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Response to stimuli - A-Level Biology notes
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Response to stimuli - A-Level Biology notes
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Chapter 14: response to stimuli14.1 Survival and response
2 types of communication:
- A stimulus is a detectable change in the
external or internal environment of an
organism that leads to a response which
increases the chances of survival if the
response is appropriate.
- Organisms that survive have a greater
chance of raising offspring and passing
their alleles to the next generation
- Selection pressure favours organisms with
more appropriate responses
stimulus receptor coordinator
effecter response
Stimuli are detected by receptors which are
specific to different types of stimuli. A
coordinator may be at a molecular level or
involve a large organ. A response is produced
by a factor involved at a molecular level or
the behaviour of a whole organism.
Taxes - A taxi is a simple response whose direction is determined by the direction of the stimulus.
o Positive taxis: movement towards the stimulus
o Negative taxis: movement away from the stimulus
- Light stimulus: phototaxis
- Chemical stimulus: chemotaxis
Kinesis - A kinesis is a form of response where the organism doesn’t move towards or away from the stimulus instead
it changes the speed of its movement and the rate at which it changes its direction
- Rate of turning increases as the organism across the sharp line dividing a favourable and an unfavourable
condition
- If the organism is far into the unfavourable area they will move in a straight line and make sharp turns
occasionally which tends to bring the organism into a new region with favourable conditions
Tropism - A tropism is the growth of parts of a plant in response to a directional stimulus
s - Shoots: positive phototropism, negative gravitropism
- Roots: negative phototropism, positive gravitropism
14.2 Plant growth factors Plants respond to: light,
gravity, water.
Control of tropisms by IAA Indole-3-acetic acid (controls plant cell
growth):
1. Positive phototropism in flowering plants:
o Cells in the tip produce IAA which moves down the shoe and gets evenly distributed throughout the plant
o Light causes movement of IAA from the light side to the shaded side
o A greater concentration of IAA builds up on the shaded side of the shoot
o The cells on the shaded side elongates faster causing the shoot tip to bend towards light
2. Gravitropism in flowering plants:
o Cells in the tip of the root produce IAA which then moves along the root
o Gravity influences movement of IAA to the lower side of the root
o IAA inhibits elongation of root cells and since there is a greater concentration of IAA on the lower side the
upper side elongates quicker causing the root to bend downwards by the force of gravity
IAA also increases plasticity of young cell walls as mature cell walls develop greater rigidity. The acid growth
hypothesis is that the active transport of H ions from the cytoplasm into spaces in the cell wall causes them to
become more plastic lounge cell elongation by expansion.
, Chapter 14: response to stimuli
14.3 A reflex arc
The spinal cord consists of a column of nervous tissue along the back that lies inside the vertebral column for
protection. Emerging at intervals along the spinal-cord or pairs of nerves:
A reflex arc consists of only 3 neurons: sensory, relay, motor. The main stages of a spinal reflex arc (ex. withdrawal
of the hand from a hot object):
1. Stimulus (heat from the object) Importance of reflex arcs:
2. Receptor (temperature receptors on the skin which generate a nerve - It doesn’t overload the brain as
impulse via the sensory neuron) it’s a subconscious response
3. Sensory neuron (passes nerve impulse to the spinal-cord) - Protects the body from harm
4. Coordinator (relay neuron) - Quick due to the short neuron
5. Motor neuron (carries nerve impulses from the spinal-cord to the muscle in pathway and absence of the
the arm) decision-making process
6. Effector (the muscle in the arm is stimulated to contract)
7. Response (pulling away from the hot object)
14.4 Receptors
Pacinian capsule (sensory receptor) respond to changes in mechanical pressure
- Specific to a single type of stimulus
- Produces a generator potential by acting as a transducer which converts energy from the stimulus into nerve
impulses that can be understood by the body
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