electron transport chain and oxidative phosphorylation

oxidative phosphorylation

Oxidative phosphorylation captures the energy of high-energy electrons to synthesise ATP.

The flow of electrons from NADH to and FADH2 to O2 occurs in the electron-transport chain or respiratory chain.

exergonic set of oxidation-reduction

This exergonic set of oxidation-reduction reactions generates a proton gradient and then the proton gradient is used to power the synthesis of ATP.

respiration

Collectively, the citric cycle and oxidation phosphorylation are called cellular respiration or simply respiration.

Respiration is defined as ATP-generated process in which an inorganic compound serves as the ultimate electron acceptor. The electron donor can either be an organic or inorganic compound.

outer mitochondrial membrane 

The outer mitochondrial membrane is permeable to most small ions and molecules because of the channel protein mitochondrial porin.

inner mitochondrial membrane

The inner membrane, which is folded into ridges called cristae, is impermeable to most molecules. The inner membrane is the site of electron transport and ATP synthesis. The citric acid cycle and fatty acid oxidation occur in the matrix.

overview of oxidative phosphorylation

The electron transport chain is 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 if 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

electron transport chain

The electron transport chain is a collection of membrane-embedded proteins and organic molecules, most of them organised into four large complexes labelled I to IV.

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.

need for oxygen

Cells use O2 during oxidative phosphorylation, the final stage of cellular respiration. Oxygen sits at the end of the electron transport chain, where it accepts electrons and picks up protons to form water.

If oxygen isn't there to accept electrons, the electron transport chain will stop running, and ATP will no longer be produced by chemiosmosis.

electron transport chain components

The respiratory chain consists of 4 complexes: three proton pumps and a physical link to the citric acid cycle.

• Electrons flow NADH to O2 through 3 large protein complexes embedded in the inner mitochondria membrane.

• These complexes pump protons out of the mitochondrial matrix, generating a proton gradient. Proton movement back in through ATP synthase powers substantial ATP synthesis.

the complexes are

NADH-Q oxidoreductase (complex I)

Q-cytochrome c oxidoreductase (complex III)

Cytochrome c oxidase (complex IV)

an additional complex

succinate Q-reductase (complex II), delivers electrons from FADH2 to complex III.

Succinate-Q reductase is not a proton  pump

important functions of the electron transport chain

1. Regeneration of electron carriers

NADH and FADH2 pass their electrons to the electron transport chain, turning back into NAD+ and FAD.

This is important because the oxidised forms of these electrons carriers are used in glycolysis and the citric acid cycle and must be available to keep these processes running.

2. 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 that can be used to make ATP.

NADH

NADH is very good at donating electrons in redox reactions (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

FADH2 is not as good at donating electrons as NADH (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 proton gradient) than an NADH.

NADH-Q oxidoreductase (complex I)

NADH transfers it electrons to complex I. In complex I, the part of it that receives the electrons, is a flavoproteins, meaning a protein with an attached organic molecule called flavin mononucleotide (FMN).

FMN

FMN is a prosthetic group, that actually accepts electrons from NADH. FMN passes the electrons to another protein inside complex I, one that has iron and sulphur bound to it (called an Fe-S protein), which in turn transfers the electrons to a small, mobile carrier called ubiquinone.

Succinate Q-reductase (complex II)

FADH2 deposits its electrons in the electron transport chain, but it does so via complex II, bypassing complex I entirely.

FADH2 transfers its electrons to iron-sulphur proteins within complex II, which then passes the electrons to ubiquinone (Q), the same mobile that collects electrons from complex I. Complex II is not a proton pump.

Q-cytochrome c oxidoreductase (complex III)

Complex III includes an iron-sulphur (Fe-S) protein and 2 cytochromes. Cytochromes are a family of related proteins that have haeme prosthetic groups containing iron ions. In complex III, electrons are passed from one cytochrome to an iron-sulphur protein to a second cytochrome, then finally transferred out of the complex to a mobile electron carrier (cytochrome C).

Complex III pumps protons from the matrix into the intermembrane space, contributing to the H+ concentration gradient.

Cytochrome c oxidase (complex IV)

In complex IV, electrons are passed through 2 more cytochromes. With the help of a nearby copper ion, it transfers electrons to O2, splitting oxygen to form 2 molecules of water.

electron transfers

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

several of the protein complexes use the released energy to

pump protons from the mitochondrial matrix to the intermembrane space, from a proton gradient

ATP synthase, a molecular mill

In the inner mitochondrial membrane H+ ions have just one channel available: a membrane-spanning protein known as ATP synthase, it's turned by the flow of H+ ions moving down their electrochemical gradient. As ATP synthase turns, it catalyses the addition of a phosphate to ADP, capturing energy from the proton gradient as ATP.

chemiosmosis

the movement of ions across a semipermeable membrane, down their electrochemical gradient 

ATP yield

About 4 H+ ions must flow back into the matrix through ATP synthesis of 1 ATP molecule. When electrons from NADH move through the transport chain, about 10 H+ ions are pumped from the matric to the intermembrane space, so each NADH yields about 2.5 ATP. Electrons from FADH2 drive pumping of only 6 H+, leading to production of about 1.5 ATP.