Responses to Changes in Environment

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

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

Answer 4

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

Question 5

Explain gravitropism in flowering plants

Answer 5

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 6

Explain phototropism 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 shaded 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 towards light
   whereas roots bend away from light

Question 7

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

Answer 7

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 8

Why are reflexes important?

Answer 8

They are very fast as there are only 3 neurones and therefore few synapses (synaptic transmission is slow). They are also Autonomic (doesn’t involve conscious regions of brain) so doesn’t have to be learnt. They protect us from harmful stimuli, prevents damage to body.