cell junctions

Meaning: Cell junctions let tissues act as organized units. The three classic animal types differ in function:

Tight junctions: belts of proteins (claudins, occludin) around the apical side of epithelial cells. They seal the space between cells so substances must cross through cells, and they help keep apical and basolateral membrane proteins separate.

Desmosomes: spot-like anchors that link the intermediate filaments (keratin) of adjacent cells through cadherins. They resist mechanical stress, so they are abundant in skin and cardiac muscle.

Gap junctions: channels formed by connexins that join the cytoplasm of two cells. Ions and small molecules pass, but large molecules such as proteins do not. They allow electrical coupling, for example in cardiac muscle and some smooth muscle.

Hemidesmosomes anchor a cell to the basement membrane through integrins. Plants have a functional counterpart to gap junctions: plasmodesmata.

On the MCAT: Expect a passage describing a tissue (intestinal epithelium, heart, skin) or a mutation or drug that disrupts one junction. You then predict the result: loss of tight junctions increases paracellular leakage, loss of desmosomes makes tissue fragile under stress, and loss of gap junctions impairs coordinated signaling or contraction.

Common confusion: Students mix up desmosomes and tight junctions because both hold cells together. Tight junctions seal and block passage between cells. Desmosomes provide mechanical strength but do not seal. Only gap junctions allow cytoplasmic continuity.