Biological Molecules

What kind of molecules are biological molecules?

Biological molecules are organic molecules. This means that they contain carbon and hydrogen. They might contain other elements such as oxygen, nitrogen, phosphorus or sulfur.

You might find them referred to as organic compounds. This is because they contain carbon as their backbone.

Serving as the backbone, carbon is the most important element in biological molecules. You may have heard that carbon is the foundation of life, or that all life on Earth is based on carbon. This is due to carbon's function as an essential building block for organic compounds.

Take a look at Figure 1, which shows a molecule of glucose. Glucose is composed of carbon, oxygen and hydrogen atoms.

Notice that carbon is in the middle (more precisely five carbon atoms and one oxygen atom), forming the base of the molecule.

Fig. 1 - Glucose is composed of carbon, oxygen, and hydrogen atoms. Carbon serves as the backbone of the molecule. Carbon atoms are omitted for simplicity

All biological molecules contain carbon except one: water.

Water contains hydrogen, but it does not contain carbon (remember its chemical formula H₂O). This makes water an inorganic molecule.

Chemical bonds in biological molecules

There are three important chemical bonds in biological molecules: covalent bonds, hydrogen bonds, and ionic bonds.

Before explaining each of them, it is important to recall the structure of the atoms that are the building blocks of molecules.

Fig. 2 - Atomic structure of carbon

Figure 2 shows the atomic structure of carbon. You can see the nucleus (a mass of neutrons and protons). Neutrons have no electrical charge, while protons have a positive charge. Therefore, overall a nucleus will have a positive charge.

Electrons (blue in this image) orbit the nucleus and have a negative charge.

Why is this important? It is helpful to know that electrons are negatively charged, and they orbit the nucleus, in order to understand how different molecules are bound on an atomic level.

Covalent bonds

The covalent bond is the bond most commonly found in biological molecules.

During covalent bonding, atoms share electrons with other atoms, forming single, double, or triple bonds. The type of bond depends on how many pairs of electrons are shared. For example, a single bond means a single pair of electrons is shared, etc.

Fig. 3 - Examples of single, double and triple bonds

The single bond is the weakest of the three, while the triple bond is the strongest.

When learning about biological macromolecules, you will come across polar and nonpolar molecules, which have polar and nonpolar covalent bonds, respectively. In polar molecules, electrons are not distributed evenly, for example in a molecule of water. In non-polar molecules, electrons are evenly distributed.

Most organic molecules are non-polar. However, not all biological molecules are non-polar. Water and sugars (simple carbohydrates) are polar, as well as certain parts of other macromolecules, such as the backbone of DNA and RNA, which is composed of sugars deoxyribose or ribose.

Interested in the chemistry side of this? For more details on covalent bonds, explore the article on Covalent bonding in the chemistry hub.

The importance of carbon bonding

Carbon can form not only one, but four covalent bonds with atoms. This fantastic ability allows for the formation of large chains of carbon compounds, which are very stable as covalent bonds are the strongest. Branched structures can be formed as well, and some molecules form rings that can attach to each other.

This is highly significant since different functions of biological molecules depend on their structure.

Thanks to carbon, large molecules (macromolecules) that are stable (due to covalent bonds) are able to build cells, facilitate different processes and overall constitute all living matter.

Fig. 4 - Examples of carbon bonding in molecules with ring and chain structures

Ionic bonds

Ionic bonds form when electrons are transferred between atoms. If you compare this to covalent bonding, electrons in covalent bonding are shared between the two bonded atoms, while in ionic bonding they are transferred from one atom to another.

You will come across ionic bonds while studying proteins since they are important in protein structure.

To read more about ionic bonds, check out the chemistry hub and this article: Ionic bonding.

Hydrogen bonds

Hydrogen bonds form between a positively charged part of one molecule and a negatively charged part of another.

Let's take water molecules as an example. After oxygen and hydrogen have shared their electrons and covalently bonded to form a water molecule, oxygen tends to “steal” more electrons (oxygen is more electronegative) which leaves hydrogen with a positive charge. This uneven distribution of electrons makes water a polar molecule. Hydrogen (+) is then attracted to negatively charged oxygen atoms of another water molecule (-).

Individual hydrogen bonds are weak, in fact, they are weaker than both covalent and ionic bonds, but strong in large quantities. You will find hydrogen bonds between nucleotide bases in DNA's double helix structure. So, hydrogen bonds are important in water molecules.

Fig. 5 - Hydrogen bonds between water molecules

Four types of biological macromolecules

The four types of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids (DNA and RNA).

All four types share similarities in structure and function, but have individual differences that are crucial for the normal functioning of living organisms.

One of the biggest similarities is that their structure affects their function. You will learn that lipids are able to form bilayers in cell membranes because of their polarity and that, due to the flexible helical structure, a very long chain of DNA can fit perfectly neatly into the tiny nucleus of a cell.

1. Carbohydrates

Carbohydrates are biological macromolecules that are used as an energy source. They are especially important for the normal functioning of the brain, and in cellular respiration.

There are three types of carbohydrates: monosaccharides, disaccharides, and polysaccharides.

• Monosaccharides are composed of one molecule of sugar (mono- means 'one'), such as glucose.

• Disaccharides are composed of two molecules of sugar (di- means 'two'), such as sucrose (fruit sugar), which is composed of glucose and fructose (fruit juice).

• Polysaccharides (poly- means 'many') are composed of many smaller molecules (monomers) of glucose, ie individual monosaccharides. Three very important polysaccharides are starch, glycogen and cellulose.

Chemical bonds in carbohydrates are covalent bonds called glycosidic bonds, which form between monosaccharides. You will come across hydrogen bonds here as well, which are important in the structure of polysaccharides.

2. Lipids

Lipids are biological macromolecules that serve as energy storage, build cells, and provide insulation and protection.

There are two major types: triglycerides, and phospholipids.

• Triglycerides are built of three fatty acids and alcohol, glycerol. Fatty acids in triglycerides can be saturated or unsaturated.

• Phospholipids are composed of two fatty acids, one phosphate group and glycerol.

Chemical bonds in lipids are covalent bonds called ester bonds, which form between fatty acids and glycerol.

3. Proteins

Proteins are biological macromolecules with various roles. They are the building blocks of many cell structures, and act as enzymes, messengers and hormones, carrying out metabolic functions.

Monomers of proteins are amino acids. Proteins come in four different structures:

• Primary protein structure

• Secondary protein structure

• Tertiary protein structure

• Quaternary protein structure

Primary chemical bonds in proteins are covalent bonds called peptide bonds, that form between amino acids. You will come across three other bonds as well: hydrogen bonds, ionic bonds and disulfide bridges. They are important in the tertiary protein structure.

4. Nucleic acids

Nucleic acids are biological macromolecules that carry the genetic information in all living things and viruses. They direct protein synthesis.

There are two types of nucleic acids: DNA and RNA.

• DNA and RNA are made up of smaller units (monomers) called nucleotides. A nucleotide is made up of three parts: a sugar, a nitrogenous base, and a phosphate group.

• DNA and RNA are neatly packed inside the nucleus of a cell.

Primary chemical bonds in nucleic acids are covalent bonds called phosphodiester bonds, that form between nucleotides. You will come across hydrogen bonds as well, which form between DNA strands.