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Cells in multicellular organisms must coordinate their activities: hormones must reach their targets, neurones must tell muscles to contract, and immune cells must respond to infection at specific sites. The system that achieves this is cell signalling, and it depends on the membrane proteins introduced in Lesson 1. This lesson covers OCR A-Level Biology A specification point 2.1.5 (f) — the roles of membranes in cell signalling.
Key Definition — Cell Signalling: The processes by which cells communicate with each other, using signalling molecules (such as hormones and neurotransmitters) that are released by one cell and detected by receptor proteins on or in a target cell, producing a specific response.
Signalling allows cells to coordinate function across a whole organism. It is the reason a glucose meal causes insulin release; the reason a stubbed toe produces pain; the reason a wound triggers inflammation.
Any signalling pathway has three common stages:
graph LR
A[Signalling cell<br/>releases ligand] --> B[Ligand travels]
B --> C[Target cell receptor]
C --> D[Signal transduction]
D --> E[Cellular response]
For a signalling pathway to work cleanly, three conditions are essential:
Receptor proteins are typically membrane-bound glycoproteins — intrinsic proteins that span the plasma membrane. Their extracellular domain has a specific shape that binds only to a particular ligand; the intracellular domain initiates the response inside the cell.
Specificity means that:
Exam Tip: When asked "why do only certain cells respond to a hormone?", the answer is "because only those cells have the specific receptor for that hormone".
Several types are examinable at A-Level:
Hormones are chemical messengers produced by endocrine glands and transported in the bloodstream to distant target cells. Examples:
Most protein hormones (e.g. insulin, glucagon, ADH) are water-soluble and cannot cross the phospholipid bilayer. They bind to membrane-bound receptors. In contrast, steroid hormones (e.g. oestrogen, testosterone, cortisol) are lipid-soluble and can cross the membrane to bind receptors inside the cell.
Neurotransmitters are short-range signals released at synapses by presynaptic neurones, crossing a narrow synaptic cleft and binding to receptors on the postsynaptic membrane. Examples include:
Neurotransmitter signalling is fast and localised: the distance is tiny, the response is nearly instantaneous, and the neurotransmitter is rapidly broken down or taken back up.
Some signalling molecules act over short distances without entering the blood. These include histamine (released in inflammation), prostaglandins and cytokines. They are important in wound healing and immune responses.
Cells also signal by direct contact — for example, antigen-presenting cells showing peptides to T-cells during an immune response, or plasmodesmata linking plant cells.
Signalling molecules that cannot enter the cell must have their message "transduced" — converted into an intracellular signal. A full treatment of G-protein and kinase cascades is beyond A-Level, but you should understand two concepts.
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