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Thermo Regulation Notes

Natural Sciences Notes > Environmental Physiology of Animals Notes

This is an extract of our Thermo Regulation document, which we sell as part of our Environmental Physiology of Animals Notes collection written by the top tier of University Of Manchester students.

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Thermoregulation Why is temperature so important?
Thermal balance is necessary for all life on Earth. The earth itself is composed of a molten core, with a thin crust of land that is separated by oceans. Temperature is responsible for the geology of our planet, and allows us to have water existing in all three states. Large fluctuations in the earth's temperature have happened throughout the history of the planet. Climate change over a geological time scale is thought to play a large role in adaptive radiation and mass extinctions. It has led to periods of glaciation, it influences sea levels and plate tectonics. Temperature fluctuations also occur over a seasonal timescale and a daily timescale. Thermal rhythms Seasons are caused by the tilt of the earth's axis and are measured in months. Seasonal climate changes are most important in temperate and high-latitude regions, and less so towards the poles. Diurnal or tidal temperature changes are most influencial on inter-tidal or desert species. Physical effects of temperature

Animals typically experience a narrow range of temperatures, with each group having its own thermal limit in which it can live. Active endotherms especially have a very narrow thermal limit, and few organisms bar some unicellular ones (bacteria, algae) can tolerate above temperatures of 60degC, with hot springs protests (eukaryotes) peaking at around 55degC. Ectotherms appear to have a broader thermal limit than endotherms.

Heat sensitivity of different classes of organisms

As seen above, increased organisational complexity of an organism is negatively correlated with its heat tolerance. Prokaryotes can, therefore, tolerate over twice the maximum temperature that metazoans (45degC) can. Simple eukaryotes, including fungus and algae, start to break down at around 55-60degC, an intermediate temperature between prokaryotes and metazoan organisms. Thermal tolerance This is a schematic representation of the ranges of thermal tolerance in marine ectotherms (invertebrates and fish) in a latitudinal cline (bottom of the graph represents south polar regions whilst top represents tropical equatorial regions and the centre representing temperate regions). The diagram ignores the different between northern and southern hemispheres. Seasonal shifts of tolerance windows occur in temperate zone species. There is a narrowing of tolerance range in polar species, especially Antarctic ones. Tropical species appear to have a wider thermal limit than polar species, which suggests there is increased phenotypic plasticity in tropical zone species. Physical effects of temperature The thermal environment pauses a large challenge to animals as all physical (and thus biophysical and physiological) processes are affected by temperature. Temperature is a

measure of the average molecular motion within a material. Molecules vibrate faster with increases in temperature which can increase the number of molecules that will reach the activation energy threshold for a given reaction. In physiology, many reactions are usually a series of steps, each with their own DG (the difference between activation energy and terminal energy). Circles in red on the graph. However, more often than not, physiologists are more interested in the overall rate of the reaction, and often express temperature using the term Q 10. This is given (10/T2-T1) by the equation: Q10 =(K2/K1) , where T=
temperature and K is the variable to be measured, for example heart rate. The value calculated then gives the difference in rate of reaction at a 10degC temperature difference. Q10 can vary over different temperature ranges. For example, the crayfish heart rate does not exist constantly at different temperatures. Between 5 and 15, there is a Q10 of 2,4, but between 15 and 25 it's

0.8, so different Q10s for a range of processes. The reason is mainly because of the effects of temperature on biological processes. Temperature and biochemistry Enzymes are proteins and thus their performance is subject to environmental temperature change. However, as temperatures rise and fall, the function of enzymes may be modified in an adaptive way to extend the thermal range. The nature of these modifications change with the time scale of the temperature change:

1. Short-term (acute, hormonal, HSP)

2. Medium-term (acclimatory, phenotypic plasticity)

3. Evolutionary (genetics) Short term - acute Changes occur from seconds to hours and act on enzyme activity via the hormones, nervous system or accessory proteins. They primarily act to increase the effective concentration or affinity of an enzyme. The changes may include:

1. Phosphorylation

2. Compartmentalisation within the cell

3. Change in the intracellular environment (eg a-stat, the relationship between temperature and pH that acts to cancel out the effects of both). For example, the ICa calcium channel shows a temperature-dependant response to adrenaline. At 7degC, the receptor reacted with a 7-fold response compared with a 1.6-fold response at 21degC. This suggests that part of the adrenergic signalling cascade is temperature sensitive; this makes sense, as you would need a greater response to adrenaline at low temperatures to stop the heart from slowing down, for example if you are a cold-blooded fish diving. This change is caused by acute phosphorylation. Short-term - Heat Shock Proteins Heat Shock Proteins (HSPs) vary with biological complexity and environment (for example, the relative temperatures on land and in aquatic systems). Heat shock proteins act as molecular chaperones. During heat or cooling stress, some proteins tend to unfold, opening themselves up for unwanted interactions between other native proteins or cell constituents and this renders them useless. HSPs limit these interactions by binding to sites in the native protein, stabilizing them until the conditions change and the proteins can re-fold in the correct state. This can confer tolerance in minutes. One example in nature is found in sea turtles, which use them to stabilise tertiary protein structures, which protects them from temperature changes. As previously mentioned, HSPs differ depending on a number of factors. The temperature at which HSPs are induced is related to the animal's normal thermal routine - for example, warm species undergo induction at substantially higher temperatures than cold species. This also changes depending on terrestrial or aquatic lifestyles and with the biological complexity of the protein itself. From the chart it can be seen that the annelids have the lowest induction temperature, followed by echinoderms, crustaceans and amphibians, followed by reptiles, birds, arachnids and molluscs; arachnids follow and the highest induced temperatures are found in prokaryotes. HSPs may also be recruited for osmotic shock, pH and hypoxia. Medium-term - Acclimation and phenotypic plasticity These changes act over days, weeks and months. They may include changes in protein synthesis and degradation (for example, an increase in cell mitochondria by up to 50%); altered gene expression (otherwise known as phenotypic plasticity) or another process called isozyme switching. Isozymes are variants of an enzyme (from different loci) that catalyse the same reaction, but each isozyme performs best at in specific conditions. Isozymes can be distinguished during gel electrophoresis owing to differences in amino acids comprising the protein.

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