2015 AP Biology free response 2 a b

All right, cellular respiration includes the metabolic pathways of glycolysis, the Krebs cycle, and the electron transport chain, as represented in the figures. So we have the figures here of glycolysis, the Krebs cycle, and the electron transport chain. If all of this looks completely foreign to you, I encourage you to watch the videos on KH Academy on glycolysis, the Krebs cycle, and the electron transport chain, and on cellular respiration in general.

All right, but let's tackle this problem. It is a nice review of all of those things in cellular respiration. Carbohydrates and other metabolites are oxidized, meaning electrons are taken away from them. The resulting energy transfer reactions support the synthesis of ATP. All right, using the information above, describe one contribution of each of the following in ATP synthesis.

All right, the first one: metabolism of glucose in glycolysis and pyruvate oxidation. The second was oxidation of intermediates in the Krebs cycle, and the third one is formation of a proton gradient by the electron transport chain. Now, each of these statements seem kind of intimidating, but they're really just saying describe how glycolysis and pyruvate oxidation contribute to ATP synthesis. Describe how the Krebs cycle produces ATP because the Krebs cycle is essentially nothing but the oxidation of these intermediates to produce things that are useful for ATP synthesis.

Then the formation of the proton gradient by the electron transport chain—that's what the electron transport chain does. It takes high-energy electrons from NADH or FADH2, and then as those high-energy electrons go to lower and lower energy states, it's pumping these hydrogen protons across the membrane. When they come back in, that's used to synthesize ATP.

So let’s just answer it. We have to describe one contribution of each. So let's first focus on glycolysis. If we look here, there's more than one contribution. You see that it can phosphorylate these two ADPs to two ATPs, so that's one contribution we could list. We could say that it's producing these NADHs, which provide both the hydrogen protons and, more importantly, the high-energy electrons for the electron transport chain later on.

You could say that it's producing the acetyl CoA, which can enter the Krebs cycle, which is used to produce GTP or more NADHs or FADH2s. So all of these are contributions, and since we only have to list one, I'll list the most direct and obvious one, although you could list any of these. I will say that since they say one contribution, I'll say the phosphorylation of two ADPs to two ATPs.

Now, let's move on to oxidation of intermediates in the Krebs cycle. The oxidation of the intermediates—that's just talking about each of these things keeps getting oxidized, and as they get oxidized, we can use that to reduce other things, including NAD+ to NADH. So when you reduce it, you're gaining electrons. Notice NAD+ is positive, and it becomes NADH, which is neutral. These NADHs are used later on in the electron transport chain to pump hydrogen protons across the membrane, which are then used in oxidative phosphorylation to produce the ATP as they go back through the membrane.

So you could talk about the NADHs or FADH2s, or you could talk about the direct creation of GTPs, which could be used to create ATPs. So any of that is fine, so I'll just list one of them. I’ll write reduction of NAD+ to NADH, which is used in the electron transport chain to pump hydrogen protons to create a proton gradient.

And once again, I could talk about the GTP being created. I could talk about the FADH2 being created.

Now, for the formation of a proton gradient by the electron transport chain: As the protons flow with the gradient back across the membrane, they power ATP synthase, which creates ATP from ADP. So as protons flow across the membrane with the gradient, they drive ATP synthesis, which could be described as oxidative phosphorylation.

So I'll just write it as protons flow with the gradient across the membrane; they drive ATP synthesis, which takes ADP and phosphorylates it to ATP.

And once again, if everything I’m saying here sounds foreign, and if these diagrams don’t make a lot of sense to you, if they don’t trigger a pleasurable memory in your mind, I encourage you to watch the videos on KH Academy. Hopefully, you'll get a little bit more intuition for the things that I am talking about.

All right, now let's see if we can tackle part B. Use each of the following observations to justify the claim that glycolysis first occurred in a common ancestor of all living organisms.

So, nearly all existing organisms perform glycolysis, so it's much more likely—let me write this in order—so for this one right over here: in order for this to be the case, it is much more likely that this evolved in a common ancestor and was passed down, or you could say selected for, rather than independently arising in multiple branches of the evolutionary tree.

The fact that nearly all existing organisms perform glycolysis suggests that it evolved at a very early stage, at a kind of primitive ancestor organism, and that was selected for. It continues to be selected for, and that's why we continue to see it.

All right, glycolysis occurs under anaerobic conditions. So actually, let me create some space here so things don’t get too jumbled up. Glycolysis occurs under anaerobic conditions. Early Earth had little oxygen. Early life had to produce ATP or metabolize sugars or carbohydrates in an anaerobic environment, so that seems to once again justify the claim that glycolysis first occurred in a common ancestor of all living organisms.

Glycolysis only occurs in the cytosol. The earliest life didn't have membrane-bound organelles. So the fact that it only occurs in the cytosol is consistent with the idea that early life first occurred in a common ancestor.

I'll then tackle parts C and D in the next video.