Membrane carbohydrate and their significance in cellular recognition

Carbohydrate in the plasma membrane and the role they perform in cell-cell recognition process.

1. Membrane Carbohydrate And Their Significance In Cellular Recognition Girish Kumar K V MSc. Biomedical Science Bharathidasan University

2. Membrane Carbohydrate Carbohydrates are the third major component of plasma membranes. The extracellular surface of the cell membrane is decorated with carbohydrate groups attached to lipids and proteins . These carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched . Carbohydrates are added to lipids and proteins by a process called glycosylation , and are called glycolipids or glycoproteins .

3. These short carbohydrates or oligosaccharides are usually chains of 15 or fewer sugar molecules . Oligosaccharides give a cell identity (i.e., distinguishing " self " from " nonself ") and are the distinguishing factor in human blood types and transplant rejection . Along with membrane proteins, these carbohydrates form distinctive cellular markers , that allow cells to recognize each other. These markers are very important in the immune system, allowing immune cells to differentiate between body cells, which they shouldn’t attack, and foreign cells or tissues, which they should.

4. Types of carbohydrate in Membrane Glycoprotein – Covalently bound to protein i). Exoplasmic face of plasma membrane. ii). They functions to increase the proteins’ solubility. iii). Improper for proper folding. Glycolipid – Covalently bound to lipid. i). Found on exoplasmic leaflet. ii). Carbohydrate portion faces the outside. iii). Glucosylcerebroside is an example for glycolipid. iv). Gangliosides in membrane of many nerve cells. Blood group antigens are glycolipids or glycoproteins.

5. Proteoglycans – Proteins that are heavily glycosylated. Glycosphingolipids – Subtype of glycolipids containing the amino alcohol sphingosine. Glycoprotein –  Proteins which contain oligosaccharide chains covalently attached to amino acid side-chains

6. Asymmetric Distribution Of Membrane Carbohydrate The distribution of carbohydrates in the cell membrane is asymmetric . The extracellular face of the membrane is in contact with the extracellular fluid . The extracellular side of the membrane contains oligosaccharides that distinguish the cell as " self ." It also contains the end of integral proteins that interact with signals from other cells and sense the extracellular environment . The inner membrane is in contact with the contents of the cell and tends to have few carbohydrates . Instead, this intracellular side of the membrane anchors to the cytoskeleton and contains the end of integral proteins that relay signals received on the external side.

7. Cell Membranes are Generally Asymmetrical

8. Cell-cell recognition is a cell’s ability to distinguish one type of neighboring cells from another. Cells recognize other cells by binding to molecules , often containing carbohydrates, on the extracellular surface of the plasma membrane. Membrane carbohydrate usually plays an important role in cell-cell recognition. They are usually short , branched chains of fewer than 15 sugar units . Some are covalently bonded to lipids , forming molecules called glycolipids . Most are covalently bonded to proteins , which are thereby glycoproteins . The carbohydrates on the extracellular side of the plasma membrane vary from species to species , among individuals of the same species , and even from one cell type to another in a single individual. They are used to identify a type of cell , if exploited by virus, then virus gains the entry. The Role of Membrane Carbohydrates in Cell-Cell Recognition

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