Internal and External Stimuli Detected and Response Flashcards
what are plant growth factors and where are they produced
- chemicals that regulate plant growth response to directional stimuli
- produced in plant growing regions (apical meristems)
- diffuse from cell to cell/phloem mass transport
explain why shoots show positive phototropism
- Indoleacetic acid (IAA) diffuses to shaded side of shoot tip
- As IAA diffuses down shaded side, it causes active transport of H+ ions into cell wall
- Disruption to H-bonds between cellulose molecules and action of expansins make cell more permeable to water (acid growth hypothesis)
- cells on shaded side elongate faster due to higher turgor pressure
- shoot bends towards light
explain why roots show positive gravitropism
- gravity causes IAA to accumulate on lower side of the root
- IAA inhibits elongation of root cells
- cells on the upper side of the root elongate faster, so the root tip bends downwards
contrast mammalian hormones and plant growth factors on concentration
mammalian: response not always dependent on concentration
plant growth factor: response proportional to concentration
contrast mammalian hormones and plant growth factors on action
mammalian: bind to complementary proteins in/on target cells
plant growth factor: can affect all cells
contrast mammalian hormones and plant growth factors on synthesis
mammalian: specialised glands
plant growth factor: various tissues in growing regions
contrast mammalian hormones and plant growth factors on transport
mammalian: circulatory system
plant growth factor: diffusion or phloem translocation
contrast mammalian hormones and plant growth factors on speed
mammalian: faster-acting (homeostasis)
plant growth hormones: slower-acting (plant growth)
define taxis
directional movement in response to external stimuli
define kinesis
non-directional response to presence and intensity of external stimuli
advantage of taxis and kinesis
maintain mobile organism in optimum environment e.g. to prevent dessication
many organisms respond to temperature and humidity via kinesis rather than taxis. why?
less directional stimuli; often no clear gradient from one extreme to the other
how could a student recognise kinesis in an organisms movement
- organism crosses sharp division between favourable and unfavourable environment: turning increases (return to the original favourable environment)
- if organism moves considerable distance into unfavourable environment: turning slowly decreases; begins to move in long, straight lines; sharper turns (lead organisms to new environment)
outline what happens in a simple reflex arc
receptor detects stimulus
sensory neuron
relay neuron in CNS coordinates response
motor neuron
response by effector
give advantages of a simple reflex
- rapid response to potentially dangerous stimuli since only 3 neurons involved
- instinctive