|Lecture Notes: 29 August||
Let's look at a "typical" plant cell for a moment. All of the organelles we saw in animals are here as well, but with a few additions: (text Figure 1-7b)
Eukaryotes also have dynamic cytoskeletons. (text Figure 1-9)
In summary then, we can look at an overview of the hierarchy of eukaryote cell organization. (text Figure 1-11), and compare the prokaryote and eukaryote levels of cellular organization. (text Table 1-3)
The Elements of Life
First, we will start with the basic requirements of an idealized, simplest life form and ask why life should use the particular atoms and molecules we see dominating in living organisms. (see also text Figure 1-12)
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 particularly 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 character 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 they 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.
Last modified 29 August 2008