Chemiosmosis
All life from the simplest bacteria to the most complex life forms rely on cellular chemical activity. The number of biochemical pathways and associated compounds has been estimated at well in excess of 100,000. To run all of these processes and make all of these compounds, cells need energy.
The energy coinage for most organisms is adenosine triphosphate (ATP). The cleavage of the high energy phosphate linkage provides about 7.5 kcal or 30 kJ per mole of ATP. The adult human body makes and uses about 77 lb (35 kg) of ATP per day. This is slightly over 40% of the average male adult body mass. And, all of this ATP is synthesized in a simple organelle within the cells called the mitochondria.
The process for the synthesis of ATP is respiration. The end products are the consumption of glucose and oxygen to produce water and carbon dioxide. However, it is by no means as simple as combustion. Both glucose and oxygen are involved in their own enzymatic cycle, located within the mitochondria.
In particular, oxygen is consumed via a series of trans-membrane protein complexes in the inner wall or membrane of the mitochondria. Roughly speaking, the pathway involves the consumption of reducing equivalents at one end and the reduction of oxygen to water at the other. Along the way, this results in the shuttling of protons from the inner parts of the mitochondria to the inter-membrane fluid. This has two effects. It creates a chemical potential or concentration gradient between the opposite sides of the membrane while also creating an electrical potential. It is this concentration gradient that is used by two other trans-membrane proteins, labeled the F1 and F0 complexes, that is used to generate ATP from adenosine diphosphate and inorganic phosphate ions.
At one point, there were competing theories as to how this chemical and electrical potential was created. The chemical hypothesis postulated that the chemical components were coupled at all stages of the process but the absence of any high energy intermediates linking oxidation with phosphorylation has been seen as telling evidence against this theory. The other theory and the one presently accepted by most biochemists, is the chemiosmotic theory, in which the oxidation and phosphorylation are uncoupled pathways. That is, the oxidation of chemical species in the respiratory chain results in the evolution and ejection of protons from the inner mitochondrial matrix separately from the inner rush of protons that drives the ATP synthesis.
The evidence for chemiosmosis is extensive and the experimental findings are consistent with this pathway. For example, the outer membrane of the mitochondria can be stripped away leaving the working inner membrane intact. Addition of protons to a treated mitochondria results in the increased synthesis of ATP without a concurrent increase in the oxidative pathways. The use of uncouplers (compounds that increase the permeability of the mitochondrial membranes to proton) effectively shut down the synthesis of ATP despite continued operation of the oxidative pathway. The experimental evidence demonstrates the absence of a link between the two biochemical pathways, consisting with the chemiosmotic theory.
Ion pumping seems to be the active mechanism in many biochemical paths including neuronal transmission and muscular movement. Pumping against a gradient is like pushing a cart up hill. It requires work but does allow for a ride back down.
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