12.2 Respiration
- Syllabus
- 9700–2028–2029
- Topic
- 12.2
- Level
- A2
Learning objectives
12.2.1Four stages of aerobic respiration location• Four stages of aerobic respiration location:- Glycolysis: cytoplasm- Link reaction: mitochondrial matrix- Krebs cycle: mitochondrial matrix- Oxidative phosphorylation: inner mitochondrial membrane12.2.2Glycolysis pathway• Glycolysis: phosphorylation of glucose, splitting of fructose 1,6-bisphosphate (6C) into two triose phosphates (3C), oxidation to pyruvate (3C), producing ATP and reduced NAD12.2.3Pyruvate and the link reaction• Pyruvate enters mitochondria for link reaction when oxygen available12.2.4Link reaction• Link reaction: role of coenzyme A in acetyl (2C) group transfer12.2.5Krebs cycle carbon compounds• Krebs cycle: oxaloacetate (4C) accepts 2C from acetyl coenzyme A to form citrate (6C), converted back to oxaloacetate12.2.6Krebs cycle redox reactions• Krebs cycle involves decarboxylation, dehydrogenation, reduction of NAD and FAD12.2.7NAD and FAD in respiration• NAD and FAD transfer hydrogen to inner mitochondrial membrane carriers12.2.8Oxidative phosphorylation• Oxidative phosphorylation:- Hydrogen splits into protons and energetic electrons- Electrons release energy through electron transport chain- Energy transfers protons across inner mitochondrial membrane- Protons return via ATP synthase by facilitated diffusion, providing energy for ATP synthesis- Oxygen as final electron acceptor forms water12.2.9Mitochondria structure-function relationship• Mitochondria structure-function relationship12.2.10Lactate and ethanol fermentation• Anaerobic respiration: lactate fermentation (mammals), ethanol fermentation (yeast)12.2.11Energy yield• Energy yield: aerobic >> anaerobic respiration12.2.12Rice adaptations to flooding• Rice adaptations: aerenchyma in roots, ethanol fermentation, faster stem growth12.2.13Redox indicator investigation• Investigate using redox indicators (DCPIP, methylene blue): effects of temperature and substrate concentration on yeast respiration rate12.2.14Respirometer investigations• Investigate using respirometers: temperature effects on respiration rate
Four aerobic stages occupy three exact cell locations
| Stage | Location in a eukaryotic cell |
|---|---|
| Glycolysis | Cytoplasm |
| Link reaction | Mitochondrial matrix |
| Krebs cycle | Mitochondrial matrix |
| Oxidative phosphorylation | Inner mitochondrial membrane (cristae) |
The route is cytoplasm → matrix → matrix → inner membrane. Glycolysis supplies pyruvate; the matrix stages process its carbon and reduce coenzymes; reduced coenzymes then deliver hydrogen/electrons to the inner membrane.
The intermembrane space stores pumped protons but is not the named site of oxidative phosphorylation. Glycolysis does not occur inside mitochondria.
Glycolysis turns one phosphorylated 6C molecule into two 3C pyruvate
Glycolysis occurs in the cytoplasm: glucose is phosphorylated, the 6C intermediate fructose 1,6-bisphosphate is split, and two 3C triose phosphates are oxidised to two 3C pyruvate.
- ATP supplies phosphate to glucose-derived intermediates, forming fructose 1,6-bisphosphate (6C).
- The 6C molecule splits into two triose phosphate molecules (3C each).
- Each triose phosphate is oxidised; hydrogen is accepted by NAD to form reduced NAD.
- Substrate-linked phosphorylation transfers phosphate to ADP. ATP production exceeds the initial investment, giving a small net gain.
- Both 3C routes end as pyruvate.
Glycolysis does not release carbon dioxide and does not require oxygen directly. It produces ATP and reduced NAD, but it is not the link reaction or Krebs cycle.
Oxygen availability directs pyruvate into mitochondrial respiration
When oxygen is available, pyruvate produced by glycolysis enters mitochondria and reaches the matrix, where it takes part in the link reaction.
Oxygen is used later as the final electron acceptor. By allowing the electron transport chain to continue, it permits reduced coenzymes to be reoxidised; oxidised NAD and FAD remain available for the link reaction and Krebs cycle. Mitochondrial processing can therefore continue.
Oxygen available → pyruvate enters mitochondrial aerobic processing.
Oxygen unavailable → electron transport stops and pyruvate remains in the cytoplasm for fermentation, which regenerates NAD so glycolysis can continue.
Oxygen does not react directly with pyruvate during the link reaction; its final-acceptor role later in respiration makes the aerobic route sustainable.
The link reaction removes carbon and transfers electrons to NAD
The link reaction connects glycolysis to the Krebs cycle. For each pyruvate that enters the mitochondrial matrix under aerobic conditions, carbon is removed as carbon dioxide, hydrogen is transferred to NAD, and the remaining two-carbon acetyl group is carried by coenzyme A as acetyl-CoA.
- Entry: Pyruvate from glycolysis is transported into the mitochondrial matrix when aerobic processing can continue. The link reaction itself is a matrix process.
- Decarboxylation: Enzymes remove one carbon from pyruvate and release it as carbon dioxide. This converts the three-carbon input into a two-carbon remainder.
- Oxidation: The remaining carbon compound is dehydrogenated. Hydrogen/electrons are accepted by NAD, forming reduced NAD and storing transferable reducing power.
- Acetyl-CoA formation: Coenzyme A binds to the two-carbon acetyl group. The product acetyl-CoA is the form that carries this group into the next stage.
- Bridge: Acetyl-CoA supplies the acetyl group to the Krebs cycle. The cycle details are separate; this card ends at the hand-off.
Per-pyruvate output: one carbon dioxide, one reduced NAD and one acetyl-CoA are formed in the source-supported link-reaction account. If the question changes the basis to one glucose, remember that glycolysis produces two pyruvate, but do not confuse this doubling with a new link-reaction step.
The link reaction is a bridge, not a cycle and not glycolysis. Its defining outputs are carbon dioxide, reduced NAD and acetyl-CoA; it is not the stage that explains the full Krebs-cycle sequence or the final ATP yield.
The Krebs cycle regenerates a four-carbon acceptor while releasing carbon dioxide
The Krebs cycle is a series of enzyme-controlled reactions in the mitochondrial matrix. Acetyl-CoA supplies a two-carbon acetyl group to a four-carbon oxaloacetate acceptor, forming a six-carbon citrate that is converted back to oxaloacetate while releasing carbon dioxide and capturing transferable reducing power.
- Enter the cycle: Acetyl-CoA carries a two-carbon acetyl group into the mitochondrial matrix. Oxaloacetate, a four-carbon acceptor, combines with it to form the six-carbon citrate.
- Process the carbon compound: A sequence of enzyme-controlled reactions changes citrate through intermediate compounds. The carbon skeleton is progressively oxidised and carbon is removed as carbon dioxide.
- Capture reducing power: Dehydrogenation transfers hydrogen/electrons to coenzymes, producing reduced coenzymes that can carry energy to later respiration stages. Detailed NAD/FAD carrier accounting belongs to the next card.
- Make a small direct ATP return: A phosphate group is transferred from an intermediate to ADP, giving a small amount of ATP by substrate-level phosphorylation.
- Regenerate the acceptor: The sequence returns to four-carbon oxaloacetate. Regeneration matters because the same acceptor can combine with the next acetyl-CoA, so the pathway is a cycle rather than a one-way chain.
Counting basis: the cycle turns once for each acetyl-CoA entering. One glucose produces two pyruvate and therefore two acetyl-CoA for aerobic processing, so the cycle turns twice per glucose. Keep this doubling separate from the chemical events of one turn.
The Krebs cycle occurs in the mitochondrial matrix and does not directly use oxygen as a reactant. It releases carbon dioxide and produces reduced coenzymes plus a small direct ATP return; the later respiratory chain explains how most reducing power is used.
Krebs reactions remove carbon and hydrogen in different ways
| Reaction type | What is removed | Product or transfer |
|---|---|---|
| Decarboxylation | A carboxyl group/carbon from an intermediate | Carbon dioxide is released |
| Dehydrogenation | Hydrogen/electrons from an intermediate | NAD or FAD accepts them and becomes reduced |
As citrate is converted through small enzyme-controlled steps, repeated decarboxylation reduces the carbon skeleton and dehydrogenation oxidises intermediates. Reduced NAD and reduced FAD preserve transferable hydrogen/electron energy for the inner mitochondrial membrane. Oxaloacetate is regenerated at the end.
Decarboxylation does not reduce NAD/FAD; dehydrogenation does. The coenzymes carry hydrogen/electrons away but are not ATP and do not make the Krebs pathway itself occur on the inner membrane.
Reduced NAD and FAD shuttle hydrogen to the inner membrane
NAD and FAD are reversible coenzymes that accept hydrogen/electrons during respiratory dehydrogenation and transfer them to carriers in the inner mitochondrial membrane.
- A respiratory intermediate is dehydrogenated.
- Oxidised NAD or FAD accepts hydrogen/electrons and becomes reduced.
- Reduced NAD or reduced FAD reaches/donates to the inner-membrane electron-transfer system.
- Hydrogen separates into protons and electrons for oxidative phosphorylation.
- NAD/FAD is reoxidised and can return to accept more hydrogen in earlier reactions.
NAD and FAD are mobile/reusable coenzymes linking pathway reactions to membrane carriers; they are not ATP, not the proton gradient and not the membrane carrier chain itself.
Electron flow builds the proton gradient that powers ATP synthase
Oxidative phosphorylation on the inner mitochondrial membrane couples electron-transfer energy to a proton gradient and ATP synthesis; oxygen completes the electron pathway by forming water.
- Reduced NAD/FAD donate hydrogen, which separates into protons and energetic electrons.
- Electrons pass through an electron transport chain and release energy in steps.
- That energy transfers protons from the matrix to the intermembrane space.
- The inner membrane retains the gradient.
- Protons return to the matrix by facilitated diffusion through ATP synthase; their flow provides energy to phosphorylate ADP.
- Oxygen accepts electrons and protons at the end, forming water.
Electrons release energy for proton pumping; protons flow through ATP synthase. Oxygen is the final electron acceptor, not the enzyme that directly forms ATP. Detailed carrier names and fixed ATP totals are not required.
Mitochondrial compartments organise carbon reactions and chemiosmosis
| Visible structure | Respiratory function |
|---|---|
| Double membrane | Separates matrix, intermembrane space and cytoplasm into controlled compartments |
| Inner membrane folded as cristae | Large area for electron carriers and ATP synthase |
| Intermembrane space | Receives pumped protons so a gradient can form across the inner membrane |
| Matrix | Contains enzymes for the link reaction and Krebs cycle, plus NAD/FAD substrates/products |
| Circular DNA and 70S ribosomes | Support synthesis of some mitochondrial proteins and organelle replication |
In an electron micrograph, identify a mitochondrion by a surrounding double membrane and internal cristae enclosing matrix. Section angle can make cristae appear as separate lines or profiles, so use several features and the scale rather than one dark shape. In a diagram, label membranes and spaces before attaching functions.
Cristae are folds of the inner membrane, not free-standing compartments. More cristae increase available membrane area but do not alone prove a fixed respiration rate; substrate, oxygen and enzyme activity also matter.
Fermentation regenerates NAD by two different pyruvate routes
Without oxygen, fermentation reoxidises reduced NAD so glycolysis can continue producing a small ATP supply by substrate-linked phosphorylation.
| Cells | Pyruvate route | Products and carbon dioxide |
|---|---|---|
| Mammalian muscle | Pyruvate accepts hydrogen from reduced NAD | Lactate; no CO2 released |
| Yeast | Pyruvate is decarboxylated to ethanal, then ethanal accepts hydrogen from reduced NAD | Ethanol and CO2 |
In both routes, reduced NAD becomes oxidised NAD. NAD can return to glycolysis to accept more hydrogen, preventing glycolysis from stopping. Fermentation itself adds no further ATP beyond the small net ATP from glycolysis.
Lactate is not produced by yeast in this syllabus comparison, and mammalian lactate fermentation does not release carbon dioxide. 'Anaerobic' does not mean zero ATP; it means no oxidative phosphorylation.
Aerobic respiration extracts far more ATP from each glucose
| Aerobic conditions | Anaerobic conditions |
|---|---|
| Glycolysis, link reaction, Krebs cycle and oxidative phosphorylation continue | Glycolysis continues; fermentation only regenerates NAD |
| Glucose carbon is completely oxidised to CO2 | Glucose is only partially oxidised; lactate or ethanol retains chemical energy |
| Many reduced NAD/FAD molecules feed electron transport and chemiosmosis | Electron transport stops because oxygen cannot be the final electron acceptor |
| Substrate-linked ATP plus a much larger oxidative-phosphorylation contribution | Only the small net substrate-linked ATP from glycolysis |
Oxygen allows continuous coenzyme reoxidation and electron flow. The link and Krebs reactions can therefore extract more hydrogen/electrons, and oxidative phosphorylation transfers much more of that energy to ATP. Anaerobic end products remain energy-rich, so yield is much lower.
Anaerobic respiration does make a small amount of ATP in glycolysis. A detailed fixed total ATP yield per glucose is not required and varies with accounting conventions.
Rice combines internal air spaces, fermentation and rapid stem growth
Water contains less available oxygen and gas diffusion is slow, so submerged rice roots may not receive enough oxygen for continuous aerobic respiration.
| Limited adaptation | How it supports flooded growth |
|---|---|
| Aerenchyma develops in roots | Large connected air spaces provide a low-resistance internal route for oxygen diffusion towards submerged root tissues |
| Ethanol fermentation in roots | Regenerates NAD so glycolysis continues and supplies a small amount of ATP when oxygen remains limiting |
| Faster stem growth | Raises leaves above water sooner, restoring contact with air for gas exchange and photosynthesis |
These adaptations do not make anaerobic respiration as efficient as aerobic respiration. The required scope is limited to root aerenchyma, root ethanol fermentation and faster stem growth; additional ethanol-detoxification mechanisms are not assumed.
Redox-indicator timing compares yeast respiration rates
DCPIP and methylene blue are blue when oxidised and become colourless when reduced. In respiring yeast, faster transfer of hydrogen/electrons reduces the indicator sooner, so time to one fixed endpoint is an inverse proxy for rate.
| Investigation | Independent variable | Keep constant |
|---|---|---|
| Temperature effect | Water-bath temperature | Yeast concentration/volume, substrate type and concentration, indicator volume/concentration, pH, total volume, endpoint |
| Substrate-concentration effect | Concentration of one named substrate | Temperature, yeast concentration/volume, substrate identity, indicator volume/concentration, pH, total volume, endpoint |
Equilibrate solutions, mix in a consistent order and start timing immediately. Record time to the same defined colour endpoint; repeat independently and calculate mean time or relative rate as 1/time. Include a no-substrate or non-living-yeast control where appropriate. Plot rate against the chosen independent variable and explain enzyme/substrate effects without inventing an optimum.
Use capped tubes carefully without dangerous pressure build-up, handle indicators according to laboratory risk assessment, and use water baths rather than direct flames. Change only one independent variable; endpoint time itself is not a direct rate or ATP measurement.
A controlled respirometer measures how temperature changes oxygen uptake
With carbon dioxide absorbed, a sealed respirometer's corrected gas-volume decrease estimates oxygen uptake. Oxygen uptake per unit time and biological mass is an indirect measure of aerobic respiration rate.
- Put equal masses of comparable organisms or germinating seeds in experimental tubes; use equal-volume inert material in controls.
- Place soda lime/potassium hydroxide behind gauze or cotton so organisms cannot touch it; use the same absorbent treatment in matched controls.
- Set each water-bath temperature, allow apparatus and organisms to equilibrate, then seal/reset to the same marker start.
- Measure marker distance over a fixed time and convert to volume using calibration or capillary area; subtract control drift.
- Express rate as corrected oxygen volume per time per mass. Repeat independently at every temperature, calculate means and plot rate against temperature.
Keep species/developmental stage, mass, apparatus gas volume, absorbent amount, acclimation and measurement time constant. Temperature can change gas pressure as well as enzyme activity, which is why equilibration and inert controls are essential. Explain rising rate through kinetic energy/collision frequency and falling rate at high temperature through enzyme/protein disruption only when data support it.
Prevent contact with corrosive absorbent, use eye/skin protection and approved handling, avoid harmful temperatures or prolonged confinement, and treat living material ethically. One displacement is not a rate, and a soda-lime respirometer does not directly measure anaerobic respiration or ATP.