Questions on thermoregulation in endotherms and ectotherms

Question 1

A biologist claims that ectotherms will cope better than endotherms with a warming climate, because ectotherms do not need to spend energy producing their own heat. Evaluate this claim.

Accepted Answer

B1 in support: as air temperature rises, an ectotherm needs to spend less time on behaviour that maintains a workable body temperature and may reach an active temperature for more of the day B1 in support: an ectotherm already converts a much larger share of the food it eats into new tissue than an endotherm does, so a longer active season without the fixed cost of internally generated heat could increase its growth and reproduction B1 against: an ectotherm's body temperature depends directly on its surroundings, so if air temperature rises above the optimum for its enzymes it has far less physiological ability to cool itself than an endotherm's hypothalamus and effectors provide B1 against: an endotherm holds its core temperature near the optimum for its enzymes largely independently of the surroundings, so it is buffered against a change in air temperature in a way an ectotherm is not, at the continuing cost of needing more food B1 judgement: the claim is too simple; whether warming helps or harms a species depends on how far conditions move from that species' own optimum and on whether food supply changes as well, not on being an ectotherm or an endotherm as such

Question 2

Describe the responses of the skin when the body becomes too warm, and describe how each of those responses increases the loss of heat.

Accepted Answer

B1 the arterioles supplying the capillary networks near the surface dilate, which is vasodilation B1 the shunt vessels beneath them constrict, so more blood is diverted through the surface capillaries and more heat is lost by radiation and convection B1 the sweat glands secrete sweat onto the surface of the skin, and its evaporation takes latent heat from the skin B1 the hair erector muscles relax so the hairs lie flat, trapping a thinner layer of still air and so reducing the insulation

Question 3

Describe how a lizard raises and then limits its body temperature during a hot day in a desert, giving named examples of the behaviour involved.

Accepted Answer

B1 in the cool of the morning it basks in the sun, absorbing heat by radiation, and presses its underside against rock the sun has warmed so that heat is also gained by conduction B1 it flattens its body and turns its flank towards the sun, presenting the largest possible surface area to the radiation, and may darken in colour so that more is absorbed B1 as the day becomes hot it moves into the shade of a rock or into a burrow, turns to face the sun so that less of it is exposed, and raises its body clear of the hot ground B1 it may also gape to evaporate water from the mouth, and shift its activity to the cooler hours of the morning and evening

Question 4

Discuss the advantages and disadvantages of being an endotherm rather than an ectotherm of the same body mass.

Accepted Answer

B1 an endotherm holds its core temperature near the optimum for its enzymes whatever the surroundings are doing, so it can be fully active at night, in winter and in cold places on land where an ectotherm is slow or dormant B1 that independence widens the range of habitats available to it and means it is not immobilised at dawn, so it can hunt or escape at any hour B1 against that, much of the energy it assimilates is spent on producing heat, so it needs perhaps ten times as much food as an ectotherm of the same mass and must find it regularly B1 an ectotherm converts a far greater share of its food into new tissue and can survive long periods without eating, so where food is scarce and the climate is warm the ectotherm's strategy is the better one

Question 5

Describe the responses of the skin and of skeletal muscle when the body becomes too cold, and describe how each response reduces heat loss or increases heat production.

Accepted Answer

B1 the arterioles supplying the surface capillary networks constrict, vasoconstriction, while the shunt vessels beneath them dilate, so less blood reaches the surface and less heat is lost by radiation and convection B1 the hair erector muscles contract, pulling each hair upright and trapping a thicker layer of still air next to the skin, which insulates, although this effect is small in a human B1 sweat glands secrete little or no sweat, so little heat is lost by evaporation B1 skeletal muscle begins to shiver, contracting rapidly without doing useful work, so nearly all of the extra energy released by the increased respiration appears as heat, raising the rate of heat production

Question 6

Explain why a mammal controls its core temperature within narrow limits, referring to enzymes in your answer.

Accepted Answer

B1 below the optimum, molecules have less kinetic energy, so there are fewer successful collisions between substrate and active site and the rate of every enzyme-controlled reaction falls B1 above the optimum, increased vibration breaks the hydrogen bonds and ionic bonds holding the tertiary structure, so enzymes denature and the active site is no longer complementary to the substrate B1 denaturation is not reversed by cooling, so a core temperature much above 37 °C is far more dangerous than one the same distance below it

Question 7

Compare the peripheral thermoreceptors of the skin with the central thermoreceptors of the hypothalamus.

Accepted Answer

B1 peripheral thermoreceptors are nerve endings in the skin and detect the temperature of the surface and so of the surroundings, whereas central thermoreceptors lie in the hypothalamus and detect the temperature of the blood B1 the blood temperature the central receptors detect is the core temperature, which is the variable actually being regulated, whereas the skin temperature is not B1 the peripheral receptors respond to a change and so allow correction to begin before the core has altered at all, which the central receptors cannot do

Question 8

A student writes that capillaries move closer to the surface of the skin when a person is hot. Explain why this is wrong, and explain what actually changes.

Accepted Answer

B1 capillaries are vessels one cell thick held in position by the tissue around them, and there is no mechanism that could move them through the dermis B1 what changes is the diameter of the arterioles supplying the surface capillary networks, which dilate when the body is warm and constrict when it is cold B1 the shunt vessels running beneath the surface network do the opposite, so the volume of blood flowing near the surface changes while the vessels themselves stay where they are

Question 9

A person working in still air at 35 °C and high humidity overheats far more quickly than a person working at the same temperature in dry air. Suggest why.

Accepted Answer

B1 at an air temperature close to body temperature there is little gradient for heat loss by radiation and convection, so evaporation of sweat is doing nearly all of the cooling B1 in humid air the water vapour concentration outside is already high, so sweat evaporates only slowly and instead runs off the skin, taking little latent heat with it B1 heat production from respiration continues while heat loss has effectively stopped, so the core temperature rises, and the sweat lost still costs the person water and ions

Question 10

Explain how non-shivering thermogenesis raises heat production in a person exposed to cold for several weeks, without any muscle contraction being involved.

Accepted Answer

B1 adrenaline and thyroxine raise the metabolic rate of the liver and other tissues, so more heat is released as a by-product of increased respiration B1 in an infant, brown adipose tissue respires in a way that releases the energy directly as heat rather than using it to make ATP B1 because this route relies on thyroxine, released over days and weeks rather than a nerve impulse, its effect builds up slowly and accounts for acclimatisation after prolonged cold exposure

Question 11

An ectotherm and an endotherm of the same body mass are kept in the same conditions. Over a week the endotherm eats 2100 g of food and the ectotherm eats 190 g, figures constructed for this question rather than measured. Calculate the ectotherm's food intake as a percentage of the endotherm's, and calculate how many times greater the endotherm's intake is.

Accepted Answer

M1 percentage = ectotherm's intake ÷ endotherm's intake × 100 = 190 ÷ 2100 × 100 A1 9.0 per cent A1 the endotherm eats 2100 ÷ 190 = about 11 times as much food as the ectotherm over the same week

Question 12

A well-furred mammal gains far more insulation from erecting its hairs in the cold than a human does. Explain why.

Accepted Answer

B1 erecting hairs traps a layer of still air next to the skin, and still air is a poor conductor of heat, so trapping more of it increases insulation B1 a furred mammal has hair dense enough that erecting it traps a substantial, continuous layer of still air over the whole body surface B1 human body hair is too sparse to trap a significant layer of air even when the hair erector muscles contract fully

Question 13

A small mammal such as a hedgehog lowers its own body temperature and metabolic rate for weeks at a time during winter, a state called torpor, rather than maintaining a normal core temperature throughout. Suggest the advantage of doing this, given what this lesson states about the energy cost of being an endotherm.

Accepted Answer

B1 maintaining a normal core temperature is energetically expensive, since much of the energy an endotherm assimilates is spent simply producing heat B1 in winter, food such as insects and other invertebrates is scarce, so a small mammal may not be able to eat enough to sustain its normal high metabolic rate B1 by lowering its body temperature and metabolic rate it greatly reduces its energy expenditure during the period when food cannot be found, surviving on stored fat until conditions improve, at the cost of being unable to respond quickly to danger while torpid

Question 14

Compare the nervous and hormonal components of the response to cold, referring to their speed and how long each takes to have its full effect.

Accepted Answer

B1 the nervous component, impulses from the hypothalamus along the autonomic nervous system to effectors such as the arterioles, and along motor neurones to skeletal muscle, acts within seconds of a temperature change B1 the hormonal component is slower: adrenaline raises the metabolic rate within minutes, while thyroxine's effect builds up over days to weeks of continued cold exposure B1 the nervous route therefore handles an immediate cold snap, such as stepping outside, while the hormonal route underlies acclimatisation to a cold season

Question 15

State the difference between core temperature and skin temperature, and state which one the body actually regulates.

Accepted Answer

B1 core temperature is the temperature of the blood and the deep organs, whereas skin temperature is the temperature of the body surface and extremities B1 it is core temperature that is regulated within narrow limits; skin temperature is allowed to vary considerably, since reducing blood flow to the surface is itself one of the mechanisms that conserves core heat