B1- Response to External Stimuli

Kinesis definition

actual definition = a non- directional response to a stimulus

explanation: behavioural pattern where an animals responds to a change in stimulus by increasing or decreasing activity.

it is a change in rate of movement

Taxis definition

directional response to a stimulus

positive taxis = towards the stimulus

negative taxis = away from the stimulus

give an example of taxes

earthworms move away from light

= negative phototaxis

give an example of kinesis

flatworms favour dark environments and usually live underneath rocks. if you lift up said rock they will start moving around quickly in random directions in order to increase their chance of reaching a darker environment again

once they have found the correct direction and therefore reach this darker environment, they turn more frequently to remain in this darker environment

KINESIS

what happens to the rate of moment and rate of change in direction when an organism is in an environment they don’t like

in an environment an organism doesn’t like, their rate of movement increases and their rate of turning/ changing direction decreases (as they are wanting to travel to a new location not stay in the same spot by turning quickly)

kinesis relates to speed of movement and frequency of turns

define the term tropism

A tropism is a directional growth response of a plant in which the direction of the growth is determined by an external stimulus

name 3 types of tropism:

• Phototropism – growth response to light

• Geotropism (gravitropism) – growth response to gravity

• Hydrotropism – growth response to water

what term is used to describe auxins?

plant growth factors

Why auxins/ plant growth factors not called hormones

Not made in glands or transported in blood to target organs — instead made in growing tips and act locally by diffusion/active transport.

Effect of IAA on shoot?

Stimulates cell elongation

Light causes IAA to move to shaded side of shoot

cells on shaded side elongate more than cells on lit side

= uneven growth → shoot bends towards light = positive phototropism.

• WORDING : refer to “light and shaded side”

6. Effect of IAA on root

high concentration of IAA inhibits growth, small concentration promotes growth

IAA inhibits growth if there is an accumulation of IAA on one side of the root resulting in a positive gravitropic response

→ root bends downwards = positive geotropism.

WORDING: refer to “upper and lower” sides

Control
Shoot bends towards unilateral light — normal positive phototropism.

Tip removed
Shoot grows straight up, no bending. Shows the tip is the site of light perception/auxin production.

Tip covered by opaque cap
Shoot grows straight up, no bending. Light can't reach the tip, so no auxin redistribution occurs — confirms the tip detects light.

Tip covered by transparent cap
Shoot bends towards light as normal. Light still reaches the tip, so this shows the cap material itself doesn't block the response (control for the opaque cap).

Base covered by opaque shield
Shoot still bends towards light. Shows that it's the tip, not the base/lower shoot, that senses light — covering the base has no effect on the response.

compare plant growth factors (auxins) to hormones

where is IAA produced

in the cells at the tip of shoots and roots

what is the main plant growth factor in plants

IAA