494
MARY BELLE ALLEN
midgut glands, kidney and muscle tissue from earthworm and potato
beetle, cell material from Escherichia coli and the yeast Torulopsis, when
the individuals or cell populations had been kept at low temperatures
(12, 45-48). On adaptation to higher temperatures, alkaline glycerophosphatase activity decreases in the potato beetle, while in yeast lactoflavin content decreases, pyridine nucleotide increases, peroxidase and
catalase increase (45, 49, 50). Amino acid decarboxylase activity in E.
coli and Micrococcus lysodeikticus decreases with increasing adaptation
temperature (51). The temperature maxima for enzyme activity in
homogenates or breis generally move up and down with the previous
temperature to which the organism was acclimated, just as do the maxima of the over-all respiratory process. Temperature maxima for cell
proliferation show no such differences, however (12, 37, 45, 52).
It should be emphasized that these phenomena are changes in activity within the normal temperature range for growth of the organism,
not alterations which permit the organism to adjust to extremes. The
latter will be discussed in Sections III and IV.
B. HOMOIOTHERMS
The transition from poikilothermy to homoiothermy, i.e., the utilization of metabolic energy to provide an internal environment of constant
temperature, is probably one of the most important evolutionary discoveries made by the higher animals. It must have had a great adaptative advantage to succeed in spite of being at first glance a very dangerous transition, involving the abandonment of the mechanism which
regulated enzymatic activity in response to temperature variation and
staking the life of the organism on its ability to maintain its temperature
within narrow limits. To quote Barcroft again (38), "Here then is a very
fine issue—the cold-blooded animal successfully adopting ingenious
mechanisms, first biochemical, then physiological, in order to adapt its
heart to the variations of its environment; the warm-blooded animal discarding what its cold-blooded predecessor has laboriously beaten out,
invoking the nervous system to reverse the normal biochemical relationship and gaining a new freedom by adapting, not itself to the internal
environment, but the internal environment to itself."
Three factors are involved in this internal temperature regulation:
(1) an increase in the rate of output of metabolic energy to cope with
the demands made upon it for heating; (2) the elaboration of structures
which act as effective heat exchangers; (3) development of a control
center which regulates the production of energy for heating and the
heat exchange devices for heating and cooling.
The differences in energy output between some representative homo-
MARY BELLE ALLEN
midgut glands, kidney and muscle tissue from earthworm and potato
beetle, cell material from Escherichia coli and the yeast Torulopsis, when
the individuals or cell populations had been kept at low temperatures
(12, 45-48). On adaptation to higher temperatures, alkaline glycerophosphatase activity decreases in the potato beetle, while in yeast lactoflavin content decreases, pyridine nucleotide increases, peroxidase and
catalase increase (45, 49, 50). Amino acid decarboxylase activity in E.
coli and Micrococcus lysodeikticus decreases with increasing adaptation
temperature (51). The temperature maxima for enzyme activity in
homogenates or breis generally move up and down with the previous
temperature to which the organism was acclimated, just as do the maxima of the over-all respiratory process. Temperature maxima for cell
proliferation show no such differences, however (12, 37, 45, 52).
It should be emphasized that these phenomena are changes in activity within the normal temperature range for growth of the organism,
not alterations which permit the organism to adjust to extremes. The
latter will be discussed in Sections III and IV.
B. HOMOIOTHERMS
The transition from poikilothermy to homoiothermy, i.e., the utilization of metabolic energy to provide an internal environment of constant
temperature, is probably one of the most important evolutionary discoveries made by the higher animals. It must have had a great adaptative advantage to succeed in spite of being at first glance a very dangerous transition, involving the abandonment of the mechanism which
regulated enzymatic activity in response to temperature variation and
staking the life of the organism on its ability to maintain its temperature
within narrow limits. To quote Barcroft again (38), "Here then is a very
fine issue—the cold-blooded animal successfully adopting ingenious
mechanisms, first biochemical, then physiological, in order to adapt its
heart to the variations of its environment; the warm-blooded animal discarding what its cold-blooded predecessor has laboriously beaten out,
invoking the nervous system to reverse the normal biochemical relationship and gaining a new freedom by adapting, not itself to the internal
environment, but the internal environment to itself."
Three factors are involved in this internal temperature regulation:
(1) an increase in the rate of output of metabolic energy to cope with
the demands made upon it for heating; (2) the elaboration of structures
which act as effective heat exchangers; (3) development of a control
center which regulates the production of energy for heating and the
heat exchange devices for heating and cooling.
The differences in energy output between some representative homo-