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1.) D-glucose is found in two forms, alpha and beta, which differ slightly in their structures. The D-glucoses that make up amylose have a hydroxyl group attached below the first carbon, and so we can conclude that the D-glucoses which make up amylose are in fact all alpha-D-glucoses. Like amylose, cellulose is also composed of D-glucoses, however the D-glucoses found in cellulose are beta-D-glucoses. Unlike alpha-D-glucose, beta-D-glucose's hydroxyl group is located above its first carbon, resulting in unique bonding behavior. While observing a chain of alpha-D-glucoses, or amylose, one would notice that the sixth carbon in each glucose is located above the ring. A chain of beta-D-glucoses would not look like this. In cellulose the sixth carbon alternates positions from ring to ring. The reason for this dichotomy, agian, is due to the difference in arrangement of the hydroxyl group on the first carbon. When it is located below the first carbon (alpha), glucose molecules are able to bond through glycosidic linkages all facing the same direction. When the hydroxyl group is located above the first carbon neighboring glucoses must flip in order to be able to form bonds, which results in the alternating bonding nature of beta-D-glucoses.
2.) When a molecule is amphipathic it means that it is both hydrophillic and hydrophobic. Most lipids are nonpolar and therefore hydrophobic (such as triaglycerols), however there are a few exceptions. Phospholipids are one example. Phospholipids are made up of two parts, a phosphate group which is polar and two fatty acid tails which are nonpolar. Since the phosphate group is polar it is hydrophillic, and since the fatty acid tails are nonpolar it is hydrophobic, making phospholipids both hydrophillic and hydrophobic. In cells, phospholipids form lipid bilayers, which are exactly what they sound like-- two layers of phospholipids that surround a cell. The two layers face eachother so that the hydrophillic phosphate 'heads' are exposed to the cell's aqueous environment and the hydrophobic tails are safely hidden. lipid bilayers are vital to a cells existence, without them cells wouldn't be able to move about the body freely. Fatty acids are also amphipathic. The hydroxyl group at the head of the molecule is polar while the tail is hydrophobic, making the molecule amphipathic. |
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