This blog post is a part of STEMTalksNC’s ever expanding General Biology Series. In the last post, we talked about how macromolecules were built. In this post, we will examine the structure and function of carbohydrates.
Carbohydrates include sugars and polymers of sugar. Monosaccharides, the simplest carbohydrates, generally have molecular formulas that are some multiple of CH₂O. Glucose, the most important monosaccharide, lays the foundation of classifying sugars. Monosaccharides are grouped as either an aldose or a ketose (depending on location of the carbonyl group). For more information on the carbonyl functional group, check this post. Glucose, for example, is an aldose because its carbonyl group is on the outermost carbon, but fructose, a structural isomer of glucose, is a ketose because its carbonyl group is inside the carbon skeleton. Another way of classifying monosaccharides is based on the size of the carbon skeleton. Glucose and fructose are called hexoses (six- carbon sugars) , while ribose (a component of RNA that we will get to in a later blog post) is a pentose (five-carbon sugar). A third way of classifying monosaccharides is the arrangement of atoms around asymmetric carbons (an asymmetric carbon is a carbon attached to four different atoms or groups of atoms). Glucose and galactose, in the picture, differ in the placement of the the hydroxyl and oxygen arranged around the asymmetric carbon.
Now that we talked about monosaccharides, you may wonder what they are actually used for. Monosaccharides, can be translated, as quick energy, They are used in cellular respiration; cells extract energy in a series of reactions starting with glucose molecules or other monosaccharides (you can learn more about cellular respiration in this post). In addition, they can be used as raw materials for the synthesis of lipids and proteins.
Now that we talked about monosaccharides, here’s a little recap:
Monosaccharides can be classified based on:
- Whether it is an aldose or ketose
- The amount of the carbons in the carbon skeleton
- The arrangement of atoms around asymmetric carbon
Monosaccharides can be used in cellular respiration, which produces ATP (energy). In addition, they can be used as raw materials for lipids and proteins.
Monosaccharides that are not immediately used in the ways we mentioned above are used as monomers for disaccharides or polysaccharides (which we will mention later). A disaccharide consists of two monosaccharides joined by a glycosidic linkage, a covalent bond formed between two monosaccharides by a dehydration reaction. For example, sucrose has the two monosaccharides glucose and fructose joined by a glycosidic linkage. When you ingest sucrose, however, your body will do the process of hydrolysis and break it into glucose and fructose.
The most complex carbohydrates are polysaccharides, which can contain a few hundred to a few thousand monosaccharides joined by glycosidic linkage. In simple terms, it is just a chain of monosaccharides. Polysaccharides are mainly used as storage material, hydrolyzed when there is a need for sugar in cells. Also polysaccharides are used as building material for structures that protect the cell or even the whole organism.
Both plants and animals store sugars for later use in the form of storage polysaccharides. Plants use starch as their storage polysaccharide, which is a polymer of glucose monomers. Plant store starch as granules within cellular structures known as plastids (such as chloroplasts). Starch is basically you stockpiling a bunch of glucose (the money) in the plastid (the bank). When the plant needs glucose, the starch will be withdrawn from this “bank” and hydrolyzed into glucose monomers for use. Most animals, including humans, have enzymes that can hydrolyze plant starch to make glucose available as a nutrient for the cell. Chemically, most of the glucose monomers in starch are joined by 1-4 linkages (number 1 carbon to number 4 carbon) with a few exceptions, and the angle that the number 1 and number 4 carbon are bonded makes the polymer helical. Also like many polysaccharides, starch can be unbranched (this starch is called amylose and is very simple) or branched (this starch is called amylopectin and is more complex form of starch that has 1-6 linkages at branch points).
Animals use glycogen as their storage polysaccharides, which is a polymer of glucose that is extensively branched. We store glycogen mainly in the liver or muscle cell. Glycogen hydrolyzes in these cells when the need for sugar increases. For example, when we exercise and we have not eaten (there is no sugar ingested to use), we break the glycogen into its monosaccharides and release it in the bloodstream. However, this glycogen does not provide a long term storage for energy because it is not very efficient (in fact glycogen storage can be depleted in one day if it is not replenished).
Organisms also use polysaccharides for structure. The polysaccharide called cellulose is a major component of cell walls that enclose plant cells. Like starch, cellulose is a polymer of glucose, but the glycosidic linkages are different. When glucose forms a ring, the hydroxyl can be attached above or below the number 1 carbon. This gives rise to two ring forms. One being alpha and the other being beta. Starch is in the alpha configuration and cellulose is in the beta configuration, making every hydroxyl opposite with respect to its neighbor.
These differing glycosidic linkages gives rise to two distinct three-dimensional shapes. The starch molecule is helical while the cellulose molecule is straight. Cellulose is never branched, and some hydroxyl groups on its glucose monomers are free to hydrogen-bond with the hydroxyls of other cellulose molecules lying parallel to it. This makes a strong building material for plant cell walls.
Another structural polysaccharide is called chitin, which is a carbohydrate used in insect exoskeletons. It provides a hard case that surrounds the soft parts of the insect. Chitin is similar to cellulose in structure, however, its glucose monomer has a nitrogen containing attachment.
Now that we have discussed carbohydrates, in the next post we will talk about lipids.