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
Practice Questions
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.
