What the exam asks
- Classify a signaling mechanism: Given an experimental scenario or diagram, identify whether the cells are communicating via direct contact, short-distance (local) signaling, or long-distance (endocrine) signaling.
- Explain specificity: Explain why a chemical signal travelling through the bloodstream or extracellular fluid affects only specific target cells and leaves neighboring non-target cells unaffected.
- Predict the effect of a disruption: Predict what happens to intercellular communication if gap junctions, plasmodesmata, extracellular fluid diffusion, or vascular transport is blocked.
Core ideas
1. Direct contact (juxtacrine and junctional signaling)
Cells in physical contact can communicate without releasing a signal into the extracellular space.
- Cell-to-cell junctions: Gap junctions in animal cells and plasmodesmata in plant cells form cytoplasmic channels between adjacent cells. Small water-soluble signaling molecules, ions (such as ), and second messengers pass directly from the cytosol of one cell into the cytosol of the next.
- Cell-surface recognition: Membrane-bound proteins or glycoproteins on one cell bind directly to complementary receptor proteins on the plasma membrane of an adjacent cell. A classic exam example is antigen presentation: an antigen-presenting immune cell displays a foreign antigen on its surface, which binds directly to a surface receptor on a helper T cell.
2. Short-distance (local) chemical signaling
Cells release chemical ligands into the immediate extracellular fluid to act on nearby target cells within a short range by simple diffusion.
- Paracrine signaling: A secretory cell releases local regulators (such as growth factors or morphogens that guide embryonic development) into the interstitial fluid, stimulating nearby target cells.
- Autocrine signaling: A cell secretes a chemical ligand that binds to receptors on its own plasma membrane (often seen in immune cell proliferation or tumor growth).
- Synaptic signaling: A specialized neuron releases neurotransmitters across a narrow microscopic gap (the synapse) to bind receptors on the target neuron or muscle cell.
- Quorum sensing: Unicellular prokaryotes (bacteria) secrete signaling molecules called autoinducers into their surroundings. As bacterial population density increases, the concentration of autoinducers rises until it crosses a threshold, binding receptors on the bacteria and triggering coordinated group behaviors such as biofilm formation or bioluminescence.