1. Receptor proteins on or in a cell determine to which extracellular signals a cell will respond.
2. Paracrine signaling has a short-lived, local effect on neighboring cells. Endocrine signals are secreted into the blood, are comparatively slow to effect a response, and can be very long-lasting. Synaptic signaling involves the transmission of a signal over a gap (synapse) between cells for an effect in another nerve or muscle cell.
3. Intracellular receptors can control cell activities enzymatically or through regulation of gene transcription.
4. Chemically gated ion channels typically consist of multi-pass helical protein domains that create "safe passage" through the hydrophobic region of the membrane for an ion.
5. Enzymic receptors typically consist of a single pass by the protein through the membrane bilayer. The extracellular portion of the receptor binds the signal, triggering activity at the intracellular end of the receptor, which is usually phosphorylation of some intracellular protein.
6. G proteins are intracellular shuttles that relay the message from the membrane signal receptor to other regions in the cell, continuing the cascade of events precipitated by binding to the signal.
7. Second messengers are intracellular intermediaries that are triggered by nonsteroid signals binding to the outside surface of the cell. Substances such as cAMP or calcium go on to trigger intracellular response to the signal.
8. Binding of the signal to a receptor may result in signal amplification within the cell via multiple protein kinase phosphorylation.
9. Cell surface markers serve to identify cells in the context of where they belong (i.e., which tissue), or serve to identify the cell as "self" to the immune system, which continually monitors the body for the presence of "nonself" invaders. Most of these markers are glycolipids.
10. Tight junctions function in holding cells tightly together at leak-proof points by a belt of fibrous proteins encircling the cell. Desmosomes "spot-weld" cells together at periodic locations using proteins called cadherins. Adherens junctions involve integrin proteins, which promote the fusion of intracellular matrix proteins with extracellular matrix material, holding cells in specific contact with other structures.
11. Gap junctions are communicating patches between cells formed by connexon proteins which, when aligned, form "pipelines" between adjacent cells. Small molecules (such as certain neurotransmitters) and ions can pass between the openings in the gap junctions.
12. Plasmodesmata are communicating patches of cytoplasm that extend from one cell to another in plants.