| Chem 431 |
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Fall 2007 |
| Lecture Notes: 27 August |
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| PREVIOUS |
Cells to Biomolecules
There are a few critically important small molecular precursors to biomolecules found in the environment. Biomolecules can be looked at in two major categories: small molecules and macromolecules.
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.
Right now we'll focus on the monomers and the associated macromolecules. There are four major categories:
1. The nitrogenous bases (purines and pyrimidines, text Figure 1-10b)) which are components of the nucleic acids (RNA and DNA-used for information storage and processing)
4. The fatty acids which, together with glycerol (text Figure 1-10c), 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 (text Figure 1-19). Also, biological systems chose a single chirality for each family (e.g. L-amino acids, D-sugars).
All of these molecules together go to make up cells.
The oldest fossil evidence for life on Earth dates to about 3.7 by (billion years ago). The Earth itself formed about 4.5 by with the formation of our solar system. It is thought that the Earth was too hot and chaotic to support life until perhaps 3.8 by (intense bombardment of the earth did not end until 3.9 by, thus life arose quite quickly, essentially as soon as possible!
How did this occur? Obviously guess work - no one was there, and there is no record in the rocks that we could even be certain of. However, we have good guesses as to Earth's early environment (atmosphere of H2O, NH3, CO2 and smaller amounts of CH4, NH3, SO2, and H2. If you treat such an atmosphere with any high energy source in the laboratory (as was first done by Miller in 1953, text Figure 1-33) you will get a mixture of organic molecules including many important to organisms today. Interestingly, we also find small precursor molecules all over the Universe - in ancient rocks, meteors, comets etc. Evidence of small precursor molecules (amino acids, nitrogenous bases etc.) also occurs in interstellar space, the atmospheres of carbon stars, gas giant planets etc.
The formation of polymers is more problematic. A major difficulty is that biopolymers are all thermodynamically unstable relative to their hydrolysis products. Some theories, but no certainty as to how polymers may have formed, though polymers have been synthesized under conditions which may have occurred on the early Earth.
 Pathway Diagrams |
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Last modified 27 August 2007
