THE CHEMI-OSMOTIC THEORY
The chemi-osmatic theory is about an electrochemical link between respiration and phosphorylation. It
was proposed by Peter Mitchell in 1961. He was awarded a Nobel prize in 1978 for his work. .
Chemi – osmosis refers to the movement of chemical ions across a semi – permeable membrane down
their electrochemical gradient (from an area of higher to lower concentration) similar to movement of
water molecules by osmosis. An example of this is the generation of ATP by the movement of hydrogen
ions across a membrane during cellular respiration or photosynthesis.
The chemi-osmotic theory deals with the generation of ATP by ATP synthase. Chemi-osmosis involves the
pumping of protons through special channels in the membranes of mitochondria from the inner to the outer
compartment.
The protons originating from the electron transport chain are translocated by the respiratory chain across the
membrane from the M – side to the C – side of mitochondrial inner membrane. According to this theory, this
translocation creates a pH change and a membrane potential.
Since the inner mitochondrial membrane is highly impermeable to protons (H+), these can reach the M-side
only through the Proton channel of the ATPase.
When H+ moves from the C-side to M-side, the F1- ATPase operating in reverse, catalyzes the ATP synthesis.
Conversely, when the F1-ATPase hydrolyzes ATP, it functions as a “proton pump” and ejects H+ from the M -
side to C-side. For each NADH that is oxidized, 6 protons are translocated through the inner membrane, these 6
H+, when returning to the M-side through the F1-ATPase, gives rise to 3 ATP molecules.
According to this theory, the inner membrane of the mitochondria acts as a transducer, converting the
energy, provided by the electrochemical gradient, into the chemical energy of ATP. The inner
mitochondrial membrane is impermeable to both H+ and OH- . Therefore if pH differences are
established across the membrane, they act as energy rich gradients.
