Respiration Overview and Mitochondrial Structure

Spec Mapping — OCR H420 Module 5.2.2 — Respiration, content statements covering the need for cellular respiration, the four stages of aerobic respiration, the structure of the mitochondrion as the site of the link reaction, Krebs cycle and oxidative phosphorylation, and the role of coenzymes NAD, FAD and coenzyme A (refer to the official OCR H420 specification document for exact wording).

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 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.

The mitochondrion's role as the site of cellular respiration was established slowly. The German biochemist Otto Warburg (1920s, Berlin) pioneered manometric techniques for measuring oxygen consumption by tissue slices, demonstrating that intact tissue could respire — paraphrasing his school of thought, the rate of cellular oxygen consumption was the master variable of biological energy metabolism. The localisation of the Krebs cycle to the mitochondrial matrix and the electron transport chain to the inner membrane was settled by Eugene Kennedy and Albert Lehninger (1948), who used differential centrifugation to isolate intact mitochondria and demonstrated that the citric-acid-cycle enzymes co-purified with the membrane fraction. Their paper (paraphrasing the landmark 1948 result) was the first rigorous demonstration of "biochemical compartmentation" — the idea that specific metabolic pathways live in specific organelles rather than being free in the cytoplasm.

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.

Learning objectives — by the end of this lesson you should be able to:

• Explain why cells respire and why the oxidation of glucose is carried out in many small steps rather than one.
• Name the four stages of aerobic respiration and state the location and principal outputs of each.
• Draw and annotate a mitochondrion, relating each ultrastructural feature to its role.
• Explain the roles of the coenzymes NAD, FAD and coenzyme A, and why reduced coenzymes are the key energy-carriers.
• Evaluate the endosymbiotic evidence for mitochondrial origin (paraphrasing Margulis).

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The Chemistry of the Coenzymes — How Hydrogen and Electrons Are Carried

At the heart of respiration is a single recurring event: a substrate is oxidised (loses hydrogen atoms, i.e. protons plus electrons) and a coenzyme is reduced (gains them). OCR requires you to know the roles of NAD, FAD and coenzyme A, but understanding how these molecules work chemically is a genuine A* discriminator and clears up several persistent misconceptions.

NAD (nicotinamide adenine dinucleotide) carries hydrogen on the nicotinamide ring, derived from the vitamin niacin (B3). When a substrate is dehydrogenated, NAD accepts two electrons and one proton (a "hydride ion", H⁻) onto the ring, while the second proton is released into solution. This is why the correct OCR notation is reduced NAD or NADH + H⁺ — never "NADH₂". The reaction is:

$\text{NAD}^+ + 2\text{H} \;(2e^- + 2\text{H}^+) \rightarrow \text{NADH} + \text{H}^+$NAD++2H(2e−+2H+)→NADH+H+

Crucially, NAD is a diffusible, water-soluble coenzyme: it is not permanently attached to any one enzyme, so it can shuttle reducing power from the enzyme that reduces it (e.g. in the matrix) to the enzyme that re-oxidises it (Complex I of the electron transport chain). This mobility is why NAD is the dominant hydrogen carrier of respiration.

FAD (flavin adenine dinucleotide) carries hydrogen on the isoalloxazine ring, derived from riboflavin (B2). Unlike NAD, FAD is usually a prosthetic group — tightly and permanently bound to its enzyme (for example, succinate dehydrogenase, which is also Complex II of the electron transport chain). Because FAD is bound within a membrane complex rather than free in solution, the electrons it accepts enter the electron transport chain at a later point than those carried by NAD — bypassing the first proton pump. This single structural fact is the reason reduced FAD ultimately yields less ATP (~1.5) than reduced NAD (~2.5), a point examined in detail in the oxidative-phosphorylation lesson. FAD can also accept electrons one at a time (via a semiquinone intermediate), making it suited to interfacing between two-electron and one-electron carriers.

Coenzyme A, derived from pantothenic acid (B5), is not an electron carrier at all — it is an acyl-group carrier. Its reactive thiol (–SH) group forms a high-energy thioester bond with acetyl groups, producing acetyl-CoA. This bond is energy-rich, which is why the transfer of the acetyl group onto oxaloacetate in the first step of the Krebs cycle is thermodynamically favourable. CoA is recycled the instant it releases its acetyl cargo.

A unifying idea: respiration does not "destroy" glucose in one violent reaction. It strips hydrogen from glucose's carbons in a controlled series of dehydrogenations, loading it onto NAD and FAD. The energy of those C–H bonds is thereby transferred into reduced coenzymes and only released — gradually, and mostly as ATP — when the coenzymes are re-oxidised by the electron transport chain. Every stage before oxidative phosphorylation is, in essence, a hydrogen-harvesting operation.

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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

• Large surface area of cristae — accommodates thousands of electron transport chains and ATP synthase enzymes for maximum ATP production.
• Small matrix volume enclosed by the inner membrane — keeps Krebs cycle enzymes and substrates close together for high reaction rates.
• Impermeable inner membrane — essential for maintaining the H⁺ proton gradient. Without impermeability, H⁺ would leak back and no ATP could be made.
• Outer membrane with porins — freely permeable to small molecules (ATP, pyruvate, O₂, CO₂) but not to large proteins.
• Own DNA and 70S ribosomes — allow mitochondria to make some of their own proteins; this is key evidence for the endosymbiotic theory, which proposes that mitochondria evolved from free-living aerobic bacteria engulfed by an ancestral eukaryotic cell.
• Numerous in metabolically active cells — liver, kidney, muscle and neurones may have hundreds to thousands of mitochondria each.

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Where Do The Stages Happen?

flowchart LR
    GLU[Glucose] --> GLY[Glycolysis - cytoplasm]
    GLY -->|Pyruvate| LR[Link reaction - matrix]
    LR -->|Acetyl CoA| KR[Krebs cycle - matrix]
    KR -->|Reduced NAD, reduced FAD| ETC[ETC - inner membrane cristae]
    ETC --> ATP[ATP synthase - inner membrane]
    ETC --> O2[O2 as final electron acceptor - forms H2O]

• Glycolysis happens in the cytoplasm — this is the only stage that does not require a mitochondrion, so anaerobic organisms can use it alone.
• Link reaction and Krebs cycle happen in the mitochondrial matrix.
• Oxidative phosphorylation happens across the inner mitochondrial membrane.

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Coenzymes: NAD, FAD and Coenzyme A

Three coenzymes play essential roles in respiration. OCR expects you to know their names and broad functions.

Coenzyme	What it does	Made from
NAD (nicotinamide adenine dinucleotide)	Accepts 2H (picks up electrons and H⁺) in glycolysis, link reaction and Krebs cycle; becomes reduced NAD	Niacin (vitamin B3)
FAD (flavin adenine dinucleotide)	Accepts 2H in the Krebs cycle; becomes reduced FAD	Riboflavin (vitamin B2)
Coenzyme A (CoA)	Carrier of acetyl groups between the link reaction and the Krebs cycle	Pantothenic acid (vitamin B5)

Reduced NAD and reduced FAD carry "reducing power" — they will donate their electrons to the electron transport chain, releasing the energy that drives ATP synthesis.

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ATP: The Universal Energy Currency

• ATP is a phosphorylated nucleotide: adenine + ribose + 3 phosphates.
• The bond between the 2nd and 3rd phosphates is a high-energy bond: when it is hydrolysed, about 30.5 kJ mol⁻¹ of energy is released.
• ATP ↔ ADP + Pi is the central reaction of cellular metabolism: energy is stored when ATP is made, and released when it is hydrolysed.
• Cells have only a small pool of ATP at any moment (~1 second's worth) — it must be continuously regenerated.
• A human adult makes and uses about their body weight in ATP every day — ~50 kg! This only works because each ATP molecule is recycled about 1000 times per day.

ATP's advantages as an energy currency:

• Small and water-soluble — can diffuse easily across the cell.
• Quickly made and broken down — regenerates and is consumed rapidly.
• Releases a useful amount of energy — not too much (wasteful as heat) and not too little (insufficient).
• Universal — used by every cell of every organism. Life has standardised on ATP.

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Exam Tip

When asked to describe mitochondrial structure, always link each feature to a function. "Has a folded inner membrane" is not enough — write "the inner membrane is folded into cristae, which increases the surface area available for the electron transport chain and ATP synthase, so more ATP can be produced per unit time." OCR mark schemes reward explicit structure-function links.

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Common Exam Mistakes

• Confusing inner and outer membranes. The inner is folded, impermeable and has the ETC. The outer is smooth, permeable and has porins.
• Saying the matrix is the fluid between the two membranes. The matrix is inside the inner membrane; the intermembrane space is between the two membranes.
• Calling reduced NAD "NADH₂". OCR uses reduced NAD (or NADH + H⁺). Do not write "NADH₂".
• Forgetting that mitochondria have their own DNA and ribosomes. This is a synoptic point linking to endosymbiotic theory and inheritance.
• Claiming glycolysis happens in mitochondria. It happens in the cytoplasm.
• Confusing NAD with NADP. NAD is used in respiration; NADP is used in photosynthesis. Same molecule with one extra phosphate on NADP.
• Saying mitochondria "produce energy". Energy cannot be produced or destroyed. Mitochondria release energy from organic molecules and transfer it to ATP.

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Mitochondrion Ultrastructure — Detailed SVG