Carbohydrates: Nature’s Most Abundant Organic Substance

Carbohydrates: Nature’s Most Abundant Organic Substance
Carbohydrates are compounds of carbon, hydrogen, and oxygen. They are usually present in the ratio of 1 C: 2 H: 1O and are grouped as H—C—OH. Carbohydrates function in protoplasm mainly as structural elements and as a source of chemical energy. Glucose is the most important of these energystoring carbohydrates. Familiar examples of carbohydrates include sugars, starches, and cellulose (the woody structure of plants). Cellulose occurs on earth in greater quantities than all other organic materials combined. Carbohydrates are synthesized by green plants from water and carbon dioxide, with the aid of solar energy. This process, called photosynthesis, is a reaction upon which all life depends, for it is the starting point in the formation of food.

Carbohydrates are usually categorized into the following three classes: (1) monosaccharides, or simple sugars; (2) disaccharides, or double sugars; and (3) polysaccharides, or complex sugars. Simple sugars are composed of carbon chains containing 4 carbons (tetroses), 5 carbons (pentoses), or 6 carbons (hexoses). Other simple sugars may have up to 10 carbons, but these sugars are not biologically important. Simple sugars, such as glucose, galactose, and fructose, all contain a free sugar group, in which the double-bonded O may be attached to the terminal or nonterminal carbons of a chain. The hexose glucose (also called dextrose) is particularly important to the living world. Glucose is often shown as a straight chain (Figure 2-2A), but in water it tends to form a cyclic compound (Figure 2-2B). The “chair” diagram (Figure 2-3) of glucose best represents its true configuration, but all forms of glucose, however represented, are the same molecule. Other hexoses of biological significance include galactose and fructose, which are compared with glucose in Figure 2-4.

Disaccharides are double sugars formed by the bonding of two simple sugars. An example is maltose (malt sugar), composed of two glucose molecules. As shown in Figure 2-5, the two glucose molecules are condensed together by the removal of a molecule of water. This condensation reaction, with the sharing of an oxygen atom by the two sugars, characterizes the formation of all disaccharides. Two other common disaccharides are sucrose (ordinary cane, or table, sugar), formed by the linkage of glucose and fructose, and lactose (milk sugar), composed of glucose and galactose.

Polysaccharides are composed of many molecules of simple sugars (usually glucose) linked together in long chains called polymers. Their empirical formula is usually written (C₆H₁₀O₅)_n, where n designates the number of simple sugar subunits contained in the polymer. Starch is the common form in which sugar is stored in most plants and is an important food for animals. Glycogen is an important form for storing sugar in animals. It is found mainly in liver and muscle cells in vertebrates. When needed, glycogen is converted to glucose and delivered by the blood to the tissues. Another polymer is cellulose, the principal structural carbohydrate of plants.


Lipids: Fuel Storage and Building Material
Lipids are fats and fatlike substances. They are composed of molecules of low polarity; consequently, they are virtually insoluble in water but are soluble in organic solvents, such as acetone and ether. The three principal groups of lipids are neutral fats, phospholipids, and steroids.

Neutral Fats
The neutral or “true” fats are major fuels of animals. Stored fat may be derived directly from dietary fat or indirectly from dietary carbohydrates that are converted to fat for storage. Fats are oxidized and released into the bloodstream as needed to meet tissue demands, especially those of active muscle.

Neutral fats include triglycerides, which are molecules consisting of glycerol and three molecules of fatty acids. Neutral fats are therefore esters, a combination of an alcohol (glycerol) and an acid. Fatty acids in triglycerides are simply long-chain monocarboxylic acids; they vary in size but are commonly 14 to 24 carbons long. Production of a typical fat by the union of glycerol and stearic acid is shown in Figure 2-6A. In this reaction, three fatty-acid molecules can be seen to have united with OH groups of the glycerol to form stearin (a neutral fat) plus three molecules of water.

Most triglycerides contain two or three different fatty acids attached to glycerol, and bear ponderous names such as myristoyl stearoyl glycerol (Figure 2-6B). The fatty acids in this triglyceride are saturated; every carbon within the chain holds two hydrogen atoms. Saturated fats, more common in animals than in plants, are usually solid at room temperature. Unsaturated fatty acids, typical of plant oils, have two or more carbon atoms joined by double bonds; the carbons are not “saturated” with hydrogen atoms and are able to form bonds with other atoms. Two common unsaturated fatty acids are oleic acid and linoleic acid (Figure 2-7). Plant fats such as peanut oil and corn oil tend to be liquid at room temperature.