Biology 12

1.2 Biological Macromolecules and Their Subunits

14 MHR • Unit 1 Metabolic Processes

The atoms of four elements make up roughly

99 percent of the mass of most cells: hydrogen,

nitrogen, carbon, and oxygen. With only a few

exceptions, molecules that contain carbon atoms

are called organiccompounds. There are millions

of different organic compounds. Nearly all organic

compounds contain hydrogen as well as carbon,

and most of these also include oxygen. Pure carbon

and carbon compounds that lack hydrogen — such

as carbon dioxide and calcium carbonate — are

considered inorganic. Inorganic compounds are,

nevertheless, integral components of living systems.

See Figure 1.9. For example, water — an inorganic

compound — provides a medium in which various

substances may be dissolved and transported

within and between cells.

Figure 1.9In what ways do living and non-living systems,

and organic and inorganic compounds interact?

The Central Atom: Carbon

The diversity of life relies greatly upon the

versatility of carbon. Recall that a carbon atom in

its most stable state has two occupied energy levels,

the second of which contains four valence electrons.

This means that, in covalent molecules, a carbon

atom can form bonds with as many as four other

atoms. In biological systems, these atoms are mainly

hydrogen, oxygen, nitrogen, phosphorus, sulfur,

and — importantly — carbon itself. Carbon’s ability

to bond covalently with other carbon atoms enables

carbon to form a variety of geometrical structures,

including straight chains, branched chains, and

rings. Figure 1.10 shows the shapes of several

simple organic molecules that contain only carbon

and hydrogen atoms. These molecules, called

hydrocarbons, comprise the fossil fuels that serve

as the main fuel source for much of the world’s

industrial activities. Hydrocarbons are themselves

not components of living systems. However,

Figure 1.10Carbon atoms can bond in several ways —

from the simple tetrahedral structure of methane, to the

short chain of carbon atoms in ethane, to the ring of carbon

atoms in benzene.

Name Molecular formula Structural formula

methane

ethane

benzene

H

H

H CH

H

H

CH

H

H

CH

CH 4

C 2 H 6

C 6 H 6

H H

H

H

H H

C

C

C

C

C

C

EXPECTATIONS

Identify functional groups and explain their effect on the properties

of molecules.

Review the types of macromolecules.

Explain the meaning of the term isomer, with examples.