Life on Earth is carbon-based

All life we know of on Earth is chiefly based on molecules containing the element carbon. Molecules containing carbon are usually referred to as organic compounds; their chemistry is organic chemistry.

Carbon has a total of six electrons. Two are buried in its inner electron shell, leaving four that can bond with other atoms. Except in very special circumstances, carbon always forms four bonds. Because the electrons in bonds repel each other as much as possible, the bonds of carbon tend to arrange themselves in a tetrahedron, like this:

carbon tetrahedron

The central carbon, in this case, is surrounded by four hydrogen (H) atoms. Except when it binds to another atom with a double or triple bond, carbon "centers" in molecules always have this tetrahedral shape.


carbon symbol


Representations of organic molecules

You will encounter many different kinds of representations of biological molecules. Each has its strengths in illustrating a particular important feature of a molecule. On the left is a nice hand sketch (a ribbon diagram) of the enzyme triose phosphate isomerase. It very nicely shows the underlying carbon-backbone


structure of many proteins, with alpha helices (the spring-looking things) and beta sheets (green). In the center are two images of short lengths of DNA molecules. The black one shows the major bonds as "sticks." While the other DNA image, a space-filling model, is more realistic, the stick model can sometimes be clearer to interpret. The image on the right is a stick diagram of sucrose (table sugar). Most small molecules are drawn in a similar fashion.

molecular models image



Many times both the full name and the drawn structure of a molecule are so complicated that we resort to abbreviations. Some common ones we'll use a lot are

DNA deoxyribonucleic acid
RNA ribonucleic acid
ATP, GTP adenosine triphosphate, guanosine triphosphate
ADP adenosine diphosphate
NADH nicotinamide adenine dinucleotide
NADPH nicotinamide adenine dinucleotide phosphate
FADH flavin adenine dinucleotide
A, T, G, C, & U adenine, thymine, guanine, cytosine, & uracil, the DNA bases


Categories of biomolecules

The table below will give you a brief outline of the five main kinds of biological molecules you'll need to know about, and what some of their functions are. Click on each to learn more.

molecules of life table

proteins button nucleic acid button lipids button


Periodic table of biologically relevant elements

This periodic table shows the elements of greatest biological significance. The elements in red are the major building blocks of all biomolecules: proteins, nucleotides, carbohydrates and lipids, and most smaller molecules of biological relevance. The blue elements are metals and other elements found in biomolecules and necessary for many functions. Yellow elements are important ions, for example, in the transmission of nervous signals. The black elements are significant bio-toxins, but many elements, with enough exposure, can harm living organisms.

biology periodic table


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Dr. Cruzan's Math and Science Web by Dr. Jeff Cruzan is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. © 2012, Jeff Cruzan. All text and images on this website not specifically attributed to another source were created by me and I reserve all rights as to their use. Any opinions expressed on this website are entirely mine, and do not necessarily reflect the views of any of my employers. Please feel free to send any questions or comments to