E

3.5.2 Respiration

Aerobic Respiration - requires oxygen → carbon dioxide, water and ATP

Anaerobic Respiration - absence of oxygen → Lactate(animals)/ ethanol and carbon dioxide(plants & fungi) and a little amount of ATP

Mitochondria

Inner mitochondrial membrane folds → cristae - contains the electron transport chain for oxidative phosphorylation and ATP synthase enzyme channels

The Matrix contains enzymes required for catalysis of the link reaction and Krebs cycle

Aerobic Respiration

Four Stages :

  • Glycolysis

  • The Link reaction

  • Krebs Cycle

  • Oxidative Phosphorylation

Glycolysis

Occurs in the cytoplasm - both aerobic and anaerobic respiration

  • Glucose is phosphorylated by ATP = ADP and Hexose Biphosphate → 2 molecules of Triose Phosphate

  • Each Triose Phosphate is oxidised → 2 molecules of Pyruvate

  • Coenzyme NAD collects H+ and is reduced → 2 reduced NAD molecules

  • 4 ATP are produced by substrate level phosphorylation

The Link Reaction

Pyruvate is actively transported into matrix of the mitochondria

  • Pyruvate is oxidised → Acetate

  • H+ ions used to form Reduced NAD(NADH)

  • Pyruvate is decarboxylated - loses CO2

  • Acetate + Coenzyme A (CoA) → Acetyl coenzyme A ( Acetyl CoA)

The Krebs Cycle

Acetyl CoA enters Krebs cycle in the matrix - cycle happens once for every pyruvate

  • Acetyl CoA combines with 4-Carbon Oxaloacetate → 6-Carbon Citrate

  • Citrate is oxidised to a 5-Carbon molecule

  • CO2 is lost (decarboxylation)

  • The H+ released produces NADH

  • 5-Carbon molecule is oxidised again → 4-Carbon Oxaloacetate

  • Co2 is lost again and H+ released used to produce NADH & FADH

  • ATP is also produced through transfer of phosphate from one molecule to another ( substrate-level phosphorylation )

  • Oxaloacetate can then combine with Acetyl CoA to begin the cycle again

Oxidative Phosphorylation

Energy carried by electrons provided by reduced coenzymes(NADH & FADH) to generate lots of ATP

Involves Electron Transport Chain and Chemiosmosis

Occurs in the inner folded membrane (cristae)

  1. NADH/FADH arrives a mitochondrial membrane and release Hydrogen → Protons & Electrons

  2. Electrons move down the electron transport chain - releases energy

  3. Energy is used to pump H+ from the matrix into the intermembrane space = electrochemical gradient

  4. H+ moves down electrochemical gradient into the matrix by ATP Synthase which provides the energy needed to combine ADP + Pi → ATP

  5. Oxygen - final electron acceptor - combines with H+ and Electrons at the end of the electron transport chain → Water

2.5 ATP = one NADH

1.5 ATP = one FADH

Anaerobic Respiration

Absence of Oxygen - only source of ATP is glycolysis

Main purpose of anaerobic respiration is to generate NAD for glycolysis which allows a small amount of ATP to be made

In Animal Cells

  • Pyruvate produced in glycolysis is reduced using NADH = lactate - lactic acid

  • This generates NAD so glycolysis can keep producing ATP

  • Lactate causes muscle fatigue - transported in the blood → liver - oxidised back into Pyruvate

In Plant and Yeast Cells

  • Pyruvate produced in glycolysis is decarboxylated - produces CO2 and Ethanal

  • Ethanal is then reduced by NADH → ethanol and regenerating NAD - glycolysis can continue to produce ATP

  • Products of Ethanol & CO2 → used in industry (fermentation)

Respiratory Quotient (RQ)

Different Respiratory substrates result in a different ratio of carbon dioxide : oxygen taken in per unit time

Respiratory Quotient = Volume of Co2 produced / Volume of O2 used

Aerobic Respiration: Requires oxygen, produces carbon dioxide, water, and ATP. Anaerobic Respiration: Occurs in the absence of oxygen, resulting in lactate (in animals) or ethanol and carbon dioxide (in plants and fungi), and produces a small amount of ATP.

Mitochondria:

  • Inner membrane folds (cristae) contain the electron transport chain and ATP synthase.

  • Matrix contains enzymes for the link reaction and Krebs cycle.

Four Stages of Aerobic Respiration:

  1. Glycolysis: Occurs in the cytoplasm; glucose is broken down to pyruvate, producing ATP and reduced NAD.

  2. Link Reaction: Pyruvate is converted to acetyl CoA in the mitochondria.

  3. Krebs Cycle: Acetyl CoA enters the cycle, generating NADH, FADH, and ATP while releasing CO2.

  4. Oxidative Phosphorylation: NADH and FADH donate electrons to the electron transport chain, producing a large amount of ATP and water.

Anaerobic Respiration:

  • In animals, lactate is produced to regenerate NAD for continued glycolysis.

  • In plants/yeasts, pyruvate forms ethanol and CO2, allowing glycolysis to continue.

Respiratory Quotient (RQ): Indicates the ratio of CO2 produced to O2 consumed, varying with different substrates used in respiration.