What do all these foods have in common? All of them consist mainly of large compounds called carbohydrates, often referred to as "carbs." Contrary to popular belief, carbohydrates are an important part of a healthy diet. They are also one of four major classes of biochemical compounds.
Chemical Compounds in Living Things
The compounds found in living things are known as biochemical compounds. Biochemical compounds make up the cells and other structures of organisms and carry out life processes. Carbon is the basis of all biochemical compounds, so carbon is essential to life on Earth. Without carbon, life as we know it could not exist.
Why is carbon so basic to life? The reason is carbon’s ability to form stable bonds with many elements, including itself. This property allows carbon to form a huge variety of very large and complex molecules. In fact, there are nearly 10 million carbon-based compounds in living things!
Most biochemical compounds are very large molecules called polymers. A polymer is built of repeating units of smaller compounds called monomers. Monomers are like the individual beads on a string of beads, and the whole string is the polymer. The strings of beads pictured below are simple models of polymers in biochemical compounds.
Classes of Biochemical Compounds
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; 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 include sugars and starches. These compounds contain only the elements carbon, hydrogen, and oxygen. Functions of carbohydrates in living things include providing energy to cells, storing energy, and forming 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 below, is a monosaccharide. It contains six carbon atoms (C) and several atoms of hydrogen (H) and oxygen (O). Thousands of glucose molecules can join together to form a polysaccharide such as starch.
Lipids include fats and oils. They contain primarily the elements carbon, hydrogen, and oxygen, although some lipids contain additional elements such as phosphorus. Functions of lipids in living things include storing energy, forming cell membranes, and carrying 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 from 4 to 28) of carbon atoms.
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 include helping cells keep their shape, making up muscles, speeding up chemical reactions, and carrying messages and materials. The monomers that make up large protein compounds are called amino acids. There are 23 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 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 the instructions from parents to 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, called the genetic code. The animation below represents the very complex structure of DNA, which consists of two chains of nucleotides.
- Why is carbon so important to life on Earth?
- What are the biochemical compounds?
- Describe the diversity of biochemical compounds, and explain how they are classified.
- Identify two types of carbohydrates. What are the main functions of this class of biochemical compounds?
- What roles are played by lipids in living things?
- The enzyme amylase is found in saliva. It helps break down starches in foods into simpler sugar molecules. What type of biochemical compound do you think amylase is?
- Explain how DNA and RNA contain the genetic code.
- What are the three elements present in every class of biochemical compound?
- For each of the following terms (nucleic acid; amino acid; monosaccharide; protein; nucleotide; polysaccharide)
- Determine whether it is a monomer or a polymer.
- Match each monomer with its correct polymer.
- Identify which class of biochemical compound is represented by each monomer/polymer pair.
- Is glucose a monomer or a polymer? Explain your answer.
- What is one element contained in proteins and nucleic acids, but not in carbohydrates?
- Describe the relationship between proteins and nucleic acids.
- Why do you think it is important to eat a diet that contains a balance of carbohydrates, proteins, and fats?
The video below discusses the importance of the element carbon.
Watch the video below to learn more about polymers and monomers.
- Grain products by Scott Bauer USDA, public domain via Wikimedia Commons
- Fio de conta by Toluaye, released into the public domain via Wikimedia Commons
- Glucose by Ben; Yikrazuul, public domain via Wikimedia Commons
- DNA cropped by Spiffistan, released into the public domain via Wikimedia Commons
- Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0