Chemiosmotic mechanism of oxidative phosphorylation

Active transport carrier proteins set up gradients which are then used to

synthesize ATP

ATP synthase

• Mitochondria are present in nearly all eucaryotic cells, plants, animals, and most eucaryotic microorganisms.

• Mitochondria metabolize acetyl groups via the citric acid cycle, producing CO2 and NADH - an activated carrier molecule that carries high-energy electrons.

• NADH donates its high-energy electrons to the electron-transport chain in the mitochondrial membrane, oxidizing NADH to NAD+.

• The electrons are quickly passed along the chain to molecular oxygen to form water.

• Energy released during the passage of electrons is used to pump protons and create a proton electrochemical gradient.

• The proton gradiant drives the synthesis of ATP by oxidative phosphorylation - the addition of a phosphate groups to ADP using this process requires oxygen.

• These events also take place in aerobic bacteria, using the plasma membrane.

Mitochondrion contains two membrane-bound compartments. They are similar in size to bacteria, contain their own DNA and RNA and a complete transcription and translation system including ribosomes (different from the cellular ribosomes). They constantly change shape and position. Form long moving chanis in association with microtubules of the cytoskeleton

The linking of electron transport, proton pumping, and ATP synthesis is called chemiosmotic coupling.

The mitochondrial electron-transport chain are grouped into three large respiratory enzyme complexes, each containing multiple individual proteins.

Each complex contains metal ions that form a pathway for the passage of electrons through the complexes. This powers the pumping of protons into the Intermembrane space

Since protons are positively charged, they will move across the membrane, since it has a excess of negative electrical charges on the other side. Therefore, the proton gradient created across the inner mitochondrial membrane is very steep, consisting of the electroattractive force and the concentration gradient set up by pumping protons into the intermembrane space - electrochemical gradient. This gradient is used to drive ATP synthesis by oxidative phosphorylation.

ATP synthase

ATP synthase is a reversible coupling device that can convert the energy of the electrochemical proton gradient into chemical bond energy or vice versa. The direction depends on the magnitude of the electrochemical proton gradient.

In many bacteria that can grow either aerobically or anaerobically, the direction in which the ATP synthase works is routinely reversed when the bacterium runs out of oxygen.