homeostasis and negative feedback

homeostasis

the maintenance of an organism’s internal environment within set limits

body temp, blood glucose, pH, water potential

what are internal conditions regulated by?

the endocrine and nervous systems, as well as behavioural patterns

negative feedback

where a change triggers a response which reduces the effect of a change

when the feedback causes the corrective measures to be turned off to return system to normal

deviation from normal set point → receptor detects the change → sensory neurone → controller co-ordinates the info → motor neurone (autonomic NS) → effector takes corrective measures → returns to normal set point → feedback loop (negative feedback)

positive feedback

• when the feedback causes the corrective measures to stay on

• large deviation from the norm

behavioural patterns

taxis and kinesis are behavioural mechanisms to maintain homeostasis, both involve movement of organisms in response to external stimuli

taxis: directional response to a stimulus

kinesis: non-directional movement in response to a stimulus

homeostasis is controlled by a reflex arc

stimulus (deviance from the optimum) → receptor (cells) → co-ordinator (CNS) → effectors (muscle/gland) → response (returning the condition back to optimum)

nervous vs endocrine

the nervous system and endocrine system are both involved in controlling the internal conditions of a mammal

NS duration: short lasting

endocrine speed: relatively slow, travelling in the blood stream

endocrine duration: long lasting → makes them ideal for controlling conditions in a more gradual manner to avoid over-compensating

why is homeostasis essential?

enzymes are sensitive to pH and temp → reactions should take place at a suitable rate

water potential → blood and tissue fluid may cause cells to shrink or burst due to osmosis → maintaining blood glucose concentration ensures a constant water potential (and provides glucose for cellular respiration)

list 2 general heat gain methods

• heat produced by respiration

• heat gained from environment

list 2 general heat loss methods

• heat lost by evaporation (sweating)

• heat lost to environment (conduction/convection/radiation)

what methods do ectotherms use to regulate their temperature?

- exposing themselves to sun
- taking shelter
- gaining warmth from ground
- generating metabolic heat
- colour variations

what methods do endotherms use to regulate their temperature?

heat gain:
- shivering
- vasoconstriction of superficial arterioles
- hair erector muscles contract (raise hair)
- increased metabolic rate

heat loss:
- sweating
- vasodilation of superficial arterioles hair erector
- hair erector muscles relax (lower hair)
- decreased metabolic rate

hair erector muscles

- raising of body hairs provides insulation and prevents heat loss
- lower reduces insulation

thermoregulation (an example of a negative feedback loop)

mammals are able to maintain their internal environment and therefore are successful in all conditions

thermoregulation in animals
- ectotherms: obtain most of their heat from environment, e.g. reptiles and amphibians
- endotherms: obtain most of their heat from metabolic activities inside their bodies, e.g. birds and mammals

body temp too high:
- detected by thermoreceptors in the hypothalamus and skin
- autonomic NS
- heat loss centre
- physiological mechanisms: sweating, vasodilation or superficial arterioles, hair erector muscles relax (lower hair), decreased metabolic rate

body temp too low:
- detected by thermoreceptors in the hypothalamus and skin
- heat gain centre
- physiological mechanisms: shivering, vasoconstriction of superficial arterioles, hair erector muscles contract (raise hair), increased metabolic rate

explain how normal core body temperature is maintained when a person moves into a cold room

1. sensors in skin and hypothalamus (thermoreceptors) detect reduced temperature

2. heat gain centre activated

3. vasoconstriction - constriction of arterioles in skin surface

4. dilation of shunt vessels, less blood to skin surface

5. increased heat gain by increased metabolic rate

6. decreased heat loss by reduced sweating

how does maintaining a constant body temperature allow metabolic reactions in cells to proceed with maximum efficiency?

1. 37 degrees Celsius is optimum temp for enzymes

2. excess heat denatures enzymes active site

3. substrate cannot bind

4. reactions slowed

5. too little reduced kinetic energy of molecules

6. fewer collisions so fewer e-s complexes formed