A2 Biology OCR June 2015 Revision Thread

Can anyone give me any tips on interpreting command words like "explain" "Suggest" Like is there a special trick?

Can someone explain the BAC method for sequencing a genome please ? thank you


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why does the krebs and link reaction stop in anaerobic conditions

Difference between Lamellae and Thylakoids?

'Cardiac muscle prefers to respire fatty acids rather than glucose. Suggest why the heart muscle is more affected by limited oxygen than other muscles?' This was a question in my revision guide and although i have the answer i'd never have been able to answer this in an exam. Does anyone know the answer to this?

This is a really good question. I don't know the answer but I can have a go.

As the heart muscle needs to contract so frequently, a lack of oxygen would lead to a large amount of anaerobic respiration occurring as Glycolysis only produces two molecules ATP. For every two molecules of ATP produced, two molecules of pyruvate are formed. These pyruvate molecules are reduced to lactic acid as to reoxidise NADH formed during Glycolysis. This means that a large amount of lactic acid is produced very quickly, which cannot be removed quickly enough as to prevent it adversely affecting the heart muscle.

That's my best guess

In aerobic conditions, oxygen acts as the 'final electron acceptor'. It accepts electrons and combines with protons used in chemiosmosis to form water.

In anaerobic conditions, there is no oxygen, so this process cannot occur. When reduced NAD and reduced FAD are produced, there needs to be a mechanism to reoxidise them back to NAD and FAD respectively as otherwise there wouldn't be enough present in the mitochondrial matrix. If there is no oxygen, the NADH and FADH2 molecules cannot lose their protons and electrons as nothing is accepting them after oxidative phosphorylation. This means that there are not enough NAD and FAD molecules present in the mitochondrial matrix for the Link Reaction and the Krebs cycle to occur (the link reaction uses NAD to form NADH and the Krebs cycle uses both NAD and FAD to produce NADH and FADH2).

I hope my answer makes sense

That was also my guess! The answer is something to do with the fact that fatty acids enter respiration in Krebs cycle which requires oxygen. If no oxygen is present then no ATP can be made in Krebs or by Oxidative phosphorylation. Also no ATP is made by glycolysis as the fatty acids bypass this part of respiration. So basically no ATP is being made at all

I understand it now but i'm not sure i'd have been able to!

Thank you! Just one thing I'm still confused about though, if there isn't enough NAD for Krebs and the link reaction, then how is there enough to maintain glycolysis? or am I missing something obvious :s

I guess great minds think alike

Oh I see! I should of focused on the part of the question talking about the heart using fatty acids. This is actually referring to the beta oxidation pathway where fatty acids and are broken down into two acetyl fragments and combined with coenzyme A. This forms NADH and FADH2. The acetyl fragments enter the Krebs Cycle as normal to produce more NADH and FADH2, which then allows oxidative phosphorylation to occur as normal. ATP is still produced (by oxidative phosphorylation) but Glycolysis is skipped.

This is mentioned on page 97 of the text book and is one of those obscure parts of the syllabus

Thank you, I'll re read that page I did the same as you, it mentioned heart muscles so assumed the heart wouldn't be able to handle lactate build up, so i went down that route instead. I guess it's trying to pick out exactly what they mean.

For Glycolysis NAD is reformed from NADH during the lactate pathway so there's enough for the process to continue. With the Link Reaction and Krebs Cycle, any NADH and FADH2 that is produced builds up as it cannot be reoxidised (there is no oxygen to accept the protons and electrons). So, in other words, without oxygen there is no mechanism to reform the NAD and FAD needed for the Link Reaction and Krebs Cycle.

I hope that helps

Thank you so much!!

Yeah, it's F215 Jan 2011 question 2e, a 9 mark question on the fight/flight response.
Yeah I agree the medulla oblongata changes heart rate in response to change in blood pressure/pH of blood.
And the heart rate also increases when adrenaline is secreted from the adrenal medulla. However note it's the hypothalamus that stimulates the adrenal medulla to secrete adrenaline.

So basically:
Textbooks say hypothalamus controls fight/flight response, which is correct in most respects.
But in addition, mark scheme gives 'AVP' mark for 'correct reference to medulla oblongata' so I presume that the medulla oblongata also acts to increase the heart rate in the fight/flight response? Oh well at least it's nothing too major, would be nice to know a definite answer though!




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Any predictions on what is likely to come up?