OxfordAQA International AS and A-level Biology: Respiration, Krebs cycle and RQ (9610) – Revision Notes

These revision notes condense section 3.3.3, Respiration, of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1 (International AS exams May/June 2017 onwards, International A-level exams May/June 2018 onwards). They cover 3.3.3.1 to 3.3.3.4 and required practical 8. The specification lists Unit 3, Populations and genes, as International A-level only.

Full explanations are in the study guide. To check your recall, try the practice questions. The course hub is OxfordAQA A-level Biology, the checklist lists every statement, and the free diagnostics find weak spots quickly.

Key terms

Term	Meaning
Glycolysis	Breakdown of glucose to pyruvate in the cytoplasm; needs no oxygen
Phosphorylation	Adding phosphate, here using ATP
Substrate-level phosphorylation	ATP made by transferring phosphate from an intermediate to ADP
Reduced NAD / reduced FAD	Coenzymes carrying hydrogen (electrons and protons)
Link reaction	Pyruvate to acetyl CoA, in the matrix
Krebs cycle	Cycle of oxidation-reduction reactions in the matrix
Oxidative phosphorylation	ATP made using the electron transfer chain and ATP synthase
ATP synthase	Enzyme in the inner mitochondrial membrane that makes ATP as protons flow through
Respiratory quotient (RQ)	Carbon dioxide produced ÷ oxygen consumed

Glycolysis (3.3.3.1)

Method in steps:

1. Glucose + ATP gives glucose phosphate; a second ATP gives a six-carbon bisphosphate.
2. This splits into 2 triose phosphate.
3. Triose phosphate is oxidised to pyruvate. NAD is reduced.
4. ATP is made by substrate-level phosphorylation.

Per glucose: 2 ATP used, 4 made, net 2 ATP, 2 reduced NAD, 2 pyruvate.

Anaerobic respiration (3.3.3.2)

Pathway	Reaction	Where
Lactate	Pyruvate + reduced NAD gives lactate + NAD	Animal cells such as muscle
Ethanol	Pyruvate gives ethanal + CO2; ethanal + reduced NAD gives ethanol + NAD	Yeast, some plant cells

The point of both: regenerate NAD so glycolysis continues. Only the 2 net ATP from glycolysis are made. Pyruvate is not oxidised any further.

Aerobic respiration (3.3.3.3)

Pyruvate enters the matrix by active transport.

Stage	Site	Inputs	Outputs per glucose
Glycolysis	Cytoplasm	Glucose, ATP, NAD	2 pyruvate, 2 reduced NAD, net gain of 2 ATP
Link reaction	Matrix	Pyruvate (2), NAD, coenzyme A	Acetyl CoA (2), CO2 (2), reduced NAD (2)
Krebs cycle	Matrix	2 acetyl CoA, 4C molecule	4 CO2, 6 reduced NAD, 2 reduced FAD, 2 ATP
Oxidative phosphorylation	Inner membrane	Reduced coenzymes, O2, ADP + Pi	Many ATP, water

Key points in the Krebs cycle: acetyl CoA (2C) + four-carbon molecule gives a six-carbon molecule and releases coenzyme A. CO2 is lost and hydrogen is removed in a series of oxidation-reduction reactions, which regenerates the four-carbon molecule.

Oxidative phosphorylation, step by step

1. Electrons arrive at the chain from reduced NAD and reduced FAD.
2. Electrons pass between carriers; the energy released moves protons out of the matrix into the space between the membranes.
3. A proton gradient forms across the inner membrane.
4. Protons flow back through ATP synthase, which makes ATP from ADP and phosphate.
5. Oxygen accepts electrons and protons to form water.

If oxygen is missing, the chain stops, the coenzymes stay reduced, and the link reaction and Krebs cycle stop too.

Respiratory substrates and RQ (3.3.3.4)

• Lipids: glycerol enters at triose phosphate; fatty acids become two-carbon units that form acetyl CoA.
• Amino acids: deaminated; the carbon skeleton enters respiration (pyruvate, acetyl CoA or a Krebs cycle intermediate).
• A gram of lipid gives more reduced coenzymes than a gram of carbohydrate, since lipid molecules carry more hydrogen.

Substrate	RQ	Reason
Lipid	near 0.7	Little oxygen in the molecule, so a lot of oxygen is needed
Protein	near 0.9	Between the other two
Carbohydrate	near 1.0	Moles of O2 used equal moles of CO2 released

RQ has no units. Use the same units for both gases, in quantities proportional to the number of molecules (moles or volumes).

RQ from an equation. Octanoic acid: C8H16O2 + 11 O2 → 8 CO2 + 8 H2O. RQ = 8 ÷ 11 = 0.73.

RQ from a respirometer. With potassium hydroxide the drop shows oxygen used. Without it the drop shows oxygen used minus carbon dioxide released. RQ = (O2 used - net change) ÷ O2 used.

Caveats to quote: a mixture of substrates is respired; other processes may use or release the gases; anaerobic respiration can release CO2 without O2 use; photosynthesis affects plants in light; temperature, leaks and gas solubility affect readings. Say conclusions are tentative.

Must-know distinctions

Compare	Point to state
NAD and reduced NAD	NAD is the oxidised form that accepts hydrogen; reduced NAD carries it to the chain or to pyruvate
Substrate-level and oxidative phosphorylation	Substrate-level transfers a phosphate directly from an intermediate (glycolysis, Krebs cycle); oxidative uses the proton gradient and ATP synthase
Link reaction and Krebs cycle	Link: 3C to 2C, one CO2 and one reduced NAD per pyruvate. Krebs: 2C joins 4C, two CO2 per turn
Lactate and ethanol pathways	Lactate gives no CO2; ethanol pathway releases CO2
Aerobic and anaerobic yield	Anaerobic respiration stops at the net 2 ATP of glycolysis; aerobic respiration continues into the mitochondrion

Worked reminders

Counting from supplied yields. Question data: reduced NAD is worth 2.5 ATP, reduced FAD 1.5 ATP. For 3 mol of acetyl CoA entering the Krebs cycle: reduced NAD = 9 mol, reduced FAD = 3 mol, ATP made directly = 3 mol. ATP total = (9 × 2.5) + (3 × 1.5) + 3 = 22.5 + 4.5 + 3 = 30 mol.

Rate from a graph or table. Yeast produces 18 cm³ of CO2 in 12 minutes. Rate = 18 ÷ 12 = 1.5 cm³ per min. Always give the unit.

Percentage change. A rate falls from 0.90 to 0.54 cm³ per min once the temperature passes the optimum. Change = (0.54 - 0.90) ÷ 0.90 × 100 = -40%.

Reading an RQ. An RQ of 0.8 lies between the typical lipid (0.7) and carbohydrate (1.0) values, so a mixture of substrates is likely. Protein alone, which is about 0.9, cannot be ruled out. So the conclusion stays tentative.

Required practical 8

Investigate a specific variable such as substrate or temperature on the rate of respiration of an organism such as yeast or a locust.

• Dependent variable: gas volume or colour-change time. Rate = volume ÷ time.
• Control variables: mass or volume of organism, substrate concentration, pH, time, temperature (water bath).
• Set up a boiled-yeast control. Do three or more repeats per condition and find the mean.
• Above the optimum, enzymes denature and the rate falls.

Quick self-test

1. State where glycolysis happens in a cell.
2. A cell extract oxidises 0.60 mol of glucose by glycolysis. Calculate the net ATP gain.
3. Explain why reduced NAD must be oxidised if glycolysis is to continue without oxygen.
4. State what is formed when acetate combines with coenzyme A.
5. Give the total number of CO2 molecules released by the link reaction and Krebs cycle for one glucose molecule.
6. 10 mmol of pyruvate pass through the link reaction and Krebs cycle. Calculate the amount of reduced NAD made.
7. Name the final electron acceptor in the electron transfer chain.
8. Name the enzyme that makes ATP using the flow of protons.
9. A tissue releases 24 mm³ of CO2 while using 30 mm³ of O2. Calculate the RQ.
10. In a respirometer the drop moves 12 mm with potassium hydroxide and 3 mm without it. Calculate the RQ.
11. Calculate the RQ of decanoic acid, C10H20O2 + 14 O2 → 10 CO2 + 10 H2O, to 2 decimal places.
12. Explain why yeast under oil, with no oxygen, can show an RQ above 1.

Answers

1. In the cytoplasm.
2. Net gain = 2 × 0.60 = 1.2 mol.
3. Reduced NAD is oxidised back to NAD, which is needed for the oxidation of triose phosphate in glycolysis. Without NAD, glycolysis stops.
4. Acetyl coenzyme A.
5. 2 + 4 = 6.
6. 4 per pyruvate (1 + 3) × 10 = 40 mmol.
7. Oxygen.
8. ATP synthase.
9. 24 ÷ 30 = 0.8.
10. (12 - 3) ÷ 12 = 0.75.
11. 10 ÷ 14 = 0.71.
12. It releases carbon dioxide by the ethanol pathway while using little or no oxygen, so the ratio of CO2 to O2 is high.

Where marks are usually lost

• Stating that glycolysis makes 2 ATP rather than a net gain of 2 from 4 made and 2 used.
• Writing “NAD” where “reduced NAD” is needed, or the other way round.
• Saying anaerobic respiration “produces ATP from pyruvate” instead of explaining that it regenerates NAD.
• Placing the Krebs cycle or link reaction in the cytoplasm or on the cristae instead of the matrix.
• Writing that oxygen takes part in the Krebs cycle, or that it makes ATP, when it accepts electrons at the end of the chain.
• Writing “ATP synthase pumps protons”. It lets them flow down the gradient.
• Calculating RQ upside down, or leaving the answer with units.
• Giving a firm conclusion from one RQ when the question asks you to comment on how tentative it is.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, section 3.3.3 (Respiration), for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards. Published by OxfordAQA.