Chemiosmosis

B. Chemiosmosis: The Energy-Coupling Mechanism

The mechanism for coupling exergonic electron flow from the oxidation of food to the endergonic process of oxidative phosphorylation is chemiosmosis.

Chemiosmosis = The coupling of exergonic electron flow down an electron transport chain to endergonic ATP production by the creation of a proton gradient across a membrane. The proton gradient drives ATP synthesis as protons diffuse back across the membrane.

Proposed by British biochemist, Peter Mitchell (1961).
The term chemiosmosis emphasizes a coupling between (1) chemical reactions (phosphorylation) and (2) transport processes (proton transport).
Process involved in oxidative phosphorylation and photophosphorylation.

The site of oxidative phosphorylation is the inner mitochondrial membrane, which has many copies of a protein complex, ATP synthase. This complex:

Is an enzyme that makes ATP.
Uses an existing proton gradient across the inner mitochondrial membrane to power ATP synthesis.

Cristae or infoldings of the inner mitochondrial membrane, increase the surface area available for chemiosmosis to occur.

Membrane structure correlates with the prominent functional role membranes play in chemiosmosis:

Using energy from exergonic electron flow, the electron transport chain creates the proton gradient by pumping H+s from the mitochondrial matrix, across the inner membrane to the inter-membrane space.
This proton gradient is maintained, because the membrane's phospholipid bilayer is impermeable to H+s and prevents them from leaking back across the membrane by diffusion.
ATP synthases use the potential energy stored in a proton gradient to make ATP by allowing H+ to diffuse down the gradient, back across the membrane. Protons diffuse through the ATP synthase complex, which causes the phosphorylation of ADP.

How does the electron transport chain pump hydrogen ions from the matrix to the intermembrane space? The process is based on spatial organization of the electron transport chain in the membrane. Note that:

Some electron carriers of the transport chain transport only electrons.
Some electron carriers accept and release protons along with electrons. These carriers are spatially arranged so that protons are picked up from the matrix and are released into the intermembrane space.

Most of the electron carriers are organized into three complexes: 1) NADH dehydrogenase complex; 2) cytochrome b-c1 complex; and 3) cytochrome oxidase complex. (See Text)

Each complex is an asymmetric particle that has a specific orientation in the membrane.
As complexes transport electrons, they also harness energy from this exergonic process to pump protons across the inner mitochondrial membrane.

Mobile carriers transfer electrons between complexes. These mobile carriers are:

1. Ubiquinone (Q). Near the matrix, Q accepts electrons from the NADH dehydrogenase complex, diffuses across the lipid bilayer, and passes electrons to the cytochrome b-c1 complex.

2. Cytochrome c (Cyt c). Cyt c accepts electrons from the cytochrome b-c1 complex and conveys them to the cytochrome oxidase complex.

When the transport chain is operating:

The pH in the intermembrane space is one or two pH units lower than in the matrix.
The pH in the intermembrane space is the same as the pH of the cytosol because the outer mitochondrial membrane is permeable to protons.

The H+ gradient that results is called a proton-motive force to emphasize that the gradient represents potential energy.

Proton motive force = Potential energy stored in the proton gradient created across biological membranes that are involved in chemiosmosis.

This force is an electrochemical gradient with two components:
1. Concentration gradient of protons (chemical gradient).

2. Voltage across the membrane because of a higher concentration of positively charged protons on one side (electrical gradient).
It tends to drive protons across the membrane back into the matrix.

Chemiosmosis couples exergonic chemical reactions to endergonic H+ transport, which creates the proton-motive force used to drive cellular work, such as:

ATP synthesis in mitochondria (oxidative phosphorylation). The energy to create the proton gradient comes from the oxidation of glucose and the ETC.
ATP synthesis in chloroplasts (photophosphorylation). The energy to create the proton gradient comes from light trapped during the energy-capturing reactions of photosynthesis.
ATP synthesis, transport processes and rotation of flagella in bacteria. The proton gradient is created across the plasma membrane. Peter Mitchell first postulated chemiosmosis as an energy-coupling mechanism based on experiments with bacteria.