Carbon

What you’ll learn to do: Discuss why it is said that life is carbon-based and the bonding properties of carbon

You’ve probably heard the term carbon-based life thrown about in scientific conversations before. But what exactly does the term mean? Possibly the quickest answer to this question is simply that all living things are reliant on molecules that include carbon. There are no living things on our planet that do not have carbon (however, there are nonliving things made up of carbon as well: e.g, diamonds and, well, carbon itself).

In this outcome we’ll learn about the importance of carbon to every living thing on earth and its unique properties that make it particularly suited for its role in living things.

Learning Outcomes

Discuss why it is said that life is carbon-based and the bonding properties of carbon.

Carbon

Diagram of a methane molecule, which made up of one carbon atoms and four hydrogen atoms bound to different sides of the carbon atom. The diagram is shaped like a plus sign.

Figure 1. Carbon can form four covalent bonds to create an organic molecule. The simplest carbon molecule is methane (CH4), depicted here.

Living things are carbon-based because carbon plays such a prominent role in the chemistry of living things. This means that carbon atoms, bonded to other carbon atoms or other elements, form the fundamental components of many, if not most, of the molecules found uniquely in living things. Other elements play important roles in biological molecules, but carbon certainly qualifies as the “foundation” element for molecules in living things. It is the bonding properties of carbon atoms that are responsible for its important role.

Carbon Bonding

The four covalent bonding positions of the carbon atom can give rise to a wide diversity of compounds with many functions, accounting for the importance of carbon in living things.

Carbon contains four electrons in its outer shell. Therefore, it can form four covalent bonds with other atoms or molecules. The simplest organic carbon molecule is methane (CH4), in which four hydrogen atoms bind to a carbon atom (Figure 1).

However, structures that are more complex are made using carbon. Any of the hydrogen atoms can be replaced with another carbon atom covalently bonded to the first carbon atom. In this way, long and branching chains of carbon compounds can be made (Figure 2a). The carbon atoms may bond with atoms of other elements, such as nitrogen, oxygen, and phosphorus (Figure 2b). The molecules may also form rings, which themselves can link with other rings (Figure 2c). This diversity of molecular forms accounts for the diversity of functions of the biological macromolecules and is based to a large degree on the ability of carbon to form multiple bonds with itself and other atoms.

Examples of three different carbon-containing molecules. The first is stearic acid, which primarily composed of a chain of fifteen carbon atoms. The first carbon atom is bound to three hydrogen atoms: one on the left, one on the top, and one on the bottom. The next thirteen carbon atoms are bound to two hydrogen atoms: one on the top and one on the bottom. The fifteenth carbon atom is double bound to an oxygen atom and bound to a hydroxyl group. The second molecules is glycine (C 2 H 5 N O 2). The backbone of the structure is a short chain of two carbon atoms and a single nitrogen atom. The first carbon atom is is double bound to an oxygen atom and bound to a hydroxyl group. The second carbon atom is bound to two hydrogen atoms: one on the top and one on the bottom. The nitrogen atom is bound to two hydrogen atoms: one on the top and one on the bottom. The third molecule is glucose, which is formed in a hexagonal shape. Five points of the hexagon are carbon atoms; the sixth (and final) point is an oxygen atom. The oxygen atom is not bound to any atoms besides the carbons in the hexagon. Four carbons are bound to a hydrogen atom and a hydroxyl group. The fifth carbon atom (which is next to the oxygen atom in the hexagon ring) is bound to a hydrogen atom and a carbon atom. This carbon atom is also bound to two hydrogen atoms and a hydroxyl group.

Figure 2. These examples show three molecules (found in living organisms) that contain carbon atoms bonded in various ways to other carbon atoms and the atoms of other elements. (a) This molecule of stearic acid has a long chain of carbon atoms. (b) Glycine, a component of proteins, contains carbon, nitrogen, oxygen, and hydrogen atoms. (c) Glucose, a sugar, has a ring of carbon atoms and one oxygen atom.

Check Your Understanding

What makes it possible for carbon to form four covalent bonds with other atoms or molecules?

  • It contains four electrons in its outer shell.
  • It has four electrons.
  • It has an atomic number of four.
Show Answer

It contains four electrons in its outer shell.