24: Survival & Response

Question 1

Define Stimulus:

Answer 1

a detectable change in the internal or external environment of an organism, that produces a response

Question 2

Define Tropisms:

Answer 2

a growth response to an external, directional stimulus
1. if the growth is towards the stimulus it is a positive response/tropism
2. if the growth is away from the stimulus it is a negative response/tropism

Question 3

What is the effect of IAA on roots + shoots?
How is IAA moved to other parts of the plant?

Answer 3

1. tropisms in the shoot and root are due to the uneven distribution of IAA, causing uneven growth (IAA is a type of auxin (plant growth regulators))
   - shoots: cell elongation
   - roots: inhibition of growth
2. diffusion (sometimes active transport

Question 4

Describe the effect of light on shoots:

Answer 4

Shoots are said to be positively phototrophic:
- IAA accumulates on the shaded side
- IAA causes cell elongation
- the shades side grows faster than the side in the light
- so the shoot bends and grows towards the light

Question 5

Describe the effect of light on roots:

Answer 5

Roots are said to be negatively phototrophic:
- IAA accumulates on shaded side
- IAA inhibits growth on root
- this causes the shaded side to grow slower than the side in light
- root grows away from light

Question 6

Describe the effect of gravity on shoots:

Answer 6

Shoots are negatively gravitropic:
- IAA accumulates on the lower side of the shoot
- IAA causes cell elongation
- lower side of root grows more than upper side, so the root grows up

Question 7

Describe the effect of gravity on roots:

Answer 7

Roots are positively gravitropic:
- IAA accumulates on the lower side of the root
- IAA inhibits root growth
- upper side grows more than underside, so root grows down

Question 8

Define Taxis:

Answer 8

• simple, innate & directional response to a stimulus, that is carried out by the whole mobile organism
• taxes could be positive (towards a favourable stimulus)
• taxes could be negative (away from an unfavourable stimulus)

Question 9

Define Kinesis:

Answer 9

• a non-directional response to a stimulus
• the rate of movement or rate of change of direction is related to the intensity of the stimulus
• kinesis results in a change in random movement patterns

Question 10

What do Neurones do?

Answer 10

• pass electrical impulses along their length and stimulate their target cells by secreting chemical neurotransmitters onto them
• this is a rapid, short-lived and localised response

Question 11

What is a reflex?

Answer 11

• rapid, short-lived, involuntary response to a specific stimulus
• simple reflexes are innate

Question 12

What is the Reflex Arc?

Answer 12

• the shortest pathway taken by the impulses in a simple reflex is known as the reflex arc

Question 13

Describe the function of:
1. motor neurone
2. sensory neurone

Answer 13

1. carries impulses from CNS to effector
2. carries impulses from receptors to CNA

Question 14

Describe a typical Reflex Arc:

Answer 14

• stimulus triggers impulses to sensory neurone
• sensory neuron enters spinal cord (CNS)
• this neurone synapses with a relay neurone, which in turn synapses with a motor neurone
• motor neurone carries impulses to the effector
• effector acts on muscle to trigger response