Temperature controls physiological activity
Key biochemical reactions important to maintenance of life are temperature sensitive. Each reaction has an optimal temperature that is related to the activity of enzymes, protein-based molecules that catalyze biochemical reactions.
Enzymes are structurally stable under a limited range of temperatures. At high temperatures, the constituent proteins lose their structural integrity, or become denatured, as their bonds break. Most enzymes become denatured at temperatures between 40°C and 70°C (104oF-158oF), but enzymes in bacteria inhabiting hot springs can remain stable at temperatures up to 100°C (212°F). The upper lethal temperature for most organisms is lower than the temperature at which their enzymes become denatured, because metabolic coordination among biochemical pathways is lost at these temperatures. The extreme lower limit for enzyme activity is about -5°C (23°F) (Willmer et al. 2005). The internal temperatures of Antarctic fishes and crustaceans may reach -2°C (28°F) because the salt concentration of the seawater in which they live lowers its freezing point. Some soil microorganisms are active at temperatures as low as -5°C (23°F).Some species can produce different forms of enzymes (called isozymes) with different temperature optima as a means of acclimatization to changes in environmental temperature. For example, some fishes (e.g., trout, carp, goldfish) and trees (e.g., loblolly pine) can produce isozymes in response to seasonal changes in temperature. However, acclimatization to temperature changes using isozymes does not appear to be a common response in animals (Willmer et al. 2005).
Temperature also determines the rates of physiological processes by influencing the properties of membranes, particularly at low temperatures. Cell and organelle membranes are composed of two layers of lipid molecules.
At low temperatures, these layers can solidify; proteins and enzymes embedded in them can lose their function, affecting processes such as mitochondrial respiration and photosynthesis, and membranes can lose their function as filters, leaking cellular metabolites. Tropical plants may suffer loss of function associated with membrane disruption at temperatures as high as 10°C (50°F), while alpine plants can function at temperatures close to freezing. The sensitivity of membrane function to low temperatures is related to the chemical composition of the membrane lipid molecules. Plants of cooler climates have a higher proportion of unsaturated membrane lipids (with greater numbers of double bonds between carbon molecules) than plants of warmer climates.Finally, temperature influences physiological processes in terrestrial organisms by affecting water availability. As we saw in Concept 2.2, the warmer the air, the more water vapor it can hold. As a result, the rate at which terrestrial organisms lose water from their bodies increases as temperature becomes warmer. We will return to this point later when we discuss how organisms cope with variations in water availability.