12.2 Respiration

Mitochondria are adapted for aerobic respiration and ATP production by separating reactions across a double membrane. The matrix, inner membrane and intermembrane space provide distinct conditions for respiratory enzymes, electron transfer and proton-gradient-driven ATP synthesis.

• Outer membrane: a smooth outer boundary that is permeable to several small molecules, helping define the organelle while allowing exchange with the cytoplasm.
• Inner membrane: a less-permeable membrane folded into cristae. It contains electron-transfer proteins and ATP synthase, so the folds provide a large working surface for oxidative phosphorylation.
• Intermembrane space: the compartment in which protons accumulate during electron transfer. Keeping it separated from the matrix allows a proton concentration gradient to form across the inner membrane.
• Matrix: the aqueous internal compartment containing enzymes and other components needed for mitochondrial reactions. Its separation from the inner membrane keeps matrix chemistry distinct from the membrane-based proton gradient.

Structure → function: the double-membrane arrangement maintains compartmentalisation; cristae increase inner-membrane area for electron-transfer proteins and ATP synthase; and the sealed inner membrane helps retain the proton gradient. Together these features allow electron-transfer energy to be coupled to ATP synthesis. Cells with greater ATP demand may contain more mitochondria or mitochondria with more extensive cristae, within the source-supported boundary.

This card explains mitochondrial structure and its functional consequences. It does not replace the later cards on the ordered stages of respiration, individual pathway chemistry or numerical ATP yield. Cristae are folds of the inner membrane, not separate organelles, and ATP is made by coupling a gradient to ATP synthase rather than by the folds alone.

Aerobic respiration can be mapped as four linked stages. The location and broad output of one stage determine what can enter the next, while the final stage uses reducing power and oxygen to support ATP production.

1. Glycolysis — cytoplasm: glucose is split into pyruvate. The stage also provides some ATP and reduced coenzymes. Pyruvate is the carbon product passed towards the mitochondrion.
2. Link reaction — mitochondrial matrix: pyruvate is processed into an acetyl group carried by coenzyme A. Carbon dioxide and reduced coenzyme are produced, and acetyl-CoA is the entry material for the Krebs cycle.
3. Krebs cycle — mitochondrial matrix: acetyl-CoA enters a cyclic sequence of enzyme-controlled reactions. Carbon dioxide is released, a small amount of ATP is made, and more reduced coenzymes carry transferable electrons onward.
4. Oxidative phosphorylation — inner mitochondrial membrane: electrons from reduced coenzymes pass through membrane carriers; the released energy supports ATP synthesis, and oxygen accepts electrons at the endpoint to form water.

Route cue: cytoplasm → mitochondrial matrix → mitochondrial matrix → inner mitochondrial membrane. Carbon skeletons are progressively processed, while reducing power is transferred to the membrane stage for the main ATP-synthesis mechanism. The four-stage map is a scaffold; the detailed chemistry and accounting belong to the following cards.

The four stages are not four separate versions of the same reaction. Glycolysis is outside the mitochondrion; the link reaction and Krebs cycle are in the matrix; oxidative phosphorylation is on the inner membrane. This overview does not supply detailed NAD/FAD accounting or a fixed ATP total.

Glycolysis is the cytoplasmic stage in which one six-carbon glucose molecule is phosphorylated, split into two three-carbon routes and oxidised to form two pyruvate molecules. It gives a small net ATP gain and reduces coenzyme NAD.

1. Set the location and starting molecule: Glycolysis occurs in the cell cytoplasm and begins with one glucose molecule. It can supply ATP without requiring mitochondria, although later aerobic processing uses the pyruvate.
2. Investment: Transfer phosphate groups from ATP to glucose-derived material. This investment makes the substrate more reactive for the pathway; it is not the final ATP yield.
3. Lysis: The phosphorylated six-carbon intermediate is split into two three-carbon triose-phosphate molecules, creating two parallel routes.
4. Oxidation: Each three-carbon route loses hydrogen, which is accepted by NAD to form reduced NAD. This stores transferable reducing power rather than making ATP directly.
5. Recovery: Phosphate groups are transferred from the pathway intermediates to ADP, producing more ATP by substrate-linked phosphorylation. Because the recovery exceeds the investment, glycolysis has a small net ATP gain per glucose.
6. Output and boundary: The two three-carbon routes finish as two pyruvate molecules. Pyruvate may be processed in the next stage when conditions allow; the link reaction and Krebs cycle are separate cards.

Investment → recovery: early ATP use raises the reactivity of the glucose pathway; splitting doubles the three-carbon route; oxidation reduces NAD; and later phosphate transfer returns more ATP than was invested. The pathway therefore converts one glucose into two pyruvate while producing immediately usable ATP and reducing power.

Glycolysis is not the link reaction or the Krebs cycle, does not occur in the mitochondrial matrix, and does not directly release carbon dioxide. Do not treat the ATP used in the investment phase as a net loss or replace the pathway with a fixed numerical yield beyond the syllabus-supported net-gain idea.

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.

1. Entry: Pyruvate from glycolysis is transported into the mitochondrial matrix when aerobic processing can continue. The link reaction itself is a matrix process.
2. Decarboxylation: Enzymes remove one carbon from pyruvate and release it as carbon dioxide. This converts the three-carbon input into a two-carbon remainder.
3. Oxidation: The remaining carbon compound is dehydrogenated. Hydrogen/electrons are accepted by NAD, forming reduced NAD and storing transferable reducing power.
4. 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.
5. 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 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.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

NAD and FAD are reversible coenzymes that accept hydrogen/electrons during dehydrogenation. Their reduced forms temporarily carry this transferable reducing power from earlier respiratory reactions to the electron transport chain on the inner mitochondrial membrane.

• NAD: accepts hydrogen/electrons during oxidation of respiratory intermediates, becoming reduced NAD. Reduced NAD later donates its electrons/hydrogen to membrane electron carriers and is regenerated in its oxidised form.
• FAD: accepts hydrogen/electrons in the same reversible carrier pattern, becoming reduced FAD. Reduced FAD also transfers its electrons/hydrogen to the respiratory chain and is then reoxidised.
• Shared bridge: reduction captures transferable energy at dehydrogenation steps; later oxidation of the reduced coenzymes makes that reducing power available to the membrane electron-transfer system. This links matrix/cytoplasmic reactions to oxidative phosphorylation.
• Boundary: the source supports both coenzymes as carriers, but this card does not infer an unlisted energy ranking or detailed entry-point comparison. The chain mechanism and ATP accounting belong to the following cards.

Reaction → carrier → membrane: dehydrogenation removes hydrogen/electrons from a respiratory substrate; NAD or FAD accepts them and is reduced; the reduced coenzyme transfers them later and becomes oxidised again. Reoxidation matters because the oxidised coenzyme can accept more hydrogen/electrons in continuing respiratory reactions.

NAD and FAD are not ATP and do not release their stored energy directly as a complete ATP yield. They are reversible carriers that bridge earlier oxidation reactions to the inner-membrane electron-transfer stage; the proton gradient, ATP synthase and oxygen endpoint are taught separately.

Oxidative phosphorylation occurs at the inner mitochondrial membrane. Reduced NAD and FAD donate hydrogen/electrons to membrane electron carriers; the released energy builds a proton gradient, and proton flow through ATP synthase drives ATP formation. Oxygen accepts electrons at the end and water is formed.

1. Donate reducing power: Reduced NAD and reduced FAD deliver hydrogen/electrons to electron carriers in the inner mitochondrial membrane. The hydrogen separates into protons and energetic electrons.
2. Transfer electrons: Electrons pass through a series of membrane carriers. Their energy is released in steps rather than all at once.
3. Build the gradient: The released energy drives protons from the matrix into the intermembrane space. The inner membrane restricts proton movement, so an electrochemical gradient is established.
4. Couple proton flow to ATP: Protons return down the gradient through the channel protein ATP synthase. This facilitated movement supplies the energy for ADP and inorganic phosphate to form ATP.
5. Complete the chain: Oxygen accepts electrons and protons at the end of the chain, forming water. This final acceptance allows electron transfer and coenzyme reoxidation to continue under aerobic conditions.

Electron flow → gradient → ATP: reduced coenzymes provide electrons; electron-transfer energy pumps protons; the separated protons store potential energy; ATP synthase couples their return to ATP synthesis. Oxygen is the terminal electron acceptor, not the protein that directly makes ATP.

This card explains the oxidative-phosphorylation chain. It does not give a fixed ATP yield, inhibitor examples or detailed carrier names. Do not confuse electron movement with proton movement: electrons release the energy for pumping, while protons return through ATP synthase.

Respiratory ATP yield depends on how far the substrate is oxidised and whether oxygen allows oxidative phosphorylation to continue. Aerobic respiration uses glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation; anaerobic respiration continues with glycolysis and fermentation only.

• Glycolysis: uses substrate-level phosphorylation to give a small net ATP return and reduces coenzyme NAD while glucose becomes pyruvate. This is the ATP-producing stage that can continue without oxygen.
• Link reaction: prepares pyruvate for mitochondrial respiration, releases carbon dioxide and produces reduced NAD; it is not the main direct ATP-producing stage.
• Krebs cycle: gives a small substrate-level ATP return and produces reduced coenzymes as carbon compounds are oxidised. These coenzymes carry reducing power to oxidative phosphorylation.
• Oxidative phosphorylation: produces most aerobic ATP by using electron-transfer energy and a proton gradient; oxygen is required as the final electron acceptor. Its contribution is therefore much larger than the direct ATP made in glycolysis or the Krebs cycle.
• Per-glucose comparison: under the SME-supported textbook convention, aerobic respiration gives a high total of approximately 36 ATP, whereas anaerobic respiration gives a low total of approximately 2 ATP from glycolysis alone. Treat these as approximate teaching values, not universal measured constants.

Why the difference: aerobic respiration completely oxidises glucose and can transfer reducing power through the mitochondrial stages; anaerobic respiration partially oxidises glucose and cannot use oxygen as the final electron acceptor, so the mitochondrial ATP-producing stages do not proceed. The yield comparison is qualitative first and numerical only at the supported approximate level.

Do not add a fixed exact ATP number to every stage or treat the approximate total as independent of conditions. Distinguish substrate-level phosphorylation from oxidative phosphorylation; inhibitors and detailed causes of yield variation belong outside this card.

Anaerobic respiration allows glycolysis to continue when oxygen is unavailable. It gives a small ATP yield because glycolysis continues, while fermentation reoxidises reduced NAD to regenerate the oxidised NAD needed for glycolysis.

1. Oxygen unavailable: oxygen cannot act as the final electron acceptor, so the electron transport chain and oxidative phosphorylation stop. Reduced coenzymes cannot be reoxidised through that route, so the Krebs cycle also stops.
2. Glycolysis continues: glucose is converted to pyruvate in the cytoplasm, producing a small amount of ATP and reduced NAD.
3. NAD is regenerated: fermentation transfers hydrogen from reduced NAD to an acceptor. This restores oxidised NAD, allowing the dehydrogenation step in glycolysis to continue.
4. Mammalian muscle / lactate route: pyruvate accepts hydrogen from reduced NAD and is converted to lactate. This route does not release carbon dioxide.
5. Yeast and some plants / ethanol route: pyruvate is decarboxylated to ethanal, releasing carbon dioxide; ethanal then accepts hydrogen from reduced NAD and is converted to ethanol. Ethanol is a waste product.

Anaerobic respiration has a low energy yield because only glycolysis supplies ATP; it does not sustain the mitochondrial ATP-producing stages. Fermentation is a temporary or condition-dependent way to regenerate NAD, not a higher-yield replacement for aerobic respiration. Do not infer rice adaptations, temperature effects or substrate-concentration effects from this card.

Waterlogging restricts gas diffusion to roots and can reduce oxygen available for aerobic respiration. Rice survives and grows by improving gas access through its tissues and by tolerating the limited ATP yield and toxic products of ethanol fermentation.

• Keep gas access: upward growth can keep leaves above the waterline, where stomata can exchange oxygen and carbon dioxide with air.
• Move gases through the plant: aerenchyma in roots and stems contains air spaces. These spaces allow gases entering through above-water stomata to diffuse to tissues above and below the water, including submerged roots.
• Use anaerobic ATP production when oxygen is scarce: ethanol fermentation allows glycolysis to continue, so cells still receive a small ATP supply. This is a low-yield solution because the mitochondrial aerobic stages cannot continue without sufficient oxygen.
• Limit ethanol damage: rice tolerates higher ethanol levels than some other plants and produces more ethanol dehydrogenase, which breaks down ethanol. This reduces the toxicity barrier to continuing anaerobic respiration.
• Growth boundary: these adaptations support survival and active growth in flooded conditions, but anaerobic respiration still has low energy output and can produce damaging waste products; it is not equivalent to fully aerobic respiration.

Not every rice plant or waterlogged plant must show every listed feature. Keep this card focused on gas-space transport, ethanol-fermentation tolerance and the associated energy/toxicity trade-off. Do not infer general effects of temperature or substrate concentration.

Temperature and substrate concentration can change the rate of respiration because respiratory reactions depend on enzyme activity and on the availability of substrate. The rate should be inferred from a time-based or volume-based readout under controlled conditions, not from an unsupported endpoint comparison.

• Temperature: changing temperature changes molecular kinetic energy and enzyme-catalysed collision frequency. Rate may rise as temperature increases, but very high temperature can denature respiratory enzymes and lower the rate; do not assign a universal optimum value.
• Substrate concentration: increasing the available respiratory substrate can increase rate while substrate availability limits the pathway. Comparisons must keep the substrate type and all other relevant conditions consistent.
• Redox-indicator readout: in a yeast suspension, DCPIP or methylene blue can accept hydrogen and change from blue to colourless when reduced. Faster discolouration indicates faster hydrogen release and therefore a faster respiration rate. For a fixed endpoint, rate is inversely related to the time taken.
• Oxygen-consumption readout: a respirometer can measure oxygen consumption by a suitable aerobic organism; a controlled temperature and a time-normalised gas-volume change provide a rate. This readout is not a measure of anaerobic respiration, which does not consume oxygen.

1. Choose one independent variable: a range of temperatures or a range of substrate concentrations.
2. Keep yeast-cell volume and concentration, substrate type, dye volume and concentration, temperature of added solutions, reaction time and other relevant conditions matched. Use a no-dye control when the yeast suspension has its own colour.
3. Measure the same defined readout for every condition: for example, time to the same indicator endpoint or oxygen-volume change per unit time.
4. Repeat each condition, calculate a representative rate, and compare the rates or plot rate against the independent variable.
5. Interpret the pattern cautiously: use the early, comparable rate rather than allowing a late endpoint or exhausted substrate to dominate the conclusion.

A fair test changes one independent variable at a time. Do not infer a precise optimum temperature, invent apparatus specifications or numerical values, or treat colour-change time as a rate without accounting for the inverse relationship. Distinguish the yeast indicator investigation of anaerobic respiration from an oxygen-consuming respirometer investigation of aerobic respiration.

Floodwater slows gas diffusion and can restrict oxygen reaching submerged roots. Rice survives and actively grows in waterlogged conditions by improving gas access and tolerating the low-yield, ethanol-producing conditions of anaerobic respiration.

• Upward growth: in some rice types, faster growth away from the waterline keeps leaves above the water. Stomata can then access oxygen and carbon dioxide from air.
• Aerenchyma: air spaces in specialised root and stem tissue provide a pathway for gases entering through above-water stomata. Oxygen and carbon dioxide can diffuse through the plant to tissues above and below the water, including submerged roots.
• Anaerobic ATP supply: when aerobic respiration is limited, ethanol fermentation regenerates oxidised NAD so glycolysis can continue. This preserves a small ATP supply, but it is less efficient than aerobic respiration.
• Ethanol tolerance: ethanol produced by fermentation can accumulate and damage plant tissue. Rice tolerates higher ethanol levels and produces more ethanol dehydrogenase, which breaks down ethanol; this permits anaerobic respiration to continue for longer and supports survival and growth.

The features solve different parts of the flooding problem: aerenchyma and upward growth improve gas access, whereas ethanol tolerance reduces the cost of relying on fermentation. They do not restore the full ATP yield of aerobic respiration, and not every rice type must show every feature. Temperature and substrate-concentration effects belong to the later investigation objectives.

DCPIP and methylene blue are redox indicators that can accept hydrogen and change from blue to colourless when reduced. In a yeast suspension, the speed of this colour change is a proxy for respiration rate: faster discolouration indicates faster hydrogen release, not a direct measurement of ATP production.

• Respiratory dehydrogenation removes hydrogen from substrate molecules.
• The indicator accepts hydrogen and becomes reduced, producing the visible blue-to-colourless change.
• A faster respiration rate gives faster hydrogen release and faster indicator reduction.
• For a fixed colour endpoint, rate is inferred from the inverse relationship: a shorter time means a faster rate. Use the same endpoint and time basis for every condition.
• The SME investigation uses living yeast and can compare one independent variable at a time, such as temperature or substrate concentration.

1. Prepare matched yeast suspensions and choose either a controlled temperature range or a range of concentrations of the same substrate.
2. Keep yeast volume and concentration, substrate type, dye identity and volume, dye concentration, temperature of the added dye and other relevant conditions constant.
3. Add the indicator consistently, start timing at the same point, and record the time for the same defined colour endpoint.
4. Repeat each condition, use a control containing the yeast suspension without dye when the suspension has its own colour, and calculate a representative rate from the repeated readings.
5. Compare or plot rate against the independent variable. Interpret the colour change as an indirect proxy and avoid treating endpoint time as a direct ATP or oxygen measurement.

A fair test changes one independent variable at a time. Do not compare different dye volumes, yeast amounts or colour endpoints as if they were equivalent, and do not assign an unsupported optimum temperature or numerical rate. This indicator method is distinct from a respirometer, which measures oxygen consumption during aerobic respiration.

A respirometer estimates respiration rate indirectly from gas-volume or manometer change over time. For aerobic respiration, oxygen consumption changes the gas volume; the result is a rate only when the change is related to a defined time and matched biological material.

1. Set up experimental and control tubes: place the respiring organisms in one sealed tube and an equal volume of inert material in the control. The control helps compensate for pressure or temperature changes that are not caused by respiration.
2. Control the conditions: place both tubes in a thermostatically controlled water bath. Keep organism type, mass or volume, acclimation time, temperature, seal and run duration matched. Use soda-lime in both tubes when the aim is to absorb carbon dioxide and isolate oxygen-consumption change.
3. Calibrate and reset: record the initial manometer position, allow the apparatus to equilibrate, and reset the fluid/air volume before a new temperature or replicate. Keep the apparatus sealed during the measurement.
4. Measure a rate: record the manometer movement over a known time and convert it to gas-volume change using the validated apparatus geometry; express the result as change per unit time. If a capillary radius and fluid distance are supplied, the volume relationship is (V = πr^2h).
5. Repeat and compare: repeat at each temperature or condition, calculate a representative value, and plot or compare the rates. Interpret the result as an indirect oxygen-consumption measure, not a direct ATP measurement.

A respirometer with soda-lime measures oxygen consumption during aerobic respiration; it does not measure anaerobic respiration directly. A single manometer reading is not a rate, and a change in temperature can affect the apparatus as well as the organisms. Do not invent numerical values, apparatus specifications or unsafe procedures; distinguish this investigation from the separate RQ calculation using paired soda-lime runs.