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TOPIC 6: LIPIDS AND BIOLOGICAL MEMBRANES The Importance of Selectively Permeable Membranes Every cell needs some sort of physical barrier to control the exchange between extracellular and intercellular environment. This barrier should be impermeable to most of the molecules and ions, otherwise substances can fuse freely in and out, and the cell would not really have a defined content. On the other hand, cell barrier can’t be completely impermeable then there won’t be able to get needful exchanges between extra and intracellular environment. Moreover, such barrier must be insoluble in water, so that it will not be dissolved by the aqueous medium of the cell. At the same time it should be permeable to water, since water is the basic solvent system of a cell. A membrane of a cell satisfies these criteria admirably. A membrane is essentially a hydrophobic permeability barrier consisting of phospholipids , glycolipids , and membrane proteins . Most membrane lipids and proteins are not simply hydrophobic; they have both hydrophilic and hydrophobic regions and are called amphipathic molecules . The distinguishing feature of amphipathic phospholipids is that each molecule consists of a polar “head” and two nonpolar “tails” The polarity of the hydrophilic head is due to the presence of a negatively charged phosphate group linked to a positively charged group Membranes Are Selectively Permeable Membranes are said to have trilaminar appearance (three-layers) Due to its hydrophobic interior a membrane is permeable to nonpolar molecules Impermeable to polar molecules and is highly impermeable to all ions Compounds with molecular weights below about 100 Daltons diffuse across membranes regardless if they are polar or nonpolar Water is an especially important example of a very small molecule that, although polar, diffuses rapidly across membranes Ions that are diffused are made permeable by proteins embedded in the membrane
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A hydrophilic channel that transports specific molecules or ions through a hydrophobic membrane and a carrier binds to a specific solute on one side of the membrane and then undergoes a conformational change to move the solute across membrane Due to these discussed reasons, biological membranes can be described as selectively permeable Lipids Lipids constitute a rather heterogeneous category of cellular components that resemble one another more in their solubility than in their chemical sdtructure Hydrophobic nature They have little, if any, affinity for water, but they are readily soluble in nonpolar solvents (ex. ether) Some lipids have are amphipathic (discussed before) Lipids have molecules that are both functionally and chemically diverse Functionally lipids play at least 3 main roles in cells: serve as energy storage , membrane structure , and specific biological functions such as the transmission of chemical signals Chemically lipids have 6 main classes:
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