Essay C, Student 9, Expert Marker
Describe and discuss the influence of environmental temperature on metabolism in poikilotherms and homeotherms.
Poikilotherms are animals whose internal body temperature varies along with variation in the temperature of the external environment. Homeotherms, in contrast to this, are animals that maintain a constant body temperature regardless of variation in the external temperature. Internal body temperature effects metabolic rate as it influences the rate of physical, chemical and biological processes within the body, such as diffusion, enzyme activity and the nerve transmission. In homeotherms there is an added dimension to this relationship as for homeotherms to actively raise or lower their internal temperature requires energy and hence constitutes a rise in metabolic rate.
In should be noted that while it may seem that all homeotherms must be endotherms and all poikilotherms must be ectotherms the relationship is in fact not so clear cut. It is possible, for example, for ectotherms to be homeothermic by using behavioural means to maintain a constant body temperature. Or for some poikilotherms to have a very stable body temperature simply because the temperature of the environment they inhabit varies very little, for example many deep sea fish inhabit waters that are always between -0.5 and 0.5°C.
Homeotherms have a thermal neutral zone, within which they can regulate their internal temperature just by adjusting how close to the skin blood flows by either vasoconstriction or vasodilatation of their arterioles. This process requires no additional input of energy and insulating layers e.g. blubber or fur, can be used to aid its effectiveness. However, when the ambient temperature is above or below the critical limits of this zone then additional measures are required to maintain a constant body temperature and these involve the use of additional energy and hence a rise in metabolic rate.
When the environmental temperature drops below the lower critical, this triggers a rise in metabolic rate and the heat generated by this is used to raise body temperature. This process is known as thermogenesis.
Thermogenesis can be either shivering (skeletal muscles utilise ATP and pull against each other generating heat) or non-shivering. Non-shivering thermogenesis (in mammals) relies on specialised adipose tissue called Brown Adipose Tissue or Brown Fat. Brown adipose tissue contains many more mitochondria than white adipose tissue and these are used to produce heat without shivering. The mitochondria in brown adipose tissue contain the protein thermogenin (uncoupling protein 1), which gives them the ability to uncouple proton transport from oxidative phosphorylation, allowing them to utilise substrates to generate heat rather than ATP. Brown Adipose Tissue is mainly around the neck and large blood vessels of the thorax and has a much higher blood supply than white adipose tissue to allow all heat generated to be easily spread to the rest of the body. Clearly both shivering and non-shivering thermogenesis both require energy and so increase the bodies metabolic rate.
Above the upper critical temperature active cooling mechanisms are deployed. This is generally achieved through evaporative cooling i.e. panting or sweating. Evaporation of 1g of water absorbs 580 calories of heat, so evaporative cooling can be highly effective. Its major drawback is that active cooling is often most required in environments where water is in short supply, hence water lost through evaporation may be hard to replace. Homeotherms in arid environments therefore typically exhibit other additional adaptations for heat loss, either anatomical or behavioural. However, overall, evaporative cooling is still the main method of active cooling in homeotherms, and as a process that requires energy, it also leads to a rise in metabolic rate.
Homeotherms therefore have raised metabolic rates when the temperature of their external environment differs significantly enough from their ideal internal temperature as to cause them to need to generate additional heat or to employ active cooling mechanisms.
Poikilotherms do not attempt to control their body temperature, and instead allow their body temperature to fluctuate with the environment. Therefore, rather than more extreme temperatures triggering a rise in energy use and metabolic rate, they instead tend to have an exponential relationship between increasing external temperature and increasing metabolic rate. This is caused by rising environmental temperatures (and hence rising body temperature) allowing for faster physical, chemical and biological processes such as enzyme catalysed reactions, which therefore allows the metabolic rate of the organism to increase.
This relationship can be described using the Temperature Quotient (Q10). The Temperature Quotient shows how much the organism’s metabolic rate changes for a 10°C change in environmental temperature; so a Q10 of 2 indicates a doubling of metabolic rate for a 10°C increase in temperature, while a Q10 of 3 indicates a tripling. To give an example, the Q10 of most ectothermic fish is 1.86.
The result of these two contrasting methods for reacting to changing environmental temperatures is that for homeotherms their metabolic rate will be lowest (basal metabolic rate) when resting at an environmental temperature within their thermal neutral zone, as they will then not have to expend energy on temperature regulation. Above and below this point metabolic rate steadily increases until the environmental temperature becomes too extreme for the animal to cope. When this point is reached varies between species.
For poikilotherms, on the other hand, their lowest metabolic rate will correspond with low external temperature, as low external temperature slows their body processes and therefore acts directly as a limiting factor on their metabolic rate. As external temperature increases, their metabolic rate will increase. While this relationship is always exponential, the exact rate of increase varies between species.
Overall, perhaps the most significant difference is that homeotherms always have far higher metabolic rates than poikilotherms and hence have a far higher demand for food. Even when at rest within the animal’s thermal neutral zone homeotherms have a far higher basal metabolic rate than the metabolic rate of a similar-sized poikilotherm at any temperature.
Reference List:
CAMPBELL, N. A. & Reece J. B. (2008) Biology, 8th Ed. Pearson.
CANNON, B (et al). 2004. Brown adipose tissue: function and physiological significance. PubMed, 84, (1), 277-359.
PURVES, W. K. (et al). (2002) Life, the Science of Biology (7th ed.) pg 785-790.
RANDALL D. (et al). (2001) Ekert Animal Physiology. Mechanisms and Adaptations (5th ed.) Freeman, pg 704-711.