Question

Explain the "hardware" that is present in the mitochondrion to carry out electron transport - so electrons really "flow" from the beginning to the end and What is the importance of knowing the sequence of the carriers in the process

Answer 1

The electron transport chain

The electron transport chainis a collection of membrane-embedded proteins and organic molecules, most of them organized into four large complexes labeled I to IV. In eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. In prokaryotes, the electron transport chain components are found in the plasma membrane.

As the electrons travel through the chain, they go from a higher to a lower energy level, moving from less electron-hungry to more electron-hungry molecules. Energy is released in these “downhill” electron transfers, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient.This gradient represents a stored form of energy, and, as we’ll, it can be used to make ATP.

The electron transport chainis a series of proteins and organic molecules found in the inner membrane of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of redox reactions. Energy released in these reactions is captured as a proton gradient, which is then used to make ATP in a process called chemiosmosis. Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation. The key steps of this process, shown in simplified form in the diagram above, include:

• Delivery of electrons by NADH and FADH2 ' .Reduced electron carriers (NADH and FADH2 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.

All of the electrons that enter the transport chain come from NADH and FADH2 molecules produced during earlier stages of cellular respiration: glycolysis, pyruvate oxidation, and the citric acid cycle.

• 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.

• FADH2 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.

Because of this "bypass," each FADH2 molecule causes fewer protons to be pumped (and contributes less to the proton gradient) than an NADH.

More about complexes I and II

Beyond the first two complexes, electrons from NADH and FADH2 travel exactly the same route. Both complex I and complex II pass their electrons to a small, mobile electron carrier called ubiquinone (Q), which is reduced to form QH2 and travels through the membrane, delivering the electrons to complex III. As electrons move through complex III, more H+ ions are pumped across the membrane, and the electrons are ultimately delivered to another mobile carrier called cytochrome C (cyt C). Cyt C carries the electrons to complex IV, where a final batch of H+ ions is pumped across the membrane. Complex IV passes the electrons to O2, which splits into two oxygen atoms and accepts protons from the matrix to form water. Four electrons are required to reduce each molecule of O2, and two water molecules are formed in the process.

More about complexes III and IV

• Regenerates electron carriers. NADH and FADH2 we pass their electron 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.

• Knowing the sequence of carriers helps in :

  flow of electrons in a particular sequence.

  determine the site of action of certain inhibitors of ETC. ex. Cyanide at cyt.c .and barbiturates at complex I.

  determine how the proton gradient is generated and the potential is used to generate ATP.