Atlas of Plant and Animal Histology
Español
Home » The cell » Cell membrane » Carbohydrates

The cell. 3. Cell membrane.

CARBOHYDRATES

« Proteins Permeability, fluidity »

Membrane carbohydrates are chemically linked to lipids (glycolipids), proteins (glycoproteins), and as part of proteoglycans.

Glycocalyx: all the membrane carbohydrates.

Glycolipids: glycosphingolipids, glycoglycerolipids, glycosylphosphatidylinositol.

Carbohydrates are sites cell-cell recognition.

Membrane carbohydrates are mostly covalently bound to glycolipids and glycoproteins. However, some membrane carbohydrates are part of proteoglycans, molecules that insert their amino acid chain among the lipid fatty acids. Although some carbohydrates can be found associated to intracellular membranes, the majority are located in the outer monolayer of the plasma membrane, facing the extracellular space. Membrane carbohydrates are mostly synthesized in the Golgi complex, and at less extent in the endoplasmic reticulum.

Three types of glycolipids are found in membranes: glycosphingolipids, which are the most abundant, glycoglycerolipids, and glycophosphatidylinositol. Glycoglycerolipids are more frequent in the plasma membrane of plant cells. However, most of the membrane carbohydrates are found as part of proteins, known as glycoproteins. Whereas nearly all of the membrane proteins have carbohydrates only 5 % of lipids are glycolipids. Carbohydrates of the plasma membrane as a whole are referred as glycocalyx, that can be up to 2 to 10 % of the membrane weight. Glycocalyx is so developed in some cell types that can be observed with electron microscopy.

Carbohydrates

Carbohydrates in the plasma membrane. Glycolipids are mainly sphingolipids with different carbohydrate compositions. Some proteoglycans have a part of their amino acid sequence inserted among the lipid fatty acid chains. Most of the carbohydrates are covalently linked to proteins, known as glycoproteins, either by O-linked glycosylation (via serine amino acid) or by N-linked glycosylation (via asparagine amino acid). (Modified from Fuster and Esko, 2005).

Membrane carbohydrates may work as a physical barrier that protects the cell, but they also may carry out other important functions for the cell. For example, they are molecules for recognition and binding in cell-cell signalling. Blood groups are determined by cell surface carbohydrates, which also have the ability to trigger immunological responses. After an infection, endothelial cells close to injured tissue expose a type of proteins, known as selectins, in their plasma membranes which recognize and bind to carbohydrates of the plasma membrane of lymphocytes in the bloodstream. In this way, lymphocytes get attached to the blood vessel walls, can cross the endothelium and go to the infection focus. Carbohydrates as recognition molecules are also important during embryonic development, and also during pathogen infection. Virus, such as influenza virus, pathogenic E. coli bacteria, and some protozoa need to be attached to the cell surface before entering the cell, because otherwise they will be swept by cleaning mechanisms of the body. These pathogens have proteins, known as lectins, that bind to specific carbohydrates of some cells. Thus, the type of cell to be infected depends on the carbohydrates they show in the plasma membrane. Vertebrates, invertebrates and protozoa bear different set of carbohydrates in their cells.

Bibliography

Fuster MM, Esko JD . The sweet and sour of cancer: glycans as novel therapeutic targets. Nature reviews cancer. 2005. 5(7):526-542.


« Proteins Permeability, fluidity »

Home » The cell » Cell membrane » Carbohydrates
Updated: 2016-04-06. 11:59