Topic 7A Respiration

What are the stages of aerobic respiration and where do they take place?

1. Glycolysis takes place in the cell cytoplasm

2. The Link reaction takes place in the matrix of the mitochondria

3. The Krebs cycle takes place in the matrix of the mitochondria

4. Oxidative phosphorylation occurs at the inner membrane of the mitochondria

How do intracellular enzymes regulate energy release from glucose?

Intracellular enzymes catalyse reactions within the cell to release energy from glucose gradually.

This prevents a sudden energy release that could dangerously increase body temperature and denature enzymes.

What is the structure and function of the mitochondria membranes?

• Outer membrane: Smooth and permeable to small molecules.

• Inner membrane: Folded into cristae, less permeable, and contains the electron transport chain and ATP synthase for oxidative phosphorylation.

What is the structure and function of the mitochondria inter membrane space?

• Low pH due to high proton concentration.

• Proton gradient across the inner membrane drives ATP synthesis.

What is the structure and function of the mitochondria matrix?

Aqueous solution containing ribosomes, enzymes, and circular mitochondrial DNA essential for mitochondrial function.

what are the steps of glycolysis and its energy yield?

1. Phosphorylation of Glucose:

   • Two ATP molecules donate phosphates to glucose, forming two triose phosphate molecules and two ADP.

2. Oxidation of Triose Phosphate:

   • Triose phosphate loses hydrogen, forming two pyruvate molecules.

   • NAD collects hydrogen, forming two reduced NAD (NADH).

3. ATP Yield:

   • Four ATP molecules are produced, but two are used during phosphorylation.

   • Net gain: Two ATP molecules.

What are the key features and outcomes of glycolysis?

• Oxygen Requirement: None; occurs in both aerobic and anaerobic respiration.

• Process:

  1. Phosphorylation of Glucose using 2 ATP, trapping it

  2. Oxidation of triose phosphate (loses hydrogen).

• Products:

  • 2 Pyruvate (3C): Moves to the mitochondrial matrix for the link reaction.

  • 2 ATP (net gain).

  • 2 Reduced NAD (used in oxidative phosphorylation).

• Anaerobic Conditions: Produces lactic acid or lactate instead of pyruvate.

What happens to pyruvate after glycolysis in aerobic respiration?

• pyruvate contains energy for ATP production.

• The enzymes and coenzymes for the link reaction are in the mitochondrial matrix.

• With oxygen, pyruvate enters the mitochondrial matrix via active transport, requiring ATP and a transport protein.

• In the matrix, pyruvate undergoes the link reaction.

What happens during the link reaction in respiration?

• Steps:

  1. Oxidation: Pyruvate is oxidised to form acetate (CH₃COO⁻) which combines with coenzyme A to form acetyl CoA.

  2. Decarboxylation: Carbon is removed from pyruvate as CO₂.

  3. Reduction: NAD is reduced to NADH.

• No ATP is produced.

• Glucose produces 2 pyruvate molecules during glycolysis.

• The link reaction and Krebs cycle occur twice per glucose molecule.

Where do the products of the link reaction go to?

• Products:

  • Acetyl CoA - kerbs cycle

  • CO₂ - waste product

  • NADH - oxidative phosphorylation

What happens during the Krebs cycle?

• A series of enzyme-controlled reactions.

• 2C Acetyl CoA enters the cycle and combines with 4C oxaloacetate to form 6C citrate.

• Coenzyme A is released and reused in the next link reaction.

• Citrate is converted back to oxaloacetate through redox reactions.

How is oxaloacetate regenerated in the Krebs cycle, and what are the key products?

• Decarboxylation of citrate → releases 2 CO₂ as waste.

• Oxidation (dehydrogenation) → releases H atoms, reducing NAD and FAD

• A phosphate group is transferred to ADP, forming 1 ATP.

What are the products of the Krebs cycle for one glucose molecule?

• Each glucose produces 2 acetyl-CoA, so the Krebs cycle occurs twice per glucose.

• Total products after two cycles:

  • 2 ATP

  • 6 NADH (reduced NAD)

  • 2 FADH₂ (reduced FAD)

  • 4 CO₂

why oxygen is so important for aerobic respiration?

• Oxygen is the final electron acceptor, combines with H+ to form water

• Without oxygen:

  • The electron transport chain stops as electrons have nowhere to go.

  • NADH and FADH₂ cannot be oxidised, so NAD and FAD are not regenerated.

  • Hydrogen transport and ATP production cease.

What happens during oxidative phosphorylation in aerobic respiration?

• Produces ATP and water.

  1. NADH and FADH₂ donate hydrogen atoms.

  2. Hydrogen atoms split into protons (H⁺) and electrons.

  3. Electrons enter the electron transport chain (ETC), releasing energy.

  4. Energy pumps H⁺ ions into the intermembrane space, creating a proton gradient.

  5. H⁺ ions flow back into the matrix via ATP synthase, driving ATP synthesis.

How much ATP is produced in oxidative phosphorylation?

• 3 ATP per reduced NAD.

• 2 ATP per reduced FAD.

• 38 ATP per glucose molecule.

Lactate fermentation

• In this pathway reduced NAD transfers hydrogen to pyruvate to form lactate

• NAD can now be reused in glycolysis

• Pyruvate is reduced to lactate by enzyme lactate dehydrogenase

• Pyruvate is the hydrogen acceptor

• The final product lactate can be further metabolised

• A small amount of ATP is produced 

What happens to lactate produced during exercise?

• Lactate builds up in cells during intense exercise.

• After production, lactate can:

  1. Be oxidised back to pyruvate → enters the Krebs cycle for ATP production.

  2. Be converted into glucose in the liver → used in respiration or stored as glycogen.

• These requires extra oxygen, causing an oxygen debt.

• Oxygen debt explains why animals breathe deeper and faster after exercise.

Ethanol fermentation

• In this pathway reduced NAD transfers its hydrogens to ethanal to form ethanol

• In the first step of the pathway pyruvate is decarboxylated to ethanal

  • Producing CO₂

• Then ethanal is reduced to ethanol by the enzyme alcohol dehydrogenase

• Ethanal is the hydrogen acceptor

• Ethanol cannot be further metabolised; it is a waste product

• Ethanol fermentation occurs in yeast and plant cells

respiratory quotient (RQ)

the ratio of carbon dioxide molecules produced to oxygen molecules taken in during respiration

RQ = CO₂ ÷ O₂ (mol)

What does the respiratory quotient (RQ) indicate in respiration?

• Aerobic respiration: RQ is typically < 1 because oxygen is used to break down the substrate.

• RQ > 1: May indicate anaerobic respiration, as little or no oxygen is used.