Why do we need oxygen?
Overview: oxidative phosphorylation
- Delivery of electrons by NADH and FADH. Reduced electron carriers (NADH and FADH) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain. In the process, they turn back into NAD and FAD, which can be reused in other steps of cellular respiration.
- Electron transfer and proton pumping. As electrons are passed down the chain, they move from a higher to a lower energy level, releasing energy. Some of the energy is used to pump H ions, moving them out of the matrix and into the intermembrane space. This pumping establishes an electrochemical gradient.
- Splitting of oxygen to form water. At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and takes up H to form water.
- Gradient-driven synthesis of ATP. As H ions flow down their gradient and back into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to synthesize ATP.
The electron transport chain
- NADH is very good at donating electrons in redox reactions (that is, its electrons are at a high energy level), so it can transfer its electrons directly to complex I, turning back into NAD. As electrons move through complex I in a series of redox reactions, energy is released, and the complex uses this energy to pump protons from the matrix into the intermembrane space.
- FADH is not as good at donating electrons as NADH (that is, its electrons are at a lower energy level), so it cannot transfer its electrons to complex I. Instead, it feeds them into the transport chain through complex II, which does not pump protons across the membrane.
- Regenerates electron carriers. NADH and FADH pass their electrons to the electron transport chain, turning back into NAD and FAD. This is important because the oxidized forms of these electron carriers are used in glycolysis and the citric acid cycle and must be available to keep these processes running.
- Makes a proton gradient. The transport chain builds a proton gradient across the inner mitochondrial membrane, with a higher concentration of H in the intermembrane space and a lower concentration in the matrix. This gradient represents a stored form of energy, and, as we’ll see, it can be used to make ATP.
|Stage||Direct products (net)||Ultimate ATP yield (net)|
|Glycolysis||2 ATP||2 ATP|
|2 NADH||3-5 ATP|
|Pyruvate oxidation||2 NADH||5 ATP|
|Citric acid cycle||2 ATP/GTP||2 ATP|
|6 NADH||15 ATP|
|2 FADH||3 ATP|
- Some cells of your body have a shuttle system that delivers electrons to the transport chain via FADH. In this case, only 3 ATP are produced for the two NADH of glycolysis.
- Other cells of your body have a shuttle system that delivers the electrons via NADH, resulting in the production of 5 ATP.
- Cyanide acts as a poison because it inhibits complex IV, making it unable to transport electrons. How would cyanide poisoning affect 1) the electron transport chain and 2) the proton gradient across the inner mitochondrial membrane?
- Dinitrophenol (DNP) is a chemical that acts as an uncoupling agent, making the inner mitochondrial membrane leaky to protons. It was used until 1938 as a weight-loss drug. How would DNP affect the amount of ATP produced in cellular respiration? Why do you think it is now off the market?*