Biomolecules

Though humans, roses, and viruses look different, they are built from the same matter. This matter is composed of atoms that make up molecules when chemically bonded to each other. Biomolecules are the compounds that living organisms can produce in their bodies. All the life processes depend on biomolecules, so we can call them the fundamental building blocks of life. In this topic, we will focus on the molecules that make up every living thing.

Organic molecules

All the organisms on Earth are carbon-based lifeforms, meaning that the main chemical element in the living matter is carbon. Carbon atoms can form four chemical bonds with other atoms, such as hydrogen. Hydrogen can form just one chemical bond. If you bond one carbon atom with four atoms of hydrogen, you get one molecule of methane. Methane is the simplest organic molecule. Generally, organic molecules are carbon-containing compounds with carbon-hydrogen chemical bonds. Take methane, change any of the hydrogen atoms to another carbon atom, leaving three connections to hydrogens, and you get a molecule with a more complex structure. If you continue to add carbon atoms, you will obtain large and branching macromolecules. Adding a link between the two ends of the molecule to make it a ring will cause its properties to change. Add chemical bonds with other elements, such as oxygen, and you will get something completely different. Structures of biomolecules vary greatly, and so do their properties and functions in living organisms.

Besides carbon (C) and hydrogen (H), the most abundant elements in organic compounds are oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S). In organic compounds, every element can form a certain number of chemical bonds: one for hydrogen, two for oxygen, three for nitrogen, and so on. Combinations of these six elements make most of biological molecules.

When you draw a chemical structure of an organic compound, you can omit most of the hydrogen atoms and represent carbons as line ends or vertices. However, you must show any other types of atoms and functional groups. Such skeletal formulae are easy to draw and very pretty, so scientific folks commonly use them. There is a brief guide to types of organic chemistry formulae that tells about different ways to represent an organic compound.

Monomers and polymers

Methane is a small molecule that contains one atom of carbon. Let's take two molecules of methane and join them through a carbon-carbon bond. You'll get an ethane, containing two atoms of carbon. If you continue adding one methane after another to the growing chain, you will get a very long molecule. This long molecule is a polymer, which is a molecule made up of multiple repeating building blocks (monomers). In this example, all of the monomers are methanes, and therefore, the polymer is homogeneous. However, monomers might vary in structure, just like Lego pieces vary in color and shape. Polymers made from different kinds of monomers are called heterogeneous polymers, or copolymers.

Many biomolecules are polymers. There are four major types of biomolecules: carbohydrates, lipids, proteins, and nucleic acids. We will take a closer look at proteins and nucleic acids in the following topics. Right now, let's focus on carbohydrates and lipids.

Carbohydrates

Carbohydrates, or sugars, are molecules made up from atoms of carbon, hydrogen, and oxygen. Their general chemical formula is Cn(H2O)n, where n is the number of carbon atoms in the molecule. That's where the name "carbohydrate" is from — they are hydrates of carbon. Carbohydrates can also be bonded to other types of molecules. There are three main types of carbohydrates: monosaccharides, disaccharides, and polysaccharides.

• Monosaccharides, like glucose and fructose, contain one monomeric sugar unit. They are hydrophilic, meaning they are easily dissolved in water. They are sweet and act as a primary energy source for most living organisms.

• Disaccharides are carbohydrates made up of two monosaccharide monomers joined together. Lactose, the main sugar in milk, and sucrose, also called table sugar, are disaccharides. They are also sweet and dissolve easily in water.

• Polysaccharides are homogeneous or heterogeneous polymers. They are not sweet. Most of them are far less soluble in water than monosaccharides, because their internal chemical bonds can't interact with water directly. This is the reason why most of polysaccharides are hydrophobic and are perfect for building structures. For example, wood is hard because it is mostly made up of long molecules of cellulose, a polysaccharide. Crab shells are also hard because their main component is the nitrogen-containing polysaccharide chitin. Living organisms also use polysaccharides, like plant starch and animal glycogen, to store energy. When your body is hungry, it can break down a long molecule of glycogen and get hundreds of tiny, sweet glucose molecules. Yummy!

Lipids

Lipids are a diverse group of compounds, including oils, waxes, and steroids. However, all lipids have a common feature: they are hydrophobic. Living organisms use lipids in many ways. Since lipids repel water, aquatic birds and mammals use them to stay dry. Some lipids are the building blocks of signaling molecules, so they help cells to communicate with each other in our body. Lipids also form membranes, which separate cells from their environment and divide the cell interior into isolated compartments.

The most important components of the cell membrane are phospholipids. A phospholipid consists of several components:

• A glycerol molecule that contains three hydrogen-bound carbon atoms and three hydroxyl (-OH) groups. Each hydroxyl group is a site where glycerol may attach other molecules.

• Long fatty acids that form two hydrophobic tails of the phospholipid.

• A phosphate group that creates the hydrophilic region (head) of the phospholipid.

In water, phospholipid molecules arrange themselves in such a way that the hydrophobic tails hide from water, while the hydrophilic heads face it. As a result, a membrane, composed of two layers of phospholipids, arranges spontaneously by itself.

If you change the phosphate group to another fatty acid in the phospholipid molecule, you'll get a triglyceride. Triglycerides are fully hydrophobic fat molecules that are perfect for long-term energy storage.

Conclusion

Biomolecules are organic compounds, produced by living organisms. There are four main groups of biomolecules: carbohydrates, lipids, proteins, and nucleic acids. Biomolecules carry out lots of functions in our bodies. The structure of biomolecules determines these functions.