22-23) The Chemical Composition of the Plasma Membrane. Tight Junctions, Desmosomes and Gap Junctions

Tight Junctions- a series of integral protein molecules in the plasma membranes of adjacent cells that fuse together, forming an impermeable junction that encircles the cell. Tight junctions help prevent molecules from passing through the extracellular space between adjacent cells.

Desmosomes- are anchoring junctions; mechanical couplings scattered like rivets along the sides of abutting cells that prevent their seperation. Desmosomes bind neighboring cells together and contribute to a continuous internal network. This arrangement distributes tension throughout a cellular sheet and reduces the chance of tearing when it is subjected to pulling forces. Desmosomes are abundant in tissues subjected to great mechanical stress, such as skin and heart muscle.

Gap Junctions- are communicating junctions that allow chemical substances to pass between adjacent cells. The adjacent plasma membranes are very close, and the cells are connected by hollow cylinders called connexons which are composed of transmembrane proteins. Ions, simple sugars and other small molecules pass through these water-filled channels from one cell to the next. Gap junctions are present in electrically excitable tissues, such as the heart and the smooth muscle, where ion passage from cell to cell helps synchronize their electrical activity and contraaction.

-The Plasma Membrane is made of a double layer of lipids within which proteins are embedded. Proteins may extend entirely through the lipid bilayer or protrude on only one face; externally facing proteins and some lipids have attached sugar grouos.
-The Plasma Membrane acts as a selectively permeable barrier. Substances move across the plasma membrane by passive processes, which depend on the kinetic energy of molecules or on pressure gradients, and by active processes which depend on the use of cellular energy (ATP). This is essential for functioning of excitable cells; externally facing proteins act as receptors (for hormones, neurotransmitters, etc.) and in cell-to-cell recognition.