OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    (b) the communication between cells by cell signalling

    Full specification wording

    Communication and homeostasis: (b) the communication between cells by cell signalling

    Cell signalling is how one cell releases a chemical messenger that is detected by a target cell, changing the target cell's behaviour.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 2 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Cell signalling is how one cell releases a chemical messenger that is detected by a target cell, changing the target cell's behaviour. The messenger (a ligand) binds a specific receptor, often a membrane protein with a complementary binding site. Binding triggers a response inside the target, such as opening an ion channel, activating an enzyme cascade or altering gene expression. Because receptors are specific, only cells with the correct receptor respond, which is why a hormone can travel in blood yet affect only its target tissues. Signalling can be local (for example a neurotransmitter diffusing across a synapse) or long-distance (a hormone carried in the bloodstream). The signal is transient: the messenger is broken down or removed, so the response stops.

    What earns the marks

    • A signalling molecule (ligand) binds to a specific receptor with a complementary shape on or in the target cell.
    • Receptor specificity means only target cells possessing that receptor respond to the messenger.
    • Binding triggers a response in the target cell, such as an ion channel opening, an enzyme cascade or a change in gene expression.
    • Signalling may be local (for example across a synapse) or long-distance (for example a hormone transported in the blood).
    • Removal or breakdown of the messenger ends the response, making signalling transient.

    Common mistakes and how to put them right

    • Assuming any cell can respond to any messenger.

      Put it right: Only cells with the complementary receptor respond.

    • Confusing the messenger with the receptor.

      Put it right: The messenger is the ligand; the receptor is the protein that binds it.

    • Thinking the signal continues indefinitely.

      Put it right: The messenger is removed or broken down so the response is temporary.

    Exam tips

    • Identify the four components in each option: messenger, receptor, target cell and response.
    • Reject options that imply a messenger acts without a receptor.
    • Check whether the example given is local or long-distance before choosing.

    Try the lesson

    The start of the Communication and homeostasis: (b) the communication between cells by cell signalling lesson, made from checked teaching notes for this specification point.

    Have a go first

    A cell releases a signalling molecule that travels through the blood and binds to receptors on a distant target cell. Which type of signalling does this describe?

    Listen

    Cell signalling is how one cell releases a chemical messenger that is detected by a target cell, changing the target cell's behaviour. The messenger (a ligand) binds a specific receptor, often a membrane protein with a complementary binding site. Binding triggers a response inside the target, such as opening an ion channel, activating an enzyme cascade or altering gene expression. Because receptors are specific, only cells with the correct receptor respond, which is why a hormone can travel in blood yet affect only its target tissues. Signalling can be local (for example a neurotransmitter diffusing across a synapse) or long-distance (a hormone carried in the bloodstream). The signal is transient: the messenger is broken down or removed, so the response stops. A strong answer earns marks for these points. A signalling molecule (ligand) binds to a specific receptor with a complementary shape on or in the target cell. Receptor specificity means only target cells possessing that receptor respond to the messenger. Binding triggers a response in the target cell, such as an ion channel opening, an enzyme cascade or a change in gene expression. Signalling may be local (for example across a synapse) or long-distance (for example a hormone transported in the blood). A common slip: Assuming any cell can respond to any messenger. Instead, only cells with the complementary receptor respond. Exam tip: Identify the four components in each option: messenger, receptor, target cell and response.

    See it

    A top-down flowchart. The signalling cell releases a messenger (ligand), which reaches the target cell. The ligand binds a specific receptor with a complementary shape. From this binding, one branch notes that only cells with the correct receptor respond, and another branch leads to a response being triggered in the target cell. That response is an ion channel opening, an enzyme cascade or a change in gene expression. From the messenger reaching the target cell, two further branches show local signalling, for example across a synapse, and long-distance signalling, for example a hormone in the blood. From the triggered response, a final step shows the messenger being removed or broken down, which stops the response, making it transient.
    Shows a messenger binding a specific receptor on a target cell, the responses this triggers, local versus long-distance signalling, and how removal of the messenger ends the response.

    Key words

    Say what each word means before you turn the card.

    Specificity of receptor–ligand binding

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    Target cell response

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    Local versus long-distance signalling

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    Termination of the signal

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    See every definition
    Specificity of receptor–ligand binding
    A ligand binds a specific receptor with a complementary binding site, so only cells with the correct receptor respond.
    Target cell response
    Binding triggers a response inside the target, such as opening an ion channel, activating an enzyme cascade or altering gene expression.
    Local versus long-distance signalling
    Signalling can be local, for example a neurotransmitter diffusing across a synapse, or long-distance, a hormone carried in the bloodstream.
    Termination of the signal
    The signal is transient because the messenger is broken down or removed, so the response stops.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The signalling cell releases a messenger (ligand), which reaches the target cell. The ligand binds a specific receptor with a complementary shape. From this binding, one branch notes that only cells with the correct receptor respond, and another branch leads to a response being triggered in the target cell. That response is an ion channel opening, an enzyme cascade or a change in gene expression. From the messenger reaching the target cell, two further branches show local signalling, for example across a synapse, and long-distance signalling, for example a hormone in the blood. From the triggered response, a final step shows the messenger being removed or broken down, which stops the response, making it transient.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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