AHL: Respiration extensions

IB Biology HL· Theme B: Form and Function, AHL Respiration· 45 min read

1. Mitochondrial Structure and Oxidative Phosphorylation★★★☆☆⏱ 15 min

📘 Definition

Oxidative Phosphorylation

The final stage of aerobic respiration where ATP is formed using energy released from the oxidation of NADH and FADH $_{2}$ , embedded in the inner mitochondrial membrane.

Example:

Accounts for ~90% of ATP produced during aerobic respiration of glucose.

The inner mitochondrial membrane is folded into finger-like projections called cristae, which dramatically increase the surface area available for electron transport chain (ETC) protein complexes and ATP synthase. As electrons are passed from NADH and FADH $_{2}$ through the ETC, energy is released to pump protons (H $^{+}$ ) from the matrix into the intermembrane space.

📐 Worked Example

Explain how cristae structure supports high rates of oxidative phosphorylation.

1
Folding of the inner membrane into cristae greatly increases total surface area.
2
More surface area allows more ETC complexes and ATP synthase enzymes to be embedded in the membrane.
3
More ETC complexes enable simultaneous processing of more electrons, increasing the rate of proton pumping.
4
Higher proton pumping capacity leads to higher rates of ATP synthesis via chemiosmosis.

Exam tip:

Always link mitochondrial structure directly to function in exam answers, this is a common expected marking point.

2. Chemiosmosis Theory★★★★☆⏱ 20 min

📘 Definition

Chemiosmosis

The process of ATP synthesis driven by the diffusion of protons down their electrochemical gradient across a semipermeable membrane, through the enzyme ATP synthase.

Proton pumping creates an electrochemical gradient across the inner mitochondrial membrane: higher H $^{+}$ concentration and positive charge in the intermembrane space compared to the matrix. The potential energy stored in this gradient is called proton motive force.

📐 Worked Example

Predict the effect of a leaky inner mitochondrial membrane on ATP synthesis.

1
A leaky membrane allows protons to diffuse back into the matrix without passing through ATP synthase.
2
This reduces or eliminates the electrochemical proton gradient across the membrane.
3
Without a gradient, there is no proton motive force to drive ATP synthesis.
4
Energy from the ETC is released as heat instead of being captured as ATP, the mechanism used in brown fat for thermoregulation.

3. Alternative Anaerobic Pathways★★☆☆☆⏱ 10 min

Core SL content covers two common fermentation pathways, but AHL extends this to include prokaryotic anaerobic respiration. All pathways operate without oxygen, but they differ significantly in ATP yield.

Lactate fermentation: Occurs in mammals and some bacteria, produces lactic acid, regenerates NAD $^{+}$ for glycolysis, reversible when oxygen returns
Ethanol fermentation: Occurs in yeast and some plants, produces ethanol and CO $_{2}$ , irreversible, used in industry
Prokaryotic anaerobic respiration: Uses inorganic molecules (nitrate, sulfate) as final electron acceptors in an ETC, produces more ATP than fermentation but less than aerobic respiration

📐 Worked Example

Distinguish between fermentation and anaerobic respiration in prokaryotes.

1
Both processes occur in the absence of oxygen and regenerate NAD $^{+}$ for glycolysis.
2
Fermentation does not use an electron transport chain, so only produces 2 net ATP per glucose from glycolysis.
3
Anaerobic respiration uses an ETC with an alternative final electron acceptor, so produces more than 2 ATP per glucose via oxidative phosphorylation.
4
Fermentation occurs entirely in the cytoplasm, while anaerobic respiration occurs across the prokaryotic cell membrane.

4. Respiratory Quotient Calculation and Interpretation★★★☆☆⏱ 15 min

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📘 Definition

Respiratory Quotient

RQ

The ratio of carbon dioxide produced to oxygen consumed during respiration, used to identify the type of respiratory substrate being used.

RQ = \frac{CO_2\ produced}{O_2\ consumed}

Different substrates have different RQ values because of their different oxidation states. Carbohydrates have a RQ of ~1.0, lipids ~0.7, proteins ~0.8-0.9. An RQ greater than 1.0 indicates anaerobic respiration or conversion of carbohydrate to fat.

📐 Worked Example

A respiring organism consumes 120 mmol of oxygen and produces 84 mmol of carbon dioxide. Calculate the RQ and identify the main substrate.

1
Substitute values into the RQ formula:
2
$RQ = \frac{84}{120} = 0.7$
3
Compare to reference RQ values: an RQ of 0.7 matches lipid as the main respiratory substrate.

5. Common Pitfalls

Wrong move:

Pumping protons into the mitochondrial matrix to create the gradient

Why:

Students often mix up the direction of proton pumping, reversing the gradient

Correct move:

Protons are pumped from the matrix into the intermembrane space, then diffuse back into the matrix through ATP synthase

Wrong move:

Claiming all ATP production requires oxygen

Why:

Students forget substrate-level phosphorylation occurs in glycolysis and Krebs cycle regardless of oxygen presence

Correct move:

Oxygen is only required as the final electron acceptor in the ETC; 2 ATP per glucose are produced without oxygen via substrate-level phosphorylation

Wrong move:

Calculating RQ as O $_{2}$ consumed divided by CO $_{2}$ produced

Why:

The ratio order is commonly memorized backwards by students

Correct move:

RQ is always CO $_{2}$ produced divided by O $_{2}$ consumed, never the reverse

Wrong move:

Stating all anaerobic processes produce only 2 ATP per glucose

Why:

Core content only covers fermentation, leading to incorrect generalization

Correct move:

Fermentation produces 2 ATP per glucose, but prokaryotic anaerobic respiration with an alternative final electron acceptor produces more ATP via oxidative phosphorylation

6. Quick Reference Cheatsheet

Respiratory Substrate	Typical RQ	Process	Location
Carbohydrate	~1.0	Oxidative phosphorylation	Inner mitochondrial membrane
Lipid	~0.7	Chemiosmosis	Across inner mitochondrial membrane
Protein	0.8-0.9	Krebs cycle	Mitochondrial matrix
Mixed diet	0.7-1.0	Glycolysis	Cytoplasm

7. Frequently Asked

How many ATP per glucose do I need to remember for IB?

IB accepts 30-32 ATP per glucose molecule for aerobic respiration; you do not need to memorize fine-grained differences from NADH transport.

Is chemiosmosis the same in photosynthesis and respiration?

The mechanism of ATP synthesis is identical, but the location (thylakoid vs inner mitochondrial membrane) and energy source (light vs NADH oxidation) differ.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 1
Multiple choice on RQ interpretation
2024 · 2
Explain chemiosmosis in respiration
2023 · 3
Compare anaerobic respiration pathways

Going deeper

syllabusIB Biology AHL Respiration SyllabusOfficial alignment check

What's Next

Respiration is a core unifying theme in IB Biology, connected to concepts across all themes. This AHL extension builds on your core SL knowledge of cell respiration, and prepares you for topics including energy flow in ecosystems, gas exchange, and metabolic responses to exercise. Understanding chemiosmosis is also critical for learning about photophosphorylation in photosynthesis, which shares the same core mechanism of ATP synthesis. Mastery of this sub-topic will help you answer multi-topic exam questions that connect form and function across biological scales.