Introduction – impact of global warming on membrane physiology

Consequences of poikilothermy: perturbation of membrane structure

Because temperature determines the rates of biological processes as well as the distribution of conformations assumed by both molecules and molecular aggregates (e.g. membranes) in cells (Hochachka & Somero, 1984), physiological function is markedly perturbed by fluctuations in body temperature. Thermal perturbation of function is particularly acute for aquatic poikilotherms such as fish, for the high heat capacity and relatively low oxygen content of aquatic environments combine to ensure that, as a consequence of respiration, the body tissues remain within 1 °C of ambient water temperature (Carey et al., 1971). One of the primary consequences of poikilothermy in fishes is the perturbation of membrane organization when body temperature changes, for the structural lipids of biological membranes display complex phase behaviours and physical properties that are exquisitely sensitive to temperature (Hazel, 1995). Temperature effects are most evident as altered physical properties of the phospholipid acyl domain, which in turn determine the order (time-averaged position in space) and rates of motion of membrane constituents, the molecular geometries of membrane phospholipids, and the dynamic phase behaviour of the membrane. At normal physiological temperatures (i.e. the temperature to which an animal is either acclimated or adapted), the fatty acyl chains of membrane phospholipids are moderately disordered an relatively fluid due to the presence of 3–7 gauche rotamers (rotations about carbon–carbon single bonds) per acyl chain (Casal & Mantsch, 1984); however, in spite of this disorder, the average molecular geometry of most phospholipid molecules is essentially cylindrical.