describe the relationship between the structure and function of mitochondria using diagrams and electron micrographs

Respiration – The Role of Mitochondria

1. Overall Aerobic Reaction

\[

\text{C}6\text{H}{12}\text{O}6 + 6\,\text{O}2 \;\longrightarrow\; 6\,\text{CO}2 + 6\,\text{H}2\text{O} + \approx 30\,\text{ATP}

\]

Note: The figure “≈30 ATP” is an estimate used for A‑Level. The syllabus only requires that most ATP is generated by oxidative phosphorylation, not the exact number.

2. Where Each Stage Occurs – Compartment Map (AS + A‑Level)

3. The Complete Aerobic Pathway (Key Points)

• Glycolysis – occurs in the cytosol; key enzymes: hexokinase, phosphofructokinase, pyruvate kinase.
• Link reaction – pyruvate enters the matrix via specific carriers; pyruvate dehydrogenase complex converts it to acetyl‑CoA.
• Krebs cycle – a series of matrix‑soluble reactions that generate NADH, FADH₂ and a small amount of ATP/GTP.
• Electron transport chain (ETC) – four membrane‑bound complexes and two mobile carriers (Coenzyme Q, cytochrome c) pass electrons from NADH/FADH₂ to O₂, pumping protons into the IMS.
• Oxidative phosphorylation – the proton‑motive force drives ATP synthase (Complex V) to synthesise ATP in the matrix.

4. Mitochondrial Compartments & Their Structure

• Outer membrane – smooth, contains abundant porin proteins that form large pores for free diffusion of ions, ADP, ATP, Pi and small metabolites.
• Intermembrane space (IMS) – thin aqueous layer; site where protons are accumulated by the ETC, creating part of the proton‑motive force.
• Inner membrane (IMM) – highly folded into cristae; rich in cardiolipin, low in cholesterol; houses Complex I–IV and ATP synthase.
• Cristae – increase the surface area of the IMM up to ten‑fold, allowing many copies of the ETC complexes to be embedded.
• Matrix – gel‑like interior containing soluble enzymes of the Krebs cycle, β‑oxidation, mitochondrial DNA, ribosomes and the ADP/ATP translocase.

5. Structure ↔ Function Relationships

1. Porins in the outer membrane – permit rapid diffusion of ADP, ATP, Pi and small metabolites, linking glycolysis (cytosol) to mitochondrial processes.
2. Inner membrane cristae – provide a large surface for embedding the ETC complexes and ATP synthase, maximising the rate of oxidative phosphorylation.
3. Cardiolipin‑rich IMM – stabilises the large protein complexes and maintains optimal fluidity for electron transfer.
4. IMS proton pool – the accumulation of H⁺ creates the ΔpH component of the proton‑motive force.
5. Matrix environment – soluble enzymes work efficiently in this aqueous compartment, allowing rapid turnover of Krebs‑cycle intermediates.
6. ATP synthase (F₀ + F₁) – uses the return flow of protons from IMS to matrix to drive rotation of the catalytic F₁ head and synthesis of ATP.

6. Electron Transport Chain & Chemiosmotic Coupling

Chemiosmotic coupling (Mitchell’s theory) – Electron flow through Complex I, III and IV pumps protons from the matrix into the IMS, creating:

• ΔpH – a higher H⁺ concentration in the IMS (acidic) than in the matrix (alkaline).
• Δψ – an electrical potential because positive charge is separated from the negatively charged matrix.

The combined gradient (proton‑motive force) drives protons back through the F₀ channel of ATP synthase, causing rotation of the F₁ head and synthesis of ~3 ATP per 10 H⁺.

Matrix‑produced ATP is exported to the cytosol by the ADP/ATP translocase (anti‑porter that swaps matrix ATP for cytosolic ADP + Pi).

7. Respiratory Quotient (RQ)

• Definition: RQ = CO₂ produced / O₂ consumed.
• Typical values

  • Carbohydrates: RQ ≈ 1.0
  • Fats: RQ ≈ 0.7
  • Proteins: RQ ≈ 0.8

• Worked example (glucose):

  1. Overall reaction produces 6 CO₂ and uses 6 O₂.
  2. RQ = 6 CO₂ / 6 O₂ = 1.0.

8. Anaerobic Respiration (Fermentation)

When oxygen is limiting, cells regenerate NAD⁺ by converting pyruvate into other end‑products.

Plant relevance: In water‑logged tissues, plant cells switch to ethanol fermentation to maintain glycolysis and avoid anoxia‑induced damage.

9. Biological Relevance – Why Mitochondrial ATP Matters (AO2)

• Muscle contraction – myosin‑ATPase hydrolyses ATP to generate force; high demand during intense exercise.
• Active transport – Na⁺/K⁺‑ATPase and H⁺‑ATPase maintain ion gradients essential for nerve impulse transmission and pH regulation.
• Biosynthesis – DNA replication, protein synthesis and lipid formation all require ATP as a direct energy source.

10. Suggested Experimental Investigations (AO3)

• Respirometer assay – measure O₂ consumption of a living tissue (e.g., chick heart) before and after addition of an ETC inhibitor (e.g., cyanide). Compare rates to infer the contribution of oxidative phosphorylation.
• Redox indicator test – use methylene blue or resazurin to monitor NADH oxidation in isolated mitochondria; colour change indicates electron flow.
• RQ determination – collect exhaled gases in a closed system, measure CO₂ and O₂ volumes, and calculate RQ for different substrates (glucose vs. oil).
• Effect of uncouplers – add 2,4‑dinitrophenol (DNP) to mitochondria and record the increase in O₂ consumption with a drop in ATP production, demonstrating the role of the proton gradient.

11. Electron Micrograph Description

An electron micrograph of an animal‑cell mitochondrion typically shows:

• Outer membrane – a smooth, continuous line surrounding the organelle.
• Inner membrane – dense, finger‑like projections (cristae) that extend deep into the matrix.
• Matrix – electron‑dense (dark) because of high concentrations of enzymes, mtDNA and ribosomes.
• Intermembrane space – a narrow, lighter band separating the two membranes.

12. Summary – Structural Features vs. Functional Roles

13. Key Points to Remember

• Crista‑rich inner membrane supplies the surface area needed for a high rate of oxidative phosphorylation.
• Compartmentalisation creates distinct chemical environments (cytosol, matrix, IMS) essential for establishing the proton‑motive force.
• Porins in the outer membrane permit rapid exchange of metabolites, linking glycolysis (cytosol) to mitochondrial respiration.
• Cardiolipin is a unique phospholipid of the inner membrane that stabilises the ETC complexes.
• Mitchell’s chemiosmotic theory explains how electron‑transfer energy is first stored as a proton gradient and then used to synthesise ATP.
• When O₂ is unavailable, cells switch to fermentation (lactate or ethanol) to regenerate NAD⁺, but ATP yield falls to only the 2 ATP from glycolysis.

14. Sample Examination Questions (Cambridge 9700)

1. Explain how the structure of the inner mitochondrial membrane (cristae, cardiolipin content) enhances ATP production.
2. Describe the role of the intermembrane space in establishing the proton‑motive force and how this drives oxidative phosphorylation.
3. Using a labelled diagram, illustrate the flow of electrons through the ETC, the pumping of protons, and the synthesis of ATP by ATP synthase.
4. Compare aerobic respiration with anaerobic fermentation in terms of location, ATP yield and end‑products.
5. Calculate the respiratory quotient for the oxidation of a fatty acid (e.g., palmitate: C₁₆H₃₂O₂) and interpret the result.
6. Design a simple experiment to test the effect of an ETC inhibitor on oxygen consumption in isolated mitochondria.