AQA A2 BIOLOGY UNIT 5: Response to Stimuli

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  • Created on: 18-04-14 14:28
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Stimulus
A stimulus is a detectable change in the internal or external environment of an organism that
produces a response in the organism.
The ability to respond to an organism increases the chances of survival for an organism.
For example, to be able to detect and move away from harmful stimuli, such as predators and
extremes of temperature, or to detect and move towards a source of food clearly aid survival.
Those organisms that survive have a greater chance of raising offspring and of passing their alleles to
the next generation.
There is always, therefore, a selection pressure favouring organisms with more appropriate
responses.
Stimuli are detected by cells or organs known as receptors.
Receptors transform the energy of a stimulus into some form of energy that can be processed by the
organism and leads to a response.
The response is carried out by one or more of a range of different cells, tissues, organs and systems.
These are known as effectors.
Receptors and effectors are often some distance apart and therefore some form of communication
between the two is needed if the organism is to respond effectively.
One means of communication occurs via chemicals called hormones, which is a relatively slow process
found in both animals and plants.
Animals have another, more rapid, means of communication: the nervous system.
Their nervous system usually has many different receptors and effectors.
Each receptor and effector is linked to a central coordinator of some type.
The coordinator connects information from each receptor with the appropriate effector.
Stimulus Receptor Coordinator Effector Response

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Taxes
A taxis is a simple response whose direction is determined by the direction of the stimulus.
As a result, a motile organism responds directly to environmental changes by moving its whole body
either towards a favourable stimulus or away from an unfavourable one.
Taxes are classified according to whether the movement is towards the stimulus or away from the
stimulus and also by the nature of the stimulus.…read more

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Tropisms
A tropism is a growth movement of part of a plant in response to a directional stimulus.
In almost all cases the plant part grows towards or away from the stimulus.…read more

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Nervous Organisation
The motor nervous system is subdivided into
Voluntary nervous system Carries nerve impulses to body muscles and is under
voluntary control
Autonomic nervous system Caries nerve impulses to glands, smooth muscle and
cardiac muscle and is not under voluntary control
The peripheral nervous system is divided into
Sensory neurones Carries nerve impulses from receptors towards the
central nervous system
Motor neurones Carries nerve impulses away from the central nervous
system to effectors
The nervous system has two major divisions
Central Nervous System Made…read more

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The Spinal Chord
The spinal chord is a column of nervous tissue that runs along the back and lies inside the vertebral
column for protection.
Emerging at intervals along the spinal chord are pairs of nerves.
Reflex Arc
An involuntary response to a sensory stimulus is called a reflex.
The pathway of neurones involved in a reflex is known as a reflex arc.
Reflex arcs involve just three neurones.…read more

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Importance of Reflex Arcs
Reflexes are involuntary: the actions they control do not need to be considered because there is only
one obvious course of action.…read more

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Control of the heart rate is an involuntary reflex.
The part of the nervous system responsible for this type of control is the autonomic nervous system.
The Autonomic Nervous System
The autonomic nervous system controls the involuntary activities of internal muscles and glands.…read more

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Control by Chemoreceptors
Chemoreceptors are found in the wall of the carotid arteries.
They are sensitive to changes in the pH of the blood that result from changes in carbon dioxide
concentration.
In solution, carbon dioxide forms an acid and therefore lowers pH.…read more

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Stimulus Receptor Neurone and Neurotransmitter Effector Response
High blood Pressure Impulses sent to the medulla which Cardiac Heart rate slows
pressure receptors sends impulses along muscles down to reduce blood
parasympathetic neurones. pressure back to
These secret acetylchlorine which normal
binds to receptors on SAN
Low blood Pressure Impulses sent to the medulla which Cardiac Heart rate speeds up
pressure receptors sends impulses along sympathetic muscles so blood pressure
neurones.…read more

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The central nervous system receives sensory information from its internal and external environment
through a variety of sense cells and organs called receptors.
Receptors are specific ­ they detect one particular stimulus.
Receptor cells are often part of sense organs, such as the ear, eye or skin.…read more

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