5.7.6

• Respiration in the absence of oxygen

  if absent :

  1. oxygen cannot act as the final electron acceptor at the end of oxidative phosphorylation. Protons diffusing through channels associated with ATP synthase are not able to combine with electrons + oxygen to form water.
  2. the concentration of protons increases in the matrix + reduces the proton gradient across the inner mitochondrial membrane
  3. oxidative phosphorylation ceases
  4. reduced NAD + reduced FAD are not able to unload their hydrogen atoms + cannot be oxidised
  5. the Krebs cycle stops, so does the link reaction
• Respiration in the absence of oxygen 2

  for organisms to survive these adverse conditions, glycolysis can take place, but reduced NAD generated during the oxidation of triose phosphate to pyruvate

  • these reduced coenzyme molecules cannot be reoxidised at the electron transport chain, so another metabolic pathway must operate to reoxidise them
• Reduced NAD has to be reoxidised

  eukaryotic cells have two metabolic pathways to reoxidised the reducing NAD:

  • fungi, such as yeast, and plants use the ethanol fermentation pathway
  • mammals use the lactate fermentation pathway
• The ethanol fermentation pathway

  alcoholic fermentation:

  1. each molecule of pyruvate produced during glycolysis is decarboxylated + converted to ethanal + this stage in the pathway is catalysed by pyruvate decarboxylase, which has a coenzyme, thiamine diphosphate, bound to it
  2. the ethanal accepts hydrogen atoms from reduced NAD, becoming reduced to ethanol + the enzyme ethanol dehydrogenase catalyses the reaction
  3. in the process, the reduced NAD is re-oxidised + made available to accept more hydrogen atoms from triose phosphate, thus allowing glycolysis to continue

  
• the lactate fermentation pathway

  occurs in mammalian muscle tissue during vigorous activity, such as when running fast to escape a predator, when the demand for ATP for muscle is high + there is an oxygen deficit

  
• the lactate fermentation pathway 2

  1. pyruvate accepts hydrogen atoms from the reduced NAD, also made during glycolysis. The enzyme lactate dehydrogenase catalyses the reaction leaving two outcomes:

  • pyruvate is reduced to lactate
  • the reduced NAD becomes reoxidised

  2. the reoxidised NAD can accept more hydrogen atoms from triose phosphate during glycolysis, + glycolysis can continue to produce enough ATP to sustain muscle contraction for a short period
• The fate of lactate

  the lactate produced in the muscle tissue is carried away from muscle, in the blood, to the liver

  when more oxygen is available, the lactate may be either:

  • converted to pyruvate, which may enter the Krebs cycle via the link reaction
  • recycled to glucose + glycogen

  if lactate were not removed from the muscle tissues, the pH would be lowered + this would inhibit the action of many of the enzymes involved in the glycolysis + muscle contraction
• The ATP yield from anaerobic respiration

  neither ethanol fermentation nor lactate fermentation produces any ATP

  • however because this allows glycolysis to continue, the net gain of two molecules of ATP per molecule of glucose is still obtained
  • because the glucose is only partly broken down , amny molecules can undergo glycolysis per minute, and therefore the overall yield of ATP is quite large

  → however for each molecule of glucose, the yield of ATP via anaerobic respiration is about 1/15 of that produced during aerobic respiration
• DYK?

  Fast-twitch muscle fibers have few mitochondria and rely on glycolysis for short bursts of power. They fatigue quickly and appear pale due to a lack of myoglobin and electron transport proteins. Slow-twitch fibers, in contrast, are dark red, rich in mitochondria, and fatigue slowly, supporting endurance activities. Chicken breast meat is pale, as it primarily contains fast-twitch fibers for brief flights. Ducks and geese, which fly long distances, have dark breast meat due to a higher concentration of slow-twitch fibers.