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Aerobic respiration is the metabolic process by which organic molecules (especially glucose) are broken down to release energy, which is captured in ATP. OCR specification module 5.2.2(a)–(b) requires you to understand the need for cellular respiration, the structure of mitochondria, and the location of each stage of respiration within the cell. This lesson sets up everything that follows — glycolysis, link reaction, Krebs cycle, oxidative phosphorylation and anaerobic respiration.
Key Definitions:
- Cellular respiration — the process by which cells release energy from organic molecules (carbohydrates, lipids, proteins) to form ATP.
- Aerobic respiration — respiration that requires oxygen as the final electron acceptor; yields up to 38 ATP per glucose.
- Anaerobic respiration — respiration without oxygen; yields only 2 ATP per glucose (in most eukaryotes).
- Mitochondrion — the double-membraned organelle where the link reaction, Krebs cycle and oxidative phosphorylation occur.
- ATP (adenosine triphosphate) — the universal energy currency of the cell.
Cells need energy for many essential processes:
All this energy must come from somewhere. It comes from the controlled oxidation of glucose (and other substrates) — the energy released is captured as ATP, which then drives the rest of the cell's work.
The summary equation for aerobic respiration is:
C6H12O6+6O2→6CO2+6H2O+Energy (ATP)
This is the reverse of the photosynthesis equation — but the energy flow is in the opposite direction: respiration releases energy, photosynthesis stores it.
Respiration does not happen in a single reaction. That would release far too much energy as heat, which would destroy the cell. Instead, the energy is released in many small steps, each controlled by a specific enzyme, with some of the energy captured as ATP along the way.
| Stage | Location | Main outputs per glucose |
|---|---|---|
| Glycolysis | Cytoplasm | 2 pyruvate, net 2 ATP, 2 reduced NAD |
| Link reaction | Mitochondrial matrix | 2 acetyl CoA, 2 CO₂, 2 reduced NAD |
| Krebs cycle | Mitochondrial matrix | 4 CO₂, 6 reduced NAD, 2 reduced FAD, 2 ATP |
| Oxidative phosphorylation | Inner mitochondrial membrane | ~26–28 ATP, 6 H₂O |
Total theoretical ATP yield: approximately 30–32 ATP per glucose (older textbooks say 38; the lower value reflects losses due to the cost of importing pyruvate and NADH into the mitochondrion). OCR typically accepts either value if the logic is correct.
Mitochondria are rod-shaped organelles about 0.5–10 µm long. They have a double membrane and are found in almost all eukaryotic cells.
flowchart TB
subgraph M[Mitochondrion]
OM[Outer membrane - smooth, permeable]
IMS[Intermembrane space - H+ accumulate here]
IM[Inner membrane - highly folded into cristae]
CR[Cristae - ETC, ATP synthase]
MX[Matrix - Krebs cycle, link reaction]
DNA[Circular DNA]
RB[70S ribosomes]
end
| Structure | Description | Function |
|---|---|---|
| Outer membrane | Smooth, permeable to small molecules via porins | Boundary with cytoplasm |
| Intermembrane space | Thin gap between outer and inner membranes | H⁺ accumulates here during oxidative phosphorylation |
| Inner membrane | Highly folded into cristae; impermeable to H⁺ | Site of electron transport chain and ATP synthase; cristae increase surface area |
| Cristae | Folds of inner membrane | Provide large surface area for ETC and ATP synthase |
| Matrix | Fluid enclosed by inner membrane | Site of link reaction and Krebs cycle; contains enzymes, DNA, ribosomes |
| Mitochondrial DNA | Small circular (bacterial-type) | Codes for some of the mitochondrion's own proteins |
| 70S ribosomes | Smaller than eukaryotic 80S ribosomes | Synthesise some mitochondrial proteins |
| ATP synthase | Large stalked particles on inner membrane | Synthesise ATP from ADP + Pi using proton gradient |
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