Chemiosmotic processes, thermodynamics

A principal difference between the chemiosmotic and the secretion theory is the involvement of metabolic energy in the process of the polar transport. According to the secretion theory, energy is directly coupled to carriers, in those cases, where auxin transport appears to be uphill whereas the new theory postulates that auxin transport is thermodynamically downhill, but with metabolic energy expended to maintain the pH and electrical gradients in addition to maintaining an ordered membrane with asymmetric permeability to lAA" (Goldsmith 1977, pp 446 and 491). 

The electron transfer processes of photosynthesis and carbohydrate metabolism drive the flow of protons across the membranes of specialized cellular compartments. The chemiosmotic theory, discussed in Chapter 5, describes how the energy stored in a proton gradient across a membrane can be used to synthesize adenosine triphosphate (ATP), a mobile energy carrier. Intimate knowledge of thermodynamics and chemical kinetics is required to understand the details of the theory cuid the experiments that eventually verified it. 

The energetics of the translocation process and the role of GTP therein have fascinated investigators for many years. Brot (1977) and Spirin (1978) have reviewed this process and the proposed means by which translocation is carried out. Spirin has suggested that, while the ribosome binds peptidyl-tRNA on the acceptor site (i.e., prior to translocation), the whole system is akin to a high-energy intermediate in which eventual translocation is thermodynamically ensured. This provocative hypothesis merits careful reading in view of its similarity to the role of the membrane in the chemiosmotic hypothesis for oxidative phosphorylation (Chapter 9). 

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