Although there are millions of different biochemical compounds in Earth’s living things, all biochemical compounds contain the elements carbon, hydrogen, and oxygen. Some contain only these elements, while others contain additional elements, as well. The vast number of biochemical compounds can be grouped into just four major classes: carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates
Carbohydrates include sugars and starches. These compounds contain only the elements carbon, hydrogen, and oxygen. In living things, carbohydrates provide energy to cells, store energy, and form certain structures (such as the cell walls of plants). The monomer that makes up large carbohydrate compounds is called a monosaccharide. The sugar glucose, represented by the chemical model in Figure 3.3.2, is a monosaccharide. It contains six carbon atoms (C), along with several atoms of hydrogen (H) and oxygen (O). Thousands of glucose molecules can join together to form a polysaccharide, such as starch.
Lipids
Lipids include fats and oils. They primarily contain the elements carbon, hydrogen, and oxygen, although some lipids contain additional elements, such as phosphorus. Lipids function in living things to store energy, form cell membranes, and carry messages. Lipids consist of repeating units that join together to form chains called fatty acids. Most naturally occurring fatty acids have an unbranched chain of an even number (generally between 4 and 28) of carbon atoms.
Proteins
Proteins include enzymes, antibodies, and many other important compounds in living things. They contain the elements carbon, hydrogen, oxygen, nitrogen, and sulfur. Functions of proteins are very numerous. They help cells keep their shape, compose muscles, speed up chemical reactions, and carry messages and materials. The monomers that make up large protein compounds are called amino acids. There are 20 different amino acids that combine into long chains (called polypeptides) to form the building blocks of a vast array of proteins in living things.
Nucleic Acids
Nucleic acids include the molecules DNA (deoxyribonucleic acid) and RNA(ribonucleic acid). They contain the elements carbon, hydrogen, oxygen, nitrogen, and phosphorus. Their functions in living things are to encode instructions for making proteins, to help make proteins, and to pass instructions between parents and offspring. The monomer that makes up nucleic acids is the nucleotide. All nucleotides are the same, except for a component called a nitrogen base. There are four different nitrogen bases, and each nucleotide contains one of these four bases. The sequence of nitrogen bases in the chains of nucleotides in DNA and RNA makes up the code for protein synthesis, which is called the genetic code. The animation in Figure 3.3.5 represents the very complex structure of DNA, which consists of two chains of nucleotides.
