The distinctive properties of an organic molecule depend not only on the arrangement of its carbon skeleton but also on the chemical groups attached to that skeleton. We can think of hydrocarbons, the simplest organic molecules, as the underlying framework for more complex organic molecules.
A number of chemical groups can replace one or more of the hydrogens bonded to the carbon skeleton of the hydrocarbon.These groups may participate in chemical reactions or may contribute to function indirectly by their effects on molecular shape. The number and arrangement of the groups help give each molecule its unique properties. Each of the four types of macromolecules—proteins, lipids, carbohydrates, and nucleic acids—has its own characteristic set of functional groups that contributes greatly to its differing chemical properties and its function in living organisms.
The seven chemical groups most important in biological processes are the hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl groups. The first six groups can act as functional groups; they are also hydrophilic and thus increase the solubility of organic compounds in water. The methyl group is not reactive, but instead often serves as a recognizable tag on biological molecules.
- Hydroxyl group
In a hydroxyl group (-OH), a hydrogen atom is bonded to an oxygen atom, which in turn is bonded to the carbon skeleton of the organic molecule. (Do not confuse this functional group with the hydroxide ion, OH–.)
The compounds are named as alcohols and their specific names usually end in -ol. It is polar as a result of the electrons spending more time near the electronegative oxygen atom and can form hydrogen bonds with the water molecules, helping dissolve organic compounds such as sugars.
- Carbonyl group
The carbonyl group ( > CO) consists of a carbon atom joined to an oxygen atom by a double bond. They are called ketones if the carbonyl group is within a carbon skeleton and aldehydes, if the carbonyl group is at the end of the carbon skeleton.
A ketone and an aldehyde may be structural isomers with different properties, as is the case for acetone and propanal. Ketone and aldehyde groups are also found in sugars, giving rise to two major groups of sugars: ketoses (containing ketone groups) and aldoses (containing aldehyde groups).
Example : Acetone, the simplest ketone and propanal, an aldehyde
- Carboxyl group
When an oxygen atom is double bonded to a carbon atom that is also bonded to an -OH group, the entire assembly of atoms is called a carboxyl group (-COOH). They are called as carboxylic acids, or organic acids.
The covalent bond between oxygen and hydrogen is so polar that it will donate an H+ and act as acids. They are in found in cells in the ionized form with a charge of 1– and is called a carboxylate ion.
Example: Acetic acid, which gives vinegar its sour taste.
- Amino group
The amino group (-NH2) consists of a nitrogen atom bonded to two hydrogen atoms and to the carbon skeleton and they are called as amines.
They acts as a base because they can pick up an H+ from the surrounding solution (water, in living organisms). They are found in cells in the ionized form with a charge of 1+.
Example: Glycine, a compound that is both an amine and a carboxylic acid because it has both an amino group and a carboxyl group; compounds with both groups are called amino acids
- Sulfhydryl group
The sulfhydryl group (-SH) consists of a sulfur atom bonded to an atom of hydrogen; it resembles a hydroxyl group in shape. Their compounds are called as thiols.
The two sulfhydryl groups can react, forming a covalent bond. This “cross-linking” helps stabilize protein structure. Cross-linking of cysteines in hair proteins maintains the curliness or straightness of hair. Straight hair can be “permanently” curled by shaping it around curlers and then breaking and re-forming the cross-linking bonds.
Example: Cysteine, an important sulfur containing amino acid.
- Phosphate group
In the phosphate group , a phosphorus atom is bonded to four oxygen atoms; one oxygen is bonded to the carbon skeleton; two oxygens carry negative charges (-OPO32-). They are called as organic phosphates.
Molecules containing phosphate groups have the potential to react with water, releasing energy and contributes negative charge to the molecule of which it is a part (2– when at the end of a molecule; 1– when located internally in a chain of phosphates).
Example: Glycerol phosphate, which takes part in many important chemical reactions in cells; glycerol phosphate also provides the backbone for phospholipids, the most prevalent molecules in cell membranes
- Methyl groups
A methyl group (-CH3) consists of a carbon bonded to three hydrogen atoms. The carbon of a methyl group may be attached to a carbon or to a different atom. They are called as methylated compounds.
Arrangement of methyl groups in male and female sex hormones affects their shape and function and also the addition of a methyl group to DNA, or to molecules bound to DNA, affects the expression of genes.
Example: 5-Methyl cytidine, a component of DNA that has been modified by addition of a methyl group.
Hydrogen bond between functional groups
Hydrogen bonds between functional groups (within the same molecule or between different molecules) are important to the function of many macromolecules and help them to fold properly and maintain the appropriate shape needed to function correctly. Hydrogen bonds are also involved in various recognition processes, such as DNA complementary base pairing and the binding of an enzyme to its substrate.