5.1- Communication and Homeostasis

Question 1

What does cell metabolism rely on

Answer 1

• enzymes
• These need a specific set of conditions in which to work efficiently

Question 2

Describe the conditions which living things need to maintain inside their cells, what will happen if this is not achieved

Answer 2

• A suitable temperature
• a suitable pH
• an aqueous environment that keeps the substrates and products in solution
• freedom from toxins and excess inhibitors
  If not achieved, cells will become inactive and die. Cells are specialised to rely upon each other- must be able to communicate in order to coordinate their activities

Question 3

Describe how changing external environments can affect living organisms

Answer 3

• all living organisms have an external environment that consists of the air, water or soil around them
• this external environment changes, which may place stress on the living organism
• e.g. cooler environment will cause greater heat loss
• if the organism is to remain active and survive, the changes in the environment must be monitored and the organism must change its behaviour or physiology to reduce the stress
• the environmental change is a stimulus and the way in which the organism changes its behaviour/physiology is a response
• may be gradual change which elicits a gradual response (e.g. seasons), or fatser change (e.g. appearance of predator or moving from a burrow to sunlight) stimulus must be monitored and the organism must respond to the change

Question 4

Describe an example of an animal response to a change in environment

Answer 4

• Environment may change slowly as seasons pass
• these changes elicit a gradual response
• e.g. the arctic fox (Alopex Lagopus) has a much thicker white coat in the winter and a thinner grey/brown coat in the summer
• the change in the coat provides greater insulation and camouflage in winter, ensuring the animal can survive
• yet, in the summer, the animal does not overheat

Question 5

Describe internal environments of organisms

Answer 5

• most multicellular organisms have a range of tissues and organs
• many of the cells and tissues aren’t exposed to the external environment- they are protected by epithelial tissues and organs e.g. skin and bark
• in many animals the internal cells and tissues are bathed in tissue fluid- this is the environment of the cells

Question 6

Describe changing internal environments of organisms

Answer 6

• all cells undergo various metabolic activities- they use up substrates and create new products
• some of these compounds may be unwanted or toxic
• these substances move out of the cells into the tissue fluid
• therefore, the activities of the cells alter their own environment

Question 7

Describe an example of changing internal environments in organisms

Answer 7

• carbon dioxide- waste product
• If this is allowed t build up in the tissue fluid outside the cells, it will alter the pH of the tissue fluid and could disrupt the action of enzymes and other proteins
• the accumulation of excess waste or toxins in this internal environment must act as a stimulus to cause the removal of these waste products so the cells can survive
• e.g. the reduced pH of blood stimulates greater breathing activity that expels the carbon dioxide from the body
• this build-up of waste products in the tissue fluid may also act directly on the cells, which respond by reducing their activities so that less waste is produced
• however, this response may not be good for the whole organism

Question 8

Describe how living organisms maintain their internal environment

Answer 8

• the composition of the tissue fluid is maintained by the blood
• blood flows throughout the body and transports substances to and from the cells
• any wastes/toxins accumulating in the tissue fluid are likely to enter the blood and be carried away
• in order to prevent their accumulation in the blood they must be removed from the body by excretion
• important that the concentrations of waste products and other substances in the blood are monitored closely
• this ensures that the body doesn’t excrete too much of any useful substance but removed enough of the waste products to maintain health
• it also ensured that the cells in the body are supplied with the substrates they need

Question 9

Describe the workings of multicellular organisms

Answer 9

• multicellular organism more efficient than single celled as its cells are differentiated
• mean that its cells are specialized to perform particular functions
• groups of specialized cells form tissues and organs
• the cells that monitor the blood may be in a different part of the body away from the source of the waste product, may also be some distance from the tissue/organ specialised to remove the waste from the body
• therefore, a good communication system is required to ensure these different parts of the body work together effectively

Question 10

Describe what a good communication system will do

Answer 10

• cover the whole body
• enable cells to communicate with each other
• enable specific communication
• enable rapid communication
• enable both short-term and long-term responses

Question 11

How do cells communicate with each other

Answer 11

Cell-signalling

Question 12

Describe cell signalling

Answer 12

• a process in which one cell will release a chemical that is detected by another cell
• the second cell will respond to the signal released by the first cell
• involved molecules that have a specific shape which is complimentary to that of the cell surface receptor- essential to enable signals to be specific

Question 13

Name and describe the 2 main systems of communication that work by cell signalling

Answer 13

• Neuronal system- an interconnected network of neurones that signal to eachotehr across synapse junctions - the neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing quickly
• Hormonal system- a system that uses the blood to transport its signals- cells in an endocrine organ release the signal (a hormone) directly into the blood- the hormone is transported throughout the body, but is only recognised by specific target cells- the hormonal system enables longer-term responses to be coordinated

Question 14

Outline homeostasis

Answer 14

Maintaining a constant internal environment despite changes in external and internal factors

Question 15

List aspects maintained by homeostasis

Answer 15

• Body temperature
• blood glucose concentration
• blood salt concentration
• water potential of the blood
• blood pressure
• carbon dioxide concentration

Question 16

Describe responses to changes in environment as part of homeostasis

Answer 16

• any response requires a complex mechanism, which may involve a series of tissues and organs that are coordinated through cell signalling

Standard response pathway:
Stimulus- receptor- communication pathway [cell signalling]- effector- response

Question 17

Describe the structures needed for pathways of homeostasis

Answer 17

Sesnory receptors
- may be on surface of body- e.g. temperature rece[tors in skin- monitor changes in external environment
- may be internal to monitor conditions inside the body- e.g. temperature receptors in brain
- when one of these receptors detects a change it will be stimulated to send a message to an effector

Communication system:
- e.g. neuronal or hormonal system
- acts by signalling between cells
- used to transmit a message from the receptor cells to the effector cells via a coordination centre which is usually in the brain
- input- the messages from the receptor to the coordination centre
- output- the messages sent to the effectors

Effector:
- E.g. liver or muscle cells
- bring about a response

Question 18

Describe feedback on homeostasis, name 2 types

Answer 18

• when the affecters respond to the output from the coordination centre, they bring about a response that will change the conditions inside the body
• such changes will be detected by the receptors
• this will have an effect on the response pathway- in effect, the input will change
• this effect is feedback
• Positive and Negative

Question 19

Describe negative feedback

Answer 19

• in order to maintain constant internal environment, any change away from optimum conditions must be reversed- negative feedback brings the conditions back towards the optimum
• when conditions change, the receptors detect this stimulus and sent an input to the coordination centre
• the coordination centre sends an output to the effectors and the effectors respond to this output
• when the effectors bring about a change that reverses the initial change in conditions, the system moves closer to the optimum and the stimulus is reduced
• the receptors detect the reduction in stimulus and reduce the input to the coordination centre
• the output from the coordination centre to the effectors is also reduced, so the effectors reduce their activity
• as the system gets closer to the optimum, the response is reduced
• number of processes must occur- a change to internal environment must be detected, the change must be signalled to other cells, there must be an effective response that reverses the change in conditions
• a negative feedback system can maintain a reasonably constant set of conditions, however they will never remain perfectly constant- some variation around optimum condition- may take time to respond to stimulus and made slightly overshoots however conditions will remain acceptable as long as variation is not too great

Question 20

Describe an example of negative feedback

Answer 20

• if the internal temperature rises too high, the response is to do something that brings the body back towards its optimum
• as a result the stimulus is reduced
• when negative feedback is applied to living systems, the temperature conditions will remain within a relatively narrow range- will remain warm enough to allow enzymes to continue functioning efficiently, but cool enough to avoid damage to the body’s many other proteins
• always some fluctuation around this set point

Question 21

Describe positive feedback

Answer 21

• less common than negative feedback
• when positive feedback occurs, the response is to increase the original change- destabilises the system and is usually harmful
• e.g. below a certain core body temperature enzymes become less active and the exergonic reactions that release heat are slower and release less heat- allows the body to cool further and slows the enzyme controlled reactions even more- causes body temperature to spiral downwards
• may be beneficial- e.g. As the cervix begins to stretch this causes the posterior pituitary gland to secrete the hormone oxytocin- oxytocin increases the uterine contractions which stretches the cervix more, which causes secretion of more oxytocin- once the cervix is fully dilated, the baby can be born- the birth ends the production of oxytocin,
• activity of neurones relies on positive feedback

Question 22

Describe the importance of controlling body temperature

Answer 22

• as temperature rises, molecules have more kinetic energy- move about more quickly and collide more frequently- essential chemical reactions occur more quickly
• in cooler conditions the opposite is true- chemical reactions slow down
• structure of proteins can also be affected by changes- increases- many proteins have metabolic function e.g. enzymes- specific tertiary structure- shape of active site complementary to shape of substrate- any change in shape will affect ability to function normally- if temperature allowed to increase too much, enzymes can denature and function is lost
• some enzymes very sensitive to temperature change- if body temperature drops by 10oC, the rate of enzyme controlled reactions falls by half- many reactions in cells release heat which can help maintain body temperature, but if temperature drops and reactions slow down, less heat released- cool further- positive feedback- as body cools, the Organism is less and less able to function normally, but if above optimum temp., enzymes denature
• the core temperature is the important factor, as all the vital organs are found in the centre of the body
• peripheral parts of the body may be allowed to increase or decrease in temperature to some extent without affecting the survival of the Organism

Question 23

Name two categories of organisms in terms of controlling body temperature

Answer 23

• Endotherms
• ectotherms

Question 24

Describe endotherms

Answer 24

• control their body temperature within very strict limits
• use a variety of mechanisms to control body temperature
• largely independent of external temperatures

Question 25

Describe ectotherms

Answer 25

• not able to control their body temperature as effectively as endotherms
• rely on external sources of heat and their body temperature fluctuates with the external temperature
• however, using various behavioural mechanisms, some ectotherms are able to control their body temperature in all but the most extreme conditions

Question 26

Describe temperature control in ectotherms

Answer 26

• do not use internal energy sources to maintain their body temperature went cold
• however, once they are active their muscle contractions will generate to some heat from increased respiration
• temperature regulation relies upon behavioural responses that can alter the amount of heat exchanged with the environment

Question 27

Describe responses in ectotherms if they are not warm enough

Answer 27

Try to absorb more heat from the environment- may:
- move into sunny area
- lie on warm surface
- expose a larger surface area to the sun

Question 28

Describe responses in endotherms if they are too hot

Answer 28

Try to avoid gaining more heat and try to increase heat loss to the environment- may:
- move out of the sun
- move underground
- reduce the body surface exposed to the sun

Question 29

Name four examples of ectotherms

Answer 29

Snakes, locusts, lizards, horned lizards

Question 30

Snakes- behavioural adaptation and benefit

Answer 30

Adaptation:
- basks in the sun
- in the UK, Adders can often be found lying on an exposed path beside vegetation

Benefit:
- absorbs heat directly from the sun

Question 31

Locust- behavioural adaptation and benefit (x2)

Answer 31

Adaptation:
- in the early morning, locusts sit side-on to the sun, exposing a large surface area
- at midday they face the sun head-on, exposing a smaller surface area
- they may also climb on top of a plant at midday to get away from the soil surface

Benefit:
- in the cool morning they can absorb more heat
- at midday when the sun is hotter they absorb less heat
- the soil surface gets hott and radiates heat- if moves away from soil, gains less heat from it

Adaption (2):
- increases both the rate of breathing and the depth of breathing movements when it is hot

Benefit (2):
- more water evaporates from the tracheal system, cooling the body

Question 32

Lizard- behavioural adaptation and benefit

Answer 32

Adaptation:
- many lizards use Burrows or crevices between rocks
- they will hide in the burrow during the hottest part of the day and the coolest part of the note

Benefit:
- an underground burrow tends to have a more stable temperature than the air
- in the hottest part of the day it will be cooler in the borough, but at night the borough may be warmer than the air outside

Question 33

Horned lizard- behavioural adaptation and benefit

Answer 33

Adaptation:
- can change its shape by expanding or contracting its rib cage

Benefit:
- expanding the rib cage increases the surface area exposed to the sun, so more heat can be absorbed

Question 34

Describe advantages of ectothermy

Answer 34

Rely on external sources of heat to keep warm- do not use energy to keep warm- therefore:
- less of their food is used in respiration
- more of the energy and nutrients gained from foods can be converted to growth
- they need to find less food
- they can survive for long periods without food

Question 35

Describe disadvantages of ectothermy

Answer 35

• less active in cooler temperatures
• means they are at risk from predators while they are cold and unable to escape
• they cannot take advantage of foods that are available while they are cold

Question 36

Describe temperature controll in endotherms

Answer 36

• do not rely on external sources of heat
• can use physiological adaptations and behavioural means to control their body temperature

Question 37

Describe temperature regulation mechanisms in endotherms

Answer 37

• relies on effectors in the skin and muscles
• the skin is the organ in contact with the external environment- therefore many of the physiological adaptations to control body temperature involve the skin- the changes that take place in the skin alter the amount of heat being lost to the environment
• many chemical reactions in the body are exergonic- release energy in the form of heat- endotherms can increase respiration (exergonic reaction) in the muscles and liver simply to release heat- using some of their energy intake to stay warm
• also have other physiological mechanisms such as directing blood towards or away from the skin to alter the amount of heat lost to the environment

Question 38

List body parts involved in temperature control (physiological) in endotherms

Answer 38

• skin
• gaseous exchange system
• skeletal muscles
• blood vessels

Question 39

Skin- response if body too hot

Answer 39

• sweat glands secrete fluids onto the skin surface- as this evaporates, it uses heat from the blood as the latent heat of vaporisation
• hairs and feathers lie flat to reduce insulation and allow greater heat loss
• vasodilation of arterioles and precapillary sphincters directs blood to the skin surface so more heat can be radiated away from the body

Question 40

Skin- response if body too cold

Answer 40

• less sweat is secreted, so less evaporation means less heat is lost
• hairs and feathers stand erect to trap air, which insulates the body
• vasoconstriction of arterioles and precapillary sphincters leading to skin surface- blood is diverted away from the surface of the skin and less heat is lost

Question 41

Gaseous exchange system- response if body too hot

Answer 41

• some animals pant, increasing evaporation of water from the surface of the lungs and airways
• evaporation uses heat from the blood as the latent heat of vaporisation

Question 42

Gaseous exchange system- response if body too cold

Answer 42

• less panting, so less heat is lost

Question 43

Liver- response if body too hot

Answer 43

• less respiration takes place, so less heat is released

Question 44

Liver- response if body too cold

Answer 44

• increased respiration in the liver cells means that more energy from food is converted to heat

Question 45

Skeletal muscles- response if body too hot

Answer 45

• fewer contractions mean that less heat is released

Question 46

Skeletal muscles- response if body too cold

Answer 46

• spontaneous muscle contractions (shivering) release heat

Question 47

Blood vessels- response if body too hot

Answer 47

• dilation to direct blood to the extremities so that more heat can be lost

Question 48

Blood vessels- response if body too cold

Answer 48

• constriction to limit blood flow to the extremities, so that blood isn’t cooled too much- this can lead to frostbite in extreme conditions

Question 49

Describe behavioural adaptations used by endotherms if body is too hot

Answer 49

• hide away from sun in the shade/in a burrow
• orientate body to reduce surface area exposed to sun
• remain inactive and spread limbs out to enable greater heat loss
• wet skin to use evaporation to help cool the body- cats lick themselves, elephants spray water over their bodies

Question 50

Describe behavioural adaptations used by endotherms if body is too cold

Answer 50

• lie in the sun
• orientate body towards sun to increase surface area exposed
• move about to generate heat in the muscles, or in extreme conditions roll into a ball shape to reduce surface area and heat loss
• remain dry

Question 51

Describe advantages of endothermy

Answer 51

Endotherms can:
- maintain a fairly constant body temperature whatever the temperature externally
- reman active even when external temperatures are low, which means they can take advantage of prey that may be available or escape from potential predators
- inhabit colder parts of the planet

Question 52

Describe disadvantages of endothermy

Answer 52

Endotherms:
- use a significant part of their energy intake to maintain a body temperature in the cold
- need more food
- use a lower proportion of the energy and nutrients gained from food for growth
- may overheat in hot weather

Question 53

Describe the control of temperature regulation in endotherms

Answer 53

• the maintenance of a core body temperature is important- if the core temperature changes this alters the temperature of the blood
• temperature receptors in the hypothalamus of the brain detect this change
• the hypothalamus then sends out impulses to cause different responses that will reverse the change
• some responses need to be quick in order to prevent further change in the body temperature- the neuronal system transmits the output from the hypothalamus in order to make these responses rapid
• other responses may need to be longer-term- the hormonal system transmits the output to cause these responses
• if the core temperature is too low com at the hypothalamus will bring about changes in the skin to reduce heat loss, release of heat through extra muscular contraction, increased metabolism in order to release more heat from exergonic reactions
• if the core temperature rises above the optimum, the hypothalamus will bring about the opposite changes- example of negative feedback

Question 54

Describe the role of peripheral temperature receptors in endotherms

Answer 54

• the thermoregulatory centre in the hypothalamus monitors blood temperature and detects changes in the core body temperature
• however, an early warning that the body temperature may change could help the hypothalamus to respond more quickly and reduce variation in the core body temperature
• if the extremities start to cool down or warm up this may eventually affect core body temperature
• peripheral temperature receptors in the skin monitor the temperature in the extremities- this information is fed to the thermoregulatory centre in the hypothalamus
• if the thermoregulatory centre signals to the brain that the external environment is very cold or very hot, the brain can initiate behavioural mechanisms for maintaining the body temperature, such as moving into the shade