Overview: aerobic respiration

· Respiration = enzyme-controlled release of energy from respiratory substrates, transferring energy to ATP.
· In eukaryotic cells, aerobic respiration involves four main stages: glycolysis, link reaction, Krebs cycle, and oxidative phosphorylation.
· Glycolysis occurs in the cytoplasm.
· Link reaction occurs in the mitochondrial matrix.
· Krebs cycle occurs in the mitochondrial matrix.
· Oxidative phosphorylation occurs on the inner mitochondrial membrane.
· Oxygen is required for aerobic respiration because it acts as the final electron acceptor, forming water.

Glycolysis

· Glucose is first phosphorylated, making it more reactive.
· Fructose 1,6-bisphosphate (6C) is split into two triose phosphate (3C) molecules.
· Triose phosphate is further oxidised to form pyruvate (3C).
· Glycolysis produces a small yield of ATP by substrate-level phosphorylation.
· Glycolysis also produces reduced NAD, formed when NAD accepts hydrogen.
· Key exam sequence: glucose → fructose 1,6-bisphosphate → triose phosphate → pyruvate.

This diagram shows the conversion of glucose into two pyruvate molecules. It helps visualise phosphorylation, splitting into triose phosphates, and production of ATP and reduced NAD. Source

Link reaction

· Pyruvate enters the mitochondrion when oxygen is available.
· In the link reaction, pyruvate is converted into an acetyl (2C) group.
· Decarboxylation occurs: carbon dioxide is removed.
· Dehydrogenation occurs: hydrogen is removed and accepted by NAD, forming reduced NAD.
· Coenzyme A combines with the acetyl group to form acetyl coenzyme A.
· Acetyl coenzyme A transfers the 2C acetyl group into the Krebs cycle.

Krebs cycle

· Acetyl coenzyme A combines with oxaloacetate (4C) to form citrate (6C).
· Citrate is converted back to oxaloacetate through a series of enzyme-controlled steps.
· Oxaloacetate is regenerated, so the cycle can continue.
· Reactions involve decarboxylation, releasing carbon dioxide.
· Reactions also involve dehydrogenation, reducing NAD and FAD.
· Reduced NAD and reduced FAD carry hydrogen to the inner mitochondrial membrane for oxidative phosphorylation.
· A small amount of ATP is made by substrate-level phosphorylation.

This image links the link reaction to the Krebs cycle. It shows how pyruvate is converted to acetyl CoA and how carbon dioxide and reduced coenzymes are produced. Source

Oxidative phosphorylation

· Reduced NAD and reduced FAD transfer hydrogen to carriers in the inner mitochondrial membrane.
· Hydrogen atoms split into protons (H⁺) and energetic electrons.
· Electrons pass along the electron transport chain and release energy.
· Released energy pumps protons from the matrix into the intermembrane space.
· This creates a proton gradient across the inner mitochondrial membrane.
· Protons return to the matrix by facilitated diffusion through ATP synthase.
· Movement of protons through ATP synthase provides energy for ATP synthesis.
· Oxygen is the final electron acceptor, combining with protons and electrons to form water.
· This process is an example of chemiosmosis.

This diagram shows electrons moving through carriers in the inner mitochondrial membrane and protons being pumped across the membrane. It is useful for revising chemiosmosis and the role of oxygen as the final electron acceptor. Source

Mitochondria: structure and function

· Outer membrane surrounds the mitochondrion.
· Inner mitochondrial membrane is folded into cristae, increasing surface area for electron transport chains and ATP synthase.
· Matrix contains enzymes for the link reaction and Krebs cycle.
· Intermembrane space allows accumulation of protons, forming the proton gradient.
· Cristae allow more oxidative phosphorylation to occur, increasing ATP production.
· In electron micrographs, identify double membrane, cristae, matrix, and intermembrane space.
· Structure-function link: more cristae = larger surface area = more ATP synthase and electron transport chain proteins = greater capacity for ATP synthesis.

Anaerobic respiration

· Anaerobic respiration occurs when oxygen is not available.
· Without oxygen, the electron transport chain cannot continue because there is no final electron acceptor.
· Reduced NAD must be reoxidised to NAD so that glycolysis can continue.
· In mammals, pyruvate is reduced to lactate: lactate fermentation.
· In yeast, pyruvate is converted to ethanol and carbon dioxide: ethanol fermentation.
· Anaerobic respiration produces much less ATP than aerobic respiration because only glycolysis continues.
· Aerobic respiration has a much higher energy yield because Krebs cycle and oxidative phosphorylation can occur.

This diagram shows how mammalian cells regenerate NAD during anaerobic respiration. Regenerating NAD allows glycolysis to continue even when oxygen is unavailable. Source

Rice adaptations to submerged roots

· Rice can grow with roots submerged in water where oxygen availability is low.
· Roots develop aerenchyma, tissue with air spaces that allow oxygen movement to submerged parts.
· Rice roots can carry out ethanol fermentation under anaerobic conditions.
· Faster growth of stems helps leaves reach air above the water surface.
· Exam focus: link submerged roots → low oxygen → anaerobic respiration → adaptations for oxygen supply and survival.

Practical: redox indicators and yeast respiration

· Use DCPIP or methylene blue as redox indicators.
· These indicators change colour when reduced, showing respiration is occurring.
· Yeast respiration rate can be investigated by changing temperature or substrate concentration.
· A faster colour change indicates a faster rate of respiration.
· Control variables include yeast concentration, indicator volume, substrate volume, pH, and total volume.
· At low temperatures, enzymes have less kinetic energy, so respiration is slower.
· At high temperatures, respiratory enzymes may denature, reducing respiration rate.
· Increasing substrate concentration increases respiration rate until another factor becomes limiting.

Practical: simple respirometers

· Simple respirometers can measure the effect of temperature on respiration rate.
· Respiring organisms take in oxygen, causing movement of fluid in the capillary tube.
· Use a control tube with non-respiring material or no organism to correct for pressure/temperature changes.
· Use an alkali such as soda lime or potassium hydroxide to absorb carbon dioxide if measuring oxygen uptake.
· Calculate respiration rate from distance moved by fluid per unit time or volume of oxygen used per unit time.
· Repeat readings and calculate a mean to improve reliability.
· Ethical and practical considerations: use suitable small organisms, avoid extreme temperatures, and prevent harm.

High-yield exam comparisons

· Glycolysis: cytoplasm; glucose to pyruvate; produces ATP and reduced NAD.
· Link reaction: matrix; pyruvate to acetyl CoA; releases CO₂ and forms reduced NAD.
· Krebs cycle: matrix; acetyl group joins oxaloacetate; releases CO₂ and forms reduced NAD and reduced FAD.
· Oxidative phosphorylation: inner mitochondrial membrane; uses reduced coenzymes, electron transport chain, chemiosmosis, and oxygen to make most ATP.
· Aerobic respiration: high ATP yield because Krebs cycle and oxidative phosphorylation continue.
· Anaerobic respiration: low ATP yield because only glycolysis produces ATP.