3.6.1 Stimuli , both internal and external , are detected snd lead to a response ( 3.6 Organisms respond to chanhed in their internal and external environments)

Question 1

what is a stimulus

Answer 1

a change in an organisms internal or external environment

Question 2

why is it important that organisms can respond to stimuli

Answer 2

organisms increase their chances of survival by responding to stimuli

Question 3

what is a tropism

Answer 3

growth of a plant in response to a directional stimulus
positive tropism = towards a stimulus , negative tropism = away from stimulus

Question 4

summarise the role of growth factors in flowering plants

Answer 4

• specific growth factors move from growing regions where they’re produced
• to other tissues where they regulate growth in response to directional stimuli

Question 5

describe how indoleacetic acid ( IAA) affects cells in roots and shoots

Answer 5

• in shoots , high conc of IAA stimulates cell elongation
• in roots , high conc of IAA inhibits cell elongation

Question 6

explain gravitropism in flowering plants

Answer 6

1) cells in tip of shoot / root produce IAA
2) IAA diffuses down shoot / root evenly
3) IAA moves to lower side of shoot / root
4) in shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
5) so shoots bend away from gravity whereas roots bend towards gravity

Question 7

explain phototropism in flowering plants

Answer 7

1) cells in tip of shoot / root produce IAA
2) IAA diffuses down shoot / root
3) IAA moves to shaded side of root / shoot
4) in shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
5) so shoots bend towards light whereas roots bend away from light

Question 8

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

Answer 8

1) taxes
- directional response
- movement towards or away from a directional stimulus
2) kinesis
- non directional response
- speed of movement or rate of direction change changed in response to a non directional stimulus
- depending on intensity of stimulus

Question 9

explain the protective effect of a simple reflex

Answer 9

• rapid as only 3 neurons and a few synapses
• autonomic so doesn’t have to be learnt
• protects from harmful stimuli for example escape predators / prevents damage to body tissues