Enrichment Unit 2 Anatomy

• Cell membranes are composed of a double layer of lipid molecules. In the diagram below, the "head" of a lipid molecule is a phosphate group and the "tail" is composed of two carbon chains. The middle part of the membrane composed of the "tails" is highly hydrophobic (lacking affinity for water) as opposed to the negatively charged "heads", which are hydrophilic (having a strong affinity for water).

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• Most membranes in cells are semipermeable, allowing some things to easily pass through and blocking other things. Sometimes the cell can control this process and other times it cannot. The energy involved in this process has both chemical and physical aspects and is far from completely understood. One thing is certain; membranes are a major part of cellular chemistry.

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• Fats are esters formed from glycerol and fatty acids. Fatty acids are carboxylic acids with 12 to 20 carbon atoms in the chain. Some fatty acids are saturated (having only single bonds), while other are unsaturated - having as many as four double bonds in the molecule. In general, animal fats are more saturated and plant oils are more unsaturated.

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• Steroids are another class of lipids. All steroids contain a tetracyclic ring system. One steroid produced by your body is cholesterol.

• Some important vitamins are lipids. Vitamins are substances used by living cells to aid enzyme reactions. Your body uses vitamins in ways ranging from the synthesis of pigments needed for good vision to the red pigment in hemoglobin that carries oxygen in the blood.

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• Nucleic acids

• Measured by mass, nucleic acids are the smallest group of organic chemicals in your body. However, these large polymers are the largest single molecules in the body. Together, DNA and RNA store and transfer genetic information. The diagram at the top of this page is a very small segment of the structural formula for one DNA molecule.