How do cells detect signals from outside and convert them into an internal response?
Explain the principles of cell signalling, including the roles of receptors, signal transduction and second messengers
A focused answer to the H2 Biology Cell Biology outcome on cell signalling. The stages of signalling (reception, transduction, response), membrane and intracellular receptors, second messengers such as cyclic AMP, and how signalling cascades amplify a response.
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What this dot point is asking
SEAB wants you to explain how a cell detects an extracellular signal and converts it into a response. You need the three stages (reception, transduction, response), the difference between membrane and intracellular receptors, the idea of a second messenger, and how a cascade amplifies the signal. This links membrane structure to the control of gene expression and to physiology more broadly.
The answer
The three stages of signalling
- Reception
- A signal molecule (a ligand, such as a hormone or neurotransmitter) binds to a specific receptor protein. Binding is specific because the receptor's binding site is complementary in shape to the ligand, so only target cells bearing the right receptor respond.
- Transduction
- Binding changes the receptor's shape, triggering a series of intracellular changes that relay and often amplify the signal. This frequently involves a G protein, an enzyme, and a second messenger.
- Response
- The signal produces a cellular effect: an enzyme is activated, an ion channel opens, or a gene is switched on or off.
Two kinds of receptor
- Cell surface (membrane) receptors are used by water-soluble (hydrophilic) signals that cannot cross the membrane. The ligand binds outside and the message is relayed inward.
- Intracellular receptors are used by lipid-soluble (hydrophobic) signals, such as steroid hormones, which pass through the membrane and bind a receptor inside the cell, often acting as a transcription factor.
Second messengers and amplification
A second messenger is a small intracellular molecule, such as cyclic AMP, produced in response to receptor activation. It spreads the signal through the cytoplasm and activates a cascade of enzymes (often protein kinases). Because each enzyme activates many of the next, a single signal molecule can trigger a large response: this is amplification.
Examples in context
Example 1. Steroid hormones and gene expression. Oestrogen is lipid-soluble, crosses the membrane, and binds an intracellular receptor. The hormone-receptor complex enters the nucleus and acts as a transcription factor, switching on specific genes. This directly links cell signalling to the control of gene expression.
Example 2. Insulin and glucose uptake. Insulin binds a surface receptor on muscle and fat cells, triggering a transduction pathway that moves glucose transporter proteins to the membrane, increasing glucose uptake. The example shows a response that changes membrane transport rather than gene expression.
Try this
Q1. Name the three stages of cell signalling in order. [1 mark]
- Cue. Reception, transduction, response.
Q2. Explain what is meant by a second messenger and give one example. [2 marks]
- Cue. A small intracellular molecule made in response to receptor activation that relays and amplifies the signal inside the cell; cyclic AMP is an example.
Q3. Explain why a single signal molecule can cause a large cellular response. [2 marks]
- Cue. Transduction occurs as a cascade in which each activated enzyme activates many molecules of the next, amplifying the original signal greatly.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksDescribe the three main stages of cell signalling, using a hormone that binds a cell surface receptor as your example.Show worked answer →
Examiners want the three named stages, each explained in sequence.
Reception: the hormone (the signal molecule, or ligand) binds to a specific membrane receptor protein on the target cell. Binding is specific because the receptor's shape is complementary to the ligand, so only target cells with that receptor respond.
Transduction: the binding changes the shape of the receptor, which activates a chain of intracellular events. This commonly involves a G protein activating an enzyme that produces a second messenger such as cyclic AMP, which in turn activates a cascade of protein kinases. Each step can activate many molecules, so the signal is amplified.
Response: the final activated molecules bring about the cellular response, such as activating an enzyme, opening an ion channel, or switching on transcription of a gene.
Markers reward the three correctly named and ordered stages, the specificity of receptor binding, the idea of a transduction cascade, and a sensible example of a response.
Original3 marksExplain why a lipid-soluble hormone can use an intracellular receptor whereas a water-soluble hormone must use a cell surface receptor.Show worked answer →
The answer turns on the membrane being a non-polar barrier.
A lipid-soluble (hydrophobic) hormone, such as a steroid, can dissolve in and pass through the hydrophobic phospholipid core of the cell surface membrane. It can therefore reach receptors inside the cell, often in the cytoplasm or nucleus, where the hormone-receptor complex frequently acts directly as a transcription factor.
A water-soluble (hydrophilic) hormone cannot cross the non-polar core of the membrane. It must instead bind a receptor on the outer surface of the membrane, and the message is then relayed inside by transduction without the hormone itself entering the cell.
Markers reward the link between solubility and the ability to cross the hydrophobic core, the location of the receptor in each case, and the point that the water-soluble hormone relies on a relayed signal.
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