When carbon is not bonding to other carbon atoms, it covalently bonds to a number of other atoms, notably hydrogen, oxygen and nitrogen. In fact, the basic carbon compound is a hydrocarbon , formed from carbon and hydrogen. Hydrocarbon variations that differ only in the arrangement of atoms are called isomers. Isomers are very important in biology, and we shall see many examples. Structural isomers vary in their covalent bonding arrangement.
Geometric isomers are important in vision.
Light hitting the pigment, rhodopsin, in the rods of the eye causes a change in structure, which triggers the nerve transmission. Enantiomers are formed when four molecular groups are bonded to a central carbon so that they can be arranged in two different ways that are mirror images of each other. As with other types of isomers, the different shape can dramatically alter function. The other is frequently in vitamin pills but lacks the same biological activity. Your text illustrates a similar situation with L-dopa, a drug which helps treat Parkinson's disease.