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 Auxins (like IAA) move from growing regions (e.g., shoot/root tips) 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 stimulate cell elongation; in roots, high concentrations of IAA inhibit 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; 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 concentration increases)
4. In shoots, this stimulates cell elongation; 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 (e.g., escape predators/prevents damage to body tissues)