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Describe and Discuss the Influence of Environmental Temperature on the
Metabolism of Poikilotherms and Homeotherms
Poikilotherms are animals whose body temperature tends to fluctuate, more or less
following the temperature of the environment. By contrast homeotherms are animals
that regulate their own body temperature within a narrow range, regardless of the
environmental temperature (Campbell et al., 2008). Therefore environmental
temperature has a very different effect on each of their metabolisms, which are
catalysed by enzymes and therefore temperature dependent. This essay will explore the
effect of environmental temperature on the metabolism of animals, looking first at
poikilotherms and then homeotherms.
The temperature of the environment has a direct effect on the metabolic rate of
poikilotherms as their body temperature more or less follows environmental
temperature. As environmental temperature increases, their metabolic rate increases
exponentially as the enzymes which catalyse the reactions have increased rates of
reaction (Randall et al,. 2002). The amount metabolic rate increases as temperature
increases differs in different poikilothermic animals. It can be measured by a
temperature quotient (Q10), where the change in metabolic rate of an animal is
measured for a 10°C change in temperature (Randall et al., 2002). The Q10 for the
majority of poikilotherms is between 1-3, for example most fish have a Q10 of 1.86,
which means their metabolic rate is multiplied by 1.86 with every 10°C increase in
temperature. However, if the environmental temperature increases too much it passes
the critical thermal maximum (CTM) of the poikilotherm and the metabolic rate of the
animal starts to decrease as its enzymes are denatured by the high temperatures, which
can be fatal (Randall et al., 2002).
Many poikilothermic species can become acclimatised to environmental temperature if
they have sustained exposure to it, for example a change in seasons (Purves et al.
2004). This acclimatisation includes metabolic compensation, where the organism alters
its metabolic rate to counter the effects of temperature change, for example many fish in
cold temperatures over winter increase their metabolic rate (Bullock, 1955). This is
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possible by the organism producing variants of their enzymes that have the same
function but have different optimal temperatures (Somereo, 1969).
In contrast to poikilotherms, homeotherms body temperature does not follow the
temperature of the environment, which means environmental temperature does not
affect their metabolism directly. However homeotherms maintain a constant temperature
because the majority are endothermic, meaning they use their metabolism to generate
their own heat, which means the environmental temperature does affect homeotherms
metabolism indirectly. At moderate temperatures within the thermal neutral zone (TNZ) a
homeotherm's metabolism is at the basal metabolic rate (BMR) as this produces
enough heat to balance heat loss to the environment. It can regulate its body
temperature within the TNZ through adjusting the thermal conductance of its body
surface, which is metabolically inexpensive (Randall et al., 2002).
However if the environmental temperature decreases below the TNZ it reaches the
homeotherm's lower critical temperature (LCT). Below the LCT the BMR of the animal is
not sufficient in balancing heat production with heat loss, and so in order to maintain its
body temperature the animal needs to increase its heat production by using metabolic
energy (Randall et al., 2002). This means as environmental temperature decreases
below an animals LCT, the animals metabolism increases. The ways it can produce
heat metabolically, known as thermogenesis, are by shivering and by non-shivering
thermogenesis. Shivering uses the contraction of skeletal muscles to convert ATP into
ADP, which releases energy in the form of heat. In non-shivering thermogenesis certain
hormones cause mitochondria to increase their metabolic activity and produce heat
instead of ATP. This non-shivering thermogenesis can occur throughout the body, but
mostly in specialised adipose tissue called brown fat which is specialised for rapid heat
production by having abundant mitochondria and a rich blood supply (Purves et al.
2004). If environmental temperature falls too far the animal enters a state of
hypothermia where its regulating mechanisms fail, its body temperature falls and its
metabolic rate drops, which can eventually lead to death (Randall, Burggren, French.
2002).
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If the environmental temperature increases above the TNZ of an homeotherm it reaches
its upper critical temperature (UCT). Above the UCT heat loss in the animal is not
balanced with heat production by the BMR, and so the animal must actively dissipate
heat which uses metabolic energy (Randall et al., 2002). This means as temperature
increases above the UCT of an animal, the animals metabolism increases.
Homeotherm's active heat-dissipating mechanisms are sweating and panting. These
both use the evaporation of water to cool the body, as one gram of water absorbs about
285 calories (2448 J), and so is a highly effective way of removing excess body heat if
enough water is available (Randall et al., 2002). If the environmental temperature rises
too far, not enough heat can be lost and the animal's metabolic rate drops as it enters a
state of hyperthermia which can lead to death (Randall et al., 2002).
In conclusion, the temperature of the environment has a direct effect on the metabolism
of poikilotherms, with an increase in environmental temperature causing an exponential
increase in metabolic rate in the animal. However poikilotherms can become
acclimatised to environmental temperature if exposed for a sustained time, adjusting
their metabolic rate. In homeotherms, if the environmental temperature falls within the
TNZ of the animal, its metabolism remains at BMR. However if it falls below this, it
causes the metabolic rate to rise as the animal uses thermogenesis to keep warm, and
if the environmental temperature rises above the TNZ, the metabolism also rises, as the
animal uses active heat-dissipating mechanisms to cool down. So all in all
environmental temperature has a very important and significant influence on the
metabolism of both poikilotherms and homeotherms.
Word count: 956
References
Bullock, T.H. (1955) Compensation for temperature in metabolism and activity of
poikilotherms. Biological Reviews, 30, (3) 311-342
Campbell, N. A., Reece, J.B., Urry, L.A., Cain, M. L., Wasserman, S.A., Minorsky, P. V.,
Jackson, R. B. (2008) Biology (Eighth edition). Pearson Benjamin Cummings, San
Essay C, Student 12 Expert Marker
Fransisco.
Purves, W. K., Sadava, D., Orians, G. H., Heller, H. C. (2004) Life -The Science of
Biology (Seventh edition). Sinauer Associates, Inc., Sunderland, Massachusetts.
Randall, D., Burggren, W., French, K. (2002) Ekert Animal Physiology – Mechanisms
and Adaptations (Fifth Edition). W.H. Freeman and Company, New York.
Somero, G. N. (1969) Enzymic mechanisms of temperature compensation: immediate
and evolutionary effects of temperature on enzymes of aquatic poikilotherms. The
American Naturalist, 103, (933) 517-532