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A2 Biology Unit 5 page 1

AQA A2 Biology Unit 5
Specification 2
Human Nervous system Nerve Cells 4
The Nerve Impulse 6
Synapses 0
Receptors 14
Muscle 17
Animal Responses 24
Control of Heart Rate 28
The Hormone System 30
Homeostasis 33
Temperature Homeostasis 34
Blood Glucose Homeostasis…

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A2 Biology Unit 5 page 2

Biology Unit 5 Specification
Control Systems are substances that stimulate their target cells via the blood
Organisms increase their chance of survival by responding system. This results in slow, long-lasting and widespread
to changes in their environment. responses. The second messenger model of adrenaline…

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A2 Biology Unit 5 page 3

Genetics · Small interfering RNA (siRNA) as a short, double-strand
The Genetic Code of RNA that interferes with the expression of a specific
The genetic code as base triplets in mRNA which code for gene. Interpret data provided from investigations into
specific amino acids.…

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A2 Biology Unit 5 page 4

The Human Nervous System
Humans, like all living organisms, can respond to changes in the environment and so increase survival.
Humans have two control systems to do this: the nervous system and the endocrine (hormonal) system.
We'll look at the endocrine system later, but…

Page 5

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A2 Biology Unit 5 page 5

Nerve Cells dendrites

The nervous system composed of nerve cells, or neurones. A neurone has a
cell body with extensions leading off it. Several dendrons carry nerve impulses

Sensory Neurone
towards the cell body, while a single long axon carries the nerve impulse away…

Page 6

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A2 Biology Unit 5 page 6

The Nerve Impulse
Neurones transmit simple on/off signals called impulses (never talk about nerve signals or messages). These
impulses are due to events in the cell membrane, so to understand the nerve impulse we need to revise
some properties of cell membranes.

The Membrane…

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A2 Biology Unit 5 page 7

an electrical pulse at one end of an axon and measure the voltage changes at the other end, using an

stimulating recording
stimulator oscilloscope
electrodes electrodes

squid giant axon isotonic bath
The normal membrane potential of these nerve cells is ­70mV (inside the…

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A2 Biology Unit 5 page 8

How do Nerve Impulses Start?
In the squid experiments the action potential was initiated by the stimulating electrodes. In living cells they
are started by receptor cells. These all contain special receptor proteins that sense the stimulus. The
receptor proteins are sodium channels that…

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A2 Biology Unit 5 page 9

How can Nerve Impulses convey strength?
How do impulses convey the strength of the stimulus? Since nerve impulses are all-or-nothing, they cannot
vary in size. Instead, the strength of stimulus is indicated by the frequency of nerve impulses. A weak
stimulus (such as dim…

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A2 Biology Unit 5 page 10

The junction between two neurones is called a synapse. An action potential cannot cross the gap between
the neurones (called the synaptic cleft), and instead the nerve impulse is carried by chemicals called
neurotransmitters. These chemicals are made by the cell that is…


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