Chemiosmotic theory and ATP synthesis | Botany Optional for UPSC PDF Download

The Chemiosmotic Theory

• The chemi-osmotic theory, formulated by Peter Mitchell in 1961, established an electrochemical connection between respiration and phosphorylation and earned him the Nobel Prize in 1978.
• Chemi-osmosis involves the movement of chemical ions across a semi-permeable membrane, following their electrochemical gradient, akin to the movement of water through osmosis. One illustrative example is the production of ATP during cellular respiration or photosynthesis through the movement of hydrogen ions across a membrane.
• This theory focuses on ATP generation by ATP synthase. It encompasses the pumping of protons (H+) through specialized channels in mitochondrial membranes, from the inner compartment to the outer compartment. Protons originating from the electron transport chain are transported across the membrane by the respiratory chain, moving from the M-side to the C-side of the mitochondrial inner membrane. This translocation leads to changes in pH and membrane potential.
• Since the inner mitochondrial membrane is impermeable to protons, these ions can only pass through the Proton channel of the ATPase to reach the M-side. When H+ moves from the C-side to the M-side, the F₁- ATPase, operating in reverse, facilitates ATP synthesis. Conversely, when F₁-ATPase hydrolyzes ATP, it acts as a "proton pump," expelling H+ from the M-side to the C-side. For each oxidation of NADH, six protons are transported through the inner membrane. These six H+ ions, upon returning to the M-side through the F₁-ATPase, yield three ATP molecules.
• According to this theory, the inner mitochondrial membrane serves as a transducer, converting energy from the electrochemical gradient into the chemical energy of ATP. This membrane is impermeable to both H+ and OH-, so differences in pH across the membrane create energy-rich gradients.
• When protons (H+) are expelled toward the cytoplasmic side (C-side) and OH- remains on the matrix side, a pH difference is established (lower pH on the C-side and higher pH on the M-side), resulting in an electrical potential. Under the influence of the protein motive force generated by the electron transfer through the respiratory chain and the release of protons on the C-side, the enzyme ATPase synthesizes ATP.

ATP synthase Structure

• ATP synthesis involves the conversion of its precursor, ADP, by the enzyme ATP synthase, also known as complex V, which is located in the cristae and inner mitochondrial membrane.
• ATP synthase comprises two parts: F₀ and F₁. The F₀ portion, embedded within the mitochondrial membrane, consists of a c-ring and subunits a, as well as two b subunits. This component functions as a motor powered by the movement of H+ ions across the membrane.
• The F₁ portion, located within the mitochondria, is composed of subunits alpha, beta, gamma, and delta. F₁ possesses a water-soluble segment capable of hydrolyzing ATP. This part acts as another motor employed in ATP generation.
• The F₀ region shares similarities with DNA helicase, an enzyme responsible for unzipping DNA strands during replication, while the F₁-ATPase region is akin to H+ motors that enable its movement.