CHAPTER 5
THE STRUCTURE AND FUNCTION OF LARGE BIOLOGICAL MOLECULES


SECTION 5.1
MACROMOLECULES ARE POLYMERS, BUILT FROM MONOMERS

Most large organic molecules (macromolecules) are polymers. Polymers are long chain molecules made up of repeating units linked by covalent bonds that are either the same as – or very similar – to each other. The small units that make up polymers are called monomers. What distinguishes polymers is the different identity of their monomers.

The Synthesis and Breakdown of Polymers

The classes of polymers differ in the nature of their monomers, but the chemical mechanisms by which cells make and break down polymers are basically the same in all cases.

The reaction that creates polymers from monomers is called a condensation (or dehydration) reaction. In this reaction, two monomers are combined, and one water molecule is released. The reverse reaction, in which a polymer is broken down into monomers after the addition of water, is called hydrolysis.



a) The dehydration process is facilitated by enzymes, specialized macromolecules that speed up chemical reactions in cells.


SECTION 5.2
CARBOYDRATES SERVE AS FEUL AND BUILDING MATERIAL

Carbohydrates are fuel binding and building material. There are four basic kinds of macromolecules that are biologically important: lipids, carbohydrates, proteins, and nucleic acids. The term carbohydrate refers both to the simple sugars (glucose, fructose, maltose, etc.) and to the polymers made from these and other subunits.

The simplest sugars are monosaccharide. Monosaccharides are simple ring structures like glucose and fructose. Both glucose and fructose have the same chemical formula C6H12O6 , but the way their atoms are arranged gives each one a different molecular formula and different chemical properties.

Most names for sugar end in –ose
a) Criteria for classifying sugars include:
a. The size of the carbon skeleton
b. The spatial arrangement of their parts around asymmetric carbons (recall that asymmetric carbon is a carbon attached to four different atoms or groups of atoms.)

Disaccharides are made up of two monosaccharide that have undergone a condensation reaction (these two monosaccharide are joined by a glycosidic linkage). Three examples of these are sucrose, maltose, and lactose.



(From left to right, the names of the disaccharides above are "Sucrose", "Maltose",
"Lactose".)

Polysaccharides (which are thousands of carbons) are basically polymers of monosaccharide. Polysaccharides are involved in the storage of carbohydrates in organisms (both plant and animal). In plants, carbohydrates stored in the form of starch, which is made up of glucose monomers. In animals, carbohydrates are stored as glucose. This too, is a polysaccharide made up of glucose, but its structure is more branched than is the structure of starch.

a) Polysaccharides are also involved in the structure of organisms. Cellulose (a polysaccharide) makes up the thick cell walls of plants. Cellulose is a polymer of glucose, but again its identity differs from that of starch and glycogen because of the way the glucose molecules are joined.

b) In animals such as arthropods (lobsters, crabs, other crustaceans, and insects), chitin is an important structural polysaccharide. Chitin is made up of a variation of glucose with a nitrogenous arm.