Cell Junction
Cell junctions are specialized regions of contact between adjacent cells or between cells and the extracellular matrix, serving roles in structural integrity, intercellular communication, and selective barrier function. In animal tissues, the three major types are tight junctions (which form impermeable seals between epithelial cells), anchoring junctions (desmosomes and hemidesmosomes, which provide mechanical strength), and gap junctions (which form channels allowing direct cytoplasmic communication between cells). Plant cells use plasmodesmata as their equivalent of gap junctions, enabling movement of water, nutrients, and signaling molecules between cells through the cell wall.
Types of Cell Junctions and Their Properties
| Junction Type | Location | Function | Key Proteins | Example Tissue |
|---|---|---|---|---|
| Tight junction (Zonula occludens) | Apical between epithelial cells | Seal to prevent paracellular leakage | Claudin, occludin, ZO proteins | Intestinal epithelium, blood-brain barrier |
| Adherens junction (Zonula adherens) | Below tight junction | Cell–cell adhesion, tension | E-cadherin, catenins, actin | Epithelial sheets, cardiac muscle |
| Desmosome (Macula adherens) | Lateral cell surfaces | Resist mechanical stress | Desmoglein, desmoplakin, keratin | Skin (epidermis), heart muscle |
| Gap junction | All cell surfaces | Cytoplasmic communication, electrical coupling | Connexins (connexons) | Heart, smooth muscle, neurons |
| Hemidesmosome | Basal surface of epithelium | Anchor cell to basement membrane | Integrin, plectin | Epidermis–basal lamina |
| Plasmodesmata (plants) | Through cell wall | Cytoplasmic continuity, signaling | Callose, actin | All plant tissues |
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Related Terms
Cytoskeleton
The cytoskeleton is a dynamic network of protein filaments and tubules that extends throughout the cytoplasm of eukaryotic cells, providing structural support, shape, and mechanical resistance. It consists of three main components: microfilaments (actin), intermediate filaments, and microtubules, each with distinct diameters and functions. The cytoskeleton also facilitates intracellular transport, cell division, and cell motility through the action of associated motor proteins such as myosin, kinesin, and dynein.
Cell Wall
The cell wall is a rigid or semi-rigid layer located outside the plasma membrane in plant cells, fungi, bacteria, and some algae, providing structural support and protection against mechanical stress and osmotic lysis. In plants, the primary cell wall is composed mainly of cellulose microfibrils embedded in a matrix of hemicellulose and pectin, while the secondary cell wall may also contain lignin for added rigidity. The cell wall is absent in animal cells, which instead rely on the extracellular matrix for structural support.
Active Transport
Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient (from low to high concentration), requiring the expenditure of cellular energy in the form of ATP. Primary active transport uses ATP directly to power transport proteins called pumps, while secondary active transport uses the electrochemical gradient established by primary active transport to drive the movement of another solute. Active transport is essential for maintaining cellular ion balances, nutrient uptake, and nerve impulse transmission.
From Latin "cella" (small room) and "junctio" (a joining), from "jungere" (to join). The structural characterization of cell junctions began in the 1950s–1960s with advances in electron microscopy, with tight junctions described by Farquhar and Palade in 1963.