Laboratory Notes for BIO 1003

© 30 August 1999, Mary Jean Holland

revised 19 August 2005 (JJ, JHW)

ORGANIC MOLECULES—BACKGROUND READING

Organic compounds contain carbon atoms linked together to form chains or rings. Four classes of organic compoundsócarbohydrates, lipids, proteins and nucleic acidsóare found in large amounts in living organisms. The chemical properties of the different classes depend on the presence of specific functional groups. In general, the larger molecules in each class are formed by joining one or more building block molecules together in a dehydration synthesis reaction during which a molecule of water is formed for each building block added. Large molecules are broken down into the smaller building block molecules by a reverse reaction called hydrolysis during which water is added. In this exercise you will learn about the structure and properties of carbohydrates, lipids and proteins and how to test for the presence of these organic molecules.

I. Lipids

Lipids are organic molecules that are insoluble in water and other polar solvents. Lipids are readily soluble in nonpolar solvents, such as chloroform, benzene, and ether. Lipids include fats and oils (important as energy storage compounds), phospholipids and glycolipids (part of the structure of cell membranes), waxes (protective surface coatings on many plants and animals), and steroids (found in some cell membranes and many hormones).

Fats and oils have similar structures, and both serve as energy storage molecules. At room temperature oils are liquid and fats are solid. Both are triglycerides formed by combining a molecule of glycerol with three molecules of fatty acid. The properties of a triglyceride depend upon the structures of the fatty acids it contains. Fatty acids are long chains containing carbon and hydrogen with a carboxyl group (COOH) on one end, which makes the molecule an acid,. The carboxyl group is involved in bonding each fatty acid to the glycerol molecule. Fatty acids differ in the length of the chain and the number of double bonds between adjacent carbon atoms. If all the carbon atoms in a fatty acid chain are bonded to four different atoms (no double bonds between carbon atoms), the fatty acid has a straight chain without bends or kinks. Double bonds between carbon atoms cause fatty acids chains to bend or kink. The fatty acids of saturated fats (such as lard, bacon fat, or butter) contain no double bonds, a maximum of hydrogen atoms, and the straight fatty acid chains pack closely together. The fatty acids of unsaturated fats contain at least one double bond, fewer hydrogen atoms, and the fatty acid chains cannot pack as closely together because at least one of the chains has a kink or bend. Monounsaturated fats, such as olive oil, have one double bond. Polyunsaturated fats, such as corn oil, have two or more double bonds. The less "orderly" structure of unsaturated fats is responsible for their lower melting point.

There are three simple tests to identify lipids. (1) Since lipids (nonpolar molecules) and water (polar molecules) don't mix, a combination of the two will separate into two layers. (2) Sudan Red dye dissolves in lipids; when added to a mixture of lipid and water, it will color only the lipid layer. (3) Lipids make grease spots on paper; water and solutions that dissolve in water do not.

II. Carbohydrates

Carbohydrates are the main energy-storage molecules in most organisms. They are also important structural components for many organisms. The building blocks of carbohydrates are small molecules called sugars, composed of carbon, hydrogen and oxygen. Carbohydrates are classified according to the number of sugar molecules they contain. Monosaccharides, such as glucose, fructose, ribose, and galactose, contain only one sugar molecule. Disaccharides, such as sucrose, maltose and lactose, contain two sugar molecules linked together. Polysaccharides, such as starch, glycogen, cellulose and chitin, contain many sugar molecules linked together.

Monosaccharides have the molecular formula (CH2O)n, where n may be any integer from 3 to 8. Monosaccharides contain hydroxyl groups and either a ketone or an aldehyde group. These polar functional groups make sugars very soluble in water. Glucose, the sugar found in the blood of most vertebrates including humans, has the molecular formula C6H12O6. Fructose, the sugar found in many fruits, has the same molecular formula as glucose, but the atoms of carbon, hydrogen and oxygen are arranged a little differently in the two monosaccharides. Glucose has an aldehyde group; fructose has a ketone group. This difference in structure gives the two monosaccharides slightly different chemical properties.

Disaccharides are formed by linking two monosaccharides together by a dehydration synthesis reaction. A molecule of water is formed in the process. Maltose (malt sugar) is formed by joining two glucose molecules together. Sucrose (cane sugar) is formed by combining glucose and fructose. Lactose (milk sugar) is formed by combining glucose and galactose.

Maltose, sucrose and lactose have the same molecular formula, C12H22O11, but slightly different structural formulae and slightly different chemical properties.

C6H12O6 + C6H12O6arrowC12H22O11 + H2O
glucose + glucosearrowmaltose + water
glucose + fructosearrowsucrose + water
glucose + galactosearrowlactose + water

A simple test for reducing sugars is to mix them with an equal amount of Benedict's Reagent (blue colored) in a test tube and heat it in boiling water. If reducing sugar is present an orange precipitate forms. A positive Benedict’s test requires an aldehyde or ketone group that is located near a hydroxyl group. All monosaccharides are reducing sugars. Some disaccharides are reducing sugars, and some are not. If the reactive aldehyde or ketone groups of both monosaccharides are involved in the bond holding the two units together, these groups are not free to react with the copper ions in the Benedict’s solution, and the disaccharide formed is not a reducing sugar. Polysaccharides do not test positive for reducing sugars unless they undergo a hydrolysis reaction (by heating or digestion) during which the polysaccharides are broken down to form monosaccharides.

Polysaccharides are formed by linking many monosaccharides together by a series of dehydration synthesis reactions. For each monosaccharide added to the polysaccharide chain, a molecule of water is formed. Polysaccharides are used as energy storage compounds by both plants and animals. Plants produce a polysaccharide called starch. Vertebrate animals, including humans, produce a polysaccharide called glycogen, which is stored in liver and muscle cells. Glycogen is sometimes called animal starch. Polysaccharides are also important as structural components in many organisms. Plant cell walls contain a polysaccharide called cellulose. Fungi cell walls and the exoskeletons of arthropods contain chitin, a polysaccharide with nitrogen.

Starch can be identified quickly by adding Lugol's Iodine. There is an immediate change of color from the brown of iodine to blue-black. The reaction only occurs with starch.

III. Proteins

Proteins are complex, specialized molecules composed of carbon, hydrogen, oxygen, and nitrogen. Many proteins also contain sulfur. The building blocks of proteins are the amino acids. There are twenty different amino acids commonly found in proteins. All of these amino acids have a similar structure. At the center of the molecule is the alpha carbon which is bonded to four different groups: (1) an amino group (NH2), (2) a carboxyl group (COOH), (3) a hydrogen atom and (4) the R group (also called the side chain). The different amino acids have different R groups; otherwise the twenty amino acids have identical structures.

Proteins are composed of one or more polypeptides. Polypeptides are formed by linking amino acids together in a long, unbranched chain. The amino acids are linked together by peptide bonds formed when the carboxyl group of one amino acid reacts with the amino group of the next amino acid in a dehydration synthesis reaction. As each amino acid is added to the growing polypeptide chain, a molecule of water is formed. Polypeptides have a free (unreacted) amino group located at one end of the molecule called the N-terminus and a free (unreacted) carboxyl group at the other end of the molecule called the C-terminus. Biochemists describe the structure of a specific polypeptide by writing down the sequence of amino acids starting at the N-terminus and proceeding along the chain to C terminus.

Proteins have many important roles in living organisms. Structural proteins, such as elastin and collagen, provide support. Regulatory proteins control cell processes. Storage proteins produced in reproductive structures are a source of amino acids for developing organisms, e.g., casein in milk, albumin in egg whites, various proteins in plant seeds. Contractile proteins are responsible for movement of cells and organisms. Transport proteins carry substances from one place to another, e.g., hemoglobin carries oxygen throughout the human body. Proteins also serve as antibodies, hormones, receptors, and enzymes.

Biuret reagent turns from blue to purple when it is mixed with a solution containing protein. Ninhydrin sprayed on dry spots made by solutions containing amino acids develops a light purple hue.


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Wahlert & Holland (Rev. 7/8/99jj)

Last updated 30 August 1999 (JHW)