18.5 - Anaerobic Respiration

Question 1

In terms of ATP production, what is the difference between anaerobic and aerobic respiration?

Answer 1

Aerobic respiration produces 38 ATP molecules compared to 2 ATP molecules from anaerobic respiration

Question 2

What are the different oxygen-dependant categories that organisms fall into?

Answer 2

Obligate anaerobes­ – cannot survive in the presence of oxygen. Almost all obligate anaerobes are prokaryotic

Facultative anaerobes – if oxygen is presence, they synthesise ATP but can switch to anerobic respiration in oxygen’s absence – e.g. yeast

Obligate aerobes – Can only synthesise ATP in the presence of oxygen.

Individual cells can be described as facultative anaerobes as they can use anaerobic respiration in low [O₂], however they eventually need O₂ making them obligate aerobes – by-products of anaerobic respiration must be broken down once oxygen is available, so the organism as a whole is an obligate aerobe

Question 3

Generally speaking, how does fermentation produce ATP?

Give 2 examples of fermentation

Answer 3

Uses substrate-level phosphorylation to produce small quantities of ATP during the breakdown of glucose

During alcoholic fermentation the end products are ethanol and carbon dioxide – occurs in plant root cells and yeast

During lactate fermentation the end product is lactate – occurs in animal cells

Question 4

Describe the process of lactate fermentation

Answer 4

Occurs in mammals

• Pyruvate acts as the hydrogen acceptor from NADH (this is catalysed by lactate dehydrogenase)
• Pyruvate is converted to lactate (lactic acid) and NAD is regenerated
  
  • The NAD is used for glycolysis to a small amount of ATP is synthesised
• In mammals, anaerobic respiration in the muscles is supported by ATP that has been synthesised by aerobic respiration elsewhere in the body
• Lactate is converted back to glucose in the liver – but requires oxygen
  
  • Therefore, there is oxygen debt after exercise

Question 5

Why can’t lactate fermentation occur indefinitely?

Answer 5

• Reduced ATP synthesis is not enough to maintain vital processes for prolonged periods of time
• Lactic acid accumulation causes a pH drop, leading to protein denaturation
  
  • Respiratory enzymes and muscle filaments (protein based) will stop functioning in highly acidic conditions

Question 6

What occurs during alcohol fermentation?

Answer 6

It is not reversible, unlike in lactate fermentation

1. Pyruvate undergoes decarboxylation to form ethanal (catalysed by pyruvate decarboxylase)
2. Ethanal is reduced (accepts H from NADH) forming ethanol
3. Regenerated NAD acts as a coenzyme to allow glycolysis to continue

This can continue indefinitely in the absence of oxygen

Ethanol is a toxic waste product to yeast products, yeast can’t survive if ethanol reaches above 15%