respiration

• 1st stage Glycolysis - CYTOPLASM

  1. phosphorylation of glucose using ATP in the cytoplasm
  2. oxidation of triose phosphate to pyruvate
  3. Net gain of ATP
  4. Gain reduced NAD

  
• 2nd stage link reaction - mitochondrial matrix

  1. if oxygen is present pyruvate molecule in the cytoplasm is actively transported into the mitochondrial matrix
  2. pyruvate is decarboxylated (looses CO2 molecule)
  3. pyruvate is oxidised into acetate
  4. acetate combines with coenzyme A to produce acetylcoenzyme A
  5. for 1 glucose molecule the link reaction occurs twice
  6. Acetylcoenzyme A added to 4c compound to make a 6c compound

  
• 3rd stage Krebs cycle - mitochondrial matrix

  1. 2c acetylcoenzyme A enters krebs cycle from link reaction
  2. acetylcoenzyme A reacts with 4c oxaloacetate in mitochondrial matrix
  3. ...to make 6c citrate
  4. ...releasing coenzyme A (reused in link reaction)
  5. series of REDOX reactions regenerates oxaloacetate from citrate
  6. produces 2x CO2 (decarboxylation), ATP (substrate level phosphorylation), reduced NAD and reduced FAD (removes hydrogen)

  
• If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD. The oxidised NAD produced in this way can be used in further glycolysis
• In anaerobic conditions, which are those without oxygen, cells can produce a small yield of ATP through glycolysis. This can only continue if the reduced NAD that is produced can be oxidised again.
• anaerobic respiration:

  The link reaction and Krebs cycle cannot continue if all the FAD and NAD are reduced, because they cannot accept any more protons and electrons. Oxidative phosphorylation cannot occur without oxygen as the final electron acceptor.
• 4th stage oxidative phosphorylation - mitochondrial matrix

  1. in mm reduced NAD and FAD loose a hydrogen as they are oxidised - hydrogen splits into H+ and e-
  2. these electrons move along electron transport chain releasing energy in inner membrane
  3. energy is used to actively transport H+ from matrix to intermembrane space by proton pump
  4. increase of H+ conc produces electrochemical gradient so H+ move back via facilitated diffusion through ATP synthase
  5. phosphorylation of ADP + Pi ---> ATP
  6. oxygen in the matrix is the final electron acceptor = combine e- and H+ = water
• in oxidative phosphorylation oxygen is the final electron acceptor picking up electrons at the end of electron transport chain and picking up hydrogens at ATP synthase channel

  • combines to form water

  2H+ + 2e- + 1/2 O2 ---> H2O
• if there isn't enough oxygen, anaerobic respiration occurs:

  • no acceptor in electron transport chain
  • so no electron transport chain and ATP via oxidative phosphorylation
  • so no oxidation of reduced NAD and FAD
  • so no NAD and FAD for Krebs cycle = no Krebs cycle
• explanation if there is no oxygen:

  1. No oxygen as the final electron acceptor = [Without oxygen, electrons can't flow through the ETC]
  2. No electron transport chain (ETC) activity and no ATP = [ETC provides energy for proton gradient that powers ATP synthase to produce ATP ---> no O2 the ETC stops working, ATP not generated]
  3. No oxidation of reduced NAD and FAD = [if the ETC stops, these molecules remain in their reduced forms they cannot offload their electrons when oxidised]
  4. No NAD and FAD available for the Krebs cycle = [in Krebs they accept electrons, but now stuck in reduced form no Krebs