Cells signal one another with chemicals:
Receptor Proteins and Signaling Between Cells
• Receptor proteins are located on or within the cell, and have three-dimensional shapes
that fit the shape of specific signal molecules.
• Binding of the signal molecule with the receptor protein induces a change in the
protein's shape and produces a cellular response.
• Immunochemistry and molecular genetics are being used to locate and characterize
receptor proteins.
Types of Cell Signaling
• Cells can communicate through any of four basic mechanisms: direct contact,
paracrine signaling, endocrine signaling, or synaptic signaling.
Proteins in the cell and on its surface receive signals from other cells:
Intracellular Receptors
• All cell-signaling pathways share certain common elements, including a chemical signal
that passes from one cell to another and a receptor that receives the signal in or on the
target cell.
• Intracellular receptors may trigger a variety of responses in the cell, dependent on the
receptor.
Cell Surface Receptors
• Cell surface receptors convert the extracellular signal to an intercellular one,
responding to the binding of the signal molecule to the cell's outside by producing a
change inside the cell.
• Many cell surface receptors either act as enzymes or are directly linked to enzymes.
• G-protein-linked receptors activate an intermediary protein, which then effects the
intercellular change.
Follow the journey of information into the cell:
Initiating the Intracellular Signal
• Second messengers, such as cAMP and calcium ions, relay messages from receptors
to target proteins.
Amplifying the Signal: Protein Kinase Cascades
• Some surface receptors generate large intracellular responses because each stage of
the pathway amplifies the next, causing a cascading effect.
Cell surface proteins mediate cell—cell interactions:
The Expression of Cell Identity
• As an organism develops, its cells acquire their specific identities by controlling gene
expression, turning on the specific set of genes that encode the particular functions of
each cell type.
• Every cell contains surface marker proteins that uniquely identify each cell type.
Intercellular Adhesion
• Cells attach to one another using cell junctions.
• Tight junctions connect the plasma membranes of adjacent cells in a sheet.
• Anchoring junctions mechanically attach the cytoskeleton of a cell to the cytoskeletons
of other cells or to the extracellular matrix.
• Communication junctions allow communication with adjacent cells through direct
connections.