Cell Signalling

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.

1. What Is 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.

2. The Basic Cell Signalling Pathway

Any signalling pathway has three common stages:

Reception — the signal molecule (ligand) binds to a specific receptor on or in a target cell.
Transduction — the signal is converted into a cellular response, often via a cascade of intracellular events.
Response — the cell changes its behaviour: gene expression, enzyme activity, secretion, movement, division.

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:

Specificity — only target cells with the correct receptor respond.
Amplification — a small signal can produce a large response via cascades.
Termination — signals must be switched off (ligand removal, receptor internalisation, enzyme degradation) so that cells can respond again.

3. Receptors and Specificity

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:

Only cells with the correct receptor are affected. For example, insulin circulates through the whole body, but only cells expressing the insulin receptor (such as liver, muscle and fat cells) respond.
A single cell can carry many different receptors and so respond to several signals simultaneously.
Changing receptor numbers (up-regulation or down-regulation) is a way of adjusting a cell's sensitivity to a signal.

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".

4. Types of Signalling Molecules

Several types are examinable at A-Level:

4.1 Hormones

Hormones are chemical messengers produced by endocrine glands and transported in the bloodstream to distant target cells. Examples:

Insulin — from the pancreas; signals liver and muscle cells to take up glucose and store it as glycogen.
Glucagon — from the pancreas; signals liver cells to break glycogen down to glucose.
Adrenaline (epinephrine) — from the adrenal medulla; triggers the "fight or flight" response.
ADH (antidiuretic hormone) — from the posterior pituitary; signals kidney collecting ducts to reabsorb water.

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.

4.2 Neurotransmitters

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:

Acetylcholine (ACh) — at the neuromuscular junction, binds to ligand-gated ion channels (nicotinic ACh receptors) on muscle fibres.
Noradrenaline — in the sympathetic nervous system.
Dopamine, serotonin, GABA, glutamate — in the brain.

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.

4.3 Local Chemical Mediators

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.

4.4 Direct Cell-Cell Contact

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.

5. Signal Transduction — Two Example Mechanisms

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.

Lesson 6: Cell Signalling