AP Syllabus focus:
‘Some hormones circulate through the bloodstream to distant targets, and ligand-gated ion channels open or close when specific ligands bind.’
Cells often need fast, coordinated changes in activity across tissues.
This figure compares major cell-signaling modes and highlights endocrine signaling as long-distance communication via the bloodstream. It visually reinforces that hormones can reach many tissues but only cells with appropriate receptors are true targets. Source
In animals and plants, long-distance hormones deliver information widely, while ligand-gated ion channels enable rapid electrical and chemical responses in target cells.
Hormones as long-distance chemical signals
What hormones are and why they matter
Hormone: A chemical messenger secreted by cells that travels through body fluids (often blood) to regulate the activity of specific distant target cells that have the appropriate receptor.
Hormones support organism-level coordination because a signal released in one location can produce effects elsewhere, often at very low concentrations.
Delivery through the bloodstream to distant targets
Key idea from the syllabus: some hormones circulate through the bloodstream to distant targets. This enables:
Endocrine-style signaling: secretory cells release hormones into blood; circulation distributes them throughout the body.
Specificity by receptors: many cell types are exposed, but only target cells with the matching receptor respond.
Persistence and timing: effects can be brief or prolonged depending on hormone stability, clearance from blood, and receptor interactions.
Types of hormones and receptor location (high-utility distinctions)
Hormones differ chemically, which influences how they travel and where they bind.
Water-soluble hormones (often peptides/proteins)
Travel readily in blood plasma.
Typically bind cell-surface receptors because they do not easily cross the lipid bilayer.
Often trigger changes in existing cell activity (e.g., enzyme regulation).
Lipid-soluble hormones (often steroids)
Often travel bound to carrier proteins in blood.
Can cross membranes more readily and bind intracellular receptors (cytoplasm or nucleus).
Commonly alter gene expression by regulating transcription after receptor binding.
This figure traces the pathway for lipid-soluble hormones: diffusion through the membrane, binding an intracellular receptor, and the hormone–receptor complex entering the nucleus to influence transcription. It reinforces why steroid-like signals often produce longer-lasting effects by changing gene expression rather than only modifying existing proteins. Source
What determines a target cell’s response
Even when the same hormone reaches many tissues, responses can differ because of:
Receptor expression level (how many receptors are present)
Receptor subtype (different proteins can bind the same hormone but produce different outcomes)
Cell state (available enzymes, ion gradients, or transcriptional machinery)
Ligand-gated ion channels
Definition and core mechanism
Ligand-gated ion channel: A membrane protein channel that changes conformation when a specific ligand binds, causing the channel to open or close and altering ion movement across the membrane.
The syllabus focus: ligand-gated ion channels open or close when specific ligands bind.
This diagram shows a ligand binding to a gated ion channel and triggering a conformational change that switches the channel between closed and open states. It emphasizes that ion movement across the membrane is directly coupled to ligand binding, enabling rapid electrical and chemical responses. Source
This directly links chemical binding to immediate changes in membrane permeability.
How ligand binding changes cell behavior
When a ligand (often a neurotransmitter or local chemical signal) binds:
The channel opens or closes due to a conformational change.
Specific ions (e.g., Na⁺, K⁺, Ca²⁺, Cl⁻) move down their electrochemical gradients.
Ion movement shifts membrane potential and/or intracellular ion concentrations.
The cell produces a rapid response, such as:
initiating or inhibiting an electrical impulse
triggering secretion
activating ion-dependent enzymes (especially with Ca²⁺ as an intracellular regulator)
Why ligand-gated channels enable rapid signalling
Compared with responses that require multi-step biochemical pathways, ligand-gated channels are fast because:
The receptor is the channel (binding is directly coupled to transport).
Opening can occur in milliseconds, allowing quick coordination in excitable tissues (e.g., neurons, muscle).
Specificity and termination of the signal
To ensure accurate communication:
Specificity comes from the channel’s ligand-binding site and ion selectivity filter.
Signal termination occurs when:
the ligand dissociates and the channel returns to its closed state
the ligand is removed (diffusion, enzymatic breakdown, or reuptake by cells)
ion gradients are restored by pumps and transporters, resetting membrane conditions
FAQ
No. Different channels are selective due to the pore’s structure.
Common selectivities include Na⁺, K⁺, Ca²⁺, or Cl⁻, which can produce very different effects on membrane potential.
Cells can express different receptor subtypes for the same hormone.
Downstream proteins present in each tissue (ion channels, enzymes, transcription factors) also differ, so the same binding event can be interpreted differently.
Key factors include hormone stability, breakdown by enzymes, uptake by the liver/kidneys, and whether it circulates bound to carrier proteins.
Receptor binding strength and receptor internalisation can also influence effective signalling duration.
Ca²⁺ can act as an intracellular regulator by binding to proteins and changing their activity.
A small Ca²⁺ influx can therefore initiate larger cellular changes, such as secretion or changes in contractile activity.
Ligands are rapidly cleared from the synaptic space.
Mechanisms include enzymatic degradation (for some neurotransmitters) and reuptake into presynaptic cells or surrounding support cells, limiting repeated channel opening.
Practice Questions
Explain how hormones can affect distant target cells without affecting every cell in the body.
Hormones are transported via the bloodstream/body fluids to many tissues (1).
Only cells with the specific receptor respond (1).
Describe how ligand-gated ion channels transmit a signal across a cell membrane and lead to a cellular response. (5 marks)
A specific ligand binds to the ligand-gated ion channel receptor on the membrane (1).
Binding causes a conformational change that opens or closes the channel (1).
Ions move across the membrane down their electrochemical gradient (1).
Ion movement changes membrane potential and/or cytosolic ion concentration (1).
This change triggers a response (e.g., electrical excitation/inhibition or activation of ion-dependent processes) (1).
