5.1.1 Communication & homeostasis

Question 1

why do multicellular organisms require internal communication systems?

Answer 1

to coordinate the activities of different specialised cells, tissues and organs in the body

to respond to changes in the internal environment, including in the context of homeostasis

to respond to changes in the external environment, e.g. in order to find food, reproduce, avoid harm etc.

Question 2

what is cell signalling?

Answer 2

Cell signalling is the communication between cells, in order to coordinate their activities.

Question 3

what is Paracrine signalling

Answer 3

Communication between adjacent cells within a tissue

Question 4

what is the mechanism of Paracrine signalling?

Answer 4

1. The secretion of ‘local hormones,’ cytokines, growth factors or neurotransmitters by some cells
2. These signalling molecules bind to complementary glycoprotein receptors, located in the plasma membrane of other cells in the same tissue
3. The consequence is a response in those cells, e.g. a change in metabolism.

Question 5

what is Endocrine signalling

Answer 5

communication between distant cells

Question 6

what is the mechanism of Endocrine signalling?

Answer 6

1. Cells in a gland produce hormones and secrete them directly into the blood
2. The hormones dissolve in the blood plasma and thus are carried around the body
3. The hormones bind to complementary glycoprotein receptors, often located in the plasma membrane of specific ‘target’ cells
4. The consequence is a response in the target cells, e.g. a change in metabolism.

Question 7

what is homeostasis?

Answer 7

The maintenance of a constant internal environment, despite changing environmental conditions.

Question 8

what are the 4 main things homeostasis controlls in humans?

Answer 8

Core body temperature
Blood (plasma) glucose concentration
Blood (plasma) water potential
Waste products

Question 9

explain breifly homeostastis and Core body temperature

Answer 9

regulated at a level close to 37°C in order to provide the optimum for enzymes – control is via various thermoregulatory processes (including sweating and shivering), coordinated by the hypothalamus

Question 10

explain breifly homeostastis and Blood (plasma) glucose concentration

Answer 10

regulated within narrow limits in order to provide an adequate supply to all body cells for respiration, without decreasing blood water potential too much – control is coordinated by the pancreas, which secretes varying levels of insulin and glucagon hormones as appropriate

Question 11

explain breifly homeostastis and Blood (plasma) water potential

Answer 11

the concentrations of salt ions and other solutes in the blood plasma – regulated at a level to equal cytoplasm, so that osmotic effects (lysis or crenation) on body cells are avoided – monitored by the hypothalamus and controlled via secretion of ADH

Question 12

explain breifly homeostastis and Waste products

Answer 12

levels are prevented from exceedingcertain thresholds
so that these wastes do not become toxic to body cells – excretion of carbon dioxide by the lungs and urea by the kidneys removes these wastes from the body.

Question 13

Describe how homeostasis generally operates

Answer 13

1. There is a change in the level of a factor, taking it above or below its optimum range
2. Receptors detect this change and pass on signals to a coordination centre (which may be within the same organ as the receptors);
3. Coordination of a response occurs, i.e. a decision is made as to the appropriate response that will correct the change detected;
4. Cell signalling takes place between the coordination centre and the effector – this typically involves secretion of hormones and/or electrical transmission.
5. The effector is the organ that now produces the response (as a consequence of the cell signalling);
6. The response produced by the effector results in the level of the factor being brought back towards the optimum – this effect is called negative feedback;
7. The deviation has been corrected – the new level of the factor is itself now detected by the receptors so the response that corrected the level does not continue inappropriately;
8. Often however, an overshoot or undershoot of the level of the factor does occur, which is detected by the receptors – the above process now repeats to achieve a correction in the opposite direction to the first.

Question 14

what does this represent?

Answer 14

The overshoot and undershoot whilst maintianing homeotastis, as the level fluctuates around the optimum

Question 15

what is a receptor?

Answer 15

extrinsict glycoproteins that bind chemical signals, triggering a response in the cell

Question 16

what is an effecotr?

Answer 16

typically a muscle or a gland – which produces a response, following stimulation from a stimulus

brings about a correction in the level of a factor back towards the optimum.

Question 17

what is Negative feedback?

Answer 17

A mechanism in which a change in the level of a factor is
detected by receptors and triggers a response by effectors that corrects the change

Question 18

Draw a diagram to summarise the negative feedback loop

Answer 18

Question 19

what is positive feedback?

Answer 19

Positive feedback is when a change in the level of a factor is detected and triggers a further change in the level, in the same direction

Question 20

Draw a diagram to summarise positive feedback

Answer 20

Question 21

compare positive and negative feedback.

Answer 21

negative feedback corrects changes and acts to bring the level of a factor back to the set point

positive feedback tends to exaggerate changes and so takes the level of a factor further and further away from the set point.

Question 22

what is thermoregulation?

Answer 22

Thermoregulation is the regulation of core body temperature; it is a key aspect of homeostasis in all animals

Question 23

why do we require thermoregulation when our body temperature is too low?

Answer 23

At temperatures below the enzyme’s optimum, activity decreases because there is less kinetic energy and hence a lower rate of successful collisions between substrates and active sites, resulting in fewer ESCs forming;

Question 24

why do we require thermoregulation when our body temperature is too high?

Answer 24

At temperatures above the enzyme’s optimum, activity decreases because the excessive kinetic energy causes hydrogen bonds in the tertiary structure to break, changing the shape of the active site; the substrate will no longer be a complementary fit and so fewer ESCs form.

Question 25

why is thermoregulation vital?

Answer 25

Thermoregulation is vital because the enzymes that catalyse metabolic processes (including respiration) have a narrow range of temperatures at which they have high activity

Question 26

what are the two response types to regulate body temperature?

Answer 26

Physiological responses are those that involve body processes (endotherms only)

Behavioural responses are those that involve changes in the way the animal is acting (both endotherms and ectotherms)

Question 27

what is an ectotherm?

Answer 27

animals which are unable to use physiological mechanisms to a great extent, but instead rely primarily on behavioural responses to control their body temperature

insects, reptiles, amphibians and fish

Question 28

what is the metabolic rate like in ectotherms?

Answer 28

lower metabolic rate than endotherms and so need less food to survive.

Question 29

what behavioural responses do ectotherms use to increase their body temperature

Answer 29

Moving out of the shade into direct sunlight – then remaining in the sunniest place to receive more heat by radiation, i.e. basking in the sun

Orientating the body (or extending parts of the body) so that the maximum surface area is warmed by the sun’s radiation

Pressing the body onto the warm surface of the ground, increasing transfer of heat into the body by conduction.

Question 30

what behavioural responses do ectotherms use to decrease their body temperature

Answer 30

Moving out of direct sunlight into the shade or into an underground burrow, decreasing the amount of radiation from the sun that reaches the body

Orientating the body (or minimising the area of parts of the body) so that the minimum surface area is warmed by the sun’s radiation

Pressing the body onto a cool surface (e.g. rocks) or submerging the body in cool mud or water, increasing transfer of heat out of the body by conduction.

Question 31

what is an endotherm?

Answer 31

animals which primarily use physiological mechanisms to control their body temperature within very narrow limits – though they may use behavioural responses too.

Birds and Mammals

Question 32

what is the metabolic rate in endotherms like?

Answer 32

higher metabolic rate than ectotherms and so need a great deal more food

Question 33

how are Physiological thermoregulatory responses are coordinated in endotherms?

Answer 33

1. There are peripheral thermoreceptors in the skin which detect body surface temperature;
2. These receptors send electrical impulses via sensory neurones to the hypothalamus in the brain;
3. The hypothalamus also has its own thermoreceptors, which detect blood temperature (corresponding to core body temperature);
4. The hypothalamus now acts as the coordination centre, collating the data from all surface and internal thermoreceptors and then initiating the cell signalling to effectors in order to trigger suitable responses.

Question 34

how do endotherms increase their body temperature?

Answer 34

The heat gain centre in the hypothalamus sends electrical impulses via motor neurones to skeletal muscles, which act as effector organs in this response:
o The muscles are stimulated to carry out repeated rapid contractions – these require an increased respiration rate;
o The higher respiration rate results in more heat being released, which warms the surrounding tissues and blood, resulting in an increase to core body temperature.

Meanwhile, the hypothalamus also sends electrical impulses via motor neurones to circular smooth muscle in the wall of the specific arterioles that supply the skin capillaries with blood:
o The circular smooth muscle contracts, constricting (narrowing) the arteriole lumen, such that less blood now flows through the skin capillaries;
o This peripheral vasoconstriction results in less blood flowing near the skin surface and hence less heat being lost from the blood to the environment by radiation;
o More heat is conserved within the body, so there is an increase in core body temperature.

Increased generation of heat can be stimulated via an increase in the release of the hormones adrenaline (from the adrenal glands) and thyroxine (from the thyroid gland), both of which stimulate an increased metabolic rate in body cells.

Contraction of erector pili muscles causes fur/feathers to stand on end, trapping a layer of insulating air and therefore reducing heat loss to the environment.

Question 35

how do endotherms decrease their body temperature?

Answer 35

The heat loss centre in hypothalamus sends electrical impulses via motor neurones to sweat glands in the skin, which act as effector organs in this response:
o The sweat glands are stimulated to secrete more sweat (water containing salt ions and urea) onto the skin surface;
o As the sweat evaporates, using heat energy from the skin surface, this cools the skin and nearby blood and so decreases core body temperature;
o The evaporation of sweat is such an effective cooling mechanism due to water having a high latent heat of vaporisation.

Meanwhile, the hypothalamus also sends electrical impulses via motor neurones to longitudinal smooth muscle in the wall of the arterioles that supply the skin capillaries with blood:
o The longitudinal smooth muscle contracts, dilating (widening) the arteriole lumen, such that more blood now flows through the skin capillaries;
o This peripheral vasodilation results in more blood flowing near the skin surface
and hence more heat being lost from the blood to the environment by radiation;
o More heat is lost from the body and so there is a decrease in core body temperature.

Endotherms, despite the above effective physiological mechanisms, do still use behavioural responses to contribute to thermoregulation too. For example, in order to increase body temperature an endotherm may bask in the sun; to decrease body temperature, it may wallow in cool mud or water, or take refuge in a burrow during the hottest part of the day.

Humans have additional behaviours. For example, if we feel too hot we will open a window, take off a jumper, have a cold drink etc; but if we feel too cold we will turn on the heating, put on extra layers of clothing, have hot drinks and food etc.