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There are two main forms of coordination in mammals: the nervous system and the hormonal
system:

Nervous system Hormonal system

Communication is by nervous impulses Communication is by chemicals called
hormones
Transmission is by neurones Transmission is by the blood system

Transmission is very rapid Transmission is usually relatively slow…

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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,…

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Paracrine Signalling
Paracrine signalling is communication between close cells using chemicals called local chemical
mediators.
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…

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Plants do not have a nervous system or endocrine system so they can't respond using neurones or
hormones.

In order for plants to survive they must respond to changes in both their external and internal
environments.

Plants must be able to:

Requirement How do they get this Tropism Why do…

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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.
This is due to the following sequence of…

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Page 7

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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…

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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…

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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…

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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…

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