AQA A-level Biology: Respiration (7402)

This guide covers section 3.5.2 Respiration of the AQA A-level Biology (7402) specification, for first teaching 2015. It is A-level only content, so it is not part of the AS course. It also covers required practical 9. The guide builds on nucleic acids and ATP, enzymes and cell structure. Use the AQA A-level Biology hub and the printable checklist to track progress. The revision notes and practice questions pair with this page, and the free diagnostics show where to start.

What the specification asks for

Statement in 3.5.2	What you must be able to do
Respiration produces ATP	Link ATP to energy-requiring processes
Glycolysis	Give its location and its three stages
Anaerobic respiration	Explain ethanol or lactate formation and NAD regeneration
Pyruvate uptake	State that it enters the matrix by active transport
Link reaction	Show oxidation to acetate and formation of acetyl CoA
Krebs cycle	Describe the cycle, with reduced coenzymes, ATP and CO₂
Oxidative phosphorylation	Explain the electron transfer chain and chemiosmotic theory
Other substrates	Say where lipid and amino acid breakdown products enter
Required practical 9	Plan, run and evaluate a rate investigation with single-celled organisms

Where each stage happens

Stage	Location	Oxygen needed?
Glycolysis	Cytoplasm	No
Link reaction	Mitochondrial matrix	Indirectly
Krebs cycle	Mitochondrial matrix	Indirectly
Oxidative phosphorylation	Inner mitochondrial membrane	Yes, as final electron acceptor

The link reaction and Krebs cycle do not use oxygen directly. They stop when oxygen is absent because reduced NAD and reduced FAD cannot be reoxidised at the electron transfer chain, so the supply of NAD and FAD runs out.

Glycolysis

Glycolysis comes first in aerobic respiration and in anaerobic respiration. It occurs in the cytoplasm and is an anaerobic process. It has three stages:

1. Phosphorylation of glucose to glucose phosphate, using ATP. Phosphorylation makes the sugar more reactive and also traps it inside the cell. In the usual account two ATP are used per glucose.
2. Production of triose phosphate. The phosphorylated six-carbon sugar is split into two three-carbon molecules of triose phosphate.
3. Oxidation of triose phosphate to pyruvate. Hydrogen is removed (oxidation) and passed to NAD, forming reduced NAD. ATP is formed directly by substrate-level phosphorylation, with a net gain of ATP overall.

Each glucose gives two triose phosphate, so stage three happens twice. Four ATP are made and two were used, a net gain of 2 ATP, plus 2 reduced NAD and 2 pyruvate.

Worked example: glycolysis arithmetic

A culture oxidises 30 glucose molecules by glycolysis. Find ATP used, ATP made, net ATP, reduced NAD and pyruvate.

• ATP used: 30 × 2 = 60
• ATP made: 30 × 4 = 120
• Net ATP: 120 − 60 = 60
• Reduced NAD: 30 × 2 = 60
• Pyruvate: 30 × 2 = 60

Anaerobic respiration and NAD regeneration

Glycolysis needs a constant supply of NAD. If respiration is only anaerobic, there is no electron transfer chain to reoxidise reduced NAD, so pyruvate takes over that job. Pyruvate accepts hydrogen from reduced NAD and is converted to ethanol or lactate. The oxidised NAD produced can be used in further glycolysis.

	Lactate pathway	Ethanol pathway
Found in	Animal cells, some bacteria	Yeast, plant cells
Steps	Pyruvate + reduced NAD → lactate + NAD	Pyruvate → ethanal + CO₂, then ethanal + reduced NAD → ethanol + NAD
CO₂ released?	No	Yes
Reversible?	Lactate can be converted back to pyruvate	No, ethanol is not converted back

Converting pyruvate to lactate or ethanol makes no ATP. The only ATP comes from glycolysis, so the net yield is 2 ATP per glucose. The pathway matters because it keeps glycolysis going, not because it makes ATP.

Entry to the mitochondrion and the link reaction

If respiration is aerobic, pyruvate enters the mitochondrial matrix by active transport, which uses a carrier protein and energy. In the matrix, the link reaction does the following:

1. Pyruvate (3C) is oxidised to acetate (2C). Hydrogen is passed to NAD, producing reduced NAD, and CO₂ is lost.
2. Acetate combines with coenzyme A to form acetylcoenzyme A (acetyl CoA).

No ATP is made directly. Per glucose, the link reaction happens twice, giving 2 reduced NAD and 2 CO₂.

The Krebs cycle

Acetyl CoA (2C) reacts with a four-carbon molecule, releasing coenzyme A and forming a six-carbon molecule that enters the cycle. Coenzyme A is then free to carry another acetate group. In a series of oxidation-reduction reactions the six-carbon molecule is converted back to the four-carbon molecule, so the cycle can turn again. The cycle:

• loses carbon dioxide (decarboxylation)
• removes hydrogen (dehydrogenation) to make reduced coenzymes
• produces ATP by substrate-level phosphorylation, where a phosphate group is transferred from an intermediate straight to ADP.

The usual figures per turn (one acetyl CoA) are 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP.

Worked example: tally for one glucose

Stage	Reduced NAD	Reduced FAD	ATP (substrate-level)	CO₂
Glycolysis	2	0	2 net	0
Link reaction (×2)	2	0	0	2
Krebs cycle (×2)	6	2	2	4
Total	10	2	4	6

Six CO₂ equals the six carbons in glucose, which confirms that all the carbon has been released.

Worked example: pyruvate counts

A cell has 40 pyruvate molecules enter the matrix and passes all of them through the link reaction and the Krebs cycle.

• Link reaction: 40 CO₂ and 40 reduced NAD, giving 40 acetyl CoA.
• Krebs cycle turns: 40, giving 40 × 2 = 80 CO₂, 40 × 3 = 120 reduced NAD, 40 reduced FAD and 40 ATP.
• Totals: 40 + 80 = 120 CO₂; 40 + 120 = 160 reduced NAD; 40 reduced FAD; 40 ATP.

Oxidative phosphorylation and chemiosmosis

Most ATP is made at the inner mitochondrial membrane, which is folded into cristae to give a large surface area. Reduced NAD and reduced FAD deliver electrons to the electron transfer chain, a series of carrier proteins in the membrane. The sequence is:

1. Reduced coenzymes are oxidised and release electrons (and protons).
2. Electrons pass from carrier to carrier in a series of oxidation-reduction reactions, releasing energy at each step.
3. The energy moves protons (H⁺) out of the matrix, across the inner membrane, and into the gap between the two mitochondrial membranes.
4. A proton concentration gradient forms. The intermembrane space has a higher proton concentration than the matrix.
5. Protons return to the matrix by facilitated diffusion through channels in ATP synthase. The flow powers ATP formation from ADP and inorganic phosphate.
6. Oxygen sits at the end of the chain and takes up the spent electrons together with protons, producing water.

This is the chemiosmotic theory (the specification spells it “chemiosomotic”). Oxygen is needed only to keep the chain moving. Without it, electrons back up, reduced NAD cannot be oxidised, and the whole pathway slows.

Worked example: ATP yield with supplied data

The ATP made per reduced coenzyme varies between textbooks, so use any figures given in the question. Suppose each reduced NAD gives 2.5 ATP and each reduced FAD gives 1.5 ATP at the chain.

• Reduced NAD: 10 × 2.5 = 25 ATP
• Reduced FAD: 2 × 1.5 = 3 ATP
• Oxidative phosphorylation total: 28 ATP
• Add 4 substrate-level ATP: 32 ATP per glucose

With figures of 3 and 2 instead, oxidative phosphorylation gives 34 ATP and the total is 38. Anaerobic respiration gives only 2 ATP, so on the 2.5 and 1.5 figures aerobic respiration yields 32 ÷ 2 = 16 times as much ATP.

Other respiratory substrates

Glucose is not the only fuel. The specification states that breakdown products of lipids and amino acids enter the Krebs cycle.

• Lipids are hydrolysed to glycerol and fatty acids. Fatty acids are broken into two-carbon units that become acetyl CoA, which enters the Krebs cycle. Lipids contain more hydrogen per gram than carbohydrate, so they give more reduced coenzymes per gram, and more ATP.
• Amino acids have their amino group removed. The remaining carbon skeleton enters respiration as a Krebs cycle intermediate or as a molecule that becomes acetyl CoA.

Worked example: a fatty acid

A saturated fatty acid has 16 carbon atoms and is fully broken into two-carbon acetyl units.

• Acetyl CoA formed: 16 ÷ 2 = 8
• In the Krebs cycle: 8 × 3 = 24 reduced NAD, 8 reduced FAD, 8 ATP and 8 × 2 = 16 CO₂

Extra reduced coenzymes are formed as the fatty acid is broken down, which is why this tally is only a minimum.

Required practical 9: rate of respiration in single-celled organisms

The practical investigates the effect of a named variable (such as temperature, pH or substrate type) on the rate of respiration of yeast or another single-celled organism. The specification links it to AT b and i. Two measures are common:

• Volume of CO₂ released over a fixed time, collected in a gas syringe or over water, or oxygen uptake in a respirometer with CO₂ absorbed.
• Redox indicator: a dye that is decolourised as it accepts hydrogen released by dehydrogenase enzymes. Rate is taken as 1 ÷ time to decolourise.

Control variables: volume and concentration of culture, substrate concentration, temperature (use a water bath), pH, and time. Controls: a boiled-yeast tube shows the change needs living cells. Repeats: at least three at each value, with a mean calculated and anomalies identified.

Worked example: pH and CO₂ volume

Volume of CO₂ (cm³) collected in 5 minutes by a yeast culture in buffers.

pH	Repeat 1	Repeat 2	Repeat 3	Mean	Rate (cm³ per min)
4.0	2.1	2.4	2.0	2.17	0.43
5.0	5.2	5.6	5.3	5.37	1.07
6.0	8.9	9.3	9.1	9.10	1.82
7.0	6.4	6.0	6.5	6.30	1.26
8.0	2.8	3.1	2.7	2.87	0.57

• Mean at pH 6.0: (8.9 + 9.3 + 9.1) ÷ 3 = 9.10; rate = 9.10 ÷ 5 = 1.82 cm³ per min.
• Percentage fall from pH 6.0 to 7.0: (9.10 − 6.30) ÷ 9.10 × 100 = 30.8%.
• Interpretation: the rate peaks near pH 6.0. Away from this, hydrogen ion concentration changes the ionic bonds in enzyme tertiary structure, the active site changes shape, fewer enzyme-substrate complexes form, and the enzymes of glycolysis and fermentation denature.

Common errors

• Writing that glycolysis “makes 2 ATP” without distinguishing 4 made and 2 used.
• Saying the Krebs cycle needs oxygen directly, or that oxygen “makes ATP”.
• Saying NAD is regenerated “to make ATP” in fermentation rather than to keep glycolysis going.
• Saying protons move through the membrane by active transport at ATP synthase. They flow down their gradient.
• Giving the pyruvate carbon count as two. Pyruvate has three, acetate two.
• Forgetting that the cycle and link reaction happen twice per glucose.
• Describing a rate investigation without a control, a time limit or repeats.

Official syllabus

AQA AS and A-level Biology (7401/7402) specification, for first teaching 2015, published by AQA. This guide follows section 3.5.2 and required practical 9. Check the current document on the AQA website for the latest version.