AQA - BIOL4 - 13th JUNE 2014

Can someone help me and explain to me the difference between the electron transport chain in photosynthesis and respiration? I don't even know if they are different or the same thing.

Can someone help me and explain to me the difference between the electron transport chain in photosynthesis and respiration? I don't even know if they are different or the same thing.

Directional selection, I believe.
There is a selection pressure against the recessive allele, meaning that the opposite extreme (compared to recessive allele extreme) is being selected for. The allelic frequency of the recessive allele decreases as time goes on. On a normal distribution curve, the mean will be moving to the side opposite the recessive allele side.
Am I right? What did the mark scheme say?

Thanks for the good luck but I've just finished a degree in Biology

To add to the explanation you're already received here is full detail.

To add to the explanation you're already received here is full detail.




Non-cyclic photophosphorylation
Non cyclic photophosphorylation produces ATP, reduced NADP and oxygen.
- Light is absorbed by photosystem II and electrons in chlorophyll become excited
- These electrons are accepted by an electron acceptor
- Consequently photosystem II is electron deficient (See photolysis)
- The electrons are passed down an ETC: the energy released is used to make ATP from ADP and Pi
- Light also hits photosystem I also exciting electrons.
- To replace the e- lost photosystem I absorbs the two electrons from photosystem II
- The energy released and the e- from photosystem II are used to reduce NADP to NADPH

Oxidative phosphorylation
Electron Transport chain
1) The ETC is formed from a series of proteins, bound in the inner mitochondrial membrane. The cristae maximises the surface area of this membrane and this increases the rate of ATP production
2) Reduced NAD (NADH), produced in the matrix during the Krebs cycle, is oxidised by the first protein in the ETC: NADH dehydrogenase. This produces a proton (H+) and NAD+ along with two electrons that bind to the protein.
3) The electrons are now passed between the ETC proteins in a series of redox reactions. As they move they lose energy. Some of this is used to pump H+ ions from the matrix into the intermembrane space; the rest is lost as heat.
4) As the inner mitochondrial membrane is impermeable to H+ ions, a concentration gradient forms
5) H+ ions move down their concentration gradient into the matrix using protein channels. These are associated with the enzyme ATP synthase, which phosphorylates one ADP for each H+ ion passing through it.
The use of energy in a chemical gradient to generate ATP by the flow of hydrogen ions through ATP synthase is called chemiosmosis.
6) The final protein in the ETC (cytochrome oxidase) donates the electron pair to the oxygen atom. Oxygen is the final proton and electron accepter as it binds with the H+ ions in the matrix to form water
1/2 O2 + 2H+ + 2e- --> H2O
7) The donation of the electrons to the oxygen molecule releases enough energy to pump another H+ ion across the membrane. This can be used to regenerate another molecule of ATP.

Reduced FAD (FADH2) is also oxidised by the ETC; however it interacts with the second protein in the chain. This means that is causes less H+ to be pumped into the intermembrane space than NADH, so regenerates less ATP



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The difference is that in photosynthesis the electrons are moving due to the energy input of light while in respiration the electrons were lent to the ETC by the NADH. Another difference is that in photosynthesis the primary electron acceptor is NADPH and in cellular respiration it is oxygen.

The similarities are that they both make ATP and they both have the process of chemiosmosis and the electrons moving down the concentration gradient.

Omg you're a genius. Till now I never really understood chemiosmosis. Just repped you.

I'm getting quite worried about BIOL4 now. How is everyone else revising at the moment?

I'm getting quite worried about BIOL4 now. How is everyone else revising at the moment?

Is reduced NAD the same as NADH?

Thank you so much

This is really helpful.

Just one more thing is reduced NAD the same as NADH?

I've written up all my notes so if you have any particularly areas you want help in feel free to ask. Now I'm just doing past papers and looking over mark schemes.


Posted from TSR Mobile

Thanks everyone, yes I said that in my question what the answer is! It is directional. I just didn't understand why until now.


Posted from TSR Mobile

I've written up all my notes so if you have any particularly areas you want help in feel free to ask. Now I'm just doing past papers and looking over mark schemes.


Posted from TSR Mobile

I'm just re-reading my notes-cum-model answers. That's it. I feel so unprepared. I have done the papers already. But tomorrow I might try some old spec papers. I just don't know how else to prepare. I don't want to focus too much on application questions because they differ every year anyway.

Can you tell me about speciation, how it actually works. And what's geographical isolation, I sometimes see them in the mark scheme, sometimes I don't.

By the way, does anyone pull a whole-nighter during exams?

Ok, I think I found another question that's similar to what you guys were inititally discussing about.

Describe how ATP is synthesised in oxidative phosphorylation. (Remember this is respiration, because the ETC in photosynthesis is photo-phosphorylation)

- Electrons release energy as they pass down the electron transport chain
- Energy used to combine ADP and Pi to form ATP
- Protons move into intermembrane space of the mitochondria by active transport
- Protons diffuse into mitochondrial matrix through ATP synthase
- By chemiosmosis

So yeah, you were both right! But only say this when they specifically ask about 'oxidative phosphorylation'. If they simply ask how ATP is made in light-dependant reaction, or how ATP is made in ETC, no need to go into this detail