5.7.6: Anaerobic respiration in eukaryotes

Question 1

How does the absence of oxygen stop the links and Krebs cycles?.

Answer 1

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 and oxygen o form water.
2. The concentration of protons increase in the matrix and reduces the proton gradient across the inner membrane.
3. Oxidative phosphorylation ceases.
4. Reduced NAD and FAD are not able to unload their hydrogen atoms and cannot be reoxidised.
5. The Krebs cycle stops, as does the link reaction.

Question 2

How can an organism survive in these adverse conditions (no oxygen)

Answer 2

• glycolysis can take place, but the reduced NAD generated during the oxidation of triose phosphate to pyruvate has to be reoxidised so that glycolysis can continue.
• These reduced coenzyme molecules cannot be reoxidised at the electron transport chain, so another metabolic pathway must operate to reoxidise them.

Question 3

What are the two metabolic pathways in eukaryotic cells that can reoxidise reduced NAD?

Answer 3

• Fungi, such as yeast, and plants use ethanol fermentation.
• Mammals use the lactate fermentation pathway.
• Both take place in the cytoplasm of the cells.

Question 4

The process of alcoholic (ethanol) fermentation has three steps.
Step 1:

Answer 4

1. Each molecule of pyruvate produced during glycolysis is decarboxylated and converted into ethanal. Catalysed by pyruvate decarboxylase.

Question 5

The process of alcoholic (ethanol) fermentation has three steps.
Step 1: Each molecule of pyruvate produced during glycolysis is decarboxylated and converted into ethanal. Catalysed by pyruvate decarboxylase.

Step 2:

Answer 5

2. The ethanal accepts hydrogen atoms from reduced NAD, becoming reduced to ethanol. Catalysed by ethanol dehydrogenase.

Question 6

The process of alcoholic (ethanol) fermentation has three steps.
Step 2: The ethanal accepts hydrogen atoms from reduced NAD, becoming reduced to ethanol. Catalysed by ethanol dehydrogenase.

Step 3:

Answer 6

3.Reduced NAD has been re-oxidised and made available to accept more hydrogen atoms from triose phosphate, thus allowing glycolysis to continue.

Question 7

What does pyruvate decarboxylase have bound to it?

Answer 7

A coenzyme thiamine diphosphate.

Question 8

When does lactate fermentation occur?

Answer 8

• In mammalian muscle tissues during vigorous physical activity
• such as when running fast to escape a predator -
• when the demand for ATP for muscle contraction and there is an oxygen deficit.

Question 9

There are two steps to the lactate fermentation pathway.

Step 1:

Answer 9

1. Pyruvate, produced during glycolysis, accepts hydrogen atoms from the reduced NAD, also made during glycolysis. The enzyme lactase dehydrogenase catalyses the reaction. There are two outcomes…
   - Pyruvate reduced to lactate
   - The reduced nAD becomes reoxidised.

Question 10

There are two steps to the lactate fermentation pathway.
Step 1: Pyruvate, produced during glycolysis, accepts hydrogen atoms from the reduced NAD, also made during glycolysis. The enzyme lactase dehydrogenase catalyses the reaction. There are two outcomes…
-Pyruvate reduced to lactate
-The reduced nAD becomes reoxidised.

Step 2:

Answer 10

2. Reoxidised NAD can accept more hydrogens from triose phosphate and glycolysis can continue to produce enough ATP to sustain muscle contraction for a short period.

Question 11

What happens to the lactate produced in the muscle tissue?

Answer 11

• It is carried away from the muscles, in the blood, to the liver. When more oxygen is available, lactate may either:
• Be converted into pyruvate, which may enter the Krebs cycle via the link reaction.
• Be recycled to glucose and glycogen.

Question 12

What would happen if lactate was not removed from the muscle tissue?

Answer 12

The pH would be lowered and this would inhibit the action of many enzymes involved in glycolysis and muscle contraction.

Question 13

How do the processes of ethanol fermentation and lactate fermentation produce two molecules of ATPper glucose molecule without actually directly producing ATP?

Answer 13

They allow glycolysis to continue so the net gain of two molecules of ATP is still obtained.

Question 14

Because the glucose molecule is only partly broken down, many more molecules of ATP can undergo glycolysis per minute, and therefore the overall yield of ATP is quite large. However, why is aerobic respiration preferred to anaerobic respiration?

Answer 14

For each molecule of glucose, the yield of ATP via anaerobic respiration is about 1/15 of that produced during aerobic respiration.