AQA A2 BIOLOGY UNIT 5: Coordination

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  • Created on: 18-04-14 14:27
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There are two main forms of coordination in mammals: the nervous system and the hormonal
Nervous system Hormonal system
Communication is by nervous impulses Communication is by chemicals called
Transmission is by neurones Transmission is by the blood system
Transmission is very rapid Transmission is usually relatively slow
Nerve impulses travel to specific parts of the Hormones travel to all parts of the body, but
body only target organs respond
Response is localised Response is widespread
Response is rapid Response is slow
Response is short-lived Response is often long-lasting
Effect is temporary and reversible Effect may be permanent and irreversible
Chemical Mediators
The nervous and hormonal systems work well in coordinating the activities of a whole organism.
At the cellular level, however, they are complemented by a further form of coordination.
This involves substances known as chemical mediators.
These are chemicals that are released from certain mammalian cells and have an effect on cells in their
immediate vicinity.
They are typically released by an infected or injured cells and cause small arteries and arterioles to
This leads to a rise in temperature and swelling of the affected area ­ the so-called `inflammatory'
Two examples of chemical mediators are:
Histamine which is stored in certain white blood cells and released following injury or in response
to an allergen, such as pollen.
It causes dilation of small arteries

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Humans have two complementary control systems that they can use to respond to their environment:
the nervous system and the endocrine (hormonal) system.
Hormones are secreted by glands into the blood stream.
There are two kinds of glands:
· Exocrine glands secrete solutions to the outside, or to body cavities, usually through ducts (tubes).
e.g. sweat glands, tear glands, mammary glands, digestive glands.
· Endocrine glands do not have ducts but secrete chemicals directly into the tissue fluid, whence they
diffuse into the blood stream.…read more

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Paracrine Signalling
Paracrine signalling is communication between close cells using chemicals called local chemical
These chemicals are released by cells into the surrounding tissue fluid, but not into the blood. Thus
they only have a local effect on the cells surrounding their release, in contrast to hormones. Like
hormones and neurotransmitters they bind to receptors on the surface of the target cells to cause an
Two such local mediators are prostaglandins and histamine, which control the inflammatory response.…read more

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Plants do not have a nervous system or endocrine system so they can't respond using neurones or
In order for plants to survive they must respond to changes in both their external and internal
environments.…read more

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Control of Tropisms by IAA
Plant growth factors are produced in small quantities.
They have their effects close to the tissue that produces them.
IAA causes plant cells to elongate.
When a plant is in light, the shoot will bend towards it.…read more

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Neurones are specialised cells adapted to rapidly
carrying electrochemical changes called nerve
impulses from one part of the body to another.
Several dendrons carry nerve impulses
towards the cell body, while a single long axon
carries the nerve impulse away from the cell body.
A nerve is a discrete bundle of several thousand
Nerve impulses are passed from the axon of one
neurone to the dendron of another at a synapse.
Numerous dendrites provide a large surface area
connecting with other neurones.…read more

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A nerve impulse is a self-propagating wave of electrical disturbance that travels along
the surface of the axon membrane.
It is not an electrical current, but a temporary reversal of the electrical potential
difference across the axon membrane.
This reversal is between two states, called the resting potential and the action potential.…read more

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The Action Potential
When a stimulus is received by a receptor or nerve ending, its energy causes a temporary reversal of
the charges on the axon of a membrane.
As a result, the negative charge inside the membrane becomes a positive charge.
This is known as an action potential and in this condition, the membrane is said to be depolarised.
This depolarisation occurs because the channels in the axon membrane change shape, and hence open
or close, depending on the voltage across the membrane.…read more

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Stimulus This excites the neurone cell membrane, causing sodium ion
channels to open
The membrane becomes more permeable to sodium so sodium
ions diffuse into the neurone down the sodium ion electrochemical
gradient and this makes the inside of a neurone less negative
Depolarisation If the potential difference reaches the threshold, more sodium ion
channels open, more sodium ions diffuse into the neurone
Repolarisation The sodium ion channels close and potassium ion channels open
The membrane is more permeable to potassium so potassium ions
diffuse…read more


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