(A-level) AQA Biology Response to Stimuli Topic Summary

Oliver Dyson


(14) Plant Responses to Stimuli:

Tropisms:

A tropism is a response a plant has to a directional stimulus

-> impacting its growth

 This can either be a positive (towards the stimulus)
 Or negative (away from the stimulus) response

They are named according to the specific stimuli

 Phototropism – response to light
 Gravitropism – response to gravity
 Hydrotropism – response to water

How plants respond:

Plants respond to external stimuli using hormone-esque substances

-> these are called plant growth factors

 They influence the plant by impacting growth
 They are made by cells all throughout the plant rather than a specific organ
 Can also affect the secreting tissue, unlike animal hormones

Control by IAA –

IAA, or indoleacetic acid, is one of the main growth factors in plants

-> belongs to a group called auxins

 This controls cell elongation
 And is involved heavily in phototropism

In flowering plants:

1. Cells in the tip of the shoot produce IAA, which moves down
the shoot (by diffusion)
2. This initially moves evenly to regions of the shoot
3. But light causes it to move to the shaded side Figure 1 – reproduced from [1]
4. This increases its concentration in this region
5. Causing greater cell elongation here than the side which is facing the sunlight

This leads to the shoot bending towards the light

-> positive phototropism

 IAA also controls the growth of roots
 However it has the opposite effect to that in the shoots
 Inhibiting elongation when it is in high concentration

This causes roots to bend away from the light as elongation is favoured on the light side

-> negative phototropism

Gravitropism in plants:

,Oliver Dyson


For a horizontal growing root

-> gravity has the following effect:

1. Cells in the tip of the root produce IAA which is transported evenly along the root
2. Gravity however, causes IAA to move to the lower side of the root
3. This increases its concentration in this region
4. Causing an inhibition of cell elongation on the lower side
5. The upper side elongating more causes the root to bend downwards

This is positive gravitropism

-> and again, the reverse is true of the shoot cells

 The IAA on the lower side increasing elongation
 Causing negative gravitropism



How IAA works:

Transport of IAA is usually in one direction – away
from the tip of the shoots and roots where it is
produced

-> and has a variety of effects

 Increases plasticity (stretchiness) of the cell
walls of young cells
 Unable to occur in the more rigid mature
cells

The explanation for how this occurs is called the
acid growth hypothesis

-> involving:

 Active transport of hydrogen ions (H+) from
the cytoplasm of cells
 In spaces between the cell wall

 Causing the cell wall to become more plastic
 And therefore be able to elongate

These changes are relatively quick

-> allowing for plants to respond to stimuli

Hydrotropism:
Figure 2 – reproduced from [1]
This is another way which roots respond to stimuli
-> this involves the response to water / moisture, and roots tend to display positive hydrotropism

 IAA moves away from areas where water is present, causing the root to bend towards it

(14+15) Response to Stimuli Notes:

, Oliver Dyson


Stimulus and Response:

A stimulus is a detectable change in the environment

-> which leads to a response for an organism

 These can be either internal or external
 But all illicit responses, e.g. moving away for survival from a predator

These stimuli are detected by receptors

-> these are specific to one type of stimulus

 These create nerve impulse or hormones that allow for a reflex / response arc
stimuli > receptor > sensory neuron > relay neuron / coordinator > motor neuron > effector > response

A coordinator formulates a suitable response, either at a micro or macroscopic level

-> for example the brain handles larger responses

 An effector is what produces the response
 This can either be a muscle or a gland (in most cases)

Responses occur through two sequences:

 Hormonal
 Nervous (quicker)



Taxes and Kinesis:

A taxis is a simple response which has a direction

-> for example a motile organism moving towards favourable conditions

 This is classified as either positive or negative
 Towards a stimulus = positive taxis
 Away = negative taxis

For example if an organism moves towards light, this could be described as positive phototaxis

Kinesis however, involves a response with no direction

-> instead speed changes and random direction changes occur (or rate of change of direction
changes)

 This still allows favourable conditions to be
found, by turning more sharply within
favourable conditions so that it is more likely to
remain in them



Nervous System:
Figure 3 – reproduced from [1]
The nervous system is made up of two main parts