Temperature dependent factors influencing nutrient uptake: an analysis of responses at different levels of organization.

Ion fluxes show a characteristically biochemical dependence on temperature when observed at the membrane level and over short periods after a perturbation of temperature. The primary active transport systems are enzymic and are dependent both on substrate supply and on changes in protein conformation. The hydrophobic parts of the proteins are surrounded by lipid molecules whose physical state may crucially affect conformation changes. These lipids may undergo transitions from a fluid to a gel state at temperatures occurring in the natural environment. It will be noted that the concepts developed in model systems of pure phospholipid/protein interactions cannot be very readily applied to the spatially heterogeneous assemblies of lipid molecules and transport proteins in real cell membranes. While it is obvious that ion transport rates are responsive to temperature changes in a given cell, it is difficult to explain exactly which components of the transport process become limiting. We will show that, on cooling, the membrane potential can initially be greatly disturbed when temperature is changed and that this may be related to ATP supply to H+-translocating ATPase. This affects the driving force for all other solutes. When temperature is lowered the permeability coefficients for most ions are reduced and yet it is commonly found that diffusive efflux of ions increases in the cold. We attempt to explain this paradox on the basis of driving forces and metabolic regulation of ion transport. Acclimatory changes occur on extended exposure of a cell or an organism to a reduced growth temperature. Some of these changes occur at the membrane level and relate to lipid composition and modulation of carrier activity. Others involve changes in the relative size and sometimes the morphology of the root system. We will show that these processes lessen the temperature dependence of ion transport and ensure that the intake of nutrients does not limit growth at low temperatures. These acclimatory changes are seen as part of the general process of regulation of nutrient uptake.