Electron Transport Chain (ETS) in Mitochondria

The electron transport chain (ETS) is a series of protein complexes located in the inner mitochondrial membrane. It plays a crucial role in oxidative phosphorylation, the process by which ATP is generated in the mitochondria. During ETS, electrons are transferred from electron donors, such as NADH, to electron acceptors, such as oxygen.

Protons Accumulation in the Intermembrane Space

As electrons pass through the electron transport chain, protons (H+) are pumped across the inner mitochondrial membrane from the matrix into the intermembrane space. This creates an electrochemical gradient, with a higher concentration of protons in the intermembrane space compared to the matrix.

Proton Accumulation from NADH

(a) 2H (2 electrons) from NADH:
- NADH is oxidized to NAD+ in the first complex of the electron transport chain, called NADH dehydrogenase complex (Complex I).
- During this process, two electrons are transferred to Complex I.
- As these electrons pass through the electron transport chain, four protons are pumped from the matrix to the intermembrane space.
- Therefore, from one molecule of NADH, four protons are accumulated in the intermembrane space.

(b) 3H* (6 electrons) from NADH*:
- Occasionally, NADH is oxidized by a different pathway, bypassing Complex I and entering the electron transport chain at a later stage.
- In this case, six electrons are transferred to the electron transport chain.
- As these electrons pass through the chain, ten protons are pumped from the matrix to the intermembrane space.
- Therefore, from one molecule of NADH*, ten protons are accumulated in the intermembrane space.

(c) 4H (8 electrons) from NADH:
- In some organisms, NADH is oxidized by a different pathway, bypassing both Complex I and Complex III.
- In this case, eight electrons are transferred to the electron transport chain.
- As these electrons pass through the chain, sixteen protons are pumped from the matrix to the intermembrane space.
- Therefore, from one molecule of NADH, sixteen protons are accumulated in the intermembrane space.

(d) 6H* (12 electrons) from NADH*:
- Similar to the previous case, NADH* can also enter the electron transport chain at a later stage, bypassing Complex I and Complex III.
- In this scenario, twelve electrons are transferred to the electron transport chain.
- As these electrons pass through the chain, twenty-four protons are pumped from the matrix to the intermembrane space.
- Therefore, from one molecule of NADH*, twenty-four protons are accumulated in the intermembrane space.

In summary, the number of protons accumulated in the intermembrane space of mitochondria from one molecule of NADH during ETS depends on the specific pathway taken by NADH in the electron transport chain. It can range from four protons (2H) to twenty-four protons (6H*) for different organisms and conditions.