Respiration Overview and Mitochondrial Structure

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.

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Why Cells Respire

Cells need energy for many essential processes:

• Active transport (Na⁺/K⁺ pumps, Ca²⁺ pumps)
• Muscle contraction (actin-myosin cross-bridges)
• Anabolic reactions (building proteins, DNA, lipids)
• Secretion (exocytosis)
• Cell division (DNA replication, chromosome movement)
• Heat production (especially in endotherms and in brown adipose tissue)
• Maintenance of membrane potential (resting potential in neurones)

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.

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The Overall Equation

The summary equation for aerobic respiration is:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{Energy (ATP)}$C6​H12​O6​+6O2​→6CO2​+6H2​O+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.

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Four Stages of Aerobic Respiration

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.

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Structure of a Mitochondrion

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

Key Structures

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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Structure-Function Relationships