Chapter 14 - Response To Stimuli

Question 1

What is a stimulus?

Answer 1

A detectable change in internal/ external environment of an organism that leads to a response

Question 2

What is taxes?

Answer 2

Movement of a motile organism in response to a stimulus (usually environmental change)

E.g. chemotaxis, phototaxis

Question 3

What is the difference between positive and negative taxes?

Answer 3

+ organism moves towards stimulus (favourable)

• organism moves away from stimulus (unfavourable)

= increased chance of survival in all cases

Question 4

What is kinesis?

Answer 4

When a stimulus is not directional and results in random patterns of movement

• organisms changes speed and rate at which it changes direction

Question 5

What occurs during favourable and unfavourable kinesis?

Answer 5

Favourable - organisms moves more slowly and turn more frequently to try and stay in those conditions

Unfavourable - organism moves rapidly in a straight line to exit those conditions

Question 6

What is movement called in plants

Answer 6

Tropism

Question 7

What is tropism?

Answer 7

The growth of plants in response to a directional stimulus

E.g. light, gravity and water

Question 8

Describe whether it is positive or negative tropism and where it occurs based on the 3 main stimuli

Answer 8

Light (phototropism) - positive in shoots, negative in roots

Gravity (gravitropism) - positive in roots, negative in shoots

Water (hydrotropism) - positive in roots

Question 9

How is the response controlled in plants?

Answer 9

By hormone-like substances known as plants growth factors

E.g. IAA that controls cell elongation by controlling flexibility/ stretchiness of plant cell walls

Question 10

What does IAA do in shoots and roots?

Answer 10

In shoots - IAA stimulates cell elongation so high concentrations promote cell growth

In roots - IAA inhibits cell elongation so high concentrations limit cell growth

Question 11

What word is used to describe direction of light for plants?

Answer 11

Unilateral - comes from one direction only

Question 12

Describe the role of IAA in plant shoots

Answer 12

1. Produced by cells at the tips of shoots called apical meristem cells
2. Then diffuses evenly away from tip, down the stem where it can stimulate cell elongation
3. A unilateral light source causes lateral diffusion of IAA to shaded side of shoot (so IAA accumulates on shaded side)
4. A higher conc of IAA on one side of shoot will cause cells to elongate faster on that side so shoot bends in a particular direction (towards light)

Question 13

Describe role of IAA in plant roots

Answer 13

• show negative phototropism (move away from light)
• and positive gravitropism (move with gravity)
• does this by inhibiting cell elongation on lower side of roots so top side can elongate to bend downwards with gravity