Cell Junctions

Most epithelial cells are tightly bound together.  Some muscle and nerve cells can be found joined together.   They perform together when joined as functional units.  There are five different types of cell junctions.

1. Tight junctions: the plasma membrane proteins are fused together. 
1. these junctions look as if someone took a sewing machine and sewed them together.
2. it looks like a web of transmembrane proteins.  
3. they tend to seal off passageways between adjacent cells
1. commonly found in the epithelial tissue of the stomach, urinary bladder and intestines.
2. They are used to slow down the passage of substances between cells and leaking into the blood or other tissues.
2. Adherons: proteins of the integral membrane are connected to the membranes of another cell.  
3. Desmosomes: attach to the intermediate filaments inside the cytoplasm.  
1. These junctions contain plaque and cadherins that extend into the intercellular space to attach adjacent cells together.  
1. The plaque attaches to intermediate filaments that contain keratin (a protein)
2. this prevents the epidermal cells from separating under stress.
4. Hemidesmosomes: only connect to extracellular material
5. Gap: The plasma membrane channels fuse together.  
1. Gap Junctions connect neighboring cells via tiny fluid-filled tunnels called connexons. 
1. Plasma membranes of gap junctions are separated by a very narrow intercellular gap
1. this allows for communication of cells within a tissue
2. ions, nutrients, waste, chemical and electrical signals travel through the connexons from one cell to another. 

(for the test, remember the ones with more information)