488
MARY BELLE ALLEN
high temperatures, the ability to repair metabolically the damaging
effects of heat may mark the difference between life and death (4, 5).
Living organisms have been able to balance these two contrasting
effects so that life in some form occurs within the whole range in which
water, that most important of all biocatalysts, is fluid. The biokinetic
zone is bounded on its lower limit by the freezing point of the cell contents and surrounding medium (which may be well below 0° in media
of high osmotic concentration), and on its upper limit by the boiling
point of water. For most individual organisms, however, the zone of
permissible temperatures is much narrower. Many forms of life die of
cold at temperatures well above freezing, and only a few specially
adapted organisms can endure temperatures above 50-55°, where many
of the usual enzymatic proteins are irreversibly damaged by heat. Temperature limits for the vital processes of a variety of organisms are
given in Table I.
It may be desirable at this point to define some of the terms which
are used in speaking of the relation of organisms to temperature. Those
which can survive and develop over a wide temperature range are referred to as eurythermal, whereas those which are limited to a narrow
temperature band are called stenothermal. Organisms which develop
at low temperatures are known as cryophilic; those which prefer moderate temperatures as mesophilic; and those which can withstand or
which require high temperatures for growth as thermophilic. The temperatures used to delimit these zones vary with the type of organism
being discussed. For example, bacteria are not usually considered
thermophilic unless they can grow at 55° or above, whereas the term
is applied to green algae which have an optimum temperature at 40°
and a maximum only slightly higher. The thermophilic organisms which
have attracted the greatest biochemical interest are those which can
survive above the coagulation temperature of many usual enzyme proteins, and which are often unable to grow at more usual temperatures.
Those animals, including the birds and mammals, which have learned
to use metabolic energy to maintain a more or less constant internal
thermic environment are known as homoiotherms, whereas all the organisms whose internal temperature reflects the temperature of their
surroundings are known as poikilotherms.
Increases in temperature generally tend to increase the rate of
chemical reactions, whether these occur within or without the living organism. In living creatures, however, influences of competing reactions,
hormonal control, etc. may act in such a way that the observed over-all
response to temperature of complex processes, e.g., growth and energy
production, is different from that predicted by physicochemical theory,
MARY BELLE ALLEN
high temperatures, the ability to repair metabolically the damaging
effects of heat may mark the difference between life and death (4, 5).
Living organisms have been able to balance these two contrasting
effects so that life in some form occurs within the whole range in which
water, that most important of all biocatalysts, is fluid. The biokinetic
zone is bounded on its lower limit by the freezing point of the cell contents and surrounding medium (which may be well below 0° in media
of high osmotic concentration), and on its upper limit by the boiling
point of water. For most individual organisms, however, the zone of
permissible temperatures is much narrower. Many forms of life die of
cold at temperatures well above freezing, and only a few specially
adapted organisms can endure temperatures above 50-55°, where many
of the usual enzymatic proteins are irreversibly damaged by heat. Temperature limits for the vital processes of a variety of organisms are
given in Table I.
It may be desirable at this point to define some of the terms which
are used in speaking of the relation of organisms to temperature. Those
which can survive and develop over a wide temperature range are referred to as eurythermal, whereas those which are limited to a narrow
temperature band are called stenothermal. Organisms which develop
at low temperatures are known as cryophilic; those which prefer moderate temperatures as mesophilic; and those which can withstand or
which require high temperatures for growth as thermophilic. The temperatures used to delimit these zones vary with the type of organism
being discussed. For example, bacteria are not usually considered
thermophilic unless they can grow at 55° or above, whereas the term
is applied to green algae which have an optimum temperature at 40°
and a maximum only slightly higher. The thermophilic organisms which
have attracted the greatest biochemical interest are those which can
survive above the coagulation temperature of many usual enzyme proteins, and which are often unable to grow at more usual temperatures.
Those animals, including the birds and mammals, which have learned
to use metabolic energy to maintain a more or less constant internal
thermic environment are known as homoiotherms, whereas all the organisms whose internal temperature reflects the temperature of their
surroundings are known as poikilotherms.
Increases in temperature generally tend to increase the rate of
chemical reactions, whether these occur within or without the living organism. In living creatures, however, influences of competing reactions,
hormonal control, etc. may act in such a way that the observed over-all
response to temperature of complex processes, e.g., growth and energy
production, is different from that predicted by physicochemical theory,