survival and response

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) (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

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. 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 (synaptic transmission is slow)
● 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