Survival and response

Question 1

What is a stimulus

Answer 1

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

Question 2

How does an organisms response to stimuli link to natural selection

Answer 2

• The ability to respond to stimuli increases an organisms chance of survival.
• Those organisms that survive have a greater change of raising offspring and and passing on their alleles.
• There is always a selection pressure favouring organisms with more appropriate responses.

Question 3

What are receptors

Answer 3

Receptors detect stimuli and are specific to one type of stimulus.

Question 4

What does a coordinator do

Answer 4

• A coordinator formulates a suitable response to a stimulus.
• Coordination may be at the molecular level or involve a large organ such as the brain.

Question 5

What does an effector do

Answer 5

• Carries out the response to the stimulus
• This response could be at the molecular level to involve the behaviour of a whole organism.

Question 6

What are the two key systems by which the body responds to stimuli

Answer 6

• The hormonal system
• The nervous system.

Question 7

List the sequence of events that occurs in either chemical control or a nervous response

Answer 7

• Stimulus
• Receptor
• Coordinator
• Effector
• Response

Question 8

What is a taxis

Answer 8

• A simple response whose direction is determined by the direction of a stimulus.
• Motile organisms respond to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one.

Question 9

How are taxes classified

Answer 9

They are classified according to whether the movement is towards the stimulus (positive taxis) or away from the stimulus (negative taxis)

Question 10

What is phototaxis

Answer 10

A taxis response to light

Question 11

What is chemotaxis

Answer 11

A taxis response to chemical changes

Question 12

What is a kinesis

Answer 12

A form of response on which the organism responds to a stimuli by changing the speed at which it moves and the rate at which it changes direction.

Question 13

Explain the advantage of kinesis

Answer 13

• In kinesis, the organism does not move towards or away from a stimulus.
• Instead it changes the speed at which it moves and the rate at which it changes direction.
• If an organism crosses a sharp dividing line between a favourable and an unfavourable environment, it’s rate of turning increases.
• This raises the changes of a quick return to a favourable environment.
• However if it moves a considerable distance into an unfavourable environment it’s rate of turning may slowly decrease so that it moves in long straight lines before it turns, often very sharply.
• This type of response tends to bring the organism into a new region with favourable conditions.
• It is important when the stimulus is less directional.

Question 14

What is a tropism

Answer 14

The growth of part of a plant in response to a directional stimulus.

Question 15

Describe the tropisms that plant shoots and roots exhibit

Answer 15

• Plant shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism) so that their leaves are in the most favourable position to capture light for photosynthesis.
• Plant roots grow away from light (negative phototropism) and towards gravity (positive gravitropism). In both cases the response increases the probability that the roots will grow into the soil, where they are better able to absorb water and mineral ions.

Question 16

Do plants have a nervous system

Answer 16

No

Question 17

What key three factors do plants respond to

Answer 17

1) Light: Shoots grow towards light to photosynthesize
2) Gravity: Plants need to be firmly anchored in the soil- roots are sensitive to gravity and grow on the direction of its pull.
3) Water: Almost all plant roots grow towards water (positively hydrotropic) in order to absorb it for photosynthesis and other metabolic processes, as well as for support.

Question 18

What are the plants responses to external stimuli not called hormones but rather plant growth factors.

Answer 18

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

Question 19

What is a key example of a plant growth factor

Answer 19

Indoleacetic acid (IAA) which belongs to a group of substances that control plant cell elongation among other things.

Question 20

Describe how the shoots of flowering plants respond to unilateral light (phototropism)

Answer 20

1. Cells in the top of the shoot produce IAA, which is then transported down the shoot.
2. The IAA is initially transported evenly throughout all regions as it begins to move down the shoot.
3. Light causes the movement of IAA from the light side to the shaded side of the shoot.
4. A greater concentration of IAA builds up on the shaded side of the shoot than on the light side
5. As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more.
6. The shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light.

Question 21

How does IAA control the bending of roots in response to light

Answer 21

• Whereas a high concentration of IAA increases cell elongation in shoots, it inhibits cell elongation in roots.
• As a result, in roots the elongation of cells is greater on the light side than on the shaded side and so roots bend away from light, so they are negatively phototropic.

Question 22

Describe how gravitropism occurs in the roots of flowering plants

Answer 22

1. Cells in the tip of the root produce IAA which is then transported along the root.
2. The IAA is initially transported to all side of the root.
3. Gravity influences the movement of IAA from the upper side to the lower side of the root.
4. A greater concentration of IAA builds up on the lower side of the roots than the upper side.
5. As IAA inhibits the elongation of root cells and their is a greater concentration on the lower side, the cells on this side elongate less than those on the upper side.
6. The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity.

Question 23

How does IAA control the shoots response to gravity

Answer 23

• In shoots, the greater concentration of IAA on the lower side increases cell elongation and causes this side to elongate more than the upper side.
• As a result the shoot grows upwards away from the force of gravity.