3.5.2 Respiration

• Respiration produces ATP.
• Glycolysis is the first stage of respiration in both aerobic and anaerobic respiration, so it can occur without oxygen.
• Glycolysis occurs in the cytoplasm of the cell.
• In the first stage of glycolysis, an enzyme adds 2 phosphate groups to glucose. This process is known as phosphorylation and produces a new molecule called glucose phosphate.

  These phosphate groups are available due to the breakdown of two molecules of ATP
• In the second stage of glycolysis, glucose phosphate breaks down into 2 molecules of triose phosphate.
• In the third stage of glycolysis, each triose phosphate is oxidised into pyruvate.

  • each TP molecule loses a H+ which is transferred to NAD
  • NAD gains a H+ which produces NADH
  • 4 molecules of ATP are formed
• The end products of glycolysis are 2 NADH molecules, 2 pyruvate molecules and an overall yield of 2 ATP.
• The second stage of aerobic respiration is the link reaction which occurs in the mitochondrial matrix.
• The link reaction

  • pyruvate is decarboxylated (loses CO2) and is oxidised (loses a H+) to form acetate.
  • the H+ is transferred to NAD to produce NADH
  • acetate combines with coenzyme A (CoA) to form acetyl-CoA
• The Krebs cycle is the third stage of aerobic respiration and it occurs in the mitochondrial matrix.
• Chemiosmosis is the movement of ions across a partially permeable membrane, from an area of a high concentration to an area of low concentration.
• Krebs cycle:

  • acetyl coA (2 carbon molecule) combines with a 4 carbon molecule to form a 6 carbon molecule - CoA is released
  • the 4 carbon molecules goes through a series of reactions where it is decarboxylated (loses CO2) and oxidised, reducing NAD to NADH and FAD to FADH2
  • one molecule of ATP is produced via substrate level phosphorylation
• The products of the Krebs cycle include 2 ATP, 6 NADH, 2 FADH2, and 4 CO2 molecules.
• Oxidative phosphorylation is the final stage of aerobic respiration that takes place in the inner mitochondrial membrane. It involves chemiosmosis and produces most of the ATP made during cellular respiration.
• Substrate level phosphorylation is where a phosphate group is directly transferred from a substrate molecule to ADP to form ATP. It occurs in glycolysis and the Krebs cycle.
• Oxidative phosphorylation:

  • NADH and FADH2 donate electrons to electron transfer chain
  • as electrons are transported through the chain, they lose energy which is used to actively transport protons (H+) from the mitochondrial matrix into the intermembrane space
  • this creates a proton gradient, so protons flow back into the matrix via ATP synthase
  • the energy from the flow drives the synthesis of ATP from ADP and Pi (chemiosmosis)
  • oxygen is the final electron acceptor - combines with electrons and protons to form water
• Oxidative phosphorylation produces up to 34 molecules of ATP and a water molecule.
• If there is no oxygen, as it is the final electron acceptor, the electrons from NADH and FADH2 cannot pass along the ETC, causing the chain to stop. This stops the flow of electrons and the active transport of protons into the intermembrane space.

  • this prevents the formation of the proton gradient necessary for ATP synthase to generate ATP in chemiosmosis.
• In anaerobic respiration in animals:

  • pyruvate is reduced to lactate (lactic acid) using NADH
  • this regenerates NAD to allow glycolysis to continue
  • 2 ATP are produced per glucose molecule
• In anaerobic respiration in plants and yeast:

  • pyruvate is decarboxylated to ethanol and carbon dioxide
  • this regenerates NAD to maintain glycolysis
  • produces 2 ATP per glucose molecule
• Describe the process of glycolysis (4)

  • Phosphorlyation of glucose using ATP
  • Oxidation of triose phosphate to pyruvate
  • Net gain of ATP - 4 produced and 2 used
  • NAD reduced to NADH+
• Malonate inhibits a reaction in the Krebs cycle.

  Explain why malonate would decrease the uptake of oxygen in a respiring cell (2)

  • less reduced NAD
  • oxygen is the terminal electron acceptor
• Anaerobic respiration does not use oxygen as its final electron acceptor so less ATP can be made compared with aerobic respiration.
• Name the substances formed from pyruvate

  • CO2
  • water
  • ATP
  • reduced NAD/FAD
• If there is a shortage of oxygen in muscle cells during exercise, some pyruvate is converted into lactate. Explain why muscles become fatigued when insufficient oxygen is available (2).

  • `build up of lactate lowers pH/increases acidity
  • enzymes inhibited
• Some of the lactate is oxidised to pyruvate by muscles when they are well-supplied with oxygen.

  Suggest an advantage of the lactate being oxidised in the muscles. (2)

  • pyruvate is an energy source
  • muscles have increased ATP supply
  • restores pH levels
• Explain why converting pyruvate to ethanol is important in allowing the continued production of ATP in anaerobic conditions. (2)

  • allows NAD to be recycled / re-formed
  • so that glycolysis can proceed / so that more glucose can be converted to pyruvate
• Give ways in which anaerobic respiration of glucose in yeast is similar to anaerobic respiration of glucose in a muscle cell.

  • ATP formed
  • pyruvate formed / reduced
  • NAD / reduced NAD
  • glycolysis involved / two step process
• Give ways in which anaerobic respiration of glucose in yeast is different from anaerobic respiration of glucose in a muscle cell

  • ethanol formed by yeast, lactic acid formed by muscle cells
  • CO2 released by yeast but not by muscle cell