6.1 Stimuli, both internal and external, are detected and lead to a respons

Question 1

What is a stimulus

Answer 1

A change in an organism’s internal or external environment

Question 2

Why is it important that organisms can respond to stimuli?

Answer 2

Organisms increase their chance 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 - hormone-like growth substances eg. Auxins (such as IAA) move via phloem or diffusion from growing regions eg. shoot / root tips where they’re produced
● To other tissues where they regulate growth in response to directional stimuli (tropisms)

Question 5

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

Answer 5

● In shoots, high concentrations of IAA stimulates cell elongation
● In roots, high concentrations of IAA inhibits cell elongation

Question 6

Explain gravitropism in flowering plants

Answer 6

Cells in tip of shoot / root produce IAA
IAA diffuses down shoot / root (evenly initially)
IAA moves to lower side of shoot / root (so concentration increases)
In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation
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 (evenly initially)
3. IAA moves to shaded side of shoot / root (so conc. ↑)
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 (tactic response)
   ○ Directional response
   ○ Movement towards or away from a directional stimulus
2. Kinesis (kinetic responses)
   ○ Non-directional response
   ○ Speed of movement or rate of direction change
   changes in response to a non-directional stimulus
   ○ Depending on intensity of stimulus

Question 9

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

Answer 9

● Rapid as only 3 neurones and few synapses
● Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt
● Protects from harmful stimuli eg. escape predators / prevents damage to body tissues