Carbon and the Molecular Diversity of Life

Hey all!

This blog post is a part of STEMTalksNC’s ever expanding General Biology Series. In this post, it would be helpful to know high school chemistry.

Although water is the universal medium for life on Earth, living organisms are made up of chemicals that are based mostly on the element carbon. Of all chemical elements, carbon is unique in its ability to form molecules that are large, complex, and diverse. This molecular diversity has established the diversity of organisms. In this post, we will explore the uniqueness of Carbon and how it connects to the diversity of life.

Compounds that contain Carbon are said to be organic. The overall percentages of the elements of life- Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, and Phosphorous- are quite the same among all organisms. However, carbon’s flexibility can be used to build a lot of distinguishable of organic molecules. Different species of organisms, and different organisms within the species, are distinguished by their difference in their organic molecules.

Carbon atoms can form diverse molecules by bonding to four other atoms 

The key to an atom’s chemical characteristics is its electron configuration. This configuration decides the varieties and the amount of bonds an atom will arrange with other atoms. As we highlighted in our post (Chapter 2 post), Carbon has 6 electrons, 4 of them being valence electrons. Having 4 electrons in a shell that can hold, carbon shares its 4 electron with other atoms in covalent bonds so that 8 electrons are present. This tetravalence, meaning it makes four bonds with other atoms, is one feature of carbon’s flexibility that makes large, complex organic molecules possible. The shape of these organic molecules often regulate its function. As we mentioned above, the electron configuration decides the varieties and the amount of bonds an atom will arrange with other atoms; the electron configuration of carbon gives it ability to bond with many different elements. The most frequent elements that bond with Carbon in organic molecules are oxygen, hydrogen and nitrogen; these are the major atoms of organic molecules. The valences of these four elements are the basics of the rules of covalent bonding in organic molecules. Valence is the number of covalent bonds an atom can form. It is generally equal to the number of electrons required to complete the valence (outermost) shell.

The valences of these four elements are as follows:

  1. Hydrogen- 1 Valence
  2. Oxygen- 2 Valence
  3. Nitrogen- 3 Valence
  4. Carbon- 4 Valence

Molecular Diversity Arises from Carbon Skeleton Variation

Carbon chains form the skeletons of most organic molecules. The variation of the carbon skeleton creates the diversity of these organic molecules. The skeletons vary in:


Image result for ethaneImage result for propane chemical formula

The two Lewis Structures differ in length of their carbon skeleton.

Branching (unbranched or branched)

Image result for butane and 2-methylpropane
Skeletons may be unbranched (left) or branched (right).


Double Bonds (the number and location of the double bonds)

Image result for 1-butene and 2-butene
The skeleton may have double bonds, which can vary in location.


Image result for cyclohexane
Some carbon skeletons are arranged in rings.


Hydrocarbons are organic molecules that contain only carbon and hydrogen. Atoms of hydrogen are attached to the carbon skeleton wherever electrons are available for covalent bonding. Many of a cell’s organic molecules have regions consisting of only carbon and hydrogen; these portions are considered hydrophobic.


The difference in the build of organic molecules is apparent in isomers, which are compounds that have the same numbers of the atoms of the same elements but different structure and properties. There are three varieties of isomers: structural isomers, geometric isomers, and enantiomers.

Structural Isomers

Structural Isomers are different because of the covalent arrangements of their atoms. The number of possible isomers increases as carbon skeletons increase in size. Structural isomers may also differ in the location of double bonds.

Image result for structural isomers
Structural isomers differ in covalent partners, as shown in this example of two isomers of C₄H₁₀: Butane (left) and Isobutane (right)

Geometric Isomers

Geometric Isomers have the same covalent partnerships, but they contrast in their spatial arrangements. These spatial arrangements are the result of the rigidness of double bonds. While single bonds are not rigid, double bonds are rigid in that they do not allow the atoms they join to rotate freely about the bond axis without changing the compound. Looking down below at the pictures, they seem as they would be classified as the isomer. However, they are considered two different types of isomers because of the rigidness of the double bonds. The Cis isomer has the Rs on the same side, but the Trans isomer has the Rs on the opposite sides.

Image result for cis and trans isomers
Geometric Isomers contrast in arrangement about a double bond. In this picture, R represents an atom or group of atoms attached to a double-bonded carbon.

This small difference in shape can significantly change the function of the organic molecule. For example, the biochemistry of vision involves a light-induced change of Rhodopsin, a chemical compound in the eye, from the Cis isomer to the Trans isomer.


Enantiomers are isomers that are mirror images of each other. Enantiomers are, in a simplistic way, left-handed and right-handed versions of the molecule. Just as your right hand won’t fit into a left-handed glove, the working molecules in a cell can distinguish the two versions by shape.

Image result for enantiomers
Enantiomers differ in spatial arrangement around an asymmetric carbon, resulting in molecules that are mirror images.

Enantiomers come in pairs, one isomer usually being active and the other being inactive. The concept of enantiomers is important in the pharmaceutical industry; the differing effects of enantiomers demonstrates that even the most subtle differences in molecules can produce drastic effects. We see that molecules have emergent properties that depend on the specific arrangement of their atoms.

In the next post, we will discuss functional groups. Functional groups also contribute to the diversity of organic molecules and hence the diversity of life.

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