By Robert Hazen, George Mason University
Think about the chemicals of life. Living organisms contain an amazing variety of materials: wood, muscle, seeds, skin, leaves, blood, bone, and countless others. Studies of life, coupled with a growing understanding of the principles of organic chemistry, eventually led to the realization that all of life’s varied chemicals, all these substances, are constructed from a few different molecular building blocks, and they share four essential characteristics.
It’s all Based on Carbon
First, life is based on carbon. Carbon atoms usually bond covalently to 4 other electrons, 4 other atoms that give carbon the extra 4 electrons it needs to achieve the magic number of 10. This occurs because carbon is element 6: it needs 4 electrons; 6 plus 4 is 10.
This feature allows carbon to form chains, rings, or branches in almost any imaginable arrangement because carbon can link to itself. Recall how hydrocarbons, the simplest of the carbon-based molecules, are compounds of carbon and hydrogen; that is, hydrogen, element 1.
Each hydrogen atom forms a covalent bond, a single bond, with carbon, and shares, therefore, a pair of electrons with the carbon. We then have carbohydrates. Carbohydrates are compounds of carbon, hydrogen, and oxygen; and oxygen, which is element 8, wants to form a double bond. For example, it forms a double bond with carbon, or it shares one pair of electrons with carbon and one with hydrogen.
Only a Few Elements Involved
The second characteristic shared by all of life’s molecules is that they’re formed from just a few different elements. Almost two-thirds of the atoms in living organisms are hydrogen; a quarter are oxygen, about one-eighth are carbon; and together, hydrogen, oxygen, and carbon comprise more than 98% of all the atoms in living things.
The next most abundant element is nitrogen, forming about 1.1% of life’s atoms. Then comes phosphorus which is essential for adenosine triphosphate (ATP), the energy molecule. That’s about 0.2% of most living things.
There are dozens of other elements that are required in very small amounts for life, but they account for less than 0.5% of all the mass of all the elements that are contained in living things.
This article comes directly from content in the video series The Joy of Science. Watch it now, on Wondrium.
Using Modular Construction
The third characteristic of life is that life’s molecules are modular—they use modular construction. It might be possible to assemble every one of the molecules we need in life independently from scratch, developing the best combination of atoms for that purpose, in such a way that the components of one molecule are completely different from the components of all the other molecules.
But in life, as in commerce, as in industry, that’s an inefficient process because custom-making every part is expensive. Rather, what life does is make most of its chemicals from a few relatively simple building blocks.
This strategy is exactly the same as that of modern architects, who rely on standard building materials to construct an almost infinite variety of different kinds of houses and stores, shops, office buildings, and so forth, just by putting them together in different ways. That’s called modular construction. Using few basic construction materials, buildings can be developed much faster, and they’re much cheaper to build and design, rather than designing every particle from scratch. Now, modularity does not imply simplicity.
Modularity is not just restricted to the molecules of life; even atoms are modular, in a sense. Atoms use protons and neutrons and electrons to build up all the different elements of the periodic table. Modularity is a fundamental aspect of nature at all different scales.
Everything Depends on Shape
The fourth characteristic is that the function of life’s molecular building blocks depends on their 3D shape. There’s a real analogy here to tools. A screwdriver, a hammer, a saw work because they have specific shapes that are suited to their function.
Life has to manipulate atoms, it has to gather energy; and so, much the same way, at a molecular level, you have molecules whose purpose is suited to their function because they have the appropriate shape. Remember, atoms themselves don’t have a shape. They’re normally considered to be more-or-less spherical collections of electrons.
But, if you cluster lots of atoms together, you can develop a 3D shape that has a specific function. For example, a long fiber, or a rounded globule, or some more complex shape serves a specific function. All chemical relations result from the rearrangement of electrons among atoms.
If a specific reaction is to take place, you need to have the atoms positioned appropriately—adjacent to each other, opposite each other—so those atoms can interact. That’s one of the ways that shape becomes so important. You can’t have atomic interactions without having appropriate shapes.
If two molecules are to link together to form a polymer, for example, then you need to have the appropriate ends of the two molecules line up and come into contact so that they can join. It’s the geometrical shape of a molecule that determines how it interacts with adjacent molecules, how chemistry proceeds.
Common Questions about Characteristics of Molecular Building Blocks
According to studies of life and the principles of organic chemistry, all these substances, are composed of a few different molecular building blocks.
These are molecular building blocks, which are all based on carbon, formed from a few different elements, use modular construction, and their functions depend on their 3D shape.
Although atoms are considered to be spherical collections of electrons, they actually don’t have a specific shape. But when there are clusters of atoms together, there will be a molecular block with a 3D shape with a specific function.
