TOPIC 6 - Internal and external stimuli are detected and lead to a response Flashcards
what is a stimuli?
a detectable change in the environment
what is response?
Action based on stimulus to restore conditions in the environment.
what are the plant growth factors?
IAA - chemicals, hormone like substances that allows plants to respond to light, water and gravity
produced in apical meristem
moves short distances by diffusion and active transport
moves long distances in the phloem mass transport
what is tropism?
the response of a plant to a directional stimulus
explain why shoots show positive phototropism?
- IAA diffuses to the shaded side of the shoot
- this causes active transport of H+ ions into the cell wall
- disruption to h- bonds on cellulose = increases permeability to water
-The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation - cell absorbs water + stores in vacuole which increases pressure = causing cell wall to stretch.
-This causes the shoot to bend towards the light
explain why roots show positive gravitropism?
- gravity causes IAA to accumulate on the lower side of the root
- IAA inhibits cell elongation in root cells
-the side with the lower concentration will grow faster, therefore the root will bend towards gravity.
define taxis?
directional movement in response to external stimulus
towards light = positive photo taxis
away from chemicals = negative chemotaxis
define kinesis?
non-directional response to presence and intensity of external stimulus
maintains mobile organisms in optimal environement
Many organisms respond to temperature and humidity by kinesis rather than taxes? Why?
- it is a less directional stimuli
- no clear gradient from one extreme to another
How can a student recognise kinesis in an organisms movement?
- organism crosses sharp division between favourable and unfavourable environment: turning increases so they can return to the original favourable environment
- turning decreases if organism moves towards unfavourable environment ( long straight lines sharper turns) = this leads the organism into 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 dangerous stimuli = helps avoid damage - doesn’t need to be learned = protects infants
what features are common to all sensory receptors ?
- act as energy transducers which establish a generator potential
- respond to specific stimuli
describe the basic structure of the Pacinian corpuscle?
- found at the ends of sensory neuron axons
- made of many layers of membrane which are separated by a gel
- The gel between the layers contains positively charged sodium ions (Na+)
-The section of axon surrounded by layers of membrane contains stretch-mediated sodium ion channels which open when sufficient pressure is applied
what stimulus does a pacinian corpuscle respond to?
pressure = The section of axon surrounded by layers of membrane contains stretch-mediated sodium ion channels which open
the influx of Na+ raises membrane to threshold potential, a generator potential is produced
action potentials move across the sensory neuron
