Tuesday, March 3, 2015

Cellular respiration - Chemiosmotic theory

The chemiosmotic theory, postulated in 1961 by Peter Mitchel explains how the transport of electrons along the mitochondrial respiratory chain is related to the synthesis of ATP. In fact, as the electrons go through the respiratory chain complexes, they behave like that are moving down an "energy staircase", always passing gradually to a lower energy level. In other words, there are small amounts of energy that are released, which individually are not useful to the cell. However, part of this energy is used to pump protons into the intermembrane space, that is, part of the energy is stored in the form of an H+ gradient. This gradient allows the accumulation of a large amount of energy, because this is an electrochemical gradient. It is a chemical gradient, because we are talking about an asymmetry of H+ concentrations between the two sides of the inner membrane. But it is also an electrical gradient, as it is also created an asymmetry of charged between both sides of the membrane, because the H+ is pumped into the intermembrane space without sending any counterion. Thus, there is an accumulation of positive charges in the intermembrane space, as compared while the matrix becomes more negative. By now many must be wondering "But what is the use of this process? Why does the cell needs a gradient of H+? "The chemiosmotic theory explains just that!

In addition to the mitochondrial respiratory chain complexes, there is also a membrane enzyme (localized in the inner membrane), called mitochondrial ATP synthase (more about this enzyme here). This enzyme has a catalytic subunit, responsible for the synthesis of ATP, and a subunit that functions as a transmembrane pore for the passage of protons. So, the idea is simple... the protons that accumulate in the intermembrane space, will cross the inner membrane through this pore, and since this transport occurs driven by the electrochemical gradient, it releases energy. This energy is used by ATP synthase to produce ATP.

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