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12.212 · Energy and Respiration

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

15 min read 30 parts Lesson 6 of 18 10 quizzes

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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12.2 Respiration — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Summarise the four stages of aerobic respiration, the mitochondrion, anaerobic respiration, RQ and the investigation of respiration rate.Uses of energy — anabolic reactions (protein and DNA synthesis),…

  • Hydrolysis
  • Making ATP
  • Link plus Krebs
  • Decarboxylation
  • Dehydrogenation
  • Coenzyme A

Part 1 of 11ATP 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)

Part 2 of 11Overall equation and location of stages

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

Part 3 of 11The four stages

StageLocationMain outputs per glucose
GlycolysisCytoplasm2 pyruvate, net 2 ATP, 2 reduced NAD
Link reactionMitochondrial matrix2 acetyl CoA, 2 CO2, 2 reduced NAD
Krebs cycleMitochondrial matrix4 CO2, 6 reduced NAD, 2 reduced FAD, 2 ATP
Oxidative phosphorylationInner mitochondrial membraneATP 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.

Part 4 of 11Glycolysis

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.

Part 5 of 11Link 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.

Part 6 of 11Oxidative 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.

Part 7 of 11Mitochondrial 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.

Part 8 of 11Anaerobic 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.

Part 9 of 11RQ 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.

Part 10 of 11Redox 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.

Part 11 of 11More 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.

Key terms

TermMeaning
GlycolysisSplitting of glucose to pyruvate in the cytoplasm.
ChemiosmosisATP synthesis using a proton gradient and ATP synthase.
Coenzyme AA coenzyme that carries the acetyl group into the Krebs cycle.
Oxygen debtExtra oxygen needed after exercise to remove lactate.
CristaeFolds of the inner mitochondrial membrane.
AerenchymaAir spaces in rice stems and roots that supply oxygen.

Avoid these mistakes

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.

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