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|Chapter Two: The Chemistry of Biology|
As mentioned in the previous section, organic molecules are classified based upon their functional groups. Here we will discuss three of the most important types of organic compounds: carbohydrates, lipids, and proteins.
Carbohydrates are molecules formed through the joining of sugar molecules. Sugars, such as glucose and fructose, always have the same number of carbon atoms and oxygen atoms, and have twice as many hydrogen atoms. The previous examples are known as monosaccharides (this is the simplest type of sugar); a sugar consisting of two monosaccharides is called a disaccharide. Monosaccharides and disaccharides can link together to form long chains which are known as polysaccharides. Carbohydrates are often used for both the storage of energy and the building blocks for the structure of many cell organelles.
Lipids, sometimes known as fats, are a class of organic molecules which do not dissolve in water and other polar compounds. Most lipids consist of molecules known as fatty acids. Lipids generally do not dissolve in water, so they are said to be hydrophobic which literally means "afraid of water." The single most important type of lipid in a unicellular organism is the phospholipid. Phospholipids contain two fatty acid molecules bonded to a glycerol molecule. The glycerol molecule is in turn bonded to a negatively charged phosphate ion. The phosphate group has a negative charge, so it is polar, but the fatty acids are themselves nonpolar. In Chapter Three, you'll learn why this fact allows phospholipids to be used as a cell membrane (the covering of a cell).
Proteins are long, complex molecules, but their structure is very easy to understand. A protein is simply a long chain of molecules called amino acids. All amino acids have two functional groups, an amino group (NH2) and a carboxyl group (COOH), but each amino acid also has a group which is often called the variable group since is varies from amino acid to amino acid. Amino acids bond with each other through what is known as a peptide bond. The OH from the carboxyl group and the H from the amino group break off and join together to form OH + H = H2O (water!). Meanwhile, the C in the carboxyl group and the N in the amino group from a bond (the peptide bond). This process, where water is removed and two molecules are joined together, is called dehydration synthesis.