tropisms, taxes and kinesis

what is a stimulus

change in an organisms internal or external environment

what is a tropism

growth of a plant in response to a directional stimulus

positive tropism

towards a stimulus

negative tropism

away from stimulus

what is directional stimulus

an environmental change that has a specific direction, e.g. light, chemicals

role of growth factors in flowering plants

Auxins move (via phloem or diffusion) from growing regions where they are produced (shoot/root tips)

to other tissues where they regulate growth in response to directional stimuli

IAA

Indoleacetic acid

effect of IAA in shoots

stimulates cell elongation

effect of IAA in roots

inhibits cell elongation

what is gravitropism

directional growth response in which a plant grows toward or away from the force of gravity

what is phototropism

the directional growth or movement of a plant or other organism in response to a light source

explain gravitropism in flowering plants

1. cells in tip of shoot/root produce IAA

2. IAA diffuses down shoot/root (evenly initially)

3. IAA moves to lower side of shoot/root so concentration increases

4. in shoots this stimulates cell elongation whereas in roots this inhibits cell elongation

5. shoots bend away from gravity whereas roots bend towards gravity

explain phototropism in flowering plants

1. cells in tip of shoot/root produce IAA

2. IAA diffuses down shoot/root (evenly initially)

3. IAA moves to shaded side of shoot/root so concentration increases

4. stimulates cell elongation in shoots whereas inhibits cell elongation in roots

5. shoots bend towards light whereas roots bend away from light

what responses keep mobile organisms (not plants) in favourable environents

• taxes

• kinesis

what is taxes

directional response

movement towards or away from a directional stimulus

what is kinesis

non-directional response

speed or movement/ rate of direction change changes in response to a non-directional stimulus

depends on intensity of stimulus

example of taxis

woodlice moving away from light to avoid predators

example of kinesis

woodlice moving faster in drier environments to increase chance of moving into an area with higher humidity