Lesson 4 Video

mcat5122018
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Joined: Sat Mar 03, 2018 3:33 pm

Lesson 4 Video

Postby mcat5122018 » Fri Mar 16, 2018 1:29 am

Hi,

During the video, Dr. Anthony talks about oxidative phosphorylation very briefly. I know substrate level phosphorylation is when a substrate is used to phosphorylate whereas oxidative phosphorylation is when an inorganic phosphate is used to phosphorylate. Just to understand this, glycolysis uses substrate level phosphorylation whereas kreb's cycle and ETC both use oxidative level phosphorylation? If Kreb's cycle is oxidative phosphorylation, can you explain why?

Thanks.
NS_Tutor_Andrew
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Re: Lesson 4 Video

Postby NS_Tutor_Andrew » Fri Mar 16, 2018 4:02 pm

Hi mcat5122018 -

The way I like to think about it is slightly different.

Oxidative phosphorylation is what ATP synthase does at the end of the electron transport chain in the mitochondria. It's called "oxidative" because the ETC involves successive redox reactions that, through somewhat complex mechanisms, "power" ATP synthase.

Substrate-level phosphorylation is basically all other cases of phosphorylation. One option is for a PO3 group to be transferred to ADP/GDP from some other phosphorylated compound - in other words, if you were to draw out the structures, you'd just see a PO3 group moving from one structure to another via enzyme catalysis. Another option is for a P_i group to be taken as a reactant and placed on ADP/GDP.

You're correct that glycolysis uses substrate-level phosphorylation, although it also produces NADH that can be shunted into the ETC, thereby indirectly contributing to oxidative phosphorylation.

The Krebs cycle mostly produces NADH/FADH2, which are fed into the ETC, indirectly contributing to oxidative phosphorylation. However, it does contain a step where substrate-level phosphorylation happens: in step 6, succinyl-CoA + GDP + P_i --> succinate + CoA-SH + GTP. The GDP + P_i --> GTP step is the specific part that corresponds to substrate-level phosphorylation.

Thanks for the excellent question, and hopefully this helps to clarify the distinction!
Andrew D.
Content Manager, Next Step Test Prep.
mcat5122018
Posts: 13
Joined: Sat Mar 03, 2018 3:33 pm

Re: Lesson 4 Video

Postby mcat5122018 » Sat Mar 17, 2018 9:54 pm

Hi Andrew,

Thanks for the explanation.

I had other questions:
During the Kreb's cycle video, Dr. Anthony mentions that the inhibition of citrate synthase by ATP is relieved by ADP and isocitrate dehydrogenase is activated by Ca2+. Why is it that isocitrate dehydrogenase cannot be activated by ADP since it is also inhibited by ATP?

During the ETC video, Dr. Anthony discusses infants having brown fat cells, which has the ATP synthase less efficient. One of the questions asks why this mechanism might have evolved. If I understood this correctly, it evolved to help the infants stay warm by burning fat cells to release heat. Is this correct?

For the attached picture, can you explain what the correct answers are? I understand why the last answer choice is correct, but I was a bit confused by the explanation and not sure if that is the only correct answer to this question or if there was another one as well.

During the practice timed passage, I realized that I am having trouble reading graphs in a time constrain and I was wondering if there are other resources or resources you guys have available that I can use to try to practice different types of graph reading skills.
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NS_Tutor_Andrew
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Re: Lesson 4 Video

Postby NS_Tutor_Andrew » Mon Mar 19, 2018 1:53 pm

Hi mcat5122018 -

At least one biochem textbook indicates that isocitrate dehydrogenase can indeed be activated by ADP. As discussed in the video, a lot of times the regulatory mechanism involving ADP/ATP is best thought of in terms of the ratio of ATP to AMP or ADP, which is an indicator of energy availability in the cell.

Re: brown fat, you've got it in a nutshell.

Re: the quiz question, only the last statement is correct. The idea here is that fructose > fructose 1-phosphate, which reduces the activity of an enzyme that inhibits glucokinase. Putting these together, we can conclude that fructose will ultimately promote the action of glucokinase, which uses ATP, eventually depleting cellular ATP stores. That said, the important thing here is to try to follow the logic -- you're not likely to be directly tested on the metabolism of non-glucose carbohydrates.

Hope this clarifies things!
Andrew D.
Content Manager, Next Step Test Prep.
NS_Tutor_Andrew
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Joined: Mon May 23, 2016 1:47 pm

Re: Lesson 4 Video

Postby NS_Tutor_Andrew » Mon Apr 16, 2018 1:31 pm

Hi mcat5122018,

It seems like you left a question about the lesson 4 video that may not be here anymore? In any case, just for future reference, for #16, I tend to think that the direction of synthesis & lagging/leading strands is one of those things that is much easier to understand when you visualize it than when you read about it. Here's a link to a video that might be useful, but by all means, feel free to keep looking -- there are TONS of videos out there on science concepts and you never know which one might present it in a way that "clicks" for you. I also recommend drawing it out a few times physically to build that kinesthetic connection with the concept.

Re: the question on dsDNA denaturation, the basic idea is that the charge-based interactions with salts help to stabilize the DNA.

Hope this helps clarify those questions, and thanks for checking in!
Andrew D.
Content Manager, Next Step Test Prep.

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