The following observations may be made regarding the elements of life:
Life is largely a phenomena of hydrogen and the second period of the Periodic Table. That is, the major component elements (red) in all known organisms are from these periods. Why these four elements?
- First, we might observe that H, O, N, and C are the smallest elements capable of forming 1, 2, 3, and 4 bonds respectively. Smallest is important because that means they can form the strongest most stable covalent bonds. So these atoms are going to be capable of forming some of the most stable molecules, an important consideration for something that needs to grow and reproduce in a hostile environment.
- C is particlarly noteworthy because it forms strong, stable bonds with itself. As a result it can form the backbone of large chain and branched structures, a unique charecter among the elements.
- Second, C, N, and O are also the only elements capable of forming strong multiple bonds (carbon and nitrogen can form triple bonds, all three can form double bonds).
The next important elements to life occur in Period 3: P and S (orange). These are the smallest elements capable of multiple covalent bonds to C, O and N, and which also have available d-shells. The d-shells allow additional transition states and reaction mechanisms. P and S are particularly important in the capture, storage, and distribution of chemical energy.
Conveniently all of these elements are among the most abundant in the Universe. None-the-less, we hypothesize that these elements were chosen for their special properties, specifically strong covalent bond formation (to enable the formation of stable biomolecules), the ability of carbon to form large branched molecules, and for C, N, and O the formation of multiple bonds which provides chemical flexibility (step-wise oxidations, different hybridization geometries etc.).
(Aside: So why not have Si based life instead of C based?)
- The "essential" elemental ions found in all studied species (blue), Ca (+2), Mg (+2), K (+1), Na (+1) and Cl (-1) were probably chosen more on the basis of availability in the primordial oceans than for any specific properties: other ions are very similar.
- The trace elements required by all studied organisms (violet), Mn, Fe, Co, Cu, and Zn, are all used as co-catalysts and/or ligands. Thus they were probably chosen for their specific redox properties and/or electronic structures as well as their availability on the early earth.
- A variety of other elements are required by at least a few organisms, and are shown on the table in black. The grayed elements are not known to be of biological importance, but are shown as "place-markers" to help us keep track on the Table.
There are a few critically important small molecular precursors to biomolecules found in the environment. Biomolecules can be looked at in two major categories:
The small molecules are going to be either metabolites or monomers from which the macromolecules are built.
First let's note the "inorganic" (sometimes called mineral) molecules and molecular ions: oxygen (O2), water (H2O), carbon dioxide (CO2) ammonia or ammonium ion (NH3 or NH4+), nitrate ion (NO3-), nitrogen (N2), phosphate ion (PO43-) and sulfate ion (SO42-). These are mostly metabolites, though the ions can also serve as counter ions along with chloride in creating the intracellular media.
These molecules and ions in turn can be made into metabolites, small organic molecules used in energy transformation and as precursors to monomers and macromolecules.
So what have we learned?
Right now we'll focus on the monomers and the associated macromolecules. There are four major categories:
1. The nitrogenous bases (purines and pyrimidines) which are components of the nucleic acids (RNA and DNA-used for information storage and processing)
4. The fatty acids which, together with glycerol, make up the fats (used mostly for energy storage) and the phospholipids (the major component of cell membranes). The 16 carbon fatty acid palmitate is shown below:
A typical phospholipid is shown here, replacement of the phosphate ester group with a third fatty acid would give a fat instead:
The amino acids, nucleotides, and sugars can all be polymerized to give the macromolecules characteristic of life: proteins, nucleic acids, and polysaccharides, respectively. We will come back to each of these macromolecules in our study, focusing particularly on proteins. Briefly, proteins comprise the machinery and much of the structure of life; nucleic acids provide the information required to specify the proteins, and polysaccharides provide structural fibers and energy storage molecules.
Note that all of these families of molecules exhibit chirality in some, and generally most, of their members. Also, biological systems chose a single chirality for each family (e.g. L-amino acids, D-sugars)
© R. A. Paselk 2010; Last modified 25 January 2013