figure 1.1 The responses to temperature in a hypothetical organism. At low temperatures, metabolism is undetectable. As the temperature increases, the rate of metabolism increases due to the increased kinetic energy supplied to reactions. Beyond the optimum temperature, however, metabolism slows and eventually ceases due to the damaging and lethal effects of high temperature. Changes in activity are associated with these changes in temperature. As the temperature increases or decreases from the optimum, the organism may become disorientated and normal processes disrupted (heat or cold stupor) and then cease altogether (heat or cold coma). Death may then result. These transitions define the ranges over which normal activity and life can occur.
tions decreases. This effect is potentially reversible. Death may result, however, from events such as irreversible changes in membrane function, although freezing, or the risk of freezing, is likely to be the major hazard. Freezing involves a change in the state of water within the organism from a liquid to a solid. This can be a sudden and violent event, and initiates a number of changes that may result in death, unless the organism has mechanisms which enable it to survive the stresses involved. The lethal effects of heat are unlikely to be due to a change of state in body water since, for most organisms, their upper lethal temperature is many degrees lower than the temperature at which water boils.
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