MCAT BIOCHEM from KA QUESTION HELP??

[futuredocno]

** The answer is B, but I cant figure out why. Does it say somewhere in the passage or is this outside knowledge. And if it is outside knowledge can someone please explain to me the answer.


The production of ATP in the mitochondria has been long known to depend on the proton gradient that is setup by the electron transport chain. Briefly, electrons from energy-rich molecules like glucose and fatty acids are transported to the inner mitochondrial membrane, which contains specialized proteins that can receive and donate electrons to the next electron acceptor in the chain. The flow of electrons eventually combine with oxygen to form water, but this flow of electrons is also coupled to the energy-requiring process of pumping protons from the inner mitochondrial matrix to the inter-membrane space. ATP Synthase, a multi-unit protein situated in the inner mitochondrial membrane, is the only passage for protons back into the mitochondrial matrix. When protons flow down their gradient through the ATP Synthase, sufficient energy is available to catalyze the phosphorylation of ADP to yield ATP.
In addition to ATP Synthase, researchers recently discovered another protein channel situated in the inner mitochondrial matrix that allows the passage of protons back into the mitochondrial matrix. They named these channels “uncoupling proteins” (UCPs) because they do not have any enzymatic activity to catalyze the production of ATP and therefore allow protons to flow down their gradient back into the mitochondrial matrix without utilizing this energy for the synthesis of ATP. In an animal model, researchers noted the following about the UCP to ATP Synthase ratio in brown fat and white fat:

Type of fat UCP:ATP Synthase (ratio)
Brown fat 90:10
White fat 20:80
Brown fat is most abundant in infants to generate body heat since newborns cannot shiver to stay warm. White fat is used to store energy in the form of triglycerides and is usually 202020-25\%25%25, percent of an adult human’s body weight.
Increasing the permeability of the inner mitochondrial membrane would have what effect on brown fat cells?
Please choose from one of the following options.

• More heat production, More ATP production
• Less heat production, Less ATP production
• More heat production, ATP production unaffected
• Less heat production, ATP production unaffected

---

Members don't see this ad.

 

[OrdinaryDO]

The answer seems to be in the passage and from external sources of knowledge.

First of all, the passage states that Brown fat is abundant in infants to generate body heat since newborns can't shiver. Therefore, since Brown fat is used primarily to create heat and it is made up of a ratio of 90:10 UCP:ATP, we know that increasing the permeability will now increase the ratio of UCP:ATP in favor is increasing UCP since it is uncoupled and doesn't use energy. If this is the case, then less heat will be produced (Less utilization of ADP --> ATP) . That answers both of the questions. What do you think?

 

[futuredocno]

I understand what you are saying. But wouldn't it not do anything to the ATP production since they do not have any enzymatic activity to catalyze the production of ATP?
But where do you get that increasing the permeability will increase the ratio of UCP:ATP ? ( I know that H+ ions are impermeable to the membrane ?)

 

[waitaminute]

The answer seems to be in the passage and from external sources of knowledge.

First of all, the passage states that Brown fat is abundant in infants to generate body heat since newborns can't shiver. Therefore, since Brown fat is used primarily to create heat and it is made up of a ratio of 90:10 UCP:ATP, we know that increasing the permeability will now increase the ratio of UCP:ATP in favor is increasing UCP since it is uncoupled and doesn't use energy. If this is the case, then less heat will be produced (Less utilization of ADP --> ATP) . That answers both of the questions. What do you think?

I don't really understand this explanation. Doesn't uncoupling itself generate heat because the energy produced by H+ ions flowing down their gradient is not utilized by the ATP synthase to make ATP? So technically if you increase the number of UCP in the membrane, the amount of energy not utilized to make ATP will increase, and thus the energy lost as heat will increase. The brown fat is used to create heat because of the large amount of UCP, which allows many H+ to flow down their gradient independently of the ATP synthase, which generates energy that can be lost as heat (instead of being used to generate ATP)

For the ATP production, I think it's because increasing the permeability of the membrane would increase the number of H+ ions flowing down their gradient independently of the ATP synthase, which would result in less ATP being produced overall.

I don't really understand the relationship between UCP and permeability/how increasing permeability would decrease heat produced, though. I would have chosen more heat produced and less ATP production, although that's not a choice.

 

[bee17]

The only thing you need to know to answer this question is that increasing the permeability of the inner mitochondrial membrane would result in less ATP production, because there wouldn't be as much of a proton gradient to power ATP synthase. Answer B is the only choice that includes "less ATP production," so B is correct.

 

[OrdinaryDO]

I don't really understand this explanation. Doesn't uncoupling itself generate heat because the energy produced by H+ ions flowing down their gradient is not utilized by the ATP synthase to make ATP? So technically if you increase the number of UCP in the membrane, the amount of energy not utilized to make ATP will increase, and thus the energy lost as heat will increase. The brown fat is used to create heat because of the large amount of UCP, which allows many H+ to flow down their gradient independently of the ATP synthase, which generates energy that can be lost as heat (instead of being used to generate ATP)

For the ATP production, I think it's because increasing the permeability of the membrane would increase the number of H+ ions flowing down their gradient independently of the ATP synthase, which would result in less ATP being produced overall.

I don't really understand the relationship between UCP and permeability/how increasing permeability would decrease heat produced, though. I would have chosen more heat produced and less ATP production, although that's not a choice.

Here is exactly what I am saying..

The only thing you need to know to answer this question is that increasing the permeability of the inner mitochondrial membrane would result in less ATP production, because there wouldn't be as much of a proton gradient to power ATP synthase. Answer B is the only choice that includes "less ATP production," so B is correct.

 

[futuredocno]

From the passage it says "therefore allow protons to flow down their gradient back into the mitochondrial matrix without utilizing this energy for the synthesis of ATP." So if there is an increase in membrane permeability for protons, that means that there will be less protons that don't utilize the energy for the synthesis of ATP , therefore less ATP?? Is that the right thinking?

 

[bee17]

From the passage it says "therefore allow protons to flow down their gradient back into the mitochondrial matrix without utilizing this energy for the synthesis of ATP." So if there is an increase in membrane permeability for protons, that means that there will be less protons that don't utilize the energy for the synthesis of ATP , therefore less ATP?? Is that the right thinking?

The proton gradient is the difference between the concentration of protons between the mitochondrial matrix and the intermembrane space. The electron transport train pumps protons from the matrix into the intermembrane space, thereby creating the gradient. The protons will flow back down their gradient if possible. Usually, most of those protons flow through ATP synthase to do this, fueling ATP synthesis. If some of the protons from the intermembrane space have an alternative route to flow down their gradient (UCPs in this case), then there will be less of a gradient to power ATP synthase. Make sense?