All living systems require a continuous supply of heat energy in order to survive. Solar radiation is used by photo-synthetic organisms and this energy becomes locked up in the chemical bonds of organic molecules such as sugars. This provides an internal source of heat energy when released by the reactions of respiration. Temperature indicates the amount of heat energy in a system.
Temperature can act as a limiting factor in the growth and development of plants by influencing the rates of processes like cell division, photosynthesis and other metabolic processes.
Adaptations of Plants to Low and High Temperature
Low temperature: In order to survive in extreme conditions such as low light intensity, low temperature, frozen soil, plants show many structural, physiological and behavioral adaptations. For example most temperate woody perennials are deciduous and lose their leaves under the influence of abscisic acid, in order to prevent water loss by transpiration during periods when water uptake is limited by low temperature.
Throughout the period of low temperature the buds are protected by scale leaves and are dormant. The conifers have needle like leaves which reduce the amount of snow which accumulate on them. Many species of plants produce low temperature resistant seeds.
Plants respond to cold stress by increasing proportion of unsaturated fatty acids, which help membrane to maintain structure at low temperature by preventing crystal formation. The plants of cold regions such as oaks, maples, rose and other plants have adapted to bring changes in solute composition of the cell, which causes cytosol to super cool without ice formation, although ice crystals may form in the cell walls. Thus cells are saved from being killed by the formation of ice crystals within the cytoplasm.
High temperature: In many regions of the world high temperature is associated with water shortage and many of the adaptations shown by the plants in these regions are related to their ability to resist desiccation. In hot regions a shiny cuticle is secreted by the epidermis which reflects much of the incident light thus preventing heat being absorbed and overheating the plant.
The large surface area contains numerous stomata openings which permit transpiration. One physiological mechanism used by plants to avoid dry conditions is to produce abscisic acid. This causes stomata to close, thus reducing loss of water by transpiration. The cells of the plants of temperate region having 40°C and above temperature synthesize large quantities of special proteins called heat shock proteins. These proteins take up enzymes and other proteins to prevent denaturation.
Mechanisms of Thermoregulation in Animals
Temperature influences the metabolic activities of animals. Aquatic organisms have a relatively stable environment with regard to temperature. Most aquatic organisms, including non-vertebrates and fish have a body temperature which varies according to the temperature of the water, though some active fishes such as tuna are able to maintain a body temperature higher than that of water.
Air temperature can fluctuate widely over a 24 hour period. To adapt and tolerate the temperature fluctuations, animals show many structural, physiological and behavioral responses.
(i) Structural adaptations: Lungs are modified for panting and the sweat glands are present in mammals. They are absent in birds. There are long term changes in the sub-dermal fatty insulation or pelage.
(ii) Physiological adaptations: These regulate blood flow to skin to dissipate heat during hot days and conserve heat during cold days. Some muscles are activated causing plumage fluffing. Sweat glands are activated, and when sweat evaporates from skin surface, energy is lost from the body as latent heat of evaporation and this reduces body temperature.
(iii) Behavioral adaptations: The animals show behavior responses as per need. For example ground squirrel moves to burrows in midday to minimize loss of heat. Lizard, marine iguana etc. increase its body temperature by basking in the sun. Some animals open their mouth to reduce heat e.g. birds. Animals also control and adjust posture to expose their body surface for the heat exchange as per requirement.
Classification of Animals on the Basis of Temperature
The extent to which different groups of animals are able to generate and conserve heat is variable.
Poikilotherms (Poikilo, various, thermo, heat) are all non-vertebrates, fishes, amphibia and reptiles are unable to maintain their body temperature within narrow limits using physiological mechanisms though many do so using behavioral mechanisms. Homeotherms are birds and mammals which are able to maintain a fairly constant body temperature by using physiological mechanisms.
It is observed that deep sea fishes maintain their body temperature due to the constant natural surroundings and lizards regulate their body temperature. In contrast, numerous birds and mammals vary their body temperature. So to overcome the problem of classification of animals as poikilotherms and homeotherms a more widely temperature classification scheme based on the source of heat production is followed. Accordingly animals are classified as Ectotherms, Endotherms and Heterotherms.
(i) Ectotherms: These animals produce metabolic heat at low level and that is also exchanged quickly with the environment. They rely more on heat derived from the environment to raise their body temperature.
Examples are most invertebrates, fishes, amphibians and reptiles.
(ii) Endotherms: These animals produce their own body heat through heat production as by-product during metabolism in muscles, or by the action of hormones that increase metabolic rate. Most land mammals’ insulation is provided by air trapped outside the skin by hair and by fat in the dermis and just below the dermis, to prevent loss of body heat during a cool day. Marine animals such as whales and seals inhabit much colder water than their body temperature have a very thick layer of fat called blubber for insulation just under the skin.
Marine mammals get rid of their excess of heat into warm seas by large number of blood vessels in the outer layer of the skin. Inland mammals sweat evaporates from skin surface which reduces body temperature.
In dogs, panting is the method to release body heat for evaporative cooling. In bats etc. the evaporative cooling methods include use of saliva and urine.
(iii) Heterotherms: These animals are capable of varying degrees of endothermic production, but generally do not regulate their body temperature within a narrow range e.g., bats, the humming birds.
Regulation of Constant Body Temperature in Man
The human body temperature is 36.8°C or 98.6°F. So there are ways in the body to control over-heating or over-cooling.
Over-heating
Vigorous activity, disease, absorption of radiation from the sun, and many other external causes may bring about overheating. It produces two marked effects in the skin; i.e. vasodilatation and sweating.
Vasodilatation: It is the expansion of blood vessels, that is, an increase in their diameter so that more blood flows through them. Whenever the body gets too hot vasodilatation occurs in the dense network capillaries which lie just beneath the epidermis of the skin. These capillaries open up and let a large volume of over-heated blood flow very close to the body surface. Here the blood rapidly loses heat by radiation through the skin, which cools the
body. This is why a person’s skin becomes a flushed pink color and feels hot to the touch when he is over-heated.
Relaxation of Hair Erector Muscles: The hair erector muscles are attached with the hair. Whenever heat is lost from the body, hair erector muscles relax and so the hairs lie more or less flat against the skin. In this position they offer the least possible obstruction to the heat loss by radiation and convection.
Sweating: Sweating is the production of watery fluid containing dissolved salt from sweat glands in the skin. As sweat evaporates from the skin, it has a cooling effect because the evaporating liquid carries away body heat. Any air movement, over the body helps to speed up the evaporation of sweat that is why fans, while not necessarily reducing the temperature of a room, have a cooling effect on the body.
Over-cooling
If the body tends to lose more heat than it is generating the following compensatory changes may take place.
Decrease in sweat production: Minimizes heat lost by evaporation.
Vasoconstriction: Constriction of the arterioles which supply the surface diminishes heat loss. Vasoconstriction makes a person look pale or blue.
Increased metabolism: It releases more heat.
Contraction of hair erector muscle: It raises the hair shafts to an almost vertical position. It forms a film of air over the skin forming insulation.
Shivering: The reflex action operates when the body temperature begins to drop. The spasmodic (spasm, violent involuntary muscular contraction) contraction of the muscles is called shivering. This contraction produces heat which helps to raise the body temperature. It is called shivering thermogenesis.
Coordinating role of brain: The body temperature regulation is based on complex homeostatic system facilitated by feedback mechanism. Hormones also trigger the heat and are termed as non-shivering thermogenesis.
Brown fats: Some mammals possess brown fats, which are specialized for rapid heat production.
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