Very hard A-level Biology question A2

The scientists compared the muscle fibres of mice after six weeks of regular exercise (trained mice) with those of mice that had not exercised (control mice).

What I don't get is that both the control mice and trained mice exercised so surely both should be able to exercise for the same period of time. Or is it because the trained mice trained beforehand, so had experience?

From what you said, both mice did not train? The trained group did train but the control group didn't.

Scientists investigated the effect of regular exercise on skeletal muscle fibres in mice. The scientists compared the muscle fibres of mice after six weeks of regular exercise (trained mice) with those of mice that had not exercised (control mice). The scientists stained the muscle fibres from both sets of mice to show succinic acid dehydrogenase activity. The darker the stain the greater the succinic acid dehydrogenase activity.

The scientists then compared the length of time that the control mice and the trained mice could carry out prolonged exercise. The trained mice were able to exercise for a longer time period than control mice.

Explain why.

Answer:
1. (More aerobic respiration) produces more ATP;
2. Anaerobic respiration delayed;
3. Less or no lactate;

What I don't get is that both the control mice and trained mice exercised so surely both should be able to exercise for the same period of time. Or is it because the trained mice trained beforehand, so had experience? And how is there less Anaerobic respiration as O2 can become a limiting factor? Are they assuming that the control mice undergoed only Anaerobic respiration because they exercised so must undergo Aerobic respiration too??

Where you've written 'answer', is this your answer or one from a mark scheme?

Mark scheme

But it says that 'then compared the length of time that the control mice and the trained mice could carry out prolonged exercise. 'So they both exercised, surely both will be able to carry out exercise for the same period of time. Also, why can't the trained mice carry out Anaerobic respiration but the control mice can?

OK, so what don't you understand about the answer? In summary.

Yeah but that is after the training. Not during the training.

Both the trained and control mice exercised so how is more ATP produced in trained mice? Why are they assuming there's no Anaerobic respiration?

They're not assuming there is no anaerobic respiration but that's not relevant to the question. The question told you the protocol involved succinate dehydrogenase: this is a enzyme in the citric acid cycle, so that tell you that we are talking about aerobic, not anaerobic respiration.

You should know that the TCA cycle takes place in the mitochondrial matrix. I think you must have missed copying out part of the question, because there is nothing about which group of mice had the 'darker' staining. But let's assume it was the trained group. Darker stain = greater amount of succinate DH = more mitochondria = greater capacity for ATP synthesis.

Okay but do the control mice exericse at all in either experiment? Also, the answer says that Anaerobic respiration was delayed.

From what you've written, yes they do. I can't see why that's a difficulty though.

Proceed from a hypothesis based on your current knowledge. An exercise programme increases the capacity for sustained exercise by, inter alia, increasing the number of mitochondria in a cell and thus increasing the amount of energy that can be produced, yes? That's fairly well known. This experiment aims to test that hypothesis (specifically, it would test the null hypothesis that there is no difference in the two groups).

So the experiment is testing whether there is a difference between the exercise capacity (and, as a proxy, the mitochondria) between the untrained 'control' group and the exercised group. If the control mice didn't take part, how could you tell whether there was a difference between your intervention group and control group?

However, I say again that I think some of this question is missing. You'd be better off taking a screenshot or photo of the question and posting it up, rather than copying it out. I think you've missed out some vital information, such as which group stained darker.

The trained mice was more darkly stained. You’re misunderstanding me, I agree with everything you’ve said but that’s not what I’m asking. According to your logic, both mice did prolonged exercise (both control and trained), so the same amount of ATP should be produced. More aerobic respiration occurs in the trained mice but that is in the first experiment, we know nothing in the second experiment?

They're not assuming there is no anaerobic respiration but that's not relevant to the question. The question told you the protocol involved succinate dehydrogenase: this is a enzyme in the citric acid cycle, so that tell you that we are talking about aerobic, not anaerobic respiration.

You should know that the TCA cycle takes place in the mitochondrial matrix. I think you must have missed copying out part of the question, because there is nothing about which group of mice had the 'darker' staining. But let's assume it was the trained group. Darker stain = greater amount of succinate DH = more mitochondria = greater capacity for ATP synthesis.

The trained mice was more darkly stained. You’re misunderstanding me, I agree with everything you’ve said but that’s not what I’m asking. According to your logic, both mice did prolonged exercise (both control and trained), so the same amount of ATP should be produced. More aerobic respiration occurs in the trained mice but that is in the first experiment, we know nothing in the second experiment?

Yes they both did prolonged exercise, but the question states that the trained group were able to exercise for a longer period of time. This is because they have had the 6 week training, so their lungs adapted to the stress of exercising by developing a more efficient uptake of oxygen (higher VO2 max). Thus, there is more O2 (the limiting reagent) at the end of the electron transport chain, so aerobic respiration can continue for longer in trained mice. This means, they will produce more succinate dehydrogenase because they are able to maintain aerobic respiration for a longer period of time. They will therefore produce more ATP, allowing muscle contraction to continue for longer. Anaerobic respiration (when the muscles run out of oxygen) can only occur for a short period of time before lactic acid builds up and glycolysis is prevented. So, the untrained mice have had to resort to anaerobic respiration more quickly because their lungs have not adapted (untrained = lower VO2 max) and thus produce more lactate and less ATP, meaning they can't exercise for as long as the trained mice.

Both control and trained will produce varying amounts of ATP. @Reality Check says it here...




The second experiment results are taken from the first experiment. They are the same...in the first one the intensity of colour is measured while in the second one, time is measured.




Here's the brief gist.
if A is controlled and B is trained for six weeks...

At the end of the six week programme, they conduct an experiment,whereby both A and B are exposed to prolonged exercise. B will produce more ATP than A thus B can exercise for longer periods of time.