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Lecture 18

Lecture 18: "Membranes & Membrane Function"

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Biology 2382B
Robert Cumming

Cell Biology Lecture No. 18: Membranes & Membrane Function th Monday March 18 , 2013 Biomembranes: -Membranes define what a cell is and allow for specialized cellular functions to occur in a localized manner. The major components of the biomembrane are lipids, sterols and proteins. Biomemebranes are made up of amphipathic components (known as phospholipids) which are part hydrophobic and part hydrophilic. Due to their amphipathicity, phospholipids spontaneously form lipid bilayers (known as micelles) in aqueous solution such that the hydrophobic regions are shielded from water, where properties of the fatty acids confer properties onto the bilayers. These bilayers are basically plasma membranes, which can include up to 50% protein on their surfaces. The functionality of the membranes is made possible by the phospholipids and proteins embedded upon their surface. Fatty Acids & The Effect Of Double Bonds On Fatty Acid Structure: -Fatty acids are biomolecules that display amphipathicity, being composed of long hydrocarbon (non- polar) chains attached to a polar carboxyl head group. They are often classified as C x : ywhere x is the number of carbon molecules and y is the number of double bonds. When fatty acids convey no double bonds between carbon atoms, it is said to be saturated. Similarly, one double bond between carbons in a fatty acid classifies it as (mono) unsaturated and for more than one, polyunsaturated. There is functionality in both the hydrophilic head group and the fatty acid chain. For fatty acids, their melting temperature increases with their chain length (e.g. 16 carbon palmitate vs. 18 carbon stearate) and decreases with greater unsaturation. -In our body, at a temperature of 37°C, saturated fatty acids will be solid. Almost all unsaturated fatty acids found in nature have double bonds in a cis configuration (trans unsaturated fatty acids are more often synthetically produced). The purpose of the cis double bond is to alter the melting temperature of the respective unsaturated fatty acid (e.g. oleate’s cis double bond forms a kink that decreases its melting temperature, making it oily in our bodies). Only a few types of fatty acids are present in biomembranes in order to establish a balance between fluidity and solidity of the cell membrane. The Three Classes Of Membrane Lipids: -There are three common classes of membrane lipids found in cell membranes: phosphoglycerides, sphingolipids, and sterols. Phosphoglycerides are the most common class of lipids and in biomembranes and consist of two fatty acid chains linked to glycerol by ester linkages. Essentially, this lipid is based off a core glycerol that is also attached to a variable head group that is one of four possible phosphate groups. Plasmologen is a less common type of phosphoglyceride that has one of its fatty acids not linked to glycerol by an ester group. Sphingolipids are not based on glycerol, but rather its starting point is an amino alcohol called sphingosine, whereby another fatty acid chain is added from an amide bond to sphingosine that forms the
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