Question

When the protein gramicidin is integrated into a membrane, an H+ channel forms and the membrane becomes very permeable to protons (H+ ions). If gramicidin is added to an actively respiring muscle cell, how would it affect the rates of electron transport, proton pumping, and ATP synthesis in oxidative phosphorylation? (Assume that gramicidin does not affect the production of NADH and FADH2 during the early stages of cellular respiration.)

Sort the labels into the correct bin according to the effect that gramicidin would have on each process.

[The following are some hints to help you approach the problem.]

You know that membranes treated with gramicidin become very leaky to protons. Consider these four questions (in this order) to help you evaluate how gramicidin alters oxidative phosphorylation

1. Is a proton gradient across the inner mitochondrial membrane required for ATP synthesis during oxidative phosphorylation?

2. What effect does a membrane that is very leaky to protons have on the ability of the mitochondrion to maintain a proton gradient across that membrane?

3. What effect does the ability of the mitochondrion to maintain a proton gradient have on the rate of proton pumping?

4. How is the rate of electron transport related to the rate of proton pumping, and are these rates affected by the membrane being leaky to protons? 

Answer 1

1) Yes. The proton gradient across the inner mitochondria is essential for the enzyme ATP synthetase for producing ATP.

2) In order to maintain the proton gradient (despite leaky membrane), there will be continuous pumping of the proton into the mitochondria membrane.

3) The proton pumping rate remains the same.

4) When an electron is transferred from one molecule to another in ETC, there is a need for a proton to further drive the process, which is provided by the proton pumping system. In the case of the leaky membrane, there will the same rate of electron transport and proton pumping rate but there will be a decrease in the proton gradient and the synthesis of ATP is inhibited.

Electron transport Rate - Remains the same (the defect is with the leaky membrane and not with the electron transport).

Proton Pumping Rate - Remains the same (Proton is pumped with regard to electron transport and it remains the same)

The rate of ATP synthesis - Decreases (failing to have a proton gradient due to leaky membrane)

The rate of Oxygen uptake - Remains the same

Size of Proton Gradient - Decreases (Due to leaky membrane)