Читать книгу Cell Biology - Stephen R. Bolsover - Страница 50

In multicellular organisms, and particularly in their epithelia, it is often necessary for neighboring cells within a tissue to be connected. This function is provided by cell junctions. In animal cells there are three types of junction. Those that form a tight seal between adjacent cells are known as tight junctions; those that allow communication between cells are known as gap junctions. A third class of cell junction anchors cells together, allowing the tissue to be stretched without tearing. These are called anchoring junctions.

Tight junctions are found wherever the flow of extracellular medium is to be restricted and are particularly common in epithelial cells, such as those lining the small intestine. The plasma membranes of adjacent cells are pressed together so tightly that no intercellular space exists between them (Figure 1.6 on page 9). Tight junctions between the epithelial cells of the intestine ensure that the only way that molecules can get from the lumen of the intestine to the blood supply that lies beneath is by passing through the cells, a route that can be selective.

Gap junctions are specialized structures that allow cell‐to‐cell communication in animals (Figure 2.6). When two cells form a gap junction, ions and small molecules can pass directly from the cytosol of one cell to the cytosol of the other cell without going into the extracellular medium. Since ions can move through the junction, changes in electrical voltage are also rapidly transmitted from cell to cell by this route. In vertebrates, the structure that makes this possible is the connexon. When two compatible connexons meet, they form a tube, about 1.5 nm in diameter, that runs through the plasma membrane of the first cell, across the small gap between the cells, and through the plasma membrane of the second cell. This hole is large enough to allow through small organic molecules as well as ions, but it is too small for proteins or nucleic acids. The limit is a relative molecular mass (Mr) of about 1000. Gap junctions are especially important in the heart, where they allow an electrical signal to pass rapidly between all the cardiac muscle cells, ensuring that they all contract at the appropriate time. Each connexon is composed of six protein subunits called connexins that can twist against each other to open and close the central channel in a process called gating (page 147). This allows the cell to control the degree to which it shares material with its neighbor.