Chapter 5 - The Structure and Function of Large Biological Molecules

Given the vast variety of life on Earth, it may come as a surprise that the most significant big molecules present in all living things—from bacteria to elephants—can be classified into four major classes: carbohydrates, lipids, proteins, and nucleic acids.

Members of three of these classes—carbohydrates, proteins, and nucleic acids—are massive on the molecular scale and are hence referred to as macromolecules.

A protein, for example, may be made up of thousands of atoms that combine to produce a molecular behemoth with a mass considerably in excess of 100,000 daltons. Given the size and complexity of macromolecules, biochemists have established the precise structure of a large number of them.

The architecture of a big biological molecule is critical to its function. Large biological molecules, including water and simple organic compounds, show distinctive emergent characteristics due to the ordered arrangement of their atoms.

In this chapter, we'll look at how macromolecules are made. The structure and function of all four types of major biological molecules will next be investigated: carbohydrates, lipids, proteins, and nucleic acids.

Polymers (from the Greek polys, many, and meros, portion) are chainlike compounds that include large carbohydrates, proteins, and nucleic acids.

A polymer is a lengthy molecule made up of numerous similar or identical building pieces connected together by covalent bonds, similar to how a railway is made up of a chain of cars. The repeating units that act as polymer building blocks are smaller molecules known as monomers (from the Greek monos, which means "single"). Some monomers, in addition to creating polymers, have functions of their own.

The image attached shows the synthesis and breakdown of polymers

Although each kind of polymer is composed of a distinct type of monomer, the chemical mechanisms by which cells produce and degrade polymers are essentially the same in all situations. Enzymes, specialized macromolecules that speed up chemical reactions, aid in these activities in cells.

The process linking monomers is an example of a dehydration reaction, which occurs when two molecules are covalently linked to each other and one water molecule is lost (as shown in the image labeled as a dehydration reaction).

When a bond develops between two monomers, each monomer contributes a portion of the water molecule produced during the reaction: one monomer contributes a hydroxyl group (OH), while the other contributes hydrogen (H).

This process is continued as monomers are added to the chain one by one, resulting in the formation of a polymer (also known as polymerization).

Hydrolysis, which is basically the inverse of the dehydration reaction, disassembles polymers to monomers (as shown in the image labeled hydrolysis). Hydrolysis is the fracturing of water (from the Greek hydro, water, lysis, break).

• The term carbohydrates refer to having Sugars and polymers of sugars

Monosaccharides, or simple sugars, are the simplest carbohydrates; they are the monomers from which more complex carbs are constructed. Disaccharides are double sugars made up of two monosaccharides linked together by a covalent connection.

Carbohydrate macromolecules are polymers known as polysaccharides that are made up of numerous sugar component units.

• The term Monosaccharides (from the Greek monos, single, and saccharin, sugar) refers to generally having molecular formulas that are multiple of the unit CH2O. Glucose (C6H12O6), the most common monosaccharide, is of central importance in the chemistry of life

The hallmarks of sugar may be seen in the structure of glucose: The molecule has a carbonyl group, l C“O, as well as numerous hydroxyl groups, OH. Sugar is either an aldose (aldehyde sugar) or a ketose depending on where the carbonyl group is located (ketone sugar).

Glucose, for example, is an aldose; fructose, a glucose isomer, is a ketose. (Most sugar names end in -ose.)

Another criterion for sugar classification is the length of the carbon skeleton, which can range from three to seven carbons. Hexoses are glucose, fructose, and other sugars with six carbons.

There are also trioses (three-carbon sugars) and pentoses (five-carbon sugars). Another source of variation for simple sugars is the spatial arrangement of their components around asymmetric carbons. (An asymmetric carbon is one that is linked to four distinct atoms or groups of atoms.)

Glucose and galactose, for example, vary solely in the arrangement of components around one asymmetric carbon (see Figure 5.3's purple boxes). What appears to be a little change is big enough to give the two sugars unique geometries and binding activity, resulting in diverse behaviors.

A disaccharide is made up of two monosaccharides linked by a glycosidic linkage, which is a covalent connection generated by a dehydration process between two monosaccharides (glucose refers to carbohydrate).

Maltose, for example, is a disaccharide produced by the joining of two molecules of glucose.