Changes in temperature have profound effects on biological processes. In general, the rate at which life processes (such as metabolism, respiration and growth) proceed declines as the temperature decreases and elevates as it increases. This is because high temperatures supply more kinetic energy to reactions than do low temperatures, allowing the reacting molecules to come into contact and to interact more frequently. If the temperature continues to increase, however, it starts to have destructive effects on organisms. The rate of metabolism, and other life processes, declines again. There is thus an optimum temperature at which metabolism is at its greatest rate and, if the temperature decreases or increases from this optimum, the metabolic rate declines (see Figure 1.1). As was pointed out in Chapter 1, the decline in metabolic rate at low temperatures (below the optimum) is due to very different causes from those producing a decline at high temperatures (above the optimum). The rate declines at low temperatures mainly because there is less kinetic energy driving the metabolic reactions, but the effect is largely reversible (unless freezing or some other event occurs which the organism cannot survive; see Chapter 5). The decline in metabolic rate at high temperatures is, however, due to destructive effects on the proteins, and the other components, of the organism. Some of these destructive effects may be reversible if the temperatures are not too high. At higher temperatures, irreversible changes occur or there is too much damage to repair and the organism will die.
The vast majority of organisms have little ability to control the temperature of their bodies and their temperature is the same as that of their immediate surroundings. Most of their heat is absorbed from outside their bodies (they are ectotherms). In most situations, their ultimate source of heat is solar radiation. Other external heat sources include biological combustion processes (such as the heat generated within a compost heap) and geothermal activity (in hot springs, hydrothermal vents and deep underground). Endotherms have their own central heating system. They generate heat via their metabolism and are able to maintain a body temperature which is above that of their surroundings. They will also, of course, absorb heat from the sun and from their environment and thus have both external and internal sources of heat. The metabolism of ectotherms will also generate some heat, but their metabolic rate is not always sufficient to raise their body temperature above that of their surroundings and the heat soon dissipates. Colouration and orientation to the sun affects the rate at which organisms gain or lose heat. They may also have insulating structures which reduce heat loss (hair, feathers or scales) or structures which shield them from the sun and reflect its rays - thus reducing heat gain (reflective spines and surfaces).
Some ectotherms have behaviours or growth forms which provide a limited degree of control over their internal temperature. They can do this simply by moving from a hot place to a cooler place (or vice versa) by seeking out shade, burrowing into the ground or by emerging into the sun. Desert lizards bask in the sun or press their bodies against the warm sand to raise their body temperature in the morning. In the heat of the day, they stand erect, adopting a posture which increases heat loss from their body, or they simply retreat into the shade. These behavioural mechanisms give lizards a remarkable degree of control over their internal temperature (for an ectotherm).
Birds and mammals are the only organisms which can maintain a constant internal temperature in the face of changing external temperatures. They are able to maintain a temperature higher than that of their surroundings since they generate heat internally by burning food through their metabolism. This is expensive in terms of energy or food use. A human at rest will consume 1300-1800 kilocalories per day at
20 °C, about 90 per cent of which is used to maintain body temperature. An endotherm will generally use about 10 times more energy at rest than does an ectotherm at the same temperature. The energy demands of metabolic heat generation are kept to a reasonable level, however, since the efficient insulation provided by fur or feathers and a layer of fat beneath the skin, together with other heat conservation measures, reduces the rate of heat loss from the body. Maintenance of internal temperature by endotherms does not, therefore, consume all their food reserves. The energy demands of maintaining a high internal temperature mean that there is a lower size limit on animals that can sustain this. A small animal loses heat faster than a large animal because of its greater surface area relative to its volume. A small animal therefore has to have a higher metabolic rate and consume more food to maintain its temperature than does a large animal. Shrews are the smallest mammals and hummingbirds are the smallest birds. These small endo-therms have to eat constantly in order to satisfy the energy demands involved in maintaining their temperature.
Mammals and birds usually maintain a core temperature of around 37°C and 41 °C, respectively. This is a relatively high temperature in the context of the range of temperatures found on Earth and, in most situations, the internal temperature of a bird or mammal is higher than that of its surroundings. Why do they maintain such a high temperature? One reason might be that, since their temperature is higher than that of their environment, they can lose heat by radiation, conduction or convection to regulate their internal temperature. The higher internal temperature also allows enzymes to catalyse biological reactions at a faster rate. This results in faster rates of growth and development. The internal temperatures of birds and mammals are about as high as they can be without incurring some of the harmful effects of high temperatures. Maintaining a constant temperature may have allowed enzymes, and biological processes, to have become optimised to function best at the body's internal temperature. A high internal temperature also enables these animals to function efficiently even though the external temperature is low. This allows activity at night and in cold
HEAT GAIN HEAT LOSS
direct solar energy indirect solar energy metabolism direct solar energy indirect solar energy metabolism evaporation
Too hot: seek shelter radiation, conduction, convection evaporation
Too hot: seek shelter
Too cold: bask in the sun or press against hot surfaces
direct solar energy indirect solar energy metabolism direct solar energy indirect solar energy metabolism evaporation
Too hot: seek shelter, decrease metabolic rate, divert blood supply to surface, increase evaporation e.g. by sweating, panting radiation, conduction, convection evaporation
Too hot: seek shelter, decrease metabolic rate, divert blood supply to surface, increase evaporation e.g. by sweating, panting
Too cold: increase metabolic rate, retain heat through insulation, divert blood supply away from surface figure 4.1 Comparing the mechanisms of heat gain and loss in an ectothermic animal, such as a lizard, and an endothermic animal, such as an Emperor penguin or a camel. For an ectotherm, solar radiation, either direct or via its heating of rocks or soil, is the most important source of heat while, for an endotherm, heat generated metabolically is more important. An ectotherm loses heat mainly by the processes of conduction, convection or radiation from its body whereas, for an endotherm, evaporation (such as sweating or panting) is the main method. Ectotherms rely mainly on behavioural mechanisms to regulate their temperature while endotherms use mainly physiological mechanisms.
environments. Endotherms are thus able to colonise habitats and environmental niches which were not open to ectotherms.
Some fishes, reptiles, invertebrates and even plants can be endother-mic, at least for a period of time and in some parts of their bodies. Some large moths, for example, can generate heat internally which enables them to warm to a temperature at which they can fly even on a cold day or at night. Honeybees, as well as generating heat, huddle together to conserve heat and can maintain a fairly constant temperature within their hive by changing the density of the huddling. The heat is circulated and distributed within the hive by individuals moving from the warm interior to its cooler outer edges.
Animals have a variety of mechanisms for regulating their temperature and/or controlling the exchange of heat with their environment (Figure 4.1). The mechanisms they use depend on whether they are capable of generating their own heat and maintaining their temperature above that of their surroundings (endothermic) or are dependent on absorbing heat from the environment (ectothermic), and whether the conditions in their environment mean they need to gain heat (or prevent it from being lost) or lose heat (or prevent it from being gained).
High temperatures have destructive effects on the molecules which make up the cells of an organism. The properties of proteins depend on their three-dimensional shape (their folding or conformation). This shape is maintained by weak bonds between parts of their structure and by the interaction of the protein molecule with water molecules. These weak bonds and interactions require only a small amount of thermal energy to disrupt them. In fact, individual weak bonds are being broken and reformed continuously, even at normal temperatures. High temperature results in the bonds being broken at a greater rate than they are reformed. The three-dimensional shape of the protein thus breaks down, and it unfolds and can no longer fulfil its biological functions. The life-supporting processes of the organisms are thus not sustained and this leads to the death of cells and the death of the organism. The breakdown in the structure of a protein is called denaturation. It is seen when we poach an egg. The white of the egg contains the protein albumin. Before cooking, it is clear and fluid. As it cooks, it gradually becomes hard and opaque due to the denaturation (unfolding) of the protein by the heat. I wonder why we prefer to eat proteins denatured (cooked)? Perhaps humans learnt by experience that cooking destroyed the disease-causing organisms that may be present in food and so cooked food came to taste better to us. Certainly, parasitologists always like their steaks well done. Or perhaps cooked food is just easier to digest.
Some proteins are more heat stable than others and the process of denaturation can be reversible. As the protein cools, the weak bonds reform and its folding and functionality are restored (renaturation). The ability of proteins to recover their activity after heating will depend on the extent to which they have been denatured (the degree of unfolding). This will increase with temperature and with the time of exposure to high temperatures. If too much damage occurs, the proteins cannot renature and recover their biological properties sufficiently quickly to prevent the death of the organism.
It may, however, be difficult to relate the destructive effects of heat at the molecular level to the death of the whole organism, particularly for multicellular plants and animals. Animals or plants may die at temperatures below that at which their enzymes become denatured, as a result of heat-induced problems at a higher level of organisation than the molecular or cellular level. For terrestrial plants and animals, it may not be the high temperatures themselves which kill them, but rather the resulting loss of water. High temperatures may also result in a lethal imbalance between the various components of the organism's metabolism. Plants, for example, respire faster than they photosynthe-sise at high temperatures. They thus consume food at a greater rate than they can produce it and starve to death. A number of studies on fish indicate that a breakdown in their osmoregulatory systems (control of water and salt balance) is an important early event in heat stress. The nervous system seems to be particularly susceptible to dis turbance by high temperatures and a loss of nervous integration and control would be fatal for an animal. It is not easy to define the temperature at which an organism dies since the lethal effects depend on how long it has been exposed to that temperature and on its thermal history - what it has experienced before exposure to a potentially lethal temperature.
Faced with rising temperatures, some organisms will try to stay cool and maintain their internal temperature at a non-lethal level by losing heat from their bodies. Heat is lost from the surface of an animal and so an animal can increase its heat loss by exposing more of its surface to the air and by diverting more blood flow to beneath the skin of exposed areas. A more upright posture will expose a greater surface area and raising feathers or hair will allow more air circulation close to the skin. An elephant loses heat by flapping its ears, which are well supplied with blood vessels. Many desert animals have large ears, such as the fennec fox of the Sahara Desert, and can lose heat by diverting blood flow to these structures. Mammals which live in hot climates generally have a greater proportion of their surface exposed to the air (free of hair) than do those in cold climates.
The evaporation of water requires the absorption of heat. Terrestrial organisms can thus achieve cooling by evaporating water from their surface or from respiratory organs. Panting increases airflow over the tongue and through the mouth and respiratory system. This raises the rate of evaporation from these surfaces and produces cooling. The incoming air is at a lower temperature than the expired air. Warming and humidifying the inspired air absorbs heat from the body. Dogs use panting as their main method of unloading heat. They pant at a rate of 300-400 times a minute when hot, compared with 10-40 times a minute under cold conditions. Throat fluttering by birds has a similar cooling effect. Some small mammals cool themselves by spreading saliva over their fur.
Sweating produces evaporative cooling from the water secreted onto the surface of the skin. This is well developed in cattle, horses, hippopotamuses and humans, who sweat over their entire bodies. Sweating is poorly developed, or absent altogether, in other mammals (such as rodents and carnivores). There is a relationship between the relative importance of panting and sweating in mammals. Those which sweat over their whole bodies do not pant (e.g. humans) while those in which sweating is poorly developed have a high capacity for panting (e.g. dogs).
All these methods of heat loss also result in greatly increased rates of water loss. Sustained exposure to high temperatures is likely to result in death through dehydration, unless the animal has access to adequate supplies of water. Water conservation measures are essential for animals in hot dry environments. Small desert mammals do not usually unload heat by sweating or panting, since they could not survive the resulting water loss. Instead, they avoid the heat of the day in burrows or rock crevices and are active at night.
Marine animals have fewer options for controlling their temperature than do terrestrial animals. Sweating and panting will not produce cooling since water cannot evaporate from their surfaces. However, the marine environment produces a much more stable temperature environment, with the bulk and high heat capacity of the water providing a buffer against temperature change. Marine organisms are thus, in general, unlikely to experience extreme high temperatures or rapid changes in temperature, except around hydrothermal vents.
If an organism is exposed to an elevated, but not fatal, temperature for a short period of time, there is a dramatic increase in its ability to survive high temperatures which would be lethal without this pretreatment. This is known as induced thermotolerance, or the heat shock response, and has been observed in almost every organism studied, from bacteria to mammals. The heat pretreatment produces, within minutes, a burst of protein synthesis. The proteins that are produced are absent, or are only present in low concentrations, in the unstressed state and are known collectively as heat shock proteins. These protect organisms against the harmful effects of heat, particularly by their effects on other proteins. Some heat shock proteins remove heat-damaged proteins from cells, preventing them from accumulating and interfering with the operation of the cell. Others act as molecular chaperones which help proteins maintain their conformation and resist denaturation by heat. Heat shock proteins also help proteins to refold and regain their function after a heat stress. Molecular chaperones are also present at normal temperatures where they assist protein folding and prevent proteins, which are present at high concentrations within cells, from sticking together and interfering with each others' formation and functions. The problems of protein folding and aggregation become worse at high temperatures and elevated levels of molecular chaperones are produced to deal with this as part of the heat shock response.
The heat shock response is an emergency system which provides organisms with a measure of protection against unpredictable stresses. Some environmental changes, such as a seasonal change in temperature, occur more gradually and with a greater degree of predictability. Over a period of days or weeks, the organism undergoes physiological changes which alter its tolerance to environmental stresses in a way which matches the changing season. The bullhead catfish, for example, can tolerate temperatures up to 36 °C during summer whereas, in winter, temperatures above 28 °C are lethal. This process of a change in tolerance is referred to as 'acclimatisation' if it occurs in nature and 'acclimation' if it is induced artificially in the laboratory. The acclimatisation process is complex and may involve a variety of changes. Some organisms produce different variants of enzymes which have the same function but different temperature optima. By shifting from one variant to another, the organism can adjust its operation to match the changing season. The properties of membranes may change by altering the proportions of different lipids in their structure. The seasonal production of some low molecular weight compounds (such as glycerol, proline and trehalose) may protect against environmental stresses. Temperature acclimatisation may even involve extensive reordering of the composition of cells. In some fish, for example, the proportion of the cell's volume that is occupied by mitochondria increases in winter and decreases in summer.
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