Enzyme reaction profile question

Hi, I'm a bit puzzled with this A level biology question. I thought both the top 2 graphs could be correct because they both have a lower activation energy with the enzyme than without, but the answer was just the first, because apparently the enzyme-controlled reaction profile can't be exothermic. But surely you can get endothermic enzyme controlled reactions, because hydrolysis of glycogen is the reverse of gycogen synthesis (as an example), so one of the directions has to be endothermic. Maybe its because when enzymes catalyse would-be-endothermic reactions they use ATP or something similar to do it, so the net enthalpy change ends-up being exothermic???

Here is the question and mark scheme. It's edexcel B btw, but I'm actually doing AQA, I was just trying out a different paper.

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Reply 1

Logic1

Original post by minwom28818

Hi, I'm a bit puzzled with this A level biology question. I thought both the top 2 graphs could be correct because they both have a lower activation energy with the enzyme than without, but the answer was just the first, because apparently the enzyme-controlled reaction profile can't be exothermic. But surely you can get endothermic enzyme controlled reactions, because hydrolysis of glycogen is the reverse of gycogen synthesis (as an example), so one of the directions has to be endothermic. Maybe its because when enzymes catalyse would-be-endothermic reactions they use ATP or something similar to do it, so the net enthalpy change ends-up being exothermic???
Here is the question and mark scheme. It's edexcel B btw, but I'm actually doing AQA, I was just trying out a different paper.
Screenshot 2025-02-21 091919.png

I have never ever encountered this question before is this As or A2 I have a feeling I know it but this is tricky?

What was ur GCSEs grades ?

Reply 2

minwom28818

Original post by Logic1

I have never ever encountered this question before is this As or A2 I have a feeling I know it but this is tricky?
What was ur GCSEs grades ?

I think the topic is in A1, but it's from an A level paper (2023), so could be A2. I got 9s in maths and sciences GCSES

Reply 3

Logic1

Original post by minwom28818

I think the topic is in A1, but it's from an A level paper (2023), so could be A2. I got 9s in maths and sciences GCSES

I mean the only reason how I know it’s an endo and exothermic is from a levle chemistry A1 that they taught us enthalpy change.

But this is actually a very good Qs cuz it’s a combination . Idk why but it’s always the b versions that r though like ocr A < OCR B