• Nerve cells use electrical impulses to send signals. They cause a rapid and localised response.
• Hormones are chemical messengers composed of proteins. They cause a long-lasting and widespread response.
• Plants use plant hormones to coordinate growth.
• Auxins are one of the most important types of plant hormone.
• Flowering plants use the auxin indole acetic acid (IAA). In the shoot tip IAA promotes growth and in the roots it inhibits growth.
• Neurones carry electrical signals.
• A neurone’s resting potential is maintained by sodium-potassium pumps which pump out three sodium ions and take in two potassium ions simultaneously.
• An action potential occurs when information is sent down the axon of a neuron and a change in membrane potential is activated.
• Depolarisation occurs when sodium flows into the axon creating an area of positive charge.
• When potassium channels open and potassium flows out the charge return to its resting potential, a process known as repolarisation.
• If not enough sodium enters, the depolarisation is not large enough and no action potential occurs: the ‘all or nothing’ law.
• After an action potential has started it’s propagated along an axon: the surrounding voltage-gated ion channels detect the reversal of the membrane potential and then open once the potential change is high enough.
• The refractory period is the time needed by an ion channel to ‘rest’ before reopening.
• During an absolute refractory period it’s not possible for a second stimulus to start a new action potential.
• The refractory period is able to limit the number of action potentials.
• Nerve impulse speed is affected by temperature, axon diameter and the presence of the myelin sheath.
• An impulse travels from one neuron to the next over a small gap called a synapse.
• An action potential travels across the synaptic cleft via neurotransmitters.
• A neurone only contains one output, the Grand Postsynaptic Potential (GPP), where all the excitatory and inhibitory potentials are summed up
• Summation is the foundation of the nervous system’s processing power.
• Synapses ensure unidirectionality, integration, summation and filtration of unnecessary stimuli.
• Drugs that simulate a synapse are known as agonists while those that inhibit a synapse are called antagonists.

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TEST IT!

1.

During an action potential the membrane potential rises then drops.

a) Why does the membrane potential drop?

b) What re-establishes the resting potential?

2.

Responses are coordinated by different substances within animals.

a) Hormones are one example. How does a hormone reach the cell it affects?

b) Neurotransmitter is another example. They travel across synapses from neurone to neurone and are unidirectional. How does acetylcholine contribute to a synapse’s unidirectionality?

c) Give two differences between a hormonal and a chemical response?

i. _______________________________________________
ii. _______________________________________________

3.

Plants use growth factors or hormones.

a) What’s the name of the growth factor found in flowering plants?

b) What is the process called by which this growth factors moves from the growing part of a plant to other tissues in the plant?

c) When a shoot has light on one side how does build-up of this growth factor on the shaded side move the leaves to a more favourable position?

ANSWERS

1.

a) The potassium channels open letting potassium leave and the sodium channels close.

b) The sodium-potassium pump then restores the ion concentration to the resting potential.

2.

a) Travels in the blood.

b) Acetylcholine is released from the presynaptic side. Neurotransmitter diffuses from a higher concentration to a lower concentration. It binds to receptors on the postsynaptic side.

c)

i. Hormones have a widespread affect while an electrical response is only affects a specific area.
ii. Hormones have a slow, long lasting effect while an electrical response is rapid and short.

3.

a) Indoleacetic acid (IAA)

b) Diffusion

c) It causes the shoot to bend towards the light (positive phototropism) which is required for photosynthesis in the leaves.