B1- response to internal and external stimuli ✅

what is a stimulus?

a change in an organisms internal or external environment

why is it important that organisms can respond to stimuli?

organisms increase their chance of survival by responding to stimuli

what is a tropism?

• growth of a plant in response to a directional stimulus

• positive tropism = towards a stimulus; negative tropism = away from stimulus

summarise the role of growth factors in flowering plants

• specific growth factors e.g. auxins (such as IAA) move via phloem/ diffusion from growing regions e.g. tips where they’re produced

• to other tissues where they regulate growth in response to directional stimuli (tropisms)

describe how IAA affects cells in roots and shoots

• in shoots, high concentrations of IAA stimulates cell elongation

• in roots, high concentrations of IAA inhibits cell elongation

explain gravitropism in roots of flowering plants

1. cells in tip of root produce IAA

2. IAA diffuses down root

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

4. high concentration of IAA inhibits cell elongation on lower side

5. cells on upper side elongate faster and so root bends towards gravity, downwards

• WORDING : refer to “upper and lower side”

explain gravitropism in shoots of flowering plants

1. cells in tip of root produce IAA

2. IAA diffuses down root

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

4. high concentration of IAA stimulates cell elongation on lower side

5. cells on lower side elongate faster and so shoot bends away from gravity, upwards

• WORDING : refer to “upper and lower side”

explain phototropism in shoots of flowering plants

1. cells in tip of shoot produce IAA

2. IAA diffuses down shoot

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

4. stimulates cell elongation on shaded side

5. so shoot bends towards light

• WORDING : refer to “light and shaded side”

explain phototropism in roots of flowering plants

1. cells in tip of root produce IAA

2. IAA diffuses down shoot

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

4. inhibits cell elongation on shaded side of root

5. so root bends away from light

• WORDING : refer to “light and shaded side”

describe the simple response that can maintain a mobile organism in a favourable environment

1. Taxes

• directional response

• movement towards or away from a directional stimulus

2. Kinesis

3. non directional response

• speed of movement and rate of directional change in response to a non direction stimulus

• depending on intensity of stimulus

explain the protective effect of a simple (eg. 3 neurone ) reflex

• rapid as only 3 neurones and few synapses

• autonomic ( doens’t involve conscious regions of the brain ) so doesn’t have to be learnt

• protects from harmful stimuli eg. escape predators / prevents damage to body tissues

give 3 examples of advantages of taxis for an animal

1. avoid competition

2. find a mate

3. avoid predators

why is the lid kept on the agar plate when measuring the growth of shoot tip on agar plate in IAA solutions of different concentrations

• prevent evaporation

• which alters IAA concentration

why are plant growth factors not classified as hormones

Not made in glands or transported in blood to target organs — instead made in growing tips and act locally by diffusion/active transport.