Question

practice mini frq

1. examining the permeability of mitochondria revealed that the outer membrane was permeable to h+ ions. after discovering this, a researcher wanted to study the effects of ph on the activity of mitochondria. mitochondria from liver cells were isolated and exposed to solutions at ph 9, 8, 7, 6, and 5. the results indicated that mitochondria were most active at ph 5. (a) explain how ph may affect the relative concentration of ions in the mitochondria. (b) propose a model to explain how a low ph could increase the productivity of mitochondria.

→use this space to reflect on topic 5
→textbook chapters/pages to review

Explanation:

(a) pH is a measure of \(H^+\) ion concentration. Since the outer mitochondrial membrane is permeable to \(H^+\) ions, a change in the external pH (which is a measure of \(H^+\) ion concentration in the surrounding solution) will lead to diffusion of \(H^+\) ions. If the external pH is lower (higher \(H^+\) concentration outside), \(H^+\) ions will diffuse into the mitochondria (down their concentration gradient). If the external pH is higher (lower \(H^+\) concentration outside), \(H^+\) ions will diffuse out of the mitochondria.
(b) Mitochondria produce ATP through processes like oxidative phosphorylation, which relies on a proton - motive force (\(H^+\) gradient). A low pH (high \(H^+\) concentration) outside could lead to a larger influx of \(H^+\) ions into the mitochondrial inter - membrane space (if we assume the inner membrane is impermeable to \(H^+\) except through ATP synthase). This larger \(H^+\) gradient (more \(H^+\) in the inter - membrane space relative to the matrix) would drive more \(H^+\) through ATP synthase. As \(H^+\) flows down its concentration gradient through ATP synthase, the enzyme phosphorylates ADP to ATP, increasing mitochondrial productivity (ATP production).

Answer:

(a) pH, as a measure of \(H^+\) ion concentration, affects the diffusion of \(H^+\) ions across the outer mitochondrial membrane (which is permeable to \(H^+\)). A lower external pH (higher \(H^+\) outside) causes \(H^+\) to diffuse into mitochondria; a higher external pH (lower \(H^+\) outside) causes \(H^+\) to diffuse out.
(b) A low pH (high \(H^+\) outside) creates a larger \(H^+\) gradient (more \(H^+\) in the inter - membrane space relative to the matrix if inner membrane is selectively permeable). This drives more \(H^+\) through ATP synthase, increasing ATP production (mitochondrial productivity).