Part 2: Chemoorganotrophic fermentation

Question 1

Why do most anaerobes lack a full array of enzymes?

Answer 1

Anoxic conditions prevent a high energy yield, meaning that anaerobes must use energy sparingly - therefore most lack a full complement of enzymes.

Question 2

Which group of organisms lack a respiratory chain?

Answer 2

Fermentors.

Question 3

List 5 economical uses for fermenting bacteria (with examples).

Answer 3

1) Lactic acid fermentation (yoghurt, salami, sauerkraut)
2) Alcoholic fermentation (beer, wine, cider)
3) Propionic acid fermentation - cheese
4) Citric acid fermentation - “lemon juice”
5) Acetone formation - industrial solvent

Question 4

Give an example of a food-spoiling fermenter and its effects.

Answer 4

Clostridium botulinum - fermentation product is a nerve toxin.

Question 5

What acts as the electron donor and electron acceptors in fermentation?

Answer 5

The fermentation acts as both donor and acceptor - some C atoms are oxidised, some reduced.

Question 6

Compare the ATP yield of lactic acid fermentation and aerobic respiration.

Answer 6

Lactic acid fermentation is very inefficient - only 2 ATP per glucose molecule. Respiration yields roughly 25 ATP.

Question 7

How much more efficient is propionic acid fermentation than lactic acid fermentation?

Answer 7

2.7 ATP per glucose for propionic compared to 2 for lactic.

Question 8

Why is conversion of pyruvate to formate a problem in fermentation?

Answer 8

An excess electron is produced which cannot be removed by formation of H2, since H2 has a lower redox potential than NADH.

Question 9

How do fermenters overcome the problem of additional electrons released during fermentation?

Answer 9

Fermenters react with methanogen spp. who are able to take up the additional H2 and who then return some energy to the fermenter.