4/30/2023 0 Comments Fluid mosaic modelAlong with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other. These carbohydrate chains may consist of 2–60 monosaccharide units and can be either straight or branched. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). Integral membrane proteins may have one or more alpha-helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3).Ĭarbohydrates are the third major component of plasma membranes. This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid. This type of protein has a hydrophilic region or regions, and one or several mildly hydrophobic regions. Some complex proteins are composed of up to 12 segments of a single protein, which are extensively folded and embedded in the membrane. Some span only part of the membrane-associating with a single layer-while others stretch from one side of the membrane to the other, and are exposed on either side. Single-pass integral membrane proteins usually have a hydrophobic transmembrane segment that consists of 20–25 amino acids. Integral proteins (some specialized types are called integrins) are, as their name suggests, integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the the phospholipid bilayer. Proteins make up the second major component of plasma membranes. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. Therefore, phospholipids form an excellent lipid bilayer cell membrane that separates fluid within the cell from the fluid outside of the cell. In contrast, the middle of the cell membrane is hydrophobic and will not interact with water. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. When placed in water, hydrophobic molecules tend to form a ball or cluster. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. This arrangement gives the overall molecule an area described as its head (the phosphate-containing group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. Hydrophobic, or water-hating molecules, tend to be non- polar. The hydrophilic or water-loving areas of these molecules are in contact with the aqueous fluid both inside and outside the cell. The main fabric of the membrane is composed of amphiphilic or dual-loving, phospholipid molecules. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane. The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. The fluid mosaic model of the plasma membrane
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |