- Cell membranes act as barriers which separate cells as well as the cellular compartments.
- Membranes are made up composed of proteins, carbohydrates, and lipids that are held together with the help of binding forces.
- Carbohydrates are linked covalently to protein (glycoproteins) or Lipids (glycolipids) as well as an essential component of cell membranes. They serve as adhesion molecules and address loci of cells.
- The Fluid Mosaic Model describes membranes as a bilayer of fluid lipids that is composed of floating proteins and carbohydrates.
- Membrane carbohydrates can be chemically bound glycoproteins and glycolipids.
- However, certain proteins are part of proteoglycans, which place their amino acid chains within the lipid fats.
- While some carbohydrates are identified as intracellular membranes. However, the majority of them reside in the outer monolayer the plasma membrane. It faces to the space beyond.
Types of Membrane Carbohydrate
Carbohydrates are found only on the surface of plasma membrane. They are attached to proteins making glycoproteins, or lipids. They form glycolipids.
The majority of membrane carbohydrates are found in glycoproteins, also known as glycoproteins. Most membrane proteins contain carbohydrates. In glycoproteins, the bulk of the molecules is composed of proteins. They have Oligosaccharides that are attached to proteins and, in general, they are branching and don’t have serial repeats. Therefore, they contain a lot of information, creating highly specific locations for recognition as well as high-affinity binding to other proteins.
As with glycolipids sugar residues are added to an area called the ER in the ER and Golgi apparatus. Because of this, oligosaccharide chains are always found on the non-cytosolic end that is the outer membrane. The sugars are attached to proteins at two places in the cell: the endoplasmic retina that produces N-linked sugars as well as the Golgi apparatus that creates O-linked sugars. The glycoproteins with N-linked links have a sugar attached the nitrogen atom. O-linked glycoproteins contain sugar that is connected to oxygen atoms. The distinct structures of NO-linked and N-linked sugars gives them distinct roles. Membrane-bound glycoproteins play a role in a variety of cell functions, such as cell recognition and antigenicity of cell surfaces and more.
Glycolipids are lipids that form membranes with head groups that are hydrophilic are oligosaccharides. Three types of glycolipids are found in membranes: glycosphingolipids, which are the most abundant in the animal cells, glycoglycerolipids, and glycophosphatidylinositol. Glycoglycerolipids are found more frequently in the plasma membranes of plant cells.
The lipids that comprise only 5 % found in membranes comprise glycolipids. Like glycoproteins, they are the specific locations to be recognized by carbohydrate-binding proteins.
Polysaccharide chains in integral membranes are known as proteoglycans. Proteoglycans consist of long polysaccharide chains that are linked covalently to a protein core are typically found outside of the cell, in the extracellular matrix. But for some proteoglycans, the protein core either extends across the lipid bilayer or is attached to the bilayer by a glycosylphosphotidylinositol (GPI) anchor.
Structure of Membrane Carbohydrate
- Carbohydrates that are present inside the plasma membrane in small, sometimes branching chains of sugars linked to external extracellular proteins (forming glycoproteins) or to the polar end of phospholipid molecules that are located in the lipid layer that is outside (forming glycolipids).
- Carbohydrate chains can be made up of monosaccharide units ranging from 2-60 and can be straight or branching.
- The oligosaccharide chains of membrane glycoproteins and glycolipids are formed by various combinations of six principal sugars D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid (also called sialic acid), N-acetyl-D-glucosamine, and N-acetyl-D- galactosamine. All of them could be made from glucose.
- The oligosaccharide side chains in glycolipids and glycoproteins are incredibly varied in their sugar arranging.
- While they typically contain less than fifteen sugars they’re often divided and sugars may be joined through a variety of covalent linkages, unlike the amino acids found in polypeptide chains which are all connected with peptide bonds of the same type.
- Just three sugars can be combined to create hundreds of trisaccharides.
- In the end, both the range of oligosaccharides and the open place of the oligosaccharides the cell’s surface make them particularly well-suited for perform particular cell-based recognition processes.
Functions of Membrane Carbohydrates
- Membrane carbohydrates play two major roles: they participate in adhesion and recognition of cells in cell-cell signaling, or cell-pathogen interactions. Additionally, they play a structural function as physical barriers.
- The blood groups can be identified by the surface carbohydrates in erythrocytes and can initiate immune reactions.
- In the aftermath of an infection, endothelial cells that surround the damaged tissue reveal a specific proteins, called selectins, within the plasma membranes of their cells. They recognize , and then bind to carbohydrates in the plasma membranes of lymphocytes which travel throughout the bloodstream. This is how lymphocytes attach to blood vessel walls, and can traverse the endothelium, and then move towards the focus of infection.
- Carbohydrates are recognized molecules and are also essential in the embryonic stage.
- Carbohydrates in the plasma membrane are important for recognition and attachment locations for pathogens in the course of infection.
- The glycocalyx can also play significant functions for humans. It helps cells interior of blood vessels resist the forceful flow of liquid that flows across their surface.
- It guards microvilli in the gut that absorb nutrients. The glycocalyx can aid to break down food to aid in the absorption process by storing digestive enzymes within its coat.
- The various plasma transporters, hormones along with enzymes comprise glycoproteins and within these proteins carbohydrate plays a crucial role in the physiological function.