Читать книгу Introduction to the Human Cell - Danton PhD O'Day - Страница 14

While human cells don’t have a cell wall, they do have a sugar coating! Many membrane proteins are covalently linked to sugar residues. The residues may consist of a few sugars or they may extend into long carbohydrate moieties. The sugar groups in membrane proteins, as well as in glycolipids, always orient towards the external environment. They always orient outside the cell, never towards the cytoplasm. The following figure shows an example of an integral membrane glycoprotein (Figure 2.7).

Figure 2.7. The basic structure of a typical glycoprotein and its association with the cell membrane.

When the carbohydrate component of the glycoprotein is extensive, typically interacting with extracellular matrix components, it can be seen in the electron microscope. For example, the extensive “sugar coating” of the intestinal epithelium is called the glycocalyx (Figure 2.8). The extensive glycocalyx in the intestine protects against injury, bacterial infections and aids in absorption of nutrients. Some use the term glycocalyx more loosely to mean the surface carbohydrate component of all cells and not just the extensive coating exemplified by intestinal epithelial cells.

Figure 2.8. The glycocalyx of intestinal epithelial cell membranes.