Chapter 5. Biochemistry
Scott Crousillac
Unit Outline
Learning Objectives
At the end of this unit, you should be able to:
I. Describe the chemistry of carbon.
II. Describe the structure and function of carbohydrates.
III. Describe the structure and function of lipids.
IV. Describe the structure and function of proteins.
V. Describe the structure and function of nucleic acids.
Organic compounds typically consist of groups of carbon atoms covalently bonded to hydrogen, oxygen, and often other elements as well. Created by living things, they are found throughout the world, in soils and seas, commercial products, and in every cell of the human body. The four types most important to human structure and function are carbohydrates, lipids, proteins, and nucleotides. Before exploring these compounds, it is essential to first understand the chemistry of carbon.
Part 1: The Chemistry of Carbon
What makes organic compounds ubiquitous is the chemistry of their carbon core. Recall that carbon atoms have four electrons in their valence shell and that the octet rule dictates that atoms tend to react in such a way as to complete their valence shell with eight electrons. Carbon atoms do not complete their valence shells by donating or accepting four electrons. Instead, they readily share electrons via covalent bonds.
Commonly, carbon atoms share with other carbon atoms, often forming a long carbon chain referred to as a carbon skeleton. It is also possible for carbon atoms to form more than one covalent bond with one another and can form double bonds and triple bonds.
In organic compounds, carbon atoms can be found to share electrons with hydrogen. Carbon and hydrogen groupings are called hydrocarbons. If you study the figures of organic compounds in the remainder of this chapter, you will see several with chains of hydrocarbons in one region of the compound.
Carbon may share electrons with oxygen or nitrogen or other atoms in a particular region of an organic compound. Moreover, the atoms to which carbon atoms bond may also be part of a functional group. A functional group is a group of atoms linked by strong covalent bonds and tending to function in chemical reactions as a single unit. You can think of functional groups as tightly knit “cliques” whose members are unlikely to be parted. Five functional groups are important in human physiology; these are the hydroxyl, carboxyl, amino, methyl, and phosphate groups (Table 5.1).
| Functional Group | Chemical Formula | Importance |
|---|---|---|
| Hydroxyl | -OH | Polar group. Components of all four major classes of organic compounds discussed in this chapter. Involved in dehydration synthesis and hydrolysis reactions and hydrogen bonding. |
| Carboxyl | -COOH | A component of the organic acids discussed in this chapter. |
| Amino | -NH2 | A component of all amino acids. |
| Methyl | -CH3 | A component of all fatty acids. |
| Phosphate | -PO42- | A component of all phospholipids and nucleotides. |
Carbon’s affinity f