Control systems operate in organisms
to cope with environmental stresses including temperature extremes.
Adaptations in
Plants to Low and High Temperature
High Temperature
High temperature denatures the
enzymes and damages the metabolism; therefore, it harms or kills the plants.
Plants use evaporative cooling to manage with high temperature. Hot and dry
weather, however, causes water deficiency resulting in closing of stomata, thus
plants suffer in such conditions. Most plants have adapted to survive in heat
stress as the plants of temperate regions face the stress of 40°C and above
temperature. The cells of these plants synthesize large quantities of special
proteins called heat-shock proteins. These proteins embrace enzymes and other
proteins thus help to prevent denaturation.
Low Temperature
In low temperature the fluidity of
the cell membrane is altered, because lipids of the membrane become locked into
crystalline structures, which affects the transport of the solutes. The
structure of the membrane proteins is also affected. Plants respond to cold
stress by increasing proportion of unsaturated fatty acids, which help
membranes to maintain structure at low temperature by preventing crystal
formation. This adaptation requires time because of this reason rapid chilling
of plants is more stressful than gradual drop in air temperature.
Freezing temperature causes ice
crystal formation. The confinement of ice formation around the cell wall does
not affect as badly and plants survive, however, formation of ice crystals
within protoplasm perforates membranes and organelles hence killing the cells.
The plants native to cold regions such as oaks, maples, roses and other plants
have adapted to bring changes in solute composition of the cells, which causes
cytosol to super cool without ice formation, although ice crystals may form in
the cell walls.
MECHANISMS IN
ANIMALS
Body Heat, Heat
Gain and Loss
Temperature of an animal depends
upon the rate of change of body heat which in turn depends on the rate of heat
production through metabolic processes and the rate of external heat gain and
rate of heat loss. This transfer of heat between an animal and its environment is
done in numerous ways. Principally, infrared thermal radiation and direct and
reflected sunlight transfer heat into the animal; whereas radiation and
evaporation transfer heat out to the environment.
Temperature
Classification of Animals
Animals deal with variation in the
thermal characteristics of their environment. There are animals in which body
temperature tends to fluctuate more or less with ambient temperature where air
or water temperatures are changed, these are poikilotherms, all invertebrates, fish
amphibians and reptiles are considered in this category. The others exposed to
changing air or water temperature maintain their body temperature are the
homeotherms and include birds and mammals. Several difficulties arise with this
terminology with studies. It is observed that deep sea fishes maintain their
body temperature due to the constant natural surroundings and lizards regulate
their body temperature and in contrast numerous birds and mammals vary their
body temperature.
Therefore, a more widely applicable
temperature classification scheme is based on the source of heat production.
According to this, animals that generate their own body heat through heat
production as a by-product during metabolism are endotherms include
birds, some fishes and flying insects. Ectotherms are the other type,
which produce metabolic heat at low level and that is also exchanged quickly
with the environment, however, absorb heat from their surroundings. Most
invertebrates, fish, amphibians and reptiles are in this category. A third
category, heterotherms is of those animals who are capable of varying degrees
of endothermic heat production but generally do not regulate their body
temperature within a narrow range e.g. bats, hummingbird etc.
Regulation of Heat
Exchange between Animals and Environment
Animals use different mechanisms for
such regulation and these are of structural, physiological and behavioral
nature. Structural Adaptations: These may be long term changes in subdermal
fatty layer insulation and pelage. The presence of sweat glands and lungs
modified for panting. Physiological Adaptations: These regulate blood flow to
the skin; specifically greater blood flow in warmth to dissipate heat and lower
in colds to economize heat loss, also the activation of certain muscles causes
plumage fluffing. Similarly activation of sweat glands is done for evaporative
cooling.
THERMOREGULATION
IN MAMMALS (HUMAN)
Regulatory
Strategies
Mammals including humans maintain
their high body temperature within a narrow range of about 36-38 °C because of
their endothermic characteristics. The origin of endothermy in birds and
mammals have provided the opportunity to keep high metabolic rate and
availability of energy round the clock, thus had acquired greater ability to
adaptations and has assisted in much of their wider diversity and distribution
in diversified regions of the earth.
These regulate the rate of metabolic
heat production, balancing it with the rate at which they gain or lose heat
from the surroundings. The rate of heat production is increased by increased
muscle contraction by movements or shivering so called as shivering
thermogenesis. Also hormones trigger the heat production as do thyroid hormones
and are termed as non-shivering thermogenesis. Some mammals possess brown fat,
which is specialized for rapid heat production. In overproduction of heat it is
dissipated through exposed surfaces by increasing blood flow or the evaporative
cooling. In mammals, it is observed that skin has been adapted as the organ of
thermoregulation. (Fig. 15.14)
In Cold Temperature
Mammals have various mechanisms that
regulate heat exchange with their environment. Vasodilation and
vasoconstriction affect heat exchange and may contribute to regional
temperature differences within an animal. On a cool day a human's temperature
may be several degrees lower in the arms and legs than in the trunk, where the
most vital glands are situated. Most land mammals respond to cold by raising
their furs thereby trapping the thicker layer of still air and it acts as a
good insulator between animal skin and the surroundings. Humans mostly rely on
a layer of fat just insulating beneath the skin as insulating material against
heat loss. Similarly marine mammals such as whales and seals inhabit much
colder water than their body temperature, and have a very thick layer of
insulating fat called blubber just under the skin.
In Warm Temperature
Marine mammals dispose-off their
excess heat into warm seas by large numbers of blood vessels in the outer layer
of the skin. This dissipates the heat from the skin surface. In terrestrial
mammals, in contrast is the mechanism of evaporative cooling. The sweat gland
activity and the evaporative cooling is one of the major temperature reducing
strategies. Panting, the evaporative cooling in the respiratory tract is the
other mechanism as represented in the dogs. Bats etc use saliva and urine for
evaporative cooling.
Thermostat
Function and Feedback Controls in Human
The body temperature regulation in
humans is based on complex homeostatic systems facilitated by feedback
mechanisms. The homeostatic thermostat is present in the hypothalamus, a brain
part. It responds to the changes in the temperature above and below a set point
which is 37°C.
In case of increase in temperature
above the set point, certain warm temperature sensitive thermoreceptors in
skin, hypothalamus and other parts of nervous systems send the signals to the
system that increase the blood flow to the skin and also cause sweat gland
activation and the sweat is evaporated for cooling.
In cold temperatures, the cold
receptors send the impulses to the hypothalamus to inhibit heat loss mechanisms
and activate the heat conservation mechanisms. This includes constriction of
superficial blood vessels and stimulating shivering and non shivering
mechanisms.
Temperature in fever (Pyrexia)
In bacterial and viral infections
mainly, leukocytes increase in number. These pathogens and the blood cells
produce chemicals called pyrogens. Pyrogens displace the set point of
hypothalamus above the normal point of 37°C. Fever or high temperature helps in
stimulating the protective mechanisms against the pathogens.
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