ATP, the energy currency
ATP is adenosine triphosphate. It is a nucleotide: the base adenine, the sugar ribose, and a chain of three phosphate groups (Figure 1a). Adenine and ribose together are called adenosine.
Energy is released when ATP is hydrolysed to ADP (adenosine diphosphate) and a phosphate. Energy is required to make ATP from ADP and phosphate. Figure 1b is the cycle this creates: respiration pays energy in to make ATP, and every energy-using process in the cell takes it out again. The amount released by one hydrolysis is not large, but it is enough for many single tasks in a cell, such as moving one ion across a membrane or joining one amino acid to a chain. You do not need a figure in kilojoules.
Why ATP and not glucose? Because ATP has the properties of a good currency.
- It is soluble in water, so it moves freely through the cytoplasm to wherever it is needed.
- It is stable at the pH of a cell: it does not break down on its own, only when an enzyme hydrolyses it, so energy is released where and when the cell chooses.
- It releases a useful amount of energy in one step. Glucose would release far more at once than any single task needs, and the surplus would be lost as heat.
- It cannot pass freely through membranes, so it stays inside the cell that made it.
- It is quickly regenerated, so a small stock is recycled over and over. A cell holds very little ATP at any moment and turns it over continuously.
3What ATP pays for
The guide names three kinds of work. Learn them as a list with an example of each.
| Process | Example |
|---|---|
| Active transport across membranes | pumping ions against a concentration gradient, as in the sodium–potassium pump |
| Synthesis of macromolecules (anabolism) | joining amino acids into proteins; building DNA, glycogen and cell walls |
| Movement of the whole cell or its parts | beating of cilia and flagella; muscle contraction; moving chromosomes during cell division |
4Respiration: the system that makes ATP
Cell respiration is the controlled release of energy from organic compounds in cells, used to produce ATP. It happens in every living cell, all the time, because every cell is always spending ATP.
The main respiratory substrates are glucose and fatty acids, but a wide range of carbon compounds can be used: amino acids from proteins when other supplies run low, and other sugars.
A frequent error is to confuse respiration with breathing. Keep them apart.
| Cell respiration | Gas exchange | |
|---|---|---|
| What it is | chemical reactions that release energy to make ATP | diffusion of oxygen and carbon dioxide between an organism and its environment |
| Where | inside every living cell (cytoplasm and mitochondria) | at a gas exchange surface, such as the alveoli or a leaf's spongy mesophyll |
| Kind of process | chemical, catalysed by enzymes | physical: diffusion |
Gas exchange supplies the oxygen that respiration uses and removes the carbon dioxide it makes, which is why the two are linked. But breathing is not respiration, and a question that asks about one scores nothing for an answer about the other.
5Aerobic and anaerobic respiration in humans
Aerobic respiration uses oxygen. Anaerobic respiration does not. Figure 2 shows where each happens.
Both begin the same way, in the cytoplasm. Then they part.
aerobic: glucose + oxygen → carbon dioxide + water
anaerobic (humans): glucose → lactate
| Aerobic | Anaerobic (humans) | |
|---|---|---|
| Oxygen | required | not required |
| Substrates | glucose, fatty acids, other carbon compounds | glucose (carbohydrate) only |
| Where | begins in the cytoplasm, finishes in the mitochondria | cytoplasm only |
| Mitochondria needed? | yes | no |
| ATP per glucose | large: about 30 | small: 2 |
| Waste products | carbon dioxide and water | lactate |
Why would a cell ever use the anaerobic route, when it yields so little? Because it is fast and needs no oxygen. During a sprint, muscle uses ATP faster than the blood can deliver oxygen for aerobic respiration, so anaerobic respiration makes up the difference for a short time. Lactate builds up, and after the sprint extra oxygen is needed to deal with it, which is why you keep breathing hard after you stop.
6Measuring the rate of respiration
Rate of respiration can be measured as oxygen used, carbon dioxide produced, or glucose used, per unit time, often also per unit mass of organism. In school the commonest tool is a respirometer, which measures oxygen uptake (Figure 3).
How it works. Germinating seeds (or small invertebrates) respire in a sealed tube. They take in oxygen and give out carbon dioxide. Soda lime or potassium hydroxide solution absorbs the carbon dioxide as fast as it is made. So the only change in the gas in the tube is the loss of oxygen: the volume of gas falls, the pressure falls, and the drop of coloured liquid in the capillary tube is pushed towards the organisms by the air outside. How far it moves in a set time measures the oxygen used.
A control tube, identical but with glass beads of the same volume in place of the seeds, shows any movement caused by temperature or pressure changes rather than respiration. Both tubes sit in a water bath so the temperature is constant.
Variables that affect the rate and can be investigated:
- Temperature: respiration is enzyme-catalysed, so rate rises with temperature up to the enzymes' optimum and falls if they denature.
- Availability of oxygen: without it, only anaerobic respiration is possible, at a much lower ATP yield.
- Substrate availability: dry seeds respire far more slowly than germinating ones, which are mobilising their food stores.
- Level of activity: an active organism, or a growing tissue, respires faster.
Figure 4 shows invented readings for 2.5 g of germinating peas at two temperatures, in a capillary of internal diameter 1.0 mm.
Turn the distance into a volume, then into a rate.
The rate at 25 °C is about twice the rate at 15 °C, as the effect of temperature on enzymes predicts. Dividing by mass lets you compare samples of different sizes, and every rate needs a unit that includes time.
7HLOxidation, reduction and NAD
SL students can skip to section 13.
Everything from here is easier once you see respiration as the removal of hydrogen. Oxidation is the loss of electrons. Reduction is the gain of electrons. A hydrogen atom is a proton and an electron, so when hydrogen is removed from a substrate, an electron goes with it and the substrate has been oxidised. Removing hydrogen is called dehydrogenation, and it is how respiratory substrates are oxidised.
Oxidation and reduction always happen together, in a redox reaction: whatever takes the electrons is reduced. In respiration the hydrogen is taken by NAD (nicotinamide adenine dinucleotide), a carrier molecule. When NAD accepts hydrogen it becomes reduced NAD. Reduced NAD carries the hydrogen, and with it the energy of its electrons, to where it can be used, and gives it up there, becoming NAD again. Say "reduced NAD" throughout; the guide uses that term.
8HLGlycolysis
Glycolysis converts one glucose into two pyruvate in the cytoplasm, in a series of steps each catalysed by a different enzyme. No oxygen is needed. You do not need the names of the intermediates, but you must know the four stages in Figure 5.
- Phosphorylation. Two phosphate groups are added to glucose, taken from two ATP. This costs energy, but it makes the sugar less stable and ready to split.
- Lysis. The 6-carbon sugar phosphate splits into two 3-carbon triose phosphates.
- Oxidation. Hydrogen is removed from each triose phosphate and accepted by NAD, giving two reduced NAD.
- ATP formation. Phosphate groups are transferred to ADP, making four ATP, and two pyruvate are formed.
9HLAnaerobic respiration: regenerating NAD
A cell holds only a small amount of NAD. Glycolysis turns it into reduced NAD, and once all of it is reduced glycolysis must stop, because step 3 has nothing to hand its hydrogen to. With oxygen, reduced NAD is reoxidised in the mitochondrion (section 11). Without oxygen, the cell needs another way to take the hydrogen back off, and anaerobic respiration is that way. Figure 6 shows the two versions.
In humans pyruvate accepts the hydrogen from reduced NAD and becomes lactate. NAD is regenerated, glycolysis continues, and the cell keeps its net yield of 2 ATP per glucose. That is the whole purpose of making lactate: it is not an energy step, it is a recycling step.
In yeast the pathway is the same up to pyruvate. Only the regeneration of NAD differs, so only the final products differ: pyruvate loses carbon dioxide and the remaining 2-carbon compound accepts the hydrogen from reduced NAD to become ethanol. Anaerobic respiration in yeast therefore produces ethanol and carbon dioxide.
The two products are why humans have used yeast for thousands of years.
- Brewing wants the ethanol. Yeast respires anaerobically on sugars in the fermenting mixture, and the ethanol accumulates; carbon dioxide can be kept in to carbonate the drink.
- Baking wants the carbon dioxide. Yeast in dough respires anaerobically once the oxygen in the dough is used up, the gas forms bubbles that make the dough rise, and the ethanol evaporates in the oven.
10HLThe link reaction and the Krebs cycle
If oxygen is present, pyruvate is carried into the matrix of the mitochondrion. Figure 7 follows one pyruvate.
The link reaction. Pyruvate (3C) is decarboxylated (a carbon is removed as carbon dioxide) and oxidised (hydrogen is removed, reducing NAD). The 2-carbon acetyl group left over is attached to coenzyme A, making acetyl CoA. Coenzyme A is the carrier that delivers acetyl groups to the Krebs cycle. Both carbohydrates and lipids are broken down to acetyl groups, which is where their pathways meet (section 12).
The Krebs cycle. The acetyl group (2C) is transferred to oxaloacetate (4C), forming citrate (6C), and coenzyme A is released to collect another. The rest of the cycle converts citrate back into oxaloacetate, ready for the next acetyl group. You need only these two names. On the way round there are:
- two decarboxylations, releasing two carbon dioxide (6C → 5C → 4C);
- four oxidations, each a dehydrogenation: three reduce NAD, and one reduces a second carrier, FAD;
- one ATP made directly.
Count the totals per glucose, remembering there are two pyruvates and so two turns of the cycle.
Notice how little ATP the cycle makes directly. Its real product is reduced NAD, and the energy in that is cashed in next.
11HLThe electron transport chain, chemiosmosis and oxygen
The inner mitochondrial membrane carries a chain of electron carriers, proteins that pass electrons from one to the next, and ATP synthase. Figure 8 shows the arrangement.
Follow the energy in four steps.
1. Reduced NAD hands over its electrons. Reduced NAD from glycolysis, the link reaction and the Krebs cycle passes a pair of electrons to the first carrier in the chain. This converts reduced NAD back to NAD, and it transfers energy into the chain.
2. The electrons flow along the chain and a proton gradient is built. Each carrier passes the electrons to the next, and at each transfer a little energy is released. The carriers use that energy to pump protons (H⁺) from the matrix across the inner membrane into the intermembrane space. The intermembrane space is narrow, so the proton concentration there rises well above that in the matrix. That difference is a proton gradient, a store of potential energy, like water held behind a dam.
3. Chemiosmosis makes ATP. Protons can only get back into the matrix through a channel in ATP synthase. As they flow down their concentration gradient through it, the enzyme uses the energy released to join ADP and phosphate into ATP. This coupling of proton flow to ATP synthesis is chemiosmosis, and it makes most of the ATP of aerobic respiration.
4. Oxygen is the terminal electron acceptor. At the end of the chain, the electrons must go somewhere, or the chain would fill up and stop. Oxygen accepts them, together with protons from the matrix, forming water (metabolic water). By removing electrons from the end of the chain, oxygen keeps electrons flowing along it.
No oxygen, no electron flow. Without oxygen the chain backs up, reduced NAD cannot give up its electrons, NAD is not regenerated, and the link reaction and Krebs cycle stop too. Only glycolysis, with anaerobic regeneration of NAD, can continue.
That is the answer to this subtopic's question. Hydrogen, stripped from the fuel, carries the energy; oxygen is where it finally ends up.
12HLLipids and carbohydrates as respiratory substrates
Figure 9 shows where each fuel enters.
Lipids yield more energy per gram: about 37 kJ g⁻¹, against about 17 kJ g⁻¹ for carbohydrates. The reason is composition. A fatty acid is a long chain of carbon and hydrogen with very little oxygen, while a carbohydrate such as glucose already contains a lot of oxygen (C₆H₁₂O₆). A fuel with more hydrogen and carbon to be oxidised, and less oxygen already in it, has more oxidation left to do, so more reduced NAD is made per gram and more ATP follows.
Only carbohydrate can be respired anaerobically. Fatty acids are broken into 2-carbon acetyl groups, which enter the pathway as acetyl CoA, after glycolysis. They never pass through glycolysis, and they can only be oxidised by the Krebs cycle and electron transport chain, which need oxygen. Glycolysis, and so anaerobic respiration, happens only if the substrate is carbohydrate. That is why a sprinter depends on glucose and glycogen, and why fat is a fuel for long, steady, aerobic exercise.
13Where marks are lost
"Respiration is breathing." Respiration is the chemical release of energy in cells; breathing is ventilation and gas exchange is diffusion. Never use one word for the other.
"Respiration produces energy." Energy cannot be produced. Respiration releases energy from carbon compounds and uses it to make ATP.
"ATP stores a lot of energy." A single ATP hydrolysis releases a small, useful amount. ATP is a currency for moving energy around, not a long-term store; that is what glycogen and fat are for.
Giving lactate as a product of yeast, or ethanol as a product of humans. Humans: lactate only, no carbon dioxide. Yeast: ethanol and carbon dioxide.
Forgetting the control in a respirometer, or what the alkali is for. The alkali absorbs carbon dioxide so the reading measures oxygen alone. The control corrects for temperature and pressure changes.
HL · "Oxygen is used in the Krebs cycle." Oxygen is used only at the end of the electron transport chain. The Krebs cycle stops without oxygen, but only because NAD is not regenerated.
HL · Protons pumped the wrong way. Protons are pumped from the matrix into the intermembrane space, and flow back into the matrix through ATP synthase. Getting either direction wrong loses the chemiosmosis marks.
HL · Counting "38 ATP" as fact. Current estimates are around 30 per glucose, and the syllabus asks only for relative yields. Write "much more than anaerobic respiration" or "about 30".
14Draw it right
- ATP: adenine, ribose and three phosphates in that order, labelled; the bond broken in hydrolysis is between the second and third phosphate.
- Respirometer: sealed tube, organisms on a gauze above an alkali, a capillary with a coloured drop and a scale, a water bath, and a note of the control. Say which way the drop moves: towards the organisms.
- Respirometer graph: time on the x-axis, distance or volume on the y-axis, both with units; the rate is the gradient.
- HL glycolysis: glucose (6C) → hexose bisphosphate (6C) → 2 triose phosphate (3C) → 2 pyruvate (3C), with 2 ATP in, 2 reduced NAD and 4 ATP out.
- HL Krebs cycle: acetyl (2C) + oxaloacetate (4C) → citrate (6C), two CO₂ released, reduced NAD shown at three points, one ATP, and back to oxaloacetate.
- HL mitochondrion: carriers in the inner membrane, protons pumped into the intermembrane space, protons returning through ATP synthase, ATP made in the matrix, and oxygen forming water on the matrix side.
15Try it
Marks in brackets. Answers and marker's notes are at the end.
Q1. Which is produced by anaerobic respiration in human muscle? 1 mark
A. Lactate and carbon dioxide
B. Lactate only
C. Ethanol and carbon dioxide
D. Carbon dioxide and water
Q2. Outline three properties of ATP that make it suitable as the energy currency of cells. 3 marks
Q3. Compare and contrast aerobic and anaerobic respiration in humans. 4 marks
Q4. A student placed 5.0 g of germinating seeds in a respirometer containing potassium hydroxide solution. The capillary tube had an internal diameter of 0.8 mm. In 12 minutes the drop moved 36 mm towards the seeds. (Invented data.)
(a) Calculate the volume of oxygen absorbed, in mm³. 2 marks
(b) Calculate the rate of oxygen uptake per gram of seeds per minute. 1 mark
(c) Explain why the drop moved towards the seeds. 2 marks
(d) State the purpose of a control tube containing glass beads. 1 mark
Q5 (HL). Explain how ATP is produced in the mitochondrion by chemiosmosis, including the role of oxygen. 7 marks
Q6 (HL). Explain why yeast in bread dough can continue to respire when no oxygen is available, and why the dough rises. 4 marks
16In one breath
ATP, adenosine triphosphate, is a nucleotide that releases a small, useful amount of energy when hydrolysed to ADP and phosphate, and needs energy to be rebuilt; it is soluble, stable, stays in the cell and is recycled constantly, and it pays for active transport, anabolism and movement. Respiration makes ATP from carbon compounds, mainly glucose and fatty acids; it is chemistry in every cell, not gas exchange. Aerobic respiration needs oxygen and mitochondria, uses many substrates, gives about 30 ATP per glucose and releases carbon dioxide and water; anaerobic respiration in humans uses only glucose, happens in the cytoplasm, gives 2 ATP and makes lactate. A respirometer measures oxygen uptake because alkali absorbs the carbon dioxide; convert distance to volume with πr², then divide by time and mass. HL: oxidation is loss of electrons, and substrates are oxidised by removing hydrogen, which NAD accepts. Glycolysis (phosphorylation, lysis, oxidation, ATP formation) nets 2 ATP and 2 reduced NAD; lactate or ethanol plus CO₂ is made only to regenerate NAD. The link reaction and Krebs cycle decarboxylate and oxidise, joining acetyl to oxaloacetate to make citrate; reduced NAD gives electrons to the chain, which pumps protons into the intermembrane space; they return through ATP synthase, making ATP; oxygen takes the electrons and protons at the end, making water. Lipids give more energy per gram but can only be respired aerobically.
Answers
Q1. B. Anaerobic respiration in humans makes lactate only. C is yeast; D is aerobic respiration; A mixes the two. B only.
Q2. Any three: ATP is soluble, so it moves freely to where energy is needed in the cell; it is stable and is only hydrolysed by enzymes, so energy is released where and when needed; hydrolysis of one ATP releases an amount of energy suited to a single task, so little is wasted; it cannot cross membranes freely, so it stays in the cell; it is rapidly regenerated from ADP and phosphate. 1 for each property with its benefit. A property with no reason scores 0 for that point.
Q3. Similarities: both begin with glycolysis in the cytoplasm; both use glucose as a substrate; both produce ATP. Differences: aerobic respiration requires oxygen, whereas anaerobic does not; aerobic finishes in mitochondria, whereas anaerobic takes place entirely in the cytoplasm; aerobic yields about 30 ATP per glucose, whereas anaerobic yields 2; aerobic produces carbon dioxide and water, whereas anaerobic produces lactate; aerobic can use lipids, whereas anaerobic uses only carbohydrate. at least one similarity and at least one difference needed for full marks; 1 per valid point, maximum 4. Differences must be stated as comparisons.
Q4. (a) Radius = 0.4 mm. Volume = π × 0.4² × 36 = 0.503 × 36 = 18.1 mm³. M1 for πr² × distance with the radius, not the diameter; A1 for 18.1 mm³. Using the diameter gives 72.4 mm³ and scores M0 A0. (b) 18.1 ÷ 12 ÷ 5.0 = 0.30 mm³ g⁻¹ min⁻¹. value and unit. (c) The seeds absorbed oxygen for aerobic respiration, and the carbon dioxide they released was absorbed by the potassium hydroxide. The volume of gas in the tube, and so its pressure, fell, and air pressure outside pushed the drop towards the seeds. 1 for oxygen used and carbon dioxide absorbed, 1 for the fall in volume or pressure. (d) To show any movement caused by changes in temperature or atmospheric pressure, not by respiration, so the readings can be corrected. [1.]
Q5 (HL). Model answer, one idea per mark point: Reduced NAD from glycolysis, the link reaction and the Krebs cycle donates a pair of electrons to the first carrier of the electron transport chain on the inner mitochondrial membrane, and is converted back to NAD. The electrons pass from carrier to carrier, releasing energy at each step. This energy is used to pump protons from the matrix into the intermembrane space. A high concentration of protons builds up in the small intermembrane space: a proton gradient. Protons diffuse back into the matrix through ATP synthase. The flow of protons provides the energy for ATP synthase to phosphorylate ADP, producing ATP; this is chemiosmosis. Oxygen is the terminal electron acceptor: it accepts electrons at the end of the chain together with protons, forming water, which keeps electrons flowing. 1 each for reduced NAD donating electrons; electrons passing along carriers releasing energy; protons pumped into the intermembrane space; a proton gradient; protons returning through ATP synthase; ADP phosphorylated to ATP by that flow (chemiosmosis); oxygen accepting electrons and protons to form water. Protons pumped "into the matrix" loses the pumping and gradient marks.
Q6 (HL). Glycolysis produces ATP but reduces NAD. Without oxygen, reduced NAD cannot be reoxidised by the electron transport chain. In yeast, pyruvate is converted to ethanol and carbon dioxide, and in this step reduced NAD passes its hydrogen on, regenerating NAD so glycolysis continues with a net gain of 2 ATP. The carbon dioxide forms bubbles in the dough, which make it rise. 1 for NAD being reduced in glycolysis, 1 for no oxygen to reoxidise it, 1 for conversion to ethanol and carbon dioxide regenerating NAD, 1 for carbon dioxide bubbles raising the dough. Lactate as the product in yeast scores 0 for the third mark.
Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section C1.2 Cell respiration. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.
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