AP Syllabus focus:
‘At short distances, local regulators are released and bind to nearby target cells to change their behavior.’
Short-distance signalling lets cells coordinate rapidly and precisely within a tissue. Instead of travelling through the bloodstream, signals diffuse locally, creating targeted responses such as growth, movement, secretion, or changes in gene expression.
Core idea: local regulators act nearby
This diagram contrasts major cell-signaling ranges, emphasizing that paracrine signals act on nearby target cells and autocrine signals act back on the signaling cell. It reinforces the core AP Biology idea that local regulators produce localized responses because they do not travel through the bloodstream. Source
What counts as “short distance”
Local regulators are chemical signals released by a cell that affect nearby target cells by binding to specific receptors, triggering a change in cell behaviour.
Local regulator: A secreted signalling molecule that diffuses a short distance to bind receptors on nearby cells, producing a specific cellular response.
A local regulator’s impact depends on:
Concentration gradient (highest near the source cell)
Receptor presence on neighbouring cells (only target cells respond)
Signal stability (how fast it is broken down or taken up)
Major types of short-distance signalling
Paracrine signalling: a cell signals other nearby cells (common in animal tissues).
Autocrine signalling: a cell signals itself (or same-type neighbours), reinforcing a response.
Synaptic signalling: neurons release neurotransmitters across a tiny gap (synapse) to a specific target cell; diffusion distance is extremely short and highly directed.
Steps in short-distance signalling
1) Release of a local regulator
Cells release signals by:
Exocytosis (common for peptide signals and neurotransmitters)
Diffusion across the membrane (some small or lipid-soluble signals)
Processing from precursors (inactive forms converted to active signals near release)
2) Movement through extracellular space
Local regulators typically move by diffusion, so proximity matters. Short-distance signals are well-suited for tissues because they can:
Coordinate neighbouring cell activities during development and repair
Create regional differences in cell behaviour within the same tissue
Factors shaping diffusion and reach:
Extracellular matrix composition (can slow or channel diffusion)
Cell density and tissue architecture (affects paths and gradients)
Enzymatic breakdown that limits how far the signal persists
3) Reception by nearby target cells
A local regulator binds a specific receptor, and this specificity determines which cells respond. Reception can lead to:
Changes in enzyme activity (fast responses)
Changes in gene expression (slower, longer-lasting responses)
Changes in cytoskeleton and motility (e.g., directed movement)
Common examples of local regulators (AP-relevant)
Growth factors
Growth factors stimulate cell division, survival, or differentiation in nearby cells, helping coordinate tissue-level outcomes.
This figure illustrates receptor tyrosine kinase (RTK) activation, where ligand binding drives receptor dimerization followed by tyrosine autophosphorylation. The phosphorylated RTK then initiates downstream signaling cascades that can alter gene expression and cell-cycle progression—core mechanisms behind growth factor effects on proliferation and differentiation. Source
They can promote cell cycle entry or cell growth in neighbouring cells.
Overproduction or inappropriate receptor activation can cause excessive local proliferation.
Nitric oxide (NO)
Nitric oxide is a small gas that diffuses readily across membranes and often acts briefly.
Because NO is short-lived, it is well-suited for local, transient signalling.
Its effects depend strongly on local concentration and rapid breakdown.
Neurotransmitters (synaptic signalling)
Neurons release neurotransmitters into the synaptic cleft, affecting a very nearby target cell (another neuron, muscle cell, or gland cell).
This diagram shows the key steps of synaptic signaling: calcium-triggered vesicle fusion (exocytosis), neurotransmitter diffusion across the synaptic cleft, and receptor binding on the postsynaptic cell. It also highlights signal termination mechanisms (reuptake and enzymatic breakdown), explaining why synaptic signals are rapid and spatially precise. Source
High spatial precision comes from targeted release at synapses.
Binding can rapidly alter ion flow or intracellular signalling, changing target cell behaviour.
Why short-distance signalling is biologically useful
Short-distance signalling allows cells in a tissue to act as a coordinated unit while still producing distinct local responses.
Precision: only cells within range and with the correct receptors respond
Speed: diffusion across small distances enables rapid responses
Patterning: gradients can assign different “instructions” to cells at different distances
Flexibility: cells can integrate multiple local regulators at once, producing a combined response (e.g., stronger activation only when multiple signals are present)
FAQ
They can bind, slow, or channel molecules through the tissue.
This can sharpen gradients, extend persistence by “reservoir” binding, or restrict movement to particular regions.
Autocrine signalling is favoured when the secreting cell must reinforce its own state.
It often depends on receptor expression on the secreting cell and the benefit of self-amplification versus neighbour coordination.
Short lifetimes prevent unintended spread and keep responses local.
Rapid degradation also enables quick “off” switching when secretion stops.
Different target cells may express different receptor subtypes or downstream signalling proteins.
The same ligand can therefore activate distinct gene expression programmes or enzyme responses.
They may lack the receptor, internalise it quickly, or secrete binding proteins that sequester the ligand near neighbouring cells.
Spatial separation of secretion sites and receptors within the same cell can also reduce self-response.
Practice Questions
Describe how local regulators influence only nearby target cells. (2 marks)
Local regulators are released and diffuse a short distance in extracellular fluid (1).
Only cells with the specific receptor bind the regulator and change behaviour (1).
Explain why diffusion and receptor distribution make short-distance signalling effective in tissues. (5 marks)
Signal concentration is highest near the secreting cell, forming a gradient (1).
Limited diffusion and/or rapid breakdown restricts the signal to local cells (1).
Only cells expressing the complementary receptor can respond (1).
Binding triggers intracellular changes that alter cell behaviour (1).
Different receptor patterns across a tissue allow region-specific responses (1).
