Cell Signaling Mechanisms

Direct Contact Signaling

• Cells communicate through cell junctions
• Cytoplasm passes freely between cells in animals (gap junctions) and plants (plasmodesmata)
• Small, nonpolar molecules pass easily through the cytoplasm
• Gap junctions and plasmodesmata allow materials and signaling substances to pass between adjacent cells.

Local Regulators

• Secreting cells release chemical messages (ligands) that travel short distances

• Chemical messages cause a response in a target cell

• Paracrine signaling: Secretory cells release local regulators (growth factors, hormones) via exocytosis to adjacent cells.

• Synaptic signaling: Occurs in animal nervous systems; neurons release neurotransmitters that diffuse across the synaptic cleft (space between nerve and target cell) to nerves or muscles.

• Local signaling is shorter distance than long distance signaling

• Local signaling targets nearby cells, compared to longer distance signaling

Long Distance Signaling

• Hormones are used for long-distance signaling
• Plants use vascular tissue or the air to carry hormones to target tissues
• Animals use endocrine signaling: specialized cells release hormones into the circulatory system to reach target cells.

Cell Signaling Steps

1. Reception

• Ligand (chemical message) binds to receptor in target cell
• This often triggers a conformational change (shape change) in the receptor protein
• Receptors can be in the plasma membrane (e.g., most common) or intracellular (e.g., inside the cytoplasm)

2. Transduction

• Conversion of extracellular signal to intracellular signal
• This often involves a series of changes (signal transduction pathway) that often involves phosphorylation and dephosphorylation to regulate proteins.
• Enzymes such as protein kinases add phosphate groups to proteins; protein phosphatases remove them
• Second messengers (small, non-protein molecules) help relay message
• Second messengers like cAMP increase signal amplification

3. Response

• Final molecule in the signal transduction pathway alters cell processes
• This can change membrane permeability, initiate enzyme activity, or alter gene expression
• Ligand binding to receptor initiates a series of events that leads to a cellular response

GPCRs (G Protein-Coupled Receptors)

• Largest category of cell surface receptors in animals
• Important in animal sensory systems
• Ligand binding to the GPCR activates GTP, in turn activating an enzyme or ion channel

Ion Channels

• Important for nervous system function
• Receptor acts as a gate for ions
• Specific ion movement leads to cellular response when ligand binds.