What comes to mind when you hear the word organic? Do you think about produce that is not genetically modified or grown without synthetic chemicals? In biology, you will frequently encounter the terms "organic compounds" or "organic molecules," but "organic" here takes on an entirely different meaning! So, let's talk about what organic molecules are.
- First, we will define organic molecules and distinguish them from inorganic molecules.
- Then, we will discuss organic molecules' components, structure, and formula.
- Finally, we will cite the four major groups of organic molecules found in all living organisms.
Organic Molecules Biology
Living matter is made up largely of organic molecules in biology.
Organic molecules contain carbon and hydrogen and have at least one covalent bond between carbon and hydrogen or between two carbon atoms.
Recall that a covalent bond is a Linkage formed between two atoms sharing a pair of valence electrons (or four electrons in its outermost shell).
A mnemonic you can use to remember elements found in organic molecules is CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
Organic Molecules Structure
The structure of the carbon skeleton of organic molecules can be ring-shaped, branched, or straight. Organic molecules consist of chains of carbon atoms of various lengths, with many being relatively long, allowing for a large number and variety of compounds.
Organic molecules contain
Organic molecules are made up of carbon and hydrogen atoms, and they might sometimes contain nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S) atoms.
Let's take ethane (C2H6) for example. Ethane is an organic molecule containing carbon and hydrogen.
Notice that the carbon atoms in ethane each have four covalent bonds. Carbon has four valence electrons, so it can form up to four covalent bonds with up to four atoms all at once. Because of its size and bonding properties, carbon atoms can bind together in large numbers, which allows it to produce a chain or carbon skeleton.
However, keep in mind that while all organic molecules contain carbon, not all molecules containing carbon are organic. For a carbon-containing molecule to be considered organic, its carbon atom must be reduced and not fully oxidized.
An atom such as carbon or oxygen is said to be reduced if they form covalent bonds with another atom with lower electronegativity.
An atom is said to be oxidized when it forms a covalent bond with an atom with higher electronegativity.
Recall that electronegativity refers to the tendency of an atom to attract electrons toward itself!
When an atom is reduced, it gains a majority of the electrons that form the covalent bond, while the oxidized atom gains only a small portion of the electrons.
For example, carbon dioxide (CO2) is considered an inorganic molecule because its carbon atom is bonded only to oxygen atoms and is therefore oxidized rather than reduced.
Functional Groups
In addition to carbon, organic molecules may also contain functional groups. Let's look at its definition.
Functional groups are groups of atoms in a molecule that are named according to their chemical composition and the chemical reactions they participate in.
So, when present in a molecule, a functional group will exhibit its unique chemical behavior! Functional groups have specific names typically incorporated into the names of the compounds that include them.
For example, formaldehyde (a strong-smelling gas used in many building materials and household products) contains the functional group aldehyde.
You should try to remember these functional groups because they typically determine how organic molecules will react!
Notice how formulas contain the symbol R. R stands for "residue," and it represents the other parts of the molecule. In other words, R can represent a single hydrogen atom or many atoms!
Also, notice that some functional groups are made up of just one or two atoms, while others are made up of these simpler functional groups.
For example, carbonyl is a simple functional group consisting of a carbon atom double bonded to an oxygen atom (C=O). Carbonyl can be found in many larger functional groups, such as carboxylic acids, aldehydes, and ketones.
When we are interested in something that is happening in a specific spot on a huge biological molecule, denoting the 'R' group is a convenient way to call attention to that part of the molecule and abbreviate the rest of the structure.
Organic Molecules Formula
Organic molecules can be represented through a molecular formula, which indicates what elements and how many atoms of each element are present in a compound.
The number of atoms is represented by a subscript after the element's symbol unless there is only one atom of the element present.
For example, methane can be represented through the molecular formula CH4, which tells us that it contains one carbon (C) atom and four hydrogen (H) atoms.
Molecules can also be represented by their structural formula, which is a graphic illustration of the arrangement of the atoms within the molecule.
There are different types of formulas that chemists can use to show the structure of an organic molecule. Here, we will look at four of them:
The Lewis structure is a diagram showing how a molecule's atoms are bonded together. In a Lewis structure, valence electrons are represented as dots, while bonds are represented by two dots between bonded atoms or a line. Double bonds are represented by two lines, while triple bonds are represented by three.
A condensed formula consists of elemental symbols, with the order of each element suggesting how they are connected. On the other hand, bond-line formulas consist of bonds and lines, with carbon atoms no longer drawn in and are instead indicated using the ends and bends of the lines.
The Dashed-Wedged line structure is similar to the bond-Line formula, except that the structure of the atoms is also illustrated: dashed lines show atoms and bonds facing away from the viewer, while wedged lines represent bonds and atoms facing the viewer.
Concept of Isomerism in Organic Molecules
It is important for the structure of organic molecules to be adequately represented because different compounds can have the same molecular formula.
For example, the Monosaccharides fructose, galactose, and glucose all have the same molecular formula, C6H12O6, but the atoms are bound to one another differently, as seen in figure 3 below.
Substances with identical molecular formulas but different atom bonding arrangements are called isomers. Chemistry places a great deal of emphasis on the concept of isomerism because the structure of organic molecules structures are often closely linked to their functions.
So, a small change in the structural arrangement of atoms in a molecule can result in very different properties!
Large Organic Molecules Examples
Lastly, let's look at some examples of large organic molecules. The four major types of organic molecules that make up Cells and cell structures are:
Carbohydrates
Proteins
Nucleic Acids
Lipids
Carbohydrates, Proteins, and Nucleic Acids are polymers, or large complex molecules made up of smaller, identical repeating units called Monomers. These Monomers are typically linked together through dehydration synthesis, a chemical reaction in which monomers bind end to end, resulting in the formation of Water Molecules as a byproduct.
On the other hand, polymers can be broken down into monomers through hydrolysis, where water is split into -OH and H.
To makes things simpler, let's go through each of these organic molecules more in-depth!
Carbohydrates
You've probably encountered the term carbohydrates in the context of food.
In biology, the term carbohydrates are used a bit differently.
Carbohydrates are organic molecules made up of monomers called Monosaccharides.
Monosaccharides are molecules with the composition [CH2O]n, where n is usually 3-6. For example, glucose is a 6-carbon sugar whose chemical formula is C6H12O6. Monosaccharides are linked together via glycosidic bonds to form Polysaccharides.
Carbohydrates play an important role in energy storage and structural support in Cells.
Starch, which you might be familiar with, is a common carbohydrate. Starch is found in pasta, bread, rice, and many other foods. Green Plants produce it for energy storage. Other examples of carbohydrates are cellulose (which makes up the cell walls of plants) and chitin (which makes up the exoskeleton of insects and marine invertebrates).
Proteins
You've probably also encountered the term protein before, but in biology, it's much more than the stuff bodybuilders use to increase muscle mass!
Proteins are organic molecules composed of amino acid monomers joined together by peptide bonds.
An amino acid contains an amino group (-NH2), a carboxyl group (-COOH), and a side chain (called the R group). The amino and carboxyl groups are joined to the central carbon atom of each amino acid molecule. There are 20 different amino acids, each containing a different R group.
Proteins play an essential role in enzymatic activities, cell signaling, and providing structural components that help bind cells into tissues.
Examples of proteins include:
Nucleic acids
Next, we have nucleic acids.
Nucleic acids are organic molecules that are made up of nucleotide monomers linked together by a phosphodiester Linkage.
Nucleotide monomers are made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base.
Both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which carry Genetic Information on living organisms, are nucleic acids. RNAs also play a major role in Protein Synthesis.
Lipids
Lastly, let's look at the definition of Lipids.
Lipids are large organic molecules made up of glycerol and fatty acids.
Phospholipids, which make up the cell membrane, are an example of lipids. The phospholipid bilayer creates a hydrophobic environment that separates the cytosol's internal aqueous environment from the exterior of the cell. Fats and oils which serve as energy storage are also examples of lipids.
To keep learning about lipids, nucleic acids, proteins, and carbohydrates, check out "Macromolecules"!
Organic Molecules - Key takeaways
- Organic molecules are those that contain carbon and hydrogen and have at least one covalent bond between carbon and hydrogen or between two carbon atoms.
- While all organic molecules contain carbon, not all molecules containing carbon are organic.
- The carbon atom in an organic molecule must be reduced.
- Organic molecules can also contain functional groups.
- The four major types of organic molecules that make up cells and cell structures are carbohydrates, proteins, nucleic acids, and lipids.
References
- OpenStax, 7.1: Organic Molecules - Biology LibreTexts, 10 July 2016.
- Molecular Formula (Chemical Formula), Illustrated Glossary of Organic Chemistry, (n.d.).
- General College Chemistry, Organic Molecules, (n.d.).
- Biological Principles, Biological Molecules, (n.d.).
- Choo, Ezen, Structure of Organic Molecules, Chemistry LibreTexts, 2 Oct. 2013.
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