6.1 Survival and response

Question 1

What is a stimulus?

Answer 1

• A change in an organism’s internal or external environment
• Organisms respond to stimuli to increase chance of survival

Question 2

What is a tropism?

Answer 2

• Growth of a plant in response to a directional stimulus
• Positive tropism is towards a stimulus, negative is away

Question 3

How does phototropism in flowering plants occur?

Answer 3

• Cells in shoot/root tip produce IAA
• IAA diffuses down shoot/root
• IAA moves to shaded side of shoot/root
• Stimulates cell elongation in shoots / inhibits cell elongation in roots
• So shoots bend towards light / roots bend away from light

Question 4

How does gravitropism in flowering plants occur?

Answer 4

• Cells in shoot/root tip produce IAA
• IAA diffuses down shoot/root
• IAA moves to lower side of shoot/root
• Stimulates cell elongation in shoots / inhibits cell elongation in roots
• So shoots bend away from gravity / roots bend towards gravity

Question 5

Taxes

Answer 5

Directional response
Organism moves towards / away from a directional stimulus

Question 6

Kinesis

Answer 6

Non-directional response
Organism changes speed of movement / rate of direction change in response to a non-directional stimulus

Question 7

What are the advantages of a simple reflex?

Answer 7

• Rapid as only 3 neurones
• Autonomic so doesn’t have to be learnt
• Protects from harmful stimuli / prevents damage to body tissues