AP Syllabus focus:
‘Cells communicate via direct cell contact or chemical signaling that allows them to coordinate activities over varying distances.’
Cells survive as coordinated systems, not isolated units. Communication lets cells detect changes, share information, and synchronise behaviours like growth, movement, secretion, and gene activity across tissues and entire organisms.
Core idea: what “cell communication” means
Cell communication (cell signalling): The transmission of information between cells that produces a specific response in a target cell.
Communication requires a signal source, a target that can detect the signal, and an appropriate response; the major difference among modes is how the signal travels.
Two broad modes of communication
This diagram summarizes the major modes of cell signaling by showing how far a released ligand travels (autocrine: same cell; paracrine: nearby cells; endocrine: via bloodstream to distant targets) and contrasts these with contact-dependent signaling, which requires membrane-to-membrane interaction. The labels make it easy to connect “range” to “delivery route,” a key idea for predicting how quickly and broadly a signal can act. Source
1) Direct contact (contact-dependent signalling)
Direct cell contact occurs when cells physically touch, allowing information transfer without releasing a freely diffusible chemical over long distances.
Common direct-contact strategies include:
Cell junctions that create cytoplasmic connections
Gap junctions (animals): small molecules/ions pass between adjacent cells for rapid coordination.
This diagram shows how gap junction channels (connexons) align between adjacent animal cell membranes to create a continuous passageway. The structure explains the functional consequence emphasized in your notes: ions and small molecules can move directly between cells, enabling rapid coordination across a tissue. Source
Plasmodesmata (plants): channels through cell walls connect cytoplasm for transport and signalling.
Membrane-bound signals: a signalling molecule remains attached to one cell’s membrane and binds a receptor on a neighbouring cell only when they contact.
Cell–cell recognition via surface markers (glycoproteins): helps cells distinguish “self” vs. “non-self” and organise into tissues.
Direct contact is especially effective when:
signals must be localised to immediate neighbours,
rapid synchronisation is needed across a cell layer,
the organism needs precise spatial control (e.g., maintaining tissue boundaries).
2) Chemical signalling (information carried by released molecules)
In chemical signalling, a cell releases signalling molecules that travel to target cells. The same basic strategy can coordinate activities over varying distances, depending on how far the chemical travels and how it is delivered.
Ligand: A signalling molecule that binds specifically to a receptor on or in a target cell.
Major distance-based categories of chemical signalling:
Autocrine signalling (self-signalling)
A cell releases a ligand that binds to receptors on the same cell (or same cell type).
Useful for reinforcing a cellular state or coordinating a group of identical cells.
Paracrine signalling (local signalling)
Ligands diffuse through extracellular fluid to nearby cells.
Range is limited by diffusion, ligand breakdown, and uptake by surrounding cells.
Synaptic signalling (neuron-to-target)
This figure depicts synaptic signaling at a chemical synapse, with neurotransmitters released from the presynaptic cell into the synaptic cleft and binding receptors on the postsynaptic cell. It also highlights signal termination via neurotransmitter breakdown, helping explain why synaptic signals are rapid, local, and precisely directed. Source
A neuron releases neurotransmitters into a synaptic cleft; diffusion distance is tiny, but delivery is highly targeted.
Enables fast, specific communication to particular cells in a network.
Endocrine signalling (long-distance)
Hormones are released into body fluids (often the bloodstream) to reach distant targets.
Signals can be widespread, but only cells with the appropriate receptor respond.
What determines signalling range and coordination power?
Whether communication is direct contact or chemical signalling over short or long distances, effectiveness depends on:
Delivery route
Contact-dependent: requires adjacency.
Chemical: relies on diffusion, bulk flow (circulation), or directed release (synapses).
Signal persistence
Short-lived ligands favour local effects; stable ligands can act farther.
Target access and specificity
Chemical signals can reach many cells, but only receptor-bearing cells respond, allowing coordinated activity across different tissues.
Concentration effects
Chemical signals often form gradients; different concentrations can coordinate different responses across space.
FAQ
Hydrophobic hormones often bind carrier proteins in plasma, which improves solubility, reduces loss in urine, and extends half-life.
Different cells can express different receptor subtypes or coupling proteins, so the same ligand–receptor binding can trigger distinct downstream responses.
The electrical impulse travels far, but the chemical step (neurotransmitter diffusion) occurs only across the tiny synaptic cleft, keeping targeting precise.
They use rapid ligand degradation, uptake by neighbouring cells, binding to extracellular matrix, and restricted release sites to confine diffusion.
It is chemical signalling in microbes where secreted molecules accumulate with population density, triggering coordinated gene expression once a threshold concentration is reached.
Practice Questions
A plant root cell sends small signalling molecules directly into an adjacent root cell through cytoplasmic channels. Identify the mode of communication and name the structure involved. (2 marks)
Mode: direct contact / contact-dependent signalling (1)
Structure: plasmodesmata (1)
Compare direct contact signalling with endocrine signalling in terms of (i) distance travelled, (ii) speed and targeting, and (iii) how a multicellular organism ensures only specific cells respond. (5 marks)
Direct contact requires adjacent cells; endocrine can act at long distance via body fluids/bloodstream (1)
Direct contact is local and often rapid between neighbours; endocrine is typically slower due to transport through circulation (1)
Synaptic-like targeting not required for credit; credit targeting comparison: direct contact inherently localised; endocrine reaches many tissues but is less spatially restricted (1)
Specificity: only cells with the correct receptor respond (1)
Coordination statement linking mode to organism-wide regulation vs local tissue coordination (1)
