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AQA A2 Biology Unit 4
9 Response to Stimuli…read more

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3.5.1 Stimuli, both internal and external, are detected and lead to a response.
· Receptors only
Survival and
Control of heart rate Receptors respond to specific
Organisms increase their The basic · Stimulation of
The role of structure of a
chance of survival by receptor membranes
chemoreceptors Pacinian
responding to changes in produces
and pressure corpuscle as an
their environment. deformation of
receptors, the example of a
autonomic stretch-mediated
receptor. The sodium channels,
Tropisms as responses to nervous system creation of a
and effectors in leading to the
directional stimuli that can generator establishment of a
maintain the roots and controlling potential on
heart rate. generator potential.
shoots of flowering plants in stimulation.
a favourable environment.
The Pacinian Differences in
Taxes and kineses as simple corpuscle sensitivity and visual
responses that can maintain should be used acuity as explained by
a mobile organism in a as an example differences in the
favourable environment. to illustrate the distribution of rods
following. and cones and the
connections they
A simple reflex arc involving make in the optic
three neurones. The nerve.
importance of simple
reflexes in avoiding damage
to the body.…read more

Slide 3

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A stimulus is a detectable change in the internal or external environment of an organism that
produces a response
Ability to respond to stimuli increases survival
Individuals that survive have a greater chance of passing on alleles to the next generation due to
section pressure
Stimuli are detected by receptors which transform energy from the stimuli into a form of energy that
leads to a response
Effectors carry out the response
The nervous system is used to form a response using a coordinator to connect information from
receptors to effectors
Hormones could also be used for communication however their process is relatively slow…read more

Slide 4

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Simple response whose direction is determined
by the direction of the stimulus
· As a result a motile organism moves towards or
Taxes away from the stimulus
· Negative phototaxis: Earthworms move away
from light
· Positive phototaxis: Algae move towards light
· The more unpleasant the stimulus the more
rapid the movement/change in direction
Kinesis · The increased random movement is designed to
bring the organism back to favourable
conditions for survival
· Growth movement of part of a plant in
response to a directional stimulus
· Positive hydrotropism: plant roots grow
towards water…read more

Slide 5

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The Nervous Organisation
Central Nervous System (CNS): made p of the brain and spinal cord
Peripheral Nervous System: made up of pairs of nerves that originate from either the brain or spinal
· Sensory neurones carry nerve impulses from receptors
Peripheral Nervous System: towards the CNS
· Motor neurones carry nerve impulses from CNS to
· Voluntary nervous system: carries nerve impulses to body
Motor Neurone System: muscles and is under voluntary control
· Autonomic nervous system: carries nerve impulses to
glands and cardiac muscles which is involuntary
The Spinal Cord · Column of nervous tissue that runs along the back and lies
inside the vertebral column for protection…read more

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