Ion gradients

The method of preparation of a support material has a tremendous effect on its properties (11). For example, zeoHtes, which are highly stmctured aluminosihcates, are known to be extremely sensitive to the conditions employed both during and after crystallization (12). Also, when siUca—titania is precipitated by a coprecipitation method using ammonia, in which localized hydroxide ion gradients are estabUshed by the precipitation process itself, the product is much more acidic than when it is precipitated using urea, which suppHes hydroxide ion slowly and uniformly during precipitation (13). 

Cells need a certain amount of energy for maintenance. The maintenance energy is, for instance, needed for maintaining the proton motive force which is, among other purposes, used for maintaining the ion gradients across the cell membrane. Furthermore, energy is needed for the turnover of proteins and mRNA, for repair and for movement (if mobile). 

Both secondary active transport and positive cooperativity effects enhance carrier-mediated solute flux, in contrast to negative cooperativity and inhibition phenomena, which depress this flux. Most secondary active transport in intestinal epithelia is driven by transmembrane ion gradients in which an inorganic cation is cotransported with the solute (usually a nutrient or inorganic anion). Carriers which translocate more than one solute species in the same direction across the membrane are referred to as cotransporters. Carriers which translocate different solutes in opposite directions across the membrane are called countertransporters or exchangers (Figs. 10 and 11). 

Possible driving forces for solute flux can be enumerated as a linear combination of gradient contributions [Eq. (20)] to solute potential across the membrane barrier (see Part I of this volume). These transbarrier gradients include chemical potential (concentration gradient-driven diffusion), hydrostatic potential (pressure gradient-driven convection), electrical potential (ion gradient-driven cotransport), osmotic potential (osmotic pressure-driven convection), and chemical potential modified by chemical or biochemical reaction. 

Carrier-mediated transport of nutrients in small intestinal epithelia is often promoted by the maintenance of transmucosal ion gradients. A mathematical descrip-... 

JS Patton. Is the intestinal membrane bilayer freely permeable to lipophilic molecules In F Alvarado, CH van Os, eds. Ion Gradient-Coupled Transport. INSERM Symposium No. 26. Amsterdam Elsevier Science, 1986, pp 33-36. 

Note that it is possible to use proton or sodium ion gradients in signalling and a possible example is provided by the reactions of the proteins of the eye-spot of green algae (see Nagel et al. in Further Reading). 

Calcium ions (gradient) Signalling, chemical and physical... 

There is a family of antibiotics (ionophores) whose effectiveness results from disrupting this crucial ion gradient. 

Secondary transporters employ energy stored in ion gradients to transport other ions and molecules 84... 

Excitable membranes maintain and rapidly modulate substantial transmembrane ion gradients in response to stimuli 576 Specific lipid messengers are cleaved from reservoir phospholipids by phospholipases upon activation by various stimuli 576 Phospholipids in synaptic membranes are an important target in seizures, head injury, neurodegenerative diseases and cerebral ischemia 576 Some molecular species of phospholipids in excitable membranes are reservoirs of bioactive lipids that act as messengers 576 Mammalian phospholipids generally contain polyunsaturated fatty acyl chains almost exclusively esterified to the second carbon of glycerol 577... 

Excitable membranes maintain and rapidly modulate substantial transmembrane ion gradients in response to... 

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