Aerobic respiration: the overall reaction and a stepped, enzyme-controlled process
This unit is A Level (IA2) content, and it assumes everything from the AS units, especially ATP and glucose from the 1a notes and enzymes and membranes from the 2a notes.
All of it is examined on the Unit 5 paper, WBI15, and its two core practicals (CP15 and CP16) are also examined as methods on the Unit 6 practical paper, WBI16.
This first section gives you the big picture that every later section fills in.
The idea: a slow, controlled "burn"
Every living cell needs a steady supply of ATP: to contract a muscle fibre, to pump sodium ions out of a neurone, to build proteins on ribosomes, to copy DNA before mitosis.
ATP cannot be stored in large amounts, so cells make it continuously from the energy stored in organic molecules such as glucose.
Respiration is the process that does this.
If you burn glucose in a flame, it reacts with oxygen to give carbon dioxide and water, and all its energy comes out at once as heat and light.
Respiration ends with the same products and releases the same total energy, but it gets there in dozens of small steps inside the cell.
Because the energy comes out a little at a time, a large part of it can be captured in ATP instead of being lost as a flash of heat.
Learn the overall reaction in two halves, and it is worth seeing both in the equation:
- Splitting the respiratory substrate. The glucose molecule is taken apart. Its carbon atoms leave, step by step, as carbon dioxide, which is a waste product that diffuses out of the cell.
- Reuniting hydrogen with atmospheric oxygen. Hydrogen atoms are stripped off the substrate and carried away by coenzymes. At the very end they are recombined with oxygen from the air to form water. This last step releases large amounts of energy, and it is where most of the ATP is made.
So the oxygen you breathe in is not used to make carbon dioxide: the oxygen atoms in the carbon dioxide you breathe out came from the glucose and from water.
Your inhaled oxygen ends up in water.
That surprises most students, and it is the key to understanding the rest of the unit.
The diagram below is the map for the whole unit.
Each box is a later section.
Stage
Where it happens
What goes in
What comes out (per glucose)
Glycolysis
cytoplasm
glucose, 2 ATP, NAD
2 pyruvate, 4 ATP (net 2), 2 reduced NAD
Link reaction
mitochondrial matrix
2 pyruvate, NAD, coenzyme A
2 acetyl coenzyme A, 2 CO₂, 2 reduced NAD
Krebs cycle
mitochondrial matrix
2 acetyl coenzyme A, NAD, FAD
4 CO₂, 2 ATP, 6 reduced NAD, 2 reduced FAD
Oxidative phosphorylation
inner mitochondrial membrane
reduced NAD, reduced FAD, O₂
NAD, FAD, H₂O, most of the ATP
Notice the pattern: the first three stages split the substrate and collect hydrogen; the last stage reunites that hydrogen with oxygen.
Carbon dioxide comes only from the link reaction and the Krebs cycle.
A stepped process, each step with its own enzyme
The second point is that respiration is a stepped process, with each step controlled and catalysed by a specific intracellular enzyme.
Three ideas sit inside that phrase.
1. Specific. Each step changes one molecule into the next, and only an enzyme whose active site is complementary to that molecule can catalyse it (the lock-and-key and induced-fit ideas from the 2a notes). So a pathway of twenty steps needs twenty different enzymes. If one is missing or inhibited, its substrate builds up and everything after it stops.
2. Intracellular. These enzymes work inside the cell: the glycolysis enzymes are dissolved in the cytoplasm, the link reaction and Krebs cycle enzymes are in the mitochondrial matrix, and the electron carriers and ATP synthase are built into the inner mitochondrial membrane. Where an enzyme is decides where its step happens.
3. Controlled. Because every step depends on an enzyme, the cell can speed respiration up or slow it down by changing enzyme activity. When ATP is plentiful, ATP itself inhibits an enzyme near the start of glycolysis; when ATP is used up, the inhibition is lifted. (Going further, not examined: this is end-product inhibition, a form of negative feedback.)
Why so many steps?
One large step would release far more energy than one ATP can hold, and the rest would be lost as heat; small steps release it in packets matched to making ATP.
Steps also give intermediates the cell can use to build amino acids and lipids.
Because each step is enzyme-catalysed, everything you learned about enzymes in the 2a notes applies to respiration as a whole.
Rate rises with temperature up to an optimum (more kinetic energy, more enzyme-substrate complexes) and falls above it as enzymes denature.
That is exactly what core practical 15 measures in yeast.
You do not need the names of any of the enzymes.
- Each marking point is worth one mark, shown as (1) in these notes. A part worth (3) needs three separate, correct points, usually from a longer list ("any three from").
- A "/" in a scheme separates alternative wordings for one point. ACCEPT lines widen a point, IGNORE lines mean the words neither gain nor lose, and REJECT lines name wording that loses the point ("produces energy", "ATPase").
- In a list answer only the first answers asked for are marked, and a wrong answer can cancel a right one. Give exactly the number asked for, one per line. Naming lists can be banded (for example three correct for 2 marks, two correct for 1).
- In a calculation a correct answer alone gains full marks, but working earns the method mark when the answer is wrong. A stated format (two significant figures, standard form, a whole number) is worth the answer mark, and counts of cells or people must be whole numbers even when the stem does not say so. Error carried forward (ecf) is decided part by part, and only where working is shown.
- The one asterisked 6-mark item on WBI15 is marked in three levels on the quality and linkage of your answer, not by counting points.
Learn these word for word.
Learn the first two word for word.
- The overall reaction of aerobic respiration: splitting of the respiratory substrate to release carbon dioxide as a waste product and reuniting hydrogen with atmospheric oxygen with the release of large amounts of energy.
- Respiration as a stepped process: respiration is a stepped process, with each step controlled and catalysed by a specific intracellular enzyme.
- Respiratory substrate: an organic molecule broken down in respiration to release energy, such as glucose, fatty acids or amino acids.
- ATP: the immediate source of energy in cells; its hydrolysis to ADP and inorganic phosphate releases energy.
1a notes: ATP (adenine, ribose, three phosphates) and glucose, a hexose.
2a notes: enzyme specificity, active sites, enzyme-substrate complexes, denaturation, and membranes that keep each stage in its own compartment.
A student writes that cells would be more efficient if glucose reacted with oxygen in one step.
Explain why respiration is a stepped process.
(3)
Full-mark answer:
- energy is released in small amounts at each step (1)
- so more of it can be used to make ATP / less is lost as heat (1)
- each step is catalysed by a specific enzyme, so the rate of each step can be controlled to match the cell's demand for ATP (1)
Also creditworthy: intermediates can be used to make other molecules; enzymes allow the reactions at body temperature.
A weaker answer: "Respiration has steps because it needs enzymes and enzymes produce energy."
This scores 0.
It restates the stem, gives no reason why steps help, and "produce energy" is rejected wording.
Describe what happens to the carbon atoms and the hydrogen atoms of a glucose molecule during aerobic respiration.
(4)
Full-mark answer:
- the carbon atoms are removed as carbon dioxide (1)
- in the link reaction and the Krebs cycle (by decarboxylation) (1)
- the hydrogen atoms are removed by dehydrogenation and picked up by NAD and FAD (1)
- the hydrogen is finally combined with oxygen to form water, at the end of the electron transport chain (1)
A weaker answer: "Glucose reacts with oxygen to make carbon dioxide and water."
This scores 0 or 1 at most.
It is the overall equation, not a description of what happens to each kind of atom, and it suggests the oxygen ends up in the carbon dioxide.
Respiring one mole of glucose releases 2870 kJ.
Hydrolysing one mole of ATP releases 30.5 kJ.
A liver cell makes 32 moles of ATP for each mole of glucose respired.
Calculate the percentage of the energy in glucose that is transferred to ATP.
Give your answer to two significant figures.
(2)
1. Energy held in ATP: kJ (1)
2. Percentage: , so 34 % (1)
Writing 34.0 % or 34.006 % ignores the instruction and earns only 1 of the 2 marks.
What the number means: about a third of the energy ends up in ATP.
The other two thirds is released as heat.
That heat is not wasted in a mammal or a bird: it is what keeps the body at about 37 °C.
When a question asks what happens to "the substrate", answer in the two halves: carbon leaves as carbon dioxide, hydrogen is carried to oxygen and forms water, with most of the energy released at that last step.
Q8 on WBI15 is built on a pre-released article: where the article mentions a poison, a disease or an organism that changes energy supply, link it to a named stage here (glycolysis, link reaction, Krebs cycle or the electron transport chain) and follow the effect through to ATP.
- "Respiration produces energy." It is tempting because we talk loosely about "getting energy from food", but energy is never produced. Write "releases energy" and "makes ATP".
- "The oxygen we breathe in is turned into carbon dioxide." The carbon dioxide comes from the substrate (its carbon, and oxygen already in the substrate and in water). Inhaled oxygen accepts hydrogen and becomes water.
- Confusing respiration with breathing. Breathing (ventilation) moves air in and out of the lungs. Respiration is a set of chemical reactions in every living cell, including plant cells, day and night.
- Naming enzymes you do not need. Enzyme names are not required. Credit comes from saying each step is catalysed by a specific enzyme, and from the enzyme ideas (active site, complexes, denaturation) applied to the step.
Check your understanding
1. Which describes what happens to the hydrogen removed from the respiratory substrate in aerobic respiration? (1)
- A it is released from the cell as hydrogen gas
- B it combines with carbon to form carbon dioxide
- C it is carried by coenzymes and combined with oxygen to form water
- D it is used to phosphorylate ADP directly
Total [1]
2. Which row shows where each stage of aerobic respiration takes place? (1)
Glycolysis
Krebs cycle
Electron transport chain
A
matrix
cytoplasm
inner membrane
B
cytoplasm
inner membrane
matrix
C
cytoplasm
matrix
outer membrane
D
cytoplasm
matrix
inner membrane
Total [1]
3. A student says: "Burning glucose and respiring glucose are the same reaction, because both release carbon dioxide, water and the same total amount of energy." Explain two ways in which respiration differs from burning. (2)
Total [2]
4. Mitochondria were isolated from liver cells and placed in a solution containing glucose and oxygen. They took up no oxygen. When pyruvate was added in place of glucose, they took up oxygen rapidly. Explain these results. (3)
Total [3]
5. A sleeping adult takes up about 15 dm³ of oxygen each hour. Assume all of it is used to respire glucose, that 1 mole of glucose needs 6 moles of oxygen and that 1 mole of oxygen occupies 24 dm³. The relative molecular mass of glucose is 180.
(a) Calculate the mass of glucose respired in one hour. Give your answer to two significant figures. (2)
(b) Part of the energy released by this respiration is not transferred to ATP. Explain why this is useful to the adult. (2)
(c) The adult's breath contains less carbon dioxide than this calculation predicts. Give one reason why. (1)
Total [5]
6. A rare inherited condition means that one enzyme of the Krebs cycle has a changed active site and does not work. Explain why this reduces the production of ATP by the cell, even though every other enzyme of respiration works normally. (3)
Total [3]
Answers
1. C (1). A: hydrogen is never released as a gas in respiration. B: carbon dioxide comes from decarboxylation of the substrate; hydrogen is not part of it. D: ADP is phosphorylated using energy released by the electron carriers, not by hydrogen directly.
2. D (1). A puts glycolysis in the matrix; it is cytoplasmic. B swaps the Krebs cycle and the chain. C puts the chain on the outer membrane; the carriers are on the inner membrane (cristae).
3. Any two from (1 each):
- respiration releases energy in small steps / burning releases it all at once
- respiration transfers part of the energy to ATP / burning releases it only as heat and light
- each step of respiration is catalysed by a specific enzyme / burning needs no enzymes and a high temperature
- respiration happens at body temperature inside cells
4. Any three from (1 each):
- glycolysis does not take place in mitochondria / its enzymes are in the cytoplasm
- so glucose cannot be broken down to pyruvate by isolated mitochondria
- so no reduced NAD or reduced FAD is made, and the electron transport chain does not run, so no oxygen is used
- pyruvate enters the link reaction and Krebs cycle in the matrix
- producing reduced NAD and reduced FAD, whose hydrogen is passed to oxygen, so oxygen is taken up
5. (a) moles of oxygen ; moles of glucose (1); mass , so 19 g (1). An answer of 18.75 g or 18.8 g gains 1 mark.
(b) Any two from (1 each): the energy not captured is released as heat; this keeps body temperature at about 37 °C; so enzymes work close to their optimum / metabolic reactions stay fast.
(c) The adult also respires fat / not all of the substrate is glucose (1). Fat respiration uses more oxygen for each carbon dioxide made; see the respiratory quotient section.
6. Any three from (1 each):
- the substrate no longer fits / binds the active site, so no enzyme-substrate complexes form
- that step of the Krebs cycle stops, and the substrate of that enzyme builds up
- each enzyme is specific, so no other enzyme can catalyse the step
- the cycle cannot continue / the acceptor is not regenerated
- so fewer reduced NAD and reduced FAD are made and less ATP is made by oxidative phosphorylation (only glycolysis continues)