Metabolism and adverse conditions

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Maintaining an active metabolic rate may not be possible at all times. Adverse conditions mean that energy would be wasted on surviving in situations that don't allow for reasonable living activities for a period of time. Therefore, metabolic rate must decrease to allow for this.


Dormancy

Dormancy occurs in organisms sporadically or as part of their lifecycle, and can be used as a response to cold, heat, lack of food or other environmental stresses that prevent the upkeep of an active metabolic state.

When dormancy takes place proactively and the period of e.g. cold is anticipated, dormancy is predictive. Predictive dormancy takes place in some plants. When it's done retroactively once the stressor has begun, it's consequential.

Predictive dormancy is a good idea because it avoids the damage of the environmental stressor, and a bad idea because it might be too preemptive and miss out on resources if the prediction is off. Hence, consequential dormancy is advantageous as it enables organisms to make the most of present resources and only decrease their metabolic rate if really necessary. A clear risk with this strategy is the sudden onset of detrimental abiotic factors that can damage organisms before they can mount a response.

One type of dormancy in animals, notably mammals, is hibernation. This is a state of lowered metabolism that takes place in cold conditions when food becomes scarce. Animals must prepare for hibernation by storing up extra energy, as a thick layer of fat tissue. Hibernation occurs as metabolism decreases along with body temperature and heart rate, and can go on for days or months, depending on the species and environment.




Dormancy that occurs in response to heat is called aestivation. Its purpose is to conserve water to prevent desiccation. Metabolic rate also goes down, and activity ceases. Snails and mosquitoes have been found to undergo aestivation.

Dormancy as seen in birds and some mammals like mice and bats is called torpor. These organisms undergo daily torpor which means that they conserve energy by lowering their body temperature at night, after being active during the day. In a well-regulated process, they drop their metabolic rate at night and increase it again the following day. Unlike hibernation and aestivation, daily torpor isn't seasonally dependent.


Migration

Another strategy to cope with adverse conditions is migration. Migration is seen in all animal groups, and is defined as a long distance movement which can be obligate or facultative. Obligate migration is required for an organism to continue its life, while facultative migration is up to them.

Migration occurs in response to adverse environmental conditions including abiotic factors, lack of food and for the purpose of mating. Animals migrate to a better suited location. Some fish migrate upstream to spawn, while birds embark on notably long migrations seasonally.

Tracking migration can be done by various methods. For example, small tags can be stuck to butterflies or rings on bird legs. Tags can use radio tracking, a GPS receiver or Platform Transmitter Terminal (PTT) to record location.




Radio tracking is suitable for stickers on butterflies, but GPS is much bigger and bulkier so has limited applications. PTT is lighter but less accurate.

Birds migrate as a response to changes in day length, and take part in migration innately or as a learned behaviour. The extent to which bird migration is innate or learnt is the subject of much research.

Birds have innate adaptations for migration including the physiological response to day length change, ability to sense the Earth's magnetic field and even olfaction.




The role of learning in migration extends to recognising landmarks along the path, imprinting early life sites and returning to them loyally, as well as teaching young birds a migration route from scratch. Migration conditioning has been accomplished by introducing new birds to a migration route through the use of a microlight aircraft.


Extremophiles

Extremophiles are organisms that are able to withstand extreme environments e.g. high temperature, high acidity, low temperature, absence of organic food sources, etc.

Many archaea are known for being extremophiles, as well as some bacteria. The bacteria that live in hot springs or seabed vents are thermophiles and have adaptations to hot temperatures.

Enzyme activity is key in metabolism, so these species have versions of common enzymes such as DNA polymerase (that synthesises DNA) which have an optimal temperature much higher than usual. In fact, this outlier has been taken advantage of in molecular biology for use in PCR which takes place at high temperatures.




The optimal temperature for its activity is a very high 70 degrees Celsius. Human enzymes typically peak at the physiological temperature of 37 degrees Celsius.

These bacteria are able to extract energy from their very barren environments by metabolising inorganic molecules instead of organic sources of food like plant and animal litter. Their ATP synthesis is based on removing high energy electrons from these inorganic substrates instead of undergoing typical cellular respiration using organic substrates (carbohydrates, lipids, proteins).

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