Unit 3.3 Respiration

A series of enzyme-controlled reactions that transfers energy from organic substrates to ATP; some energy is released as heat.
Larger organic molecules are broken down into smaller molecules.

ATP supplies immediate energy for processes such as active transport, muscle contraction, protein synthesis and cell division.

An organic molecule used in respiration to release energy, e.g. glucose, fatty acids, glycerol or amino acids.

It requires oxygen and completely oxidises glucose to CO₂ and H₂O.
They operate without oxygen and only partly break down glucose, producing lactate or ethanol + CO₂.

Glucose + oxygen → carbon dioxide + water, with energy transferred to ATP and released as heat.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

ATP = adenosine triphosphate, a nucleotide. It contains adenine + ribose + three phosphate groups. Adenine + ribose = adenosine.

ADP has two phosphate groups; ATP has three.

ATP + H₂O → ADP + Pᵢ; removal of the terminal phosphate releases energy overall. ATPase catalyses the reaction.

Energy drives ADP + Pᵢ → ATP. Adding phosphate is phosphorylation.
Inorganic phosphate.

It releases energy rapidly in small usable quantities and can be regenerated quickly.
It is soluble, its reactions are enzyme-controlled, and it is continually made and used as a temporary energy store.

Enzyme-catalysed phosphate transfer from a phosphorylated intermediate to ADP, forming ATP independently of an electron transport chain.
In glycolysis and the Krebs cycle.

ATP production using a proton gradient generated by electron transport; it occurs at the inner mitochondrial membrane.

No. However, the Krebs cycle depends indirectly on oxygen because its oxidised coenzymes must be regenerated.

In the cytoplasm of most eukaryotic cells; prokaryotes lack mitochondria.
Mature red blood cells.

Cells with high ATP demand, such as muscle cells, generally contain many mitochondria.

It encloses the organelle and separates it from the cytoplasm.

Electron carriers, proton pumps and ATP synthase.
Their folds increase the inner membrane's surface area for oxidative phosphorylation.

The space between the outer and inner membranes; protons accumulate here during electron transport.
Its small volume allows proton concentration to rise as protons are pumped into it.

The internal compartment containing enzymes and coenzymes for the link reaction and most Krebs cycle reactions.

It carries genes for some mitochondrial proteins and RNA; it is typically circular.
Synthesise some mitochondrial proteins.

ATP synthase complexes, with catalytic heads projecting into the matrix.

In the cytoplasm/cytosol.
In the mitochondrial matrix.
In the mitochondrial matrix; this is the location expected in the overview.
At the inner mitochondrial membrane/cristae.

Converts one 6C glucose into two 3C pyruvates.

It does not directly require oxygen and occurs in both aerobic respiration and anaerobic pathways.

Phosphorylation, including splitting; then oxidation.

Two ATP, each becoming ADP.
Glucose → hexose phosphate → hexose bisphosphate.
It activates the molecule for subsequent reactions, including splitting.

It splits into two 3C triose phosphate molecules.

Hydrogen is removed and accepted by NAD, forming reduced NAD; further reactions form pyruvate.
Triose phosphate is oxidised; NAD is reduced.

Phosphate is transferred from phosphorylated organic intermediates to ADP by substrate-level phosphorylation.
Four ATP.
Four ATP made − two ATP used initially = two ATP gained overall.

Per glucose: two pyruvates + two reduced NAD + two net ATP. No CO₂ is released and no reduced FAD is formed.

One 6C glucose → phosphorylated 6C compound → two 3C triose phosphates → two 3C pyruvates.
Two ATP used at the start; four ATP formed later; two NAD reduced.

It enters the mitochondrion for the link reaction.

Its reducing equivalents supply electrons for electron transport, regenerating oxidised NAD.
Glycolysis requires oxidised NAD to accept hydrogen; otherwise its limited supply runs out.

It can be reduced to lactate, or converted to ethanol + CO₂, depending on the organism.

One carbon atom leaves as CO₂, leaving a 2C fragment.
Hydrogen is accepted by NAD, forming reduced NAD.
The remaining 2C acetyl group combines with coenzyme A.

Oxidation by hydrogen removal and removal of CO₂ occur.
Dehydrogenase and decarboxylase activities.

It carries the acetyl group into the Krebs cycle; it is a coenzyme, not an enzyme.
No. Two carbons refers to the acetyl group carried by coenzyme A.

Pyruvate + NAD + coenzyme A → acetyl coenzyme A + reduced NAD + CO₂.

Per pyruvate: one acetyl CoA + one reduced NAD + one CO₂; no direct ATP. Per glucose: double these products because glycolysis makes two pyruvates; still no direct ATP.

Acetyl groups enter Krebs; reduced NAD supplies electrons for oxidative phosphorylation; CO₂ leaves the cell.

An enzyme-controlled cyclic pathway that oxidises acetyl groups, releasing CO₂ and producing reduced coenzymes and ATP.

A 2C acetyl group combines with a 4C acceptor to form a 6C compound; coenzyme A is released and reused.
Oxaloacetate = 4C acceptor; citrate = 6C product. Carbon numbers are the priority in the overview.

6C → 5C → 4C. Each step releases one CO₂: two decarboxylations and two CO₂ per turn.

Hydrogen is removed from intermediates and accepted by NAD or FAD. Four pairs are removed per turn, forming three reduced NAD + one reduced FAD.

One ATP equivalent by substrate-level phosphorylation.

So it can accept another acetyl group and the cycle can repeat.

One glucose produces two pyruvates, which give two acetyl groups.
Four CO₂ + six reduced NAD + two reduced FAD + two direct ATP equivalents.
Eight pairs.

No. Oxygen is used directly at the end of electron transport.
Electron transport needs oxygen to reoxidise reduced NAD/FAD. Without oxidised coenzymes, dehydrogenation and the cycle stop.

2C + 4C → 6C → 5C → 4C; two CO₂ lost; acceptor regenerated; three NAD and one FAD reduced; one ATP formed per turn.

Loss of electrons; in respiration, often loss of hydrogen.
Gain of electrons; in respiration, often gain of hydrogen.

Removal of hydrogen, catalysed by a dehydrogenase.
Removal of CO₂ from a molecule.

The substrate is oxidised; NAD is reduced.

An organic non-protein molecule that helps an enzyme, e.g. by transferring hydrogen, electrons or chemical groups.

Glycolysis, the link reaction and the Krebs cycle.
The Krebs cycle.

NAD/FAD carry hydrogen/electrons; coenzyme A carries acetyl groups.

NADH, or NADH + H⁺ when the accompanying proton is shown; use the notation in the question.
FADH₂.

They are oxidised again and can accept hydrogen in further reactions.
Their supply is limited; recycling allows respiration to continue.

A sequence of electron carriers in the inner mitochondrial membrane.
Reduced NAD and reduced FAD.

A carrier is reduced when it accepts electrons and oxidised when it passes them on.

Energy is released in stages and used to pump protons.
From the matrix into the intermembrane space.
Against the electrochemical gradient; it requires energy.

An electrochemical proton gradient: a difference in H⁺ concentration and electrical charge.
The intermembrane space.

It prevents unrestricted proton return and allows the gradient to be maintained.

H⁺ flows down its electrochemical gradient from the intermembrane space into the matrix through ATP synthase. This drives ADP + Pᵢ → ATP on the matrix side. Coupling proton flow to ATP formation is chemiosmosis.

Oxidation of reduced coenzymes supplies energy that ultimately drives phosphorylation of ADP.

It is the final electron acceptor.
Oxygen combines with electrons and H⁺ to form water.
½O₂ + 2e⁻ + 2H⁺ → H₂O; equivalently O₂ + 4e⁻ + 4H⁺ → 2H₂O.

It diffuses from blood through tissue fluid into cells.

Electrons cannot be passed to the final acceptor, so carriers remain reduced and electron flow stops.
Pumping stops; the gradient dissipates; oxidative ATP synthesis stops.

Reduced NAD/FAD cannot be reoxidised by electron transport, so oxidised coenzymes become unavailable.
Yes, if an anaerobic pathway regenerates NAD.

Its electrons enter later, bypassing one proton-pumping stage, so less energy is used to establish the gradient.

Matrix and intermembrane space; inner membrane; carriers; H⁺ pumped out and returning through ATP synthase; oxygen forming water; ATP formation.

Glycolysis 2 + link reaction 2 + Krebs 6 = 10.
Glycolysis 0 + link reaction 0 + Krebs 2 = 2.

Three ATP through oxidative phosphorylation.
Two ATP through oxidative phosphorylation.

Reduced NAD: 10 × 3 = 30 ATP. Reduced FAD: 2 × 2 = 4 ATP. Total oxidative ATP = 34 (theoretical model).

Two from glycolysis + two from Krebs = four ATP.
34 oxidative + 4 net substrate-level = 38 ATP.

ATP made by substrate-level phosphorylation minus ATP used in that stage.
ATP made during oxidative phosphorylation using electrons from coenzymes produced in an earlier stage.

Reduced FAD 0; reduced NAD 2; net direct ATP 2. Its reduced NAD supports 6 ATP later. Total attributed to glycolysis = 2 + 6 = 8 in the theoretical model.

Reduced FAD 0; reduced NAD 2; direct ATP 0; later oxidative ATP 6; total attributable ATP 6.
No. Its two reduced NAD supply electrons for later oxidative phosphorylation.
Three later ATP from one reduced NAD; no direct ATP.

Reduced FAD 2; reduced NAD 6; direct ATP 2; later oxidative ATP 22; total attributable ATP 24.
(6 reduced NAD × 3) + (2 reduced FAD × 2) = 22.

Reduced FAD 2; reduced NAD 10; net direct ATP 4; later oxidative ATP 34; total ATP 38.

(3 × 3) + (1 × 2) + 1 direct = 12 ATP equivalents.

Six: four in glycolysis + two in Krebs. Subtract the initial two ATP used to give four net.

Multiply the per-glucose yield by the number of glucose molecules.
Aerobic: 5 × 38 = 190 ATP. Anaerobic: 5 × 2 = 10 ATP.

38 ÷ 2 = 19 times.

The link reaction and Krebs; not glycolysis or electron transport.
Electron transport, where oxygen is the final electron acceptor.

Their reduced forms are oxidised overall when they supply electrons; this regenerates NAD and FAD.

Gross vs net; per turn vs per glucose; direct ATP vs ATP made later from reduced coenzymes.

Regenerating NAD so glycolysis can continue.

Pyruvate accepts hydrogen from reduced NAD and becomes lactate. Reduced NAD is oxidised to NAD, allowing glycolysis to continue. Pyruvate + reduced NAD → lactate + NAD.

One 6C glucose → two 3C lactates.
No.

Two net ATP per glucose, all made in glycolysis. Converting pyruvate to lactate makes no extra ATP.

It provides ATP when demand exceeds the rate supported by oxygen delivery and oxidative phosphorylation.
Its yield per glucose is low; glucose is used rapidly and much chemical energy remains in lactate.

3C pyruvate → 2C ethanal + CO₂ (decarboxylation). Ethanal accepts hydrogen from reduced NAD: ethanal + reduced NAD → ethanol + NAD. The NAD returns to glycolysis.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
Two ethanol + two CO₂ + two net ATP from glycolysis; two reduced NAD are reoxidised.

Pyruvate in the lactate pathway; ethanal in the ethanol pathway.

Reduced NAD supplies electrons for electron transport and becomes oxidised again.

Aerobic respiration: complete. Lactate and ethanol pathways: incomplete, so their products retain chemical energy.

Their breakdown supplies glucose or glucose-derived intermediates; both are α-glucose polymers.

Glucose + fructose.
It is converted to glycolytic intermediates; fructose and glucose are structural isomers.

No; humans lack cellulase.

They are broken into 2C units carried as acetyl CoA into the Krebs cycle.
No; the number depends on chain length.

They are more reduced and contain proportionally more hydrogen, supplying more electrons for oxidative phosphorylation.

It can support rapid ATP production and anaerobic glycolysis; fatty-acid oxidation depends on aerobic metabolism.

They are hydrolysed to amino acids, which are deaminated, mainly in the liver.
Removal of the amino group from an amino acid, producing ammonia and a carbon skeleton/keto acid.

Through glycolysis, acetyl CoA or Krebs intermediates, depending on the amino acid.

It is converted to less toxic urea in the liver; urea is carried to the kidneys and excreted in urine.

During severe nutrient shortage/starvation.
Loss of functional muscle tissue and production of nitrogenous waste.

Carbohydrates, lipids and proteins can feed into common pathways; intermediates can also be used in synthesis.

Condensation builds proteins; hydrolysis breaks them down.

No. Opposite directions can use different enzymes and different pathways.

Energy = mass × energy value per gram; make units consistent.
Kilojoules per gram.

Glucose 32 kJ; lipid 78 kJ; protein 34 kJ. Treat these as supplied practice values rather than universal constants.

RQ = CO₂ produced ÷ O₂ consumed, using volumes or rates. Match time interval, volume units, temperature and pressure. RQ has no units because the units cancel.

Divide the CO₂ coefficient by the O₂ coefficient.
6 CO₂ ÷ 6 O₂ = 1.0.

Fat contains proportionally less oxygen, so oxidation needs relatively more external O₂ than the CO₂ produced.

102 ÷ 145 ≈ 0.70. The equation is provided for application, not memorisation.

14 ÷ 20 = 0.70.

Multiply by 1,000; 1 dm³ = 1,000 cm³.

No; division by zero is undefined.

Gas-exchange rate = change in volume ÷ change in time, e.g. cm³ min⁻¹.
A faster gas-exchange rate.
Divide the corresponding CO₂-production gradient by the O₂-consumption gradient.

About 1 µm diameter and 10 µm long; size varies. These are not fixed dimensions to memorise.

Glyceraldehyde 3-phosphate.

No. It is an aldolase-catalysed cleavage; use 'splitting' in the overview.

As hydrogencarbonate ions, before being released at the lungs; linked transport knowledge.

Citric acid cycle; tricarboxylic acid cycle (TCA).
Sir Hans Krebs, 1937; historical background only.

It is rearranged to isocitrate; both contain six carbons.

6C isocitrate → 5C α-ketoglutarate, releasing CO₂ and reducing NAD.
Oxoglutarate.

5C α-ketoglutarate → a 4C succinyl group carried on CoA, releasing CO₂ and reducing NAD.

Succinyl coenzyme A → succinate; one ATP equivalent per turn.
Depending on the enzyme form, ATP or GTP is formed; GTP can transfer phosphate to ADP. Detailed mechanism is extra.

Energy from its conversion supports a transient phosphorylated intermediate; phosphate is relayed through the enzyme to ADP or GDP. It is not a one-step phosphate transfer from unmodified succinyl CoA.

Succinate → fumarate, by dehydrogenation.
No; water is added in this step.
Malate → oxaloacetate, by dehydrogenation.

Their hydrogen/oxygen content and bonding differ; carbon number alone does not identify a molecule.
Citrate/isocitrate: 6C; α-ketoglutarate: 5C; succinyl group, succinate, fumarate, malate and oxaloacetate: 4C.

Two CO₂-removal/NAD-reduction steps; ATP-equivalent production; FAD reduction; an intervening step; final NAD reduction.
Carbon counts of the detailed sequence including the succinyl group; optional mnemonic, not the essential overview.

Nicotinamide adenine dinucleotide; flavin adenine dinucleotide.

Adenine, ribose and two phosphate groups.
NAD contains nicotinamide and two ribose rings; FAD contains flavin, a ribose ring and a ribitol chain.

NAD⁺ + 2H → NADH + H⁺.

NADH at complex I; electrons from succinate oxidation enter through complex II.
I, III and IV; II does not.
No. It is an alternative electron-entry route.
Complex IV.

About 10 for NADH-linked entry and 6 for FAD-linked entry. Do not combine these figures mechanically with the older 3/2 ATP model.

No; it is a simplified teaching value. Keep the principle of proton flow driving ATP synthesis.

Lactate predominates at physiological pH; 'lactic acid' is often used in school diagrams.

It can be converted back to pyruvate and used metabolically; the liver can also use its carbon to remake glucose.
Extra oxygen taken in after exercise to support recovery, including metabolism of accumulated lactate and restoration of energy stores.
To supply oxygen for recovery processes.

No; it is a reusable metabolite. Avoid treating this as an explanation for all exercise fatigue.

Ethanal = acetaldehyde, CH₃CHO; ethanol = CH₃CH₂OH.

Via hexose bisphosphate; actual routes vary with tissue, so do not treat this as universal.

It has many more oxidisable carbon and hydrogen atoms: C₅₁H₉₈O₆ versus C₆H₁₂O₆.

In CO₂.

Enzyme activities, oxygen consumption and ATP-production rates; the passage also describes immediate freezing of the sample.

They enter at different points, helping identify which region of a pathway is impaired.

It has many mitochondria and is relatively accessible compared with some vital organs.

A problem before α-ketoglutarate's entry point, e.g. pyruvate transport, the link reaction or early Krebs steps; later reactions still function.
No. It narrows down a region; more evidence is needed.

Its conversion to the next compound is restricted, possibly by an inhibited or defective enzyme.

More reliance on glycolysis and pyruvate reduction to regenerate NAD.

In the supplied older model: NAD-linked route A gives 3 ATP per electron pair; FAD-linked route B gives 2. These labels belong only to that question.
Two glycolytic reduced NAD each support one fewer ATP: a loss of 2 ATP overall.

Repeated muscle contraction uses ATP.

No; shuttle pathways transfer reducing equivalents. Shuttle details are extra context.

W outer membrane; X intermembrane space; Y inner membrane; Z matrix. These labels are copied from the notes and apply only to the original diagram, which is not reproduced in this deck.
X → Z: intermembrane space → matrix. ADP + Pᵢ → ATP.

A triose phosphate; B pyruvate; C acetyl coenzyme A. Use only with the original labelled diagram.
D NAD\/reduced NAD; E CO₂; F O₂. The original diagram is needed to interpret D's direction.

Aerobic bacteria became established inside ancestral eukaryotic cells more than one billion years ago; the date is passage context.
More efficient aerobic ATP production.
Nutrients and protection; host glycolysis could provide pyruvate.

DNA, ribosomes and double-membrane organisation.

Triglyceride hydrolysis produces glycerol + fatty acids. Glycerol → triose phosphate → glycolysis. Fatty acids → 2C units carried as acetyl CoA → Krebs.

It connects the breakdown of sugars, fats and some amino acids.

Some H⁺ leaks back without passing through ATP synthase, dissipating gradient energy. Transporting metabolites also has an energetic cost; WJEC mentions pyruvate and ADP.

Around 30–32 per glucose; keep this separate from the theoretical 38-ATP accounting model.
Follow the question's stated model; do not mix older 3/2 ATP yields with modern estimates.

Electrochemical gradient.

ATP synthetase; both refer to the ATP-producing enzyme here.

Both mitochondrial and chloroplast membranes maintain proton gradients that drive ATP synthesis.
Mitochondria: accumulate in the intermembrane space, return to matrix. Chloroplasts: accumulate in thylakoid lumen, return to stroma.

They provide 'electrons and protons' for electron transport; explain how these support ATP production.

Accounting for ATP used and made under aerobic and anaerobic conditions.

Unit 3.1 excludes their names from required recall; learn their functions. Numbered complexes remain in the extra-detail section only.

Investigating factors affecting the rate of respiration in yeast.
CO₂ production per unit time.

Mix 5 cm³ yeast suspension with 10 cm³ sucrose solution in a 20 cm³ syringe; lay it in a water bath with its nozzle uppermost and hold it with a weight.
Allow about 2 minutes to equilibrate; once bubbles emerge regularly, count them for 1 minute.

To distribute cells evenly, making samples more comparable.

So its temperature reaches the chosen value.

Independent: temperature. Dependent: CO₂ production rate, estimated by bubbles per minute.
Yeast amount/concentration, sugar type/concentration/volume, pH, measurement duration and apparatus.

Changes during measurement would alter enzyme activity and confound the comparison.

To assess variation and reduce the effect of random error; examine anomalous results rather than automatically deleting them.

Bubble volumes can differ; equal numbers need not mean equal gas volumes.
Collect and measure CO₂ volume over time with a suitable gas syringe or sensor, checking the apparatus for leaks.

Restart that replicate rather than use its count.

Molecules have more kinetic energy; successful enzyme–substrate encounters occur more frequently.
Enzyme structure and active sites are disrupted by denaturation, reducing catalysis.

Vary sugar concentration while keeping yeast, temperature, pH, total volume and timing constant; measure CO₂ rate.

Baseline CO₂ production without supplied sugar; yeast may still use stored substrates.

Use buffers of different pH while controlling the other conditions and comparing CO₂ rates.

Temperature on the x-axis; mean respiration-rate estimate on the y-axis, with units.

Rate at (T + 10) °C ÷ rate at T °C; it describes the effect of a 10 °C rise over the chosen interval.
24 ÷ 12 = 2.
No. It can fit a common trend below the optimum, but accuracy requires independent assessment; Q₁₀ varies with conditions.

Avoid hot-water burns, keep electrical handling dry, and prevent yeast/solutions contacting eyes; use eye protection and follow the school risk assessment.

Investigating dehydrogenase activity with artificial hydrogen acceptors such as methylene blue, DCPIP or tetrazolium compounds.

It accepts hydrogen removed during respiration and changes colour on reduction.
Blue → colourless.

Equilibrate 10 cm³ yeast suspension at 30 °C for 5 minutes; add 1 cm³ indicator, mix once and time the colour change in the water bath.

Relative rate = 1/time; if time is in seconds, units are s⁻¹.

Indicator amount/concentration, yeast amount, endpoint judgement and all conditions except the chosen independent variable.
Oxygen can reoxidise the indicator; mixing and exposure to air must be consistent.

Live yeast versus boiled, cooled yeast treated otherwise identically; heating denatures enzymes.

Avoid skin/eye contact and wear eye protection; follow the indicator-specific school risk assessment.

H⁺ is pumped from matrix to intermembrane space against its gradient. It returns through ATP synthase down the gradient, driving ATP formation. Oxygen is the final electron acceptor.