CH14 Response to stimuli Flashcards
What are plant growth factors - where are they produced
Chemicals that regulate plant growth response to directional stimuli
Produced in plant growing regions
Diffuse from cell to cell - phloem mass transport
Explain why shoots show positive phototropism
- Cells in the tip of the shoot produce IAA, which is transported down the shoot
- The IAA is initially transported evenly throughout all regions as it begins to move down the shoot
- Light causes the movement of IAA from the light side to the shaded side of the shoot
- A greater concentration of IAA builds up on the shaded side of the shoot than on the light side
- As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells elongate more
- The shaded side of the shoot elongates faster than the right side, causing the shoot tip to bend towards the light
Explain why roots show positive gravitropism
- Cells in the tip of the root produce IAA, which is then transported along the root
- The IAA is initially transported to all sides of the root
- Gravity influences the movement of IAA from the upper side to the lower side of the root
- A greater concentration of IAA builds up on the lower side of the root than on the upper side
- As IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side
- The relatively greater elongation of cells on the upper side compared to the lower side compared to the lower side causes the root to bend downwards towards the force of gravity
Define taxis
Directional movement in response to external stimulus
Define kinesis
Non-directional response to presence and intensity of external stimulus
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 to recognise kinesis
Organsm crosses sharp division between favourable and unfavourable - turning increases
Organism moves considerable distance in unfavourable - turning decreases, long straight lines
Outline simple reflex arc
Receptor –> sensory neurone –> relay neurone in CNS –> motor neurone –> response by effector
Advantages of simple reflex
Rapid response to dangerous stimuli
Instinctive
What features are common to all sensory neurones
Act as energy transducers which establish a generator potential
Respond to specific stimuli
Describe basic structure of Pacinian corpuscle
Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule
Stretch-mediated Na+ channels on plasma membrane
Capillary runs along base layer of tissue
What stimulus does a Pacinian corpuscle respond to - how
Pressure deforms membrane, causing stretch-mediated Na+ ion channels to open
Influx of Na+ raises membrane to threshold potential, a generator potential is produced
Action potential moves along sensory neurone
2 photoreceptors in retina
Cone cells
Rod cells
Where are rod cells located in the retina
Evenly distributed around periphery but not in central fovea
Where are cone cells located in the retina
Mainly central fovea, no photoreceptors at blind spot
