(A-level) AQA Biology Response to Stimuli Topic Summary
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In-depth summary for content covered in the Response to Stimuli topic of A-level AQA Biology. Covering both plant and human responses in detail according to the specification, including nervous action potentials, muscle contraction, IAA action and more.
This will still be applicable to other exa...
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
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