In beginning and not-so-beginning texts it is sometimes stated that a pH gradien

Question

In beginning and not-so-beginning texts it is sometimes stated that a pH gradient is created by the electron transport system across mitochondrial membranes and that drives ATP synthesis. This is partly wrong. The chemisosmotic theory claims that there is both a pH gradient and a passage created across the membrane that drives ATP synthesis. Knowing what you know about membrane charging this should seem reasonable. The total free energy of the process of pumping a proton out is written below. The proton(s) moving back in would have an equal and opposite free energy and this drives ATP synthesis from ADP and Pi (-30.5 kJ/mol needed per ATP synthesized) SHOW ALL WORK:

In fact, the across the mitochondria is often as high as -200 mV.

Assume that outside the mitochondrion in the cytosol there is so much buffering power and liquid volume that these protons would not appreciably change the pH outside (pH=7.2). However, inside the mitochondrion is a different matter perhaps. Calculate how pumping this number of protons from a single mitochondrion would change the inside pH. Assume that the inside has a buffering capacity identical to a phosphate buffer (pK = 7.2) of 20 mM concentration and that the starting pH is 7.2 (i.e. 10 mM A-, 10 mM HA). What is pHin- pHout for this situation? Do you think pH gradient it is a major factor in ATP synthesis as compared to passage? Explain.

Explanation / Answer

pH inside the mitochondrial matrix is higher than that in the cytosol. pH in the intermembrane space is same as that in the cytosol.

Now, high pH means less H+. So, inside the mitochondrial matrix, less H+ ions are there under normal conditions. But the number of H+ in intermembrane space and cytosol is more. This leads to a H+ gradient. Since proton channels (called ATPase or Channel V) are present in the inner mitochondrial membrane, so protons move from a region of high concentration (intermembrane space) to a region of lower concentration (mitochondrial matrix); resulting in production of ATP.

The first channel of mitochondrial inner membrane is a pump; it moves H+ from a region of low to high concentration. This channel is named as 'Channel 1 or NADH-dehydrogenase complex. During the process of pumping of H+, NADH is converted in NAD+; electrons are released which move toward Channel 3 via the aid of ubiquinone. This further increases the pH gradient; and forces “Channel V” to transport H+ into the matrix.

Also note that pH = -log10 [H+]

So, greater the pumping of H+; less will be the [H+]; lower the pH. So, pumping protons from a single mitochondrion would increase the inside pH.

So, your answer is yes. pH gradient is extremely essential. High pH gradient will create more hydrogen ions in intermembrane space. This is essential to create proton-motive force; and is the major factor in ATP synthesis.

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