Kinds of signal at Higher Level: An AHL statement: sorting animal signalling chemicals by what they do.
Practise this as you read
- Identify which substances act as signals.
- Outline the role of calcium ions.
- Distinguish hormones from neurotransmitters.
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The big idea: Animals use four functional categories of signalling chemical. Hormones travel in the blood to distant cells. Neurotransmitters cross a synapse to the next cell. Cytokines act on nearby cells, mainly in immunity. Calcium ions enter a cell and trigger a response inside it.
The points to remember
- Hormones: gland to blood to target cells anywhere in the body.
- Neurotransmitters: neuron to the next cell, across a synapse.
- Cytokines: to nearby cells, mainly between immune cells.
- Calcium ions: flow into a cell and trigger vesicle release or contraction.
- The categories are about what the signal does, not its chemistry.
Remember it as: Blood, gap, neighbour, inside.
Real examples. Adrenaline from the adrenal glands reaches the heart through the blood. Acetylcholine crosses the gap between a motor neuron and a muscle fibre. Interleukin-2, a cytokine from helper T cells, makes nearby lymphocytes divide. Calcium ions entering the end of a neuron make vesicles release neurotransmitter.
Trap: Calcium ions are not made by cells, yet they still act as a signal: the categories are functional.
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How it is asked: Expect an MCQ asking which substances act as signals, an Outline question on calcium ions, or a Distinguish question on hormones and neurotransmitters.
Distinguish between hormones and neurotransmitters as signalling chemicals.
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See the answer broken into clear points, with guidance on how each point earns marks.