Lipid Structures, Properties, and Functions1) Lipids are the cellular components that are soluble in organic solvents (chloroform, methanol) but are sparingly soluble or insoluble in water. They have very diverse structure, chemistry, and functions. They can be hydrophobic or amphipathic. They often contain fatty acids, sterols, or isoprene. They have many functions, including energy storage, membranes, cofactors, electron carriers, and hormones and other signal molecules.Pretty much everything but lipids is soluble in water in a cell. Triacylglycerols store fat and are nonpolar.2) Fatty acids have a carboxylic acid head with a hydrocarbon tail. Saturated fatty acids contain no double bonds in the hydrocarbon tail. Monounsaturated fatty acids contain a single double bond in the hydrocarbon tail, usually at C9 and usually cis. Polyunsaturated fatty acids have double bonds separated by a methylene group (notconjugated) and are almost always cis. Fatty acids can range from 4 to 36 carbons long. Common biological fatty acids range from 12 to 24 fatty acids with an even number of carbon atoms. The properties of fatty acids are determined by their length and saturation. They are poorly soluble in water. Their melting point increases with length and degree of saturation. Double bonds cause kinks that prevent packing, therefore lowering the melting point. A wedge shape causes micelle formation. Fatty acids are usually part of larger molecules via ester or amidebonds.The melting point is the point at which half of the substance has melted. The more tightly packed the fatty acids are, the higher the melting point. You very rarely find fatty acids floating in solution freely (i.e. noncovalently bonded to serum albumin); they are almost always attached through an amide or ester bond to something.3) Fatty acids can be named using common names, shorthand carbon skeleton names, and systematic names. Shorthand carbon skeleton names follow the pattern: chain length:# double bonds ( bond position from –COOH end). Omega fatty acids have special names.Eicosa stands for 20. Adding double bonds does more to change the melting point of a fatty acid than changing chain length.7) Triacylglycerols have a glycerol backbone with three fatty acids attached by ester linkages. The fatty acids can be the same but are usually different. Triacylglycerols are the most abundant lipids in the human body but do not form membranes. They are hydrophobic and are stored in an anhydrous form. Triacylglycerols are lighter than water and their fatty acids determine their properties (liquid or solid at room temperature). Triacylglycerols are used primarily as metabolic storage molecules (fats in animals, oils in plants). The more reduced carbon provides more energy than carbohydrates. Glycogen stores last 24 hours but triacylglycerol stores last 2-3 months. They can also be used for insulation.Membrane lipids are amphipathic; they have a polar head group that is a phosphodiester and a sugar, and a hydrophobic portion made up of fatty acids and isoprene derivatives. They are very diverse due to mixing and matching of head groups and tails. They have glycerol and sphingosine backbones. Membrane lipids are amphipathic and due to a cylindrical cross section, they form bilayers and liposomes in aqueous solutions. Their polar heads are exposed to the aqueous solution while their hydrophobic tails are buried.Glycerophospholipids have a glycerol backbone. Their fatty acids are esterified to C1 and C2. Their head group is attached to C3 by a phosphodiester linkage. Some have either linkages to C1. The different head groups of glycerophospholipids alter their properties, along with their different fatty acids. C1 often holds a saturated fatty acid while C2 often holds a mono- or polyunsaturated fatty acid. Glycerophospholipids are major membrane constituents as well as precursor molecules for signaling.Triacylglycerols are not amphipathic; they are just a glycerol attached to 3 fatty acids. Fat stores more energy than sugar. Formation of bilayers is driven by hydrophobic forces. Snake venom has phospholipase A that cuts off a fatty acid tail and destroys membranes so that cells fall apart. Glycerophospholipids can be broken up to produce signaling molecules. The melting point of fatty acids affects the fluidity of the membrane.8) Galactolipids and sulfolipids have a glycerol backbone with fatty acids esterified to C1 and C2 and a head group attached to C3 by a glycosidic linkage. They are similar to glycerophospholipids. They are abundant in plant chloroplasts and are themost abundant membrane lipids on the planet. They may compensate for phosphatelimitation in plants.Sphingolipids are built on the amino alcohol sphingosine. Sphingosine has an amino group on C2 and a hydroxyl group on C3. It also contains a trans double bond.A fatty acid is attached by an amide bond to the nitrogen on C2 of the sphingosine backbone with a polar head group attached to C1. Sphingolipids are similar to glycerophospholipids and galactolipids. Sphingomyelin is prevalent in myelin and neuronal membranes. Gangliosides are involved in cell recognition and ABO blood groups.Cholesterol has a steroid nucleus with 4 rigid fused rings in a planar structure. It also has a hydrocarbon tail. There is an alcohol group on C3. Cholesterolis built from isoprene, not fatty acids. Cholesterol is amphipathic with hydrophobic rings and tail and a polar OH group. Cholesterol is very rigid. It affects membrane fluidity and is a steroid hormone and bile acid precursor.Phosphatidic acid is a precursor for synthesis. It is rare in cells because it is used up quickly. It also has a negative charge from phosphate. Ethanolamine has no charge. Choline also has no charge but has more hydrophobic parts, so it acts differently. Inositol is a derivative of sugar and can be phosphorylated. It is a precursor for signaling. Cardiolipin is important in inner mitochondrial membranes because it interacts with complex IV in the electron transport chain. Galactolipids and sulfolipids are common in plants and usually exhibit polyunsaturation. They have no phosphate group but can have a sulfate group. They are polar with nocharge. Sphingolipids have a different backbone, as well as an amino group on C2. They are amphipathic and found in membranes. Sphingomyelin has a phosphocholine head group and is neutral but charged. Cerebroside has one sugar, globoside has 2 or more sugars, and
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