AQA A-level Biology: Respiration (7402) – Revision Notes

These notes condense section 3.5.2 Respiration of the AQA A-level Biology (7402) specification, for first teaching 2015. It is A-level only content. For full explanations read the study guide, then test yourself with the practice questions. Tick off topics on the printable checklist, see the AQA A-level Biology hub, and use the free diagnostics to find weak spots. Background on ATP is in the nucleic acids and ATP notes.

Key definitions

Term	Meaning
Respiration	Series of reactions that release energy from organic molecules to make ATP
Glycolysis	Splitting of glucose to pyruvate in the cytoplasm; anaerobic
Phosphorylation	Adding a phosphate group to a molecule
Oxidation	Loss of electrons or hydrogen (or gain of oxygen)
Reduction	Gain of electrons or hydrogen
Reduced NAD	NAD that has accepted hydrogen; a coenzyme carrying hydrogen to the electron transfer chain
Substrate-level phosphorylation	ATP formed by transferring a phosphate group from a substrate to ADP
Oxidative phosphorylation	ATP formed using energy from the electron transfer chain and ATP synthase
Chemiosmotic theory	Proton movement down an electrochemical gradient through ATP synthase drives ATP synthesis
Acetyl CoA	Two-carbon acetate group carried by coenzyme A

Whole pathway at a glance

Stage	Where	Inputs	Outputs per glucose
Glycolysis	Cytoplasm	Glucose, 2 ATP, NAD	2 pyruvate, 2 net ATP, 2 reduced NAD
Link reaction	Matrix	2 pyruvate, NAD, CoA	2 acetyl CoA, 2 reduced NAD, 2 CO₂
Krebs cycle	Matrix	2 acetyl CoA	2 ATP, 2 reduced FAD, 6 reduced NAD, 4 CO₂
Oxidative phosphorylation	Inner membrane	Reduced coenzymes, O₂, ADP, Pi	ATP, water, NAD, FAD

Totals before oxidative phosphorylation: 10 reduced NAD, 2 reduced FAD, 4 ATP, 6 CO₂.

Glycolysis in steps

1. Glucose is phosphorylated to glucose phosphate, using ATP.
2. Glucose phosphate gives two three-carbon triose phosphate molecules.
3. Triose phosphate is oxidised to pyruvate. ATP is made and NAD becomes reduced NAD.
4. Net gain per glucose: 2 ATP, 2 reduced NAD, 2 pyruvate.

Reminder: ATP made = 4, ATP used = 2, net = 2. Pyruvate is a three-carbon molecule.

Anaerobic respiration

• If respiration is only anaerobic, pyruvate accepts hydrogen from reduced NAD.
• Lactate: pyruvate + reduced NAD → lactate + NAD. No CO₂.
• Ethanol: pyruvate → ethanal + CO₂; ethanal + reduced NAD → ethanol + NAD.
• Purpose: regenerate NAD so glycolysis can continue. No extra ATP is made by this step.

Link reaction and Krebs cycle

Feature	Link reaction	Krebs cycle
Starting molecule	Pyruvate (3C)	Acetyl CoA (2C) joins 4C molecule
Carbon change	3C → 2C acetate	6C → 4C
CO₂ lost	1 per pyruvate	2 per turn
Reduced NAD	1 per pyruvate	3 per turn
Reduced FAD	0	1 per turn
ATP	0	1 per turn by substrate-level phosphorylation
Coenzyme A	Combines with acetate	Released when acetyl CoA reacts

Both happen twice per glucose. Pyruvate enters the matrix by active transport.

Oxidative phosphorylation in steps

1. Electrons from reduced NAD and reduced FAD are handed to carriers in the inner mitochondrial membrane.
2. Electrons pass along the electron transfer chain by oxidation-reduction reactions, releasing energy.
3. Energy pumps protons from the matrix into the intermembrane space.
4. A proton gradient forms across the inner membrane.
5. Protons return to the matrix via ATP synthase, so ADP and Pi are joined to give ATP.
6. Oxygen accepts electrons and protons at the end of the chain, forming water.

Cristae increase the membrane area, so more chains and ATP synthase molecules fit in each mitochondrion.

Other substrates

Substrate	Breakdown product	Enters respiration as
Lipid	Fatty acids	Acetyl CoA (Krebs cycle)
Protein	Amino acids, after amino group removal	Krebs cycle intermediates or acetyl CoA

Lipids give more reduced coenzymes per gram than carbohydrate because they contain more hydrogen per gram.

Required practical 9 reminders

• Variable: a named factor such as temperature, pH or substrate; organism: yeast or another single-celled culture.
• Measure CO₂ volume in a fixed time (rate = volume ÷ time), or time for a redox indicator to decolourise (rate = 1 ÷ time).
• Keep constant: culture volume and concentration, substrate, temperature, pH, time.
• Boiled yeast is the negative control. Repeat at least three times and calculate a mean.
• Use a thermostatically controlled water bath, allow the tubes to reach temperature first, and use a stopwatch with the same start point each time.
• Safety: wash hands after handling cultures, wipe spills, and keep any ethanol away from naked flames.

Method in steps: processing practical data

1. Identify and exclude any anomaly, and say why it is anomalous.
2. Calculate the mean of the remaining repeats.
3. Convert to a rate: volume ÷ time, or 1 ÷ time.
4. Describe the trend with figures, then explain it using enzyme or ATP ideas.
5. Evaluate: say what limits confidence and give an improvement.

Small worked reminders

Pyruvate counts. A cell passes 25 pyruvate through the link reaction and Krebs cycle. CO₂ = 25 × 3 = 75. Reduced NAD = 25 × 4 = 100. Reduced FAD = 25. ATP = 25.

ATP total with supplied data. If a reduced NAD gives 2.5 ATP and a reduced FAD gives 1.5 ATP, one glucose gives 10 × 2.5 + 2 × 1.5 + 4 = 32 ATP.

Percentage change. CO₂ rises from 5.4 to 6.3 cm³ per 5 minutes: (6.3 − 5.4) ÷ 5.4 × 100 = 16.7%.

Redox indicator rate. A dye in yeast at 20 degrees C decolourises in 410 s and at 35 degrees C in 150 s. Rate = 1000 ÷ t, so 2.4 and 6.7 per 1000 s. The rate is 410 ÷ 150 = 2.7 times as fast at 35 degrees C.

Fatty acid. A 14-carbon fatty acid gives 14 ÷ 2 = 7 acetyl CoA. In the Krebs cycle these give 7 × 3 = 21 reduced NAD, 7 reduced FAD, 7 ATP and 14 CO₂.

Aerobic against anaerobic. With 32 ATP aerobic and 2 ATP anaerobic per glucose, a cell needs 16 times as much glucose to make the same ATP without oxygen.

Must-know distinctions

• Substrate-level vs oxidative phosphorylation. Substrate-level transfers a phosphate directly in glycolysis or the Krebs cycle. Oxidative uses the electron transfer chain and ATP synthase.
• Link reaction vs Krebs cycle. The link reaction makes acetyl CoA from pyruvate. The Krebs cycle oxidises the acetyl group.
• Lactate vs ethanol pathway. Ethanol production releases CO₂. Lactate production does not.
• Oxidation vs reduction of NAD. NAD is reduced when it accepts hydrogen, and oxidised when it gives it away.
• Respiration vs gas exchange. Respiration is a set of cell reactions. Gas exchange is the movement of oxygen and CO₂ between the organism and its environment.

Command words in this topic

• State: give a short fact, such as the location or number of molecules.
• Describe: say what happens in order, using the stage names.
• Explain: give a reason that links cause and effect, such as why NAD must be regenerated.
• Calculate: show the working and give units for rates.
• Suggest: apply respiration ideas to an unfamiliar situation, for example a poison or a mutation affecting ATP synthase.

Quick self-test

1. State where glycolysis happens and whether oxygen is needed.
2. How many ATP are used and made in glycolysis of one glucose?
3. Why does anaerobic respiration need pyruvate to accept hydrogen?
4. How does pyruvate enter the mitochondrial matrix?
5. Name the two products of the link reaction other than acetyl CoA.
6. How many CO₂ are lost per turn of the Krebs cycle?
7. Calculate the ATP from 6 reduced NAD and 2 reduced FAD at 2.5 and 1.5 ATP each.
8. What is the role of oxygen at the electron transfer chain?
9. Where do fatty acids enter respiration?
10. A yeast culture releases 6.3 cm³ of CO₂ in 7 minutes. Calculate the rate in cm³ per min.
11. Why is boiled yeast used as a control?

Answers

1. Cytoplasm; no.
2. 2 used and 4 made, a net of 2.
3. To oxidise reduced NAD back to NAD so glycolysis can continue.
4. Active transport.
5. Reduced NAD and CO₂.
7. (6 × 2.5) + (2 × 1.5) = 15 + 3 = 18 ATP.
8. Final electron acceptor; it forms water with electrons and protons.
9. As acetyl CoA, into the Krebs cycle.
10. 6.3 ÷ 7 = 0.90 cm³ per min.
11. It shows that CO₂ release depends on living cells, not on the solution or equipment.

Where marks are usually lost

• Quoting a glycolysis yield of 2 ATP and not showing the 4 made less 2 used.
• Giving two carbons for pyruvate.
• Writing “the Krebs cycle makes most ATP”. Most is made at the inner membrane by ATP synthase.
• Saying protons are pumped by ATP synthase. The electron transfer chain pumps them, and ATP synthase lets them flow back.
• Failing to say that NAD regeneration is the purpose of ethanol or lactate formation.
• Writing “energy is made” rather than released or transferred.
• Leaving out the oxidation (hydrogen removal) in the link reaction.
• Describing a practical with no control, no repeats or no fixed time.
• Not using figures from the data when asked to describe a trend.

Official syllabus

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