15 Macromolecules

Organic molecules are carbon containing molecules, typically containing hydrogen and oxygen. Initially, organic chemistry was the study of compounds that were obtained from living organisms (hence the name organic)  and inorganic chemistry was the study of compounds that originated from nonorganic matter.  In our study of organic molecules, we are interested in the four major groups of biomolecules:  carbohydrates, lipids, proteins, and nucleic acids. Each of these types of organic molecules  contain carbon, hydrogen, and oxygen, yet each basic group typically shares common  characteristics with others in its group.

CARBOHYDRATES

Carbohydrates are generally those molecules that have the empirical molecular formula of  CnH2nOn which may also be written as (CH2O)n , though there are exceptions. Carbohydrates  usually have the ending –ose, such as seen in sucrose, lactose, and glucose. Approximately half  of all organic matter on earth is made up of carbohydrates. The carbon bonds within the  carbohydrate, when broken down, yield energy and most living organisms have the means to  break them down. Thus, carbohydrates are an energy source for most life forms.

Saccharide (or sugar) is the term used to describe the structural unit for carbohydrates. If a  carbohydrate has only one structural unit, it is referred to as a monosaccharide. If the  carbohydrate is composed of 2 saccharide units it is called a disaccharide and it is called a  polysaccharide if it is composed of many saccharides. Monosaccharides (or simple sugars) are  the “building blocks” of all the other carbohydrates and consist of a single unit and cannot be  broken down into simpler sugars. A monosaccharide is usually composed of 3 to 7 carbon  atoms. Common 6-carbon monosaccharides (or hexoses) include glucose and fructose.

Disaccharides may be formed when 2 monosaccharides are joined together with the  elimination of a water molecule, a dehydration synthesis or condensation reaction. For  example, sucrose (common table sugar) is formed when a bond is formed between glucose and  fructose, with the elimination of a water molecule. The reverse reaction, hydrolysis, involves  the addition of a water molecule, which is lysed or split into the ions H⁺ and OH^–, to break the  disaccharide back down into the 2 monosaccharides, glucose and fructose.

Polysaccharides are formed from the combining of many monosaccharides, after dehydration.  Polysaccharides are a major storage form of carbohydrates, such as starch in plants and  glycogen in animals. Polysaccharides may be broken down into their simpler monosaccharide  “building blocks” by hydrolysis. Starch, the storage molecule in plants, is not readily soluble in water and this makes it a good way to store energy reserves within the plant cell. Glycogen  (sometimes referred to as animal “starch” but shouldn’t be confused with the starch found in  plants), is stored in the liver and skeletal muscle cells of animals.

LIPIDS

Lipids are complex organic molecules that tend to be insoluble in water (hydrophobic) because  they are nonpolar. Lipids tend to be grouped in 4 groups: neutral fats, phospholipids, steroids,  and other lipoid substances.

Neutral lipids, also called triglycerides, are composed of 2 basic “building blocks,” glycerol and  3 fatty acids. Because the neutral fats are insoluble in water they provide an excellent means  for the body to store high-energy molecules.

Phospholipids are similar to the neutral lipids except that they contain a phosphate group in  place of one of the fatty acid chains. The phosphate group is polar (and hydrophilic), providing  the phospholipids with dual chemical properties (they are both hydrophilic and hydrophobic).  Phospholipids are a major structural molecule of cell membranes. The phospholipids of the cell’s membrane arrange themselves into a bilayer of phospholipids; the hydrophilic phosphate  heads of one layer orient themselves towards the inside of the cell (where there is a lot of  water) and the second layer orients its phosphate heads closest to the outside of the cell  (where there is a lot of water), thus, sandwiching the nonpolar/hydrophobic fatty acid chains of both layers in the middle.

Although the steroids differ structurally from the other lipids, they are insoluble in water. Steroids include cholesterol, bile salts, Vit. D,  and some of the hormones. Lipoid substances include the fat-soluble vitamins, A, E, and  K.

PROTEINS

Proteins are another group of organic molecules that are both structurally and functionally  important in the human body. The “building blocks” of proteins are amino acids. There are  about 20 naturally occurring amino acids. All amino acids contain an amine group (-NH2) and a  carboxyl group (-COOH), which is the organic acid group. Each different amino acid also has a  specific “R” group (the side chain), making each amino acid unique. These groups determine the many  characteristics of amino acids and proteins. Two amino acids can be attached to each other by  dehydration synthesis reaction with the removal of a water molecule. The two amino acids are  then linked together by a peptide bond. When many amino acids are linked together by  peptide bonds it is referred to as a polypeptide chain. Proteins consist of one or more  polypeptide chains.

pH AND BUFFERS

By definition, an acid is a substance that increases the concentration of the hydrogen ions (H+)  in water. A base is a substance that decreases the concentration of the hydrogen ions, which in water is the equivalent to increasing the concentration of the hydroxyl or hydroxide ions (OH-).

How acidic or basic a solution might be is commonly measured in terms of a value known as pH.  Literally, pH refers to the “potential of the hydrogen ion.” It is the measurement of the relative  concentration of the positive hydrogen ions (H+) relative to the negative hydroxide ions (OH-) in  a solution. The pH scale is a logarithmic scale between 0 (the acidic end) and 14 (the basic end),  where 7.0 is neutral (i.e., the concentration of the hydrogen ions equals the concentration of  the hydroxide ions). If the concentration of the hydrogen ions is greater than the concentration  of the hydroxide ions, then the solution is acidic and the pH would be less than 7.0. If the  concentration of the hydrogen ions is less than the concentration of the hydroxide ions, then  the solution is basic (or alkaline) and the pH would be greater than 7.0.

The pH of whole blood is between 7.35 and 7.45, which is slightly basic. Whereas, metabolic  processes tend to result in more acidic products. If the blood’s pH were to exceed the normal  range, it could have detrimental effects. Thus, the body uses buffers to help it maintain a fairly  constant blood pH. A buffer, sometimes referred to as a “chemical sponge,” is a substance that  helps to maintain a fairly constant pH by taking up the excess hydrogen ions or giving off more  hydrogen ions when necessary.