5.1.1 Communication and Homeostasis

Question 1

What set of conditions do living things need to maintain in their cells?

Answer 1

• a suitable temperature
• a suitable pH
• an aqueous environment that keeps substrates and products in solution
• freedom from toxins and excess inhibitors

Question 2

What are the threats from different changing environments?

Answer 2

Changing external environments:

• external environment changes may place stress on living organism
• environmental change is stimulus and way organism changes is its response

Changing internal environments:

• as cells undergo various metabolic acitivities, they use up substrates and create new products, which may be unwanted or toxic e.g. CO₂
• therefore, the activities of the cells alter their own environment

Maintaining internal environment:

• blood flows throughout body and transports substances to and froom cells
• concentrations of waste products are monitored closely to ensure body does not excrete too much useful substance, but also removes waste products to maintain good health

Coordinating activities of different organs

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

Question 3

What is cell signalling?

Answer 3

• the process by which cells communicate with each other

Question 4

What are the two major systems of communication that work by cell signalling?

Answer 4

Neuronal system:

• an interconnected network of neurones that signal to each other across synpase junctioins
• 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 only recognised by specific target cells
• this system enables longer-term responses to be coordinated

Question 5

Definition of homeostasis

Answer 5

• maintaining a constant internal environment despite changes in external and internal factors

Question 6

Give examples of aspects that homeostasis may maintain

Answer 6

• body temperature
• blood glucose concentration
• blood salt concentration
• water potential of blood
• blood pressure
• CO₂ concentration

Question 7

What is the standard response pathway for the mechanism of homeostasis?

Answer 7

stimulus → receptor → communication pathway (cell signalling) → effector → response

Question 8

What type of specialised structures are required for the standard response pathway to work?

Answer 8

• sensory receptors: e.g. temperature receptors
• these may be on the surface of the body which monitors changes in the external environment
• they may also be receptors that measure conditions internally
• when they detect a change, it will be stimulated to send a message to an effector
• a communication system such as a neuronal system or hormonal system
• this acts by signalling between cells
• transmits a message from receptors cells to the effector cells via a coordination centre
• effector cells: bring about a response

Question 9

What is negative feedback and how does it work?

Answer 9

• the mechanism that reverses a change, bringing the system back to the optimum
• when conditions change, receptors detect this stimulus and send an input to the coordination centre
• coordination centre sends an output to the effectors and effectors responds to it
• the change moves the system closer to optimum and stimulus is reduced
• so, the receptors detect the reduction in stimulus and reduce input to coordination centre
• output from coordination centre to effectors also reduced, so less effector activity

Question 10

Give an example of a negative feedback loop using temperature

Answer 10

Question 11

Why can conditions never remain perfectly constant?

Answer 11

• there will be some variation around the optimum condition
• response may slightly ‘overshoot’ but remain acceptable

Question 12

What is positive feedback and how does it work?

Answer 12

• the mechanism that increases a change, taking the system further away from the optimum
• tend to be harmful as it destabilises the system

Question 13

Give a positive and negative example of positive feedback

Answer 13

Negative:

• e.g. below certain core body temperature enzymes become less active and the exergonic reactions that release heat are slower and release less heat
• this allows body to cool further, slowing enzyme-controlled reactions even more, so body temperature spirals downwards

Positive:

• at end of pregnancy, as the cervix begins to stretch, the posterior pituitary gland secretes the hormone oxytocin
• oxytocin increases the uterine contractions which stretches cervix more, which causes secretion of more oxytocin
• this allows cervix to be fully dilated

Question 14

Why is controlling body temperature important?

Answer 14

• effect upon the activity of cell processes: hen temperature rises, molecules have more kinetic energy, so more frequent collisions mean chemical reactions occur more quickly and vice versa
• may affect structures of proteins
• some enzymes are very sensitive to temperature change
• core temperature is important as all vital organs are found in the centre of body

Question 15

What is an ectotherm?

Answer 15

• an organism that relies on external sources of heat to maintain body temperature
• not able to control their body temperature as effectively as endotherms

Question 16

What are endotherms?

Answer 16

• an organism that uses heat from metabolic reactions to maintain body temperature

Question 17

How do ectotherms try to absorb heat when they are not warm enough?

Answer 17

• move into a sunny area
• lie on a warm surface
• expose a larger surface area to the sun

Question 18

How do ectotherms cool down and increase heat loss?

Answer 18

• move out of the sun
• move underground
• reduce body surface exposed to the sun

Question 19

Give some examples of behavioural adaptations of ectotherms

Answer 19

Snake:

• basks in the sun
• absorbs heat directly from sun

Locust:

• in early morning, locusts sit side on to the sun, exposing large surface area. in midday, they face head-on, exposing smaller surface area
• they can absorb more heat in the cool morning and less heat in midday
• move away from soil and climb to top of plant
• increases both rate of breathing and depth of breathing movements when it is hot
• more water evaporates from tracheal system, cooling body

Lizard:

• burrows or crevices between rocks to hide during hot times and cold times
• more stable temperature than the air

Horned lizard:

• can change its shape by expanding or contracting its ribcage
• surface area

Question 20

What are the advantages and disadvantages of ectothermy?

Answer 20

Advantages:

• they do not use up energy to keep warm
• less of their food is used in respiration
• more energy and nutrients gained from food can be converted to growth
• need to find less food
• can survive for long period without food

Disadvantages:

• less active in colder temperatures
• at risk from predators while they are cold and unable to escape
• cannot take advantage of food when cold

Question 21

What are some physiological responses from endotherms when body is too hot or cold

Answer 21

Skin:

• sweat glands secrete fluid onto skin surface, which then evaporates
• less sweat is secreted, so less evaporation means less heat is lost
• hair and feathers lie flat to reduce insulation and allow greater heat loss
• hair and feathers stand erect to trap air, insulating body
• vasodilation of arterioles and precapillary sphincters direct blood to the skin surface so more heat can be radiated away from the body
• vasoconstriction of arterioles and precapillary sphincters

Gaseous exchange system:

• some animals pant, increasing evaporation of water from surface of lungs and airways
• less panting

Liver:

• less respiration takes place, so less heat is released
• increases respiration in the liver cells, so more energy from food is converted to heat

Skeletal muscles:

• fewer contractions mean less heat is released
• spontaneous muscle contractions (shivering)

Blood vessels:

• dilation to direct blood to the extremities so more radiation
• constriction to limit blood flow to extremities

Question 22

What are some behavioural responses when body is too hot or cold?

Answer 22

• Hide away from sun in the shade or 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 body

Question 23

What are the advantages and disadvantages of endothermy?

Answer 23

Advantages:

• maintain a fairly constant body temperature whatever the external temperature
• remain active even when external temperature is low, so can escape or capture prey
• inhibit colder parts of the planet

disadvantages:

• use a significant part of energy to maintain body temperature in the cold
• need more food
• use for growth a lower proportion of energy
• may overheat in hot weather

Question 24

How is temperature control regulated in endotherms?

Answer 24

• temperature receptors in the hypothalamus of the brain detect change
• hypothalamus sends out impulses to cause different responses
• an early warning that the body temperature may change is given by the peripheral temperature receptors in the skin, monitoring the temperature of the extremities
• this is fed to the thermoregulatory centre in the hypothalamus and can initiate behavioural mechanisms