6. Organisms Respond To Changes In Their Environment

Question 1

Define stimulus

Answer 1

A stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism

Question 2

Give the sequence of events of the response to a stimulus

Answer 2

Stimulus —> receptor —> coordinator —> effector —> response

Question 3

What is a taxis

Answer 3

A simple response whose direction is determined by the direction of a stimulus.
As a result a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one

Question 4

What is meant by positive/negative taxis

Answer 4

Movement towards the stimulus = positive taxis
Movement away from stimulus = negative taxis

Question 5

What is a kinesis

Answer 5

A kinesis is a form of response in which the organism does not move toward or away from a stimulus.
Instead it changes the speed at which it moves and the rate at which it changes direction

Question 6

What is a tropism

Answer 6

A tropism is the growth of a plant in response to a directional stimulus

Question 7

What are plant growth factors referring to

Answer 7

The hormone like substances involved in the responses of plants to external stimuli

Question 8

Why do we refer to the hormone-like substances as plant growth factors over hormones ?

Answer 8

They exert their influence by affecting growth
They may be made by cells located throughout the plant rather than in particular organs
Unlike animal hormones, some plant growth factors affect the tissue that release them rather than acting on distant target organs

Question 9

Give an example of a plant growth factor and its role

Answer 9

Indoleacetic acid (IAA)
- controls plant elongation

Question 10

What is growth of a plant towards the light referred to as

Answer 10

Positive phototropism
or
Negative gravitropism

Question 11

Describe the steps that explain positive phototropism

Answer 11

1. Cells in the tip of the shoot produce IAA, which is then transported (initially evenly) down the shoot
2. Light causes the movement of IAA from the light side to the shaded side of the shoot
3. A greater conc of IAA builds up on the shaded side compared with the light side
4. As IAA causes elongation of shoot cells, cells on the shaded side (side with greatest conc of IAA) elongate faster than the light side
5. Therefore shoot tip bends towards the light

Question 12

Describe the steps that explain positive gravitropism in plants

Answer 12

1. Cells in the root tips produce IAA which is initially transported evenly to all sides of the root
2. Gravity influences the movement of IAA from the upper side to the lower side of the root
3. A greater conc of IAA builds up on the lower side of the root compared with the upper side
4. As IAA inhibits elongation of root cells, the cells on the lower side of the root (region with greater IAA conc) elongate less than those on the upper side
5. Therefore root bends downwards towards the force of gravity