The compounds found in living things are known as biochemical compounds or biological molecules. Biochemical compounds make up the cells and other structures of organisms. They also 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.
Carbon is so basic to life because of its ability to form stable bonds with many elements, including itself. This property allows carbon to create 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 individual beads (monomers) can do some jobs on their own, but sometimes you need a larger molecule, so the monomers can be connected to form polymers.
What do all of 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 biological macromolecules.
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 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 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 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 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.
Your body is full of biochemical compounds. Without them, plants and animals wouldn’t exist – they’re the compounds that make up living things.
All life on Earth is carbon-based, which means that the large molecules that make up much of our body all contain carbon. In some sci-fi stories, humans find silicon-based life on other planets. That’s because silicon has a lot of similarities with carbon. But as far as we know, all life is carbon-based. So for the moment, that’s how we define biochemical compounds.
A compound is a substance made of molecules that contain two or more elements bonded together. A biochemical compound is any compound that contains carbon and is found in living things. They’re involved in every process of life: growth, digestion, respiration, you name it. In the real world, all biochemical molecules contain hydrogen and oxygen. They might also contain nitrogen, sulfur and phosphorus.
The biggest biochemical molecule ever discovered is titin. It’s found in muscles and contains 169,723 carbon atoms, 270,464 hydrogen atoms, 45,688 nitrogen atoms, 522,243 oxygen atoms and 912 sulfur atoms. We call it titin because its real name is too hard to say. It would take a person around half an hour!