Biology Question Dispute

[MichaelShieh]

This is going to be somewhat long and maybe confusing ;)

Does anyone know of an e-mail address I can mail to over a dispute about the correct answer on a Biology question? I was taking another test during the verification period.

The question was #3 on the Regional Biology section.

It has a diagram of a mitochondrion with letters A - E pointing at various parts of the organelle. The question asks which labeled area or its surroundings is the concentration of hydrogen ions the highest.

Here's a crude remake of the diagram:


A
ooooooooo
B
oooooooooC

D

ooooooooo

oooooooooE


A - Labels the area outside of the Outer mitochondrial membrane
B - Labels the area in between the outer and inner mitochondrial membrane, aka the "Interspace membrane"
C - Labels the Inner mitochondrial membrane
D - Labels the area inside the Inner mitochondrial membrane, AKA the "Mitochondrial matrix"
E - Labels the Outer mitochondrial membrane

The correct answer is B; however I believe it to be D. In a way, both answers are correct.

The question needs to be more specific in that:
If the question had stated that the mitochondria was actively participating in cellular respiration, then choice B would be correct. Because the hydrogen ions would be more concentrated in area B than in area D due to the mitochondrial enzymes producing a hydrogen ion gradient.

If the question had stated that the mitochondria was NOT actively participating in cellular respiration, then choice D would be correct. Because the hydrogen ions originate in the mitochondrial matrix. Since in order to produce the hydrogen ion gradient, the hydrogen ions would have to be pumped from an area of higher concentration (Area D) to an area of lower concentration (Area B) and then after some time has passed Area B would have a higher concentration of hydrogen ions than Area D. Thus producing a hydrogen ion gradient needed for the production of ATP.


So... What do you think? :D

[JonathanTrinh]

Altho u tried to reason it out, it is not the correct reasoning. The Matrix of the mitochondria will never have a higher H conc because if that is the case, you will be dead. Actually, you will be dead if it even come close to quilibrium favoring the diffusion of the H in the intermembrane space vs the matrix. much less positive H conc of the matrix vs the intermembrane space. to put it in simplistic term, y would the cell need to expand energy to pump H from the matrix to the intermembrane space if the matrix originally contains higher H conc than the intermem space. Wouldnt facilitated the diffusion work best in this situation. Living organism will evolve utilizing the most effecient way of conserving energy. The reason y the intermebrane space will never even come close to reaching equilirium in terms of the H conc with the matrix is because oxygen diffuse into the matrix and serve as the final electron acceptor as the electron travel down the electron trasport chain. That in turn pich up the protons diffusing by facilatated diffusion going thru the ATP synthase from the intermembrane space into the matrix to form h20. The proton pumps embeded in the inner membrane that form the matrix will then in turn pump protons from the matrix to the intermembrane space using the hi energy electrons generated during photosynthesis. As you can see, the matrix will never contain more H conc than the intermem space under any circumstances. Even when u died, it prob reach equilibrium still prob favoring the inetrmembrane space vs the matrix. I hope this help. Jonathan Trinh / Dulles HS Math and Science Team Coach SEMPER FI

[MichaelShieh]

Are you sure about that?

Here's an excerpt from a Biology book:


Look at Line 5 under "Building an Electrochemical Gradient". Also, the text under Figure 9.17.

[Brahma Physics]

Below are two statements you made that may be leading to your misconceptions.

A) ".....Since in order to produce the hydrogen ion gradient, the hydrogen ions would have to be pumped from an area of higher concentration (Area D) to an area of lower concentration (Area B) and then after some time has passed Area B would have a higher concentration of hydrogen ions than Area D."

B) "...Thus producing a hydrogen ion gradient needed for the production of ATP."

To address A): To create a gradient in the first place, you generally would start from a relative equlibrium (Although Mr. Trinh is right, we're never truly at equilibrium in this instance..if so, death would ensue...on a related topic, check out how cyanide works, making the membrane "leaky", pretty cool biochemistry). This implies that there are already Hydronium Ions both inside AND outside the membrane....which you would expect in any aqueous environment. What make this system work is that the membrane is impermeable to the ions. The energy supplied by the electron transport chain (that received high energy electrons from NADH and FADH2) is what "pumps" the ions in only one direction -- outward from the matrix into the intermembrane space. Subsequently, you end up with the acidic environment of the intermembrane space (a higher concentration of hydroniums).

This leads to statement B): It's not DIRECTLY the hydronium ions' return to the matrix that actually allows the production of the ATP. The ion gradient (charge gradient) creates membrane potential (voltage). The movement of ions back down the gradient (chemiosmosis) through a different integral protein (ATPase) causes a change in shape of the ATPase. During this shape change, an inorganic phosphate and ADP (both of which have temporarily bound to the ATPase) are connected, creating a high energy bond between the two from the electric energy of the membrane. The newly formed ATP is simultaneously released from the ATPase.

The diagram you posted shows this. I hope this helps. On a related note, check out the mechanism of photophosphorylation in photosythesis (between photosystem I and II). This time, the hydronium gradient is created INSIDE the membrane (with the thylakoid).

[MichaelShieh]

Ah, I see now.

Thank you very much, Brahma Physics. :)

[bradp]

I'm gonna have to memorize all of this stuff before district next year...

[peterliang]

I understood about 2 words from that