Question

Briefly summarize the steps in the electron transport chain from NADH to oxygen.

Short answer

Electrons are transferred from NADH through Complexes I, III, and IV, creating a proton gradient that drives ATP synthesis. Finally, electrons combine with oxygen to form water.

Step-by-step solution

NADH Donates Electrons

NADH donates two electrons to Complex I (NADH dehydrogenase) in the electron transport chain. This process also releases protons (H+) into the mitochondrial matrix.

Electron Transfer Through Complex I

Electrons move through Complex I, which pumps protons from the mitochondrial matrix into the intermembrane space, creating a proton gradient across the inner mitochondrial membrane.

Transfer to Coenzyme Q

The electrons are then transferred from Complex I to coenzyme Q (ubiquinone), which carries them to Complex III.

Through Complex III

Electrons move through Complex III (cytochrome bc1 complex), which also pumps protons into the intermembrane space, further strengthening the proton gradient. Electrons are transferred to cytochrome c.

Transfer to Cytochrome c

Cytochrome c carries the electrons one at a time from Complex III to Complex IV (cytochrome c oxidase).

Final Transfer to Oxygen

In Complex IV, electrons combine with oxygen (O2) and protons (H+) from the matrix to form water (H2O). This final transfer helps to maintain the electrochemical gradient needed for ATP production.

Formation of ATP

The proton gradient generated by the transfer of electrons through the complexes drives the synthesis of ATP from ADP and inorganic phosphate (Pi) via ATP synthase, a process known as oxidative phosphorylation.

Key concepts

NADH Dehydrogenase

NADH dehydrogenase, also known as Complex I, is the first step in the electron transport chain.
It receives two electrons from NADH, an electron carrier. When NADH donates these electrons, protons (H+) are released into the mitochondrial matrix.
These electrons then travel through Complex I, which is capable of pumping protons from the mitochondrial matrix into the intermembrane space.
This action contributes to the establishment of the proton gradient across the inner mitochondrial membrane.

Proton Gradient

The proton gradient is essential for the electron transport chain and subsequent ATP production.
As electrons move through the complexes of the electron transport chain, protons are pumped from the matrix into the intermembrane space.
This creates an electrochemical gradient, where the concentration of protons is significantly higher in the intermembrane space compared to the matrix.
This gradient is a form of stored energy, essential for ATP synthesis, as the flow of protons back into the matrix through ATP synthase drives the production of ATP.

Coenzyme Q

Coenzyme Q, also known as ubiquinone, plays a crucial role in the electron transport chain.
It picks up electrons from Complex I (or Complex II) and transports them to Complex III.
Coenzyme Q is a lipid-soluble carrier, which means it can move freely within the inner mitochondrial membrane.
By transferring electrons between complexes, coenzyme Q helps continue the flow of electrons down the chain, enabling the maintenance of the proton gradient.

Cytochrome c Oxidase

Cytochrome c oxidase is another term for Complex IV in the electron transport chain.
This complex plays a crucial role in the final stage of electron transfer.
Complex IV receives electrons from cytochrome c, a carrier protein that carries electrons one at a time from Complex III.
Inside Complex IV, the electrons combine with molecular oxygen (O2) and additional protons (H+) to form water (H2O).
This reaction is vital, as it helps sustain the electrochemical gradient required for ATP synthesis.

Oxidative Phosphorylation

Oxidative phosphorylation is the process where ATP is produced as a result of the transfer of electrons through the electron transport chain.
The proton gradient generated by this transfer creates a potential energy difference across the inner mitochondrial membrane.
As protons flow back into the matrix through ATP synthase, this enzyme uses the energy to combine ADP with inorganic phosphate (Pi), forming ATP.
This process is essential because ATP acts as the primary energy currency in cells, fueling numerous biological processes.