Respiration — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

Section 1 of 30ATP as the energy currency

Uses of energy

• Uses of energy anabolic reactions, active transport, movement (muscle contraction, cilia) and maintaining body temperature in mammals and birds.
  • (protein and DNA synthesis)

Why ATP suits its role.

• Energy is released quickly in one hydrolysis step and in small amounts that match cell needs.
• It is soluble and moves easily within the cell.
• It cannot pass out through the cell membrane.
• It is regenerated rapidly from ADP.

Hydrolysis

• ATP + H₂O → ADP + Pi releases about 30.5 kJ per mol, catalysed by ATPase.

Making ATP

• Making ATP substrate-level phosphorylation and chemiosmosis (mitochondria in respiration, chloroplasts in photosynthesis).
  • (glycolysis, Krebs cycle)

Section 2 of 30Overall equation and location of stages

• Equation
  • C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy).

Section 3 of 30The four stages

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	ATP and water

1Link plus Krebs

• Link plus Krebs together they give 6 CO₂, 8 reduced NAD, 2 reduced FAD and 2 ATP per glucose (two pyruvates).

2Decarboxylation

• Decarboxylation removal of CO₂.

3Dehydrogenation

• Dehydrogenation removal of hydrogen, catalysed by dehydrogenase enzymes.

Section 4 of 30Glycolysis

1

Glucose is phosphorylated to hexose phosphate and then hexose bisphosphate, using 2 ATP.

2

This makes it more reactive and traps it in the cell.

3

The hexose bisphosphate splits (lysis) into two triose phosphates.

4

Each triose phosphate is oxidised, with H removed to NAD.

5

Each triose phosphate is converted to pyruvate, making 2 ATP by substrate-level phosphorylation (4 in total).

6

The net yield is 2 ATP and 2 reduced NAD per glucose.

Section 5 of 30Link reaction and Krebs cycle

1

Pyruvate is actively transported into the mitochondrial matrix.

2

In the link reaction it is decarboxylated and dehydrogenated, and the acetyl group joins coenzyme A to form acetyl CoA.

3

In the Krebs cycle acetyl (2C) combines with oxaloacetate (4C) to form citrate (6C).

4

Decarboxylations and dehydrogenations regenerate oxaloacetate.

5

Each turn releases CO₂, reduced NAD, reduced FAD and 1 ATP.

6

Coenzyme A

• Coenzyme A made from a nucleoside joined to a vitamin (pantothenic acid).
  • (adenine plus ribose)
• It picks up the 2C acetyl group, hands it to oxaloacetate and is released for reuse.

7

Other substrates

• Other substrates fatty acids are split into 2C fragments that enter as acetyl CoA.
• Amino acids are deaminated, and the keto acids enter the Krebs cycle or become pyruvate or acetyl CoA.

Section 6 of 30Oxidative phosphorylation

1

Reduced NAD and FAD are re-oxidised, releasing H atoms that split into protons and electrons.

2

Electrons move along a chain of carriers at progressively lower energy levels.

3

The energy pumps protons into the intermembrane space, creating a proton gradient.

4

Reduced FAD donates electrons further along, so fewer protons are pumped (fewer ATP).

5

Protons diffuse back through ATP synthase (chemiosmosis), driving ADP + Pi → ATP.

6

Oxygen is the final electron acceptor, combining with electrons and protons to form water.

Section 7 of 30Mitochondrial structure and function

Mitochondrial structure and function

Double membrane

• Double membrane the inner membrane is folded into cristae, giving a large surface area for the electron transport chain and ATP synthase.

Outer membrane

• Outer membrane it is smooth and lets many small molecules through.

Inner membrane

• Inner membrane it is impermeable to H⁺, so protons return only through ATP synthase.

Intermembrane space

• Intermembrane space it is small, so a steep proton gradient builds, and it has a lower pH than the matrix.

Matrix

• Matrix it contains enzymes of the link reaction and Krebs cycle, mitochondrial DNA and 70S ribosomes.

Cell activity

• Cell activity cells with high ATP demand,
  • such as muscle, have more mitochondria than fat-storage cells, often with longer, more densely packed cristae.

Section 8 of 30Anaerobic respiration and yield

Without oxygen

• Without oxygen the electron transport chain stops, reduced NAD cannot be re-oxidised, and the link reaction and Krebs cycle stop.

Mammals

• Mammals pyruvate accepts H to form lactate.
  • (regenerating NAD so glycolysis continues)
• Lactate lowers pH and is carried to the liver, where it is oxidised back to pyruvate (oxygen debt).

Yeast and plants

• Yeast and plants pyruvate → ethanal + CO₂, then ethanal + reduced NAD → ethanol + NAD.

Yield

• Yield anaerobic respiration nets 2 ATP per glucose, while aerobic respiration gives about 32 in practice.
  • (theoretical up to 38)
• Some energy is lost as heat and some protons leak.

Rice

• Rice it is adapted to flooded paddy fields.
• Aerenchyma lets oxygen diffuse to submerged tissues.
• Stems grow rapidly to keep leaves above water.
• Roots tolerate ethanol and have high alcohol dehydrogenase.

Section 9 of 30RQ and respirometer

RQ

CO₂ produced ÷ O₂ consumed (no units), with both volumes in the same units.

What the symbols mean: RQ = respiratory quotient (no unit) · about 1.0 for carbohydrate, 0.7 for lipid, 0.9 for protein

Typical values

• Typical values carbohydrate 1.0, protein about 0.9, lipid about 0.7, and above 1.0 means some anaerobic respiration.
  • (for example yeast making ethanol)

Energy values

• Energy values lipids about 39 kJ per g, carbohydrate about 16 and protein about 17.
• Lipids release most because they contain more hydrogen atoms, giving more reduced NAD and FAD and more ATP.

Respirometer

• Respirometer organisms plus soda lime or KOH, which absorbs CO₂, so the liquid moves towards the organisms as O₂ is used.
• A control tube with glass beads of equal volume compensates for temperature and pressure changes.
• A thermostatic water bath keeps temperature constant.
• Repeat with water instead of soda lime to find CO₂ production.

Volume

πr²h; rate = volume ÷ time, often per gram of organism.

Section 10 of 30Redox indicators and yeast

Colour change

• DCPIP or methylene blue are blue when oxidised and colourless when reduced.
• They accept hydrogen from dehydrogenase reactions, so they measure respiration rate in yeast.

Temperature

• Temperature time for decolourisation at different temperatures gives rate = 1 ÷ time.
• The rate rises to an optimum, then falls as enzymes denature.

Substrate concentration

• Substrate concentration compare glucose concentrations such as 0.2, 0.6 and 1.2 per cent.
• Keep temperature, yeast volume, dye volume and sugar type constant.
• A tube of yeast without dye shows the end-point colour.

Section 11 of 30More from the specification

More from the specification

Respiration

• Respiration breakdown of glucose to release ATP, where aerobic respiration takes place in the mitochondria and releases carbon dioxide and water

Fermentation

• Fermentation anaerobic process that regenerates NAD when oxygen is absent, so glycolysis can continue

Ethanol fermentation

• Ethanol fermentation in yeast, pyruvate is converted to ethanol and carbon dioxide

Function of the mitochondrion

• Function of the mitochondrion the matrix hosts the link reaction and Krebs cycle,
  • and the cristae of the inner membrane carry the electron transport chain

Fermentation

• Anaerobic respiration by yeast producing ethanol and carbon dioxide.

Ethanol fermentation

• Decarboxylation of pyruvate to ethanal, then reduction to ethanol.

Section 12 of 30The Krebs cycle

Location

• Location the mitochondrial matrix.

Entry

• Entry acetyl CoA (2C) from the link reaction joins oxaloacetate (4C) to form citrate (6C).

Regeneration

• Regeneration citrate is converted back to oxaloacetate through a series of enzyme-controlled reactions.

Decarboxylation

• 2 CO₂ are released as waste.

Dehydrogenation

• Dehydrogenation hydrogen atoms reduce NAD and FAD.

Substrate-level phosphorylation

• Substrate-level phosphorylation one ATP is made per turn.

Section 13 of 30Role of NAD and FAD

Hydrogen carriers

• NAD and FAD accept hydrogen atoms and are reduced.
  • (a proton and an electron)

Delivery

• Delivery reduced NAD and FAD carry hydrogens to the electron transport chain on the inner membrane.

Oxidation

• Oxidation they are oxidised when the hydrogens are removed, so they can be used again.

Electrons

• Electrons go to the electron transport chain.

Protons

• Protons are released and pumped across the membrane.

Reduced coenzymes per glucose	Stage
Reduced NAD, 2	Glycolysis
Reduced NAD, 2	Link reaction
Reduced NAD, 6	Krebs cycle
Reduced FAD, 2	Krebs cycle

Section 14 of 30Oxidative phosphorylation

1

Reduced NAD and FAD donate hydrogen atoms, which split into protons and electrons.

2

High-energy electrons pass along the electron transport chain, releasing energy.

3

The energy pumps protons from the matrix into the intermembrane space.

4

A proton gradient forms, because the membrane is impermeable to protons.

5

Protons flow back by facilitated diffusion through ATP synthase, driving ATP synthesis.

6

Oxygen combines with protons and electrons to form water.

Section 15 of 30Anaerobic conditions

1Electron transport chain

• Electron transport chain has no final acceptor and stops.

2ATP

• ATP no more is made by oxidative phosphorylation.

3Reduced NAD and FAD

• Reduced NAD and FAD are not oxidised.

4Krebs cycle

• Krebs cycle stops because oxidised NAD and FAD are not available.

5Glycolysis

• Glycolysis can continue if NAD is regenerated by fermentation, making small amounts of ATP.

Section 16 of 30Ethanol fermentation

1Organisms

• Organisms yeast and some microorganisms.

2Step 1

• Step 1 pyruvate is decarboxylated to ethanal, releasing CO₂.

3Step 2

• Step 2 ethanal is reduced to ethanol by alcohol dehydrogenase, with reduced NAD as hydrogen donor.

4Ethanol

• Ethanol cannot be metabolised further and is a waste product.

Section 17 of 30Lactate fermentation

Organisms

• Organisms mammalian muscle cells and some microorganisms.

Reaction

• Reaction pyruvate is reduced to lactate by lactate dehydrogenase, with reduced NAD as hydrogen donor.

Fate of lactate

• Fate of lactate oxidised back to pyruvate or converted to glycogen in the liver.

Oxygen debt

• Oxygen debt the extra oxygen needed to oxidise lactate.

Section 18 of 30More from the specification

1

Aerobic vs. Anaerobic Respiration Pathways

• Aerobic respiration requires molecular oxygen to completely break down a glucose molecule into carbon dioxide and water, yielding approximately 32 ATP,
• whereas anaerobic respiration occurs in the absence of oxygen and produces lactate or ethanol with only 2 net ATP.

2

Four Main Stages of Aerobic Respiration

• Aerobic respiration proceeds through four sequential stages: glycolysis in the cytoplasm, the link reaction, the Krebs cycle in the mitochondrial matrix,
• and oxidative phosphorylation on the inner mitochondrial membrane.

3

Effect of Temperature and Substrate Concentration

• As temperature increases towards the optimum, respiratory enzyme activity increases due to greater kinetic energy,
• but excessive heat denatures enzymes; similarly,
• increasing glucose concentration increases respiration rate until enzyme active sites are saturated.

4

Measuring Respiration Rate Using Respirometers and Indicators — A respirometer measures the rate of aerobic respiration by tracking oxygen consumption while potassium hydroxide absorbs produced CO₂; redox indicators like methylene blue monitor dehydrogenase function by changing colour from blue to colourless when reduced.

5

Substrate concentration

• The amount of substrate per unit volume.

6

Respiration

• Chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

7

Aerobic

• Using oxygen.

8

Anaerobic

• Without oxygen.

9

Aerobic respiration

• Respiration that uses oxygen to release a relatively large amount of energy.

10

Anaerobic respiration

• Respiration without oxygen, releasing much less energy per glucose molecule.

Section 19 of 30Energy yield

Anaerobic

• Anaerobic glucose is only partly oxidised, so little energy is transferred to ATP.

Only glycolysis

• Only glycolysis continues, giving about 2 ATP per glucose.

Aerobic

• Aerobic mitochondrial stages make far more ATP, roughly 30 to 36 per glucose.

Reason

• Reason without oxygen none of the mitochondrial reactions can occur.

Section 20 of 30Rice and flooding

Problem

• Problem waterlogged roots get little oxygen and submerged leaves little CO₂.

Fast upward growth

• Fast upward growth keeps leaves above water.

Aerenchyma

• Aerenchyma air-space tissue carries gases to submerged parts.

Ethanol tolerance

• Ethanol tolerance rice survives higher ethanol levels than other plants.

Ethanol dehydrogenase

• Ethanol dehydrogenase rice makes more of this enzyme, so ethanol is broken down.

Result

• Result rice can respire anaerobically for longer and keep growing.

Section 21 of 30Redox indicators and yeast

Indicators

• DCPIP and methylene blue change from blue to colourless when reduced.

Mechanism

• Mechanism they take up hydrogens released during respiration.

Rate

• Rate rate of respiration is inversely proportional to the time for colour change.

Variables

• Variables temperature or substrate concentration is varied while dye volume, yeast volume and substrate type are kept constant.

Control

• Control a tube without dye shows the colour of the yeast suspension itself.

Section 22 of 30Respirometer and temperature

1

Place the respirometer in a thermostatically controlled water bath.

2

Measure the gas volume change over a fixed time.

3

Reset the apparatus and change the temperature.

4

Allow acclimatisation and repeat at several temperatures.

Oxygen consumed

• π r² h per minute.

Trend

• Trend rate is low at low temperature, rises with temperature, and falls when enzymes denature.

Limit

• Limit a respirometer measures aerobic respiration only.

Section 23 of 30More from the specification

1

Fermentative glycolysis metabolic function

• Fermentative glycolysis metabolic function regenerating oxidized NAD under anaerobic conditions allows continuing substrate-level phosphorylation to preserve critical cellular function.

2

Mitochondrial Krebs citric acid cycle

• Within the mitochondrial matrix, acetyl-CoA combines with oxaloacetate (4C) to form citrate (6C), undergoing decarboxylations and oxidations to yield NADH, FADH2,
• and ATP.

3

Electron transport chain and proton pumping

• High-energy electrons donated by NADH and FADH2 traverse inner membrane protein complexes (I-IV), generating a steep electrochemical proton-motive force.

4

Chemiosmotic ATP synthesis

• Protons flow through ATP synthase complexes back into the matrix, driving conformational rotation that catalyzes ATP synthesis from ADP and Pi.

5

Mitochondrial Krebs cycle reactions

• Mitochondrial Krebs cycle reactions pyruvate enters the mitochondrial matrix where cyclical enzyme-controlled reactions produce reduced coenzymes for oxidative phosphorylation.

6

Electron transport chain in inner membrane

• Electron transport chain in inner membrane electron carrier complexes embedded in the inner mitochondrial membrane pump protons into the intermembrane space.

7

Anaerobic glycolysis versus aerobic pathway

• Anaerobic glycolysis versus aerobic pathway when cells lack oxygen, glycolysis regenerates NAD by reducing pyruvate to lactate or ethanol.

8

Fermentation

• Anaerobic respiration by yeast producing ethanol and carbon dioxide.

9

Oxidative phosphorylation

• ATP production driven by the electron transport chain.

10

Intermembrane space

• The gap between the two membranes where protons accumulate.

11

Ethanol fermentation

• Decarboxylation of pyruvate to ethanal, then reduction to ethanol.

12

Pyruvate

• A 3-carbon product of glycolysis.

13

Phosphorylation

• Adding a phosphate group.

14

Matrix

• The solution inside the inner membrane containing Krebs cycle enzymes.

15

Lactate

• The product of anaerobic respiration in human cells.

Section 24 of 30What the respirometer measures

Oxygen consumption

• Oxygen consumption by respiring organisms such as germinating seeds or small invertebrates.

CO₂

• CO₂ is absorbed by alkali so its production does not mask the fall in gas volume.

Manometer fluid

• Manometer fluid moves as the gas volume falls.

Control tube

• Control tube equal conditions and a control tube separate respiration from pressure and temperature changes.

Section 25 of 30Method for different temperatures

1

Place the respirometer in a temperature-controlled water bath.

2

Let it reach the chosen temperature and close the system.

3

Measure the movement of the manometer fluid over a fixed time.

4

Reset the fluid and admit air before changing the temperature.

5

Let the apparatus acclimatise and repeat at each temperature.

Controlled variables

• Controlled variables organism mass, time interval and apparatus dimensions.

Repeats

• Repeats several at each temperature.

Section 26 of 30Calculating oxygen uptake

Volume

• π r² h cm³, where r is the capillary radius and h the fluid movement.

Rate

• Rate divide by the time in minutes to give cm³ min^-1.

Graph

• Graph plot the rate of oxygen consumption against temperature.

Section 27 of 30Explaining the trend

Low temperature

• Low temperature molecules have little kinetic energy, so enzyme reactions are slow.

Rising temperature

• Rising temperature successful collisions become more frequent, so the rate increases.

Very high temperature

• Very high temperature respiratory enzymes denature and the rate falls.

Aerobic only

• Aerobic only anaerobic pathways use no oxygen, so they are not measured.

Section 28 of 30More from the specification

1

Respiratory enzyme catalytic function

• Respiratory enzyme catalytic function supra-optimal temperatures cause protein denaturation, destroying mitochondrial enzyme active sites and disrupting cellular respiratory function.

2

Glycolysis in Cellular Respiration

• In the cytoplasm, glucose is phosphorylated and split during glycolysis into two molecules of pyruvate, generating net ATP by substrate-level phosphorylation and reducing NAD+ to reduced NAD (NADH).

3

The Krebs Cycle in the Mitochondrial Matrix

• Pyruvate enters the mitochondria and undergoes the link reaction and Krebs cycle,
• where decarboxylation and dehydrogenation reactions transfer hydrogen atoms to NAD and FAD to yield reduced coenzymes.

4

The Electron Transport Chain on the Inner Mitochondrial Membrane — Reduced NAD and reduced FAD deliver high-energy electrons and hydrogen carriers to protein complexes embedded in the folded inner membrane (cristae) of the mitochondria.

5

Chemiosmosis and Hydrogen Ion Transport

• As electrons flow through the electron transport chain to the final electron acceptor oxygen, proton pumps actively transport hydrogen ions into the intermembrane space, establishing an electrochemical gradient that drives ATP synthesis.

6

Substrate

• The molecule that an enzyme acts on and which binds to the active site.

7

Matrix

• The solution inside the inner membrane containing Krebs cycle enzymes.

8

Lactate

• The product of anaerobic respiration in human cells.

9

Fermentation

• Anaerobic respiration by yeast producing ethanol and carbon dioxide.

10

Krebs cycle

• Cycle in the mitochondrial matrix producing NADH, FADH2, ATP and CO₂.

11

Oxidative phosphorylation

• ATP production driven by the electron transport chain.

12

Substrate concentration

• The amount of substrate per unit volume.

13

Intermembrane space

• The gap between the two membranes where protons accumulate.

14

Reduced

• Having gained electrons

15

Mitochondria

• Organelles where most aerobic respiration takes place.

16

Glucose

• A simple sugar; the unit that starch, glycogen and cellulose are built from.

17

Glycolysis

• Splitting of glucose to pyruvate in the cytoplasm.

18

Pyruvate

• A 3-carbon product of glycolysis.

19

Phosphorylation

• Adding a phosphate group.

20

Ethanol

• Alcohol produced by yeast in anaerobic respiration.

Section 29 of 30Key terms

Term	Meaning
Glycolysis	Splitting of glucose to pyruvate in the cytoplasm.
Chemiosmosis	ATP synthesis using a proton gradient and ATP synthase.
Coenzyme A	A coenzyme that carries the acetyl group into the Krebs cycle.
Oxygen debt	Extra oxygen needed after exercise to remove lactate.
Cristae	Folds of the inner mitochondrial membrane.
Aerenchyma	Air spaces in rice stems and roots that supply oxygen.

Term	Meaning
Dehydrogenation	Removal of hydrogen atoms from a substrate.
Decarboxylation	Removal of carbon dioxide from a substrate.
Electron transport chain	Series of carriers passing electrons and pumping protons.
ATP synthase	Channel protein and enzyme that makes ATP as protons flow through.
Fermentation	Anaerobic regeneration of NAD from reduced NAD.
Oxygen debt	Extra oxygen needed to remove lactate after anaerobic respiration.

Term	Meaning
Aerenchyma	Plant tissue with large air spaces.
Redox indicator	Dye that changes colour when reduced or oxidised.
Ethanol dehydrogenase	Enzyme that breaks down ethanol.
Partial oxidation	Incomplete breakdown of glucose in anaerobic respiration.
Thermostatically controlled water bath	Water bath that keeps a set temperature.

Term	Meaning
Respirometer	Apparatus measuring gas volume changes of respiring organisms.
Manometer	U-tube whose fluid shows gas volume change.
Kinetic energy	Energy of movement of molecules.
Denature	Change in shape of an enzyme so it no longer works.
Acclimatise	Allow apparatus to reach a new temperature.

Section 30 of 30Exam technique

1

Saying oxygen is used in glycolysis — oxygen is used only at the end of the electron transport chain.

2

Saying the Krebs cycle takes place in the cristae — it takes place in the matrix.

3

Saying anaerobic respiration makes no ATP — it nets 2 ATP from glycolysis.

4

Forgetting to double the products, as one glucose gives two pyruvates.

1

Saying NAD is used up — it is reduced and then oxidised and reused.

2

Saying oxygen is needed for glycolysis — it is needed as the final acceptor in the electron transport chain.

3

Saying lactate is a dead end — it can be oxidised to pyruvate or converted to glycogen.

4

Saying ethanol fermentation is one step — it has two.

1

Saying anaerobic respiration makes no ATP — glycolysis still makes 2 ATP.

2

Saying rice avoids ethanol — it tolerates it and breaks it down.

3

Using the respirometer for anaerobic respiration — no oxygen is used.

4

Saying a longer colour-change time means faster respiration — it means slower.

1

Measuring anaerobic respiration with this apparatus — no oxygen is used.

2

Forgetting to divide by time — rate needs cm3 per minute.

3

Not resetting the fluid between temperatures — readings would be wrong.

4

Saying enzymes die at high temperature — they denature.