Its’ A Bugs Life
In order to progress our discussion it will be necessary to focus our observations on biological entities, even with a narrow remit my knowledge and vocabulary may not be sufficient to convey the complexities of the micro world. Anything you can imagine from Jekyll & Hyde and super-powers to Frankenstein and vampires with whips and chains, probably exists in the micro world (check this one out!). They are everywhere, even in the most inhospitable environments where humans fear to tread, even with all their technology to protect them, converting complex molecules into fundamental chemicals; they are the ultimate recyclers, constantly cleaning our environment and releasing the fundamental building blocks for life to continue on this planet. So with that wonderful and expansive introduction let’s explore a bugs life...
SO, WHAT AM I?
In the context of this discussion a bug is a microscopic living organism, which may be single-celled or multicellular. Collectively bugs or microorganisms as they are referred to are a very diverse group and includes all bacteria, archaea and most protozoa. It also contains some species of fungi, algae, and certain microscopic animals, such as rotifers. Many macroscopic animals and plants have microscopic juvenile stages. Basically microorganisms can be found almost anywhere in the taxonomic organization of life on this planet. Because of this diversity microorganisms live in every part of the biosphere, including soil, hot springs, "seven miles deep" in the ocean, "40 miles high" in the atmosphere and inside rocks far down within the Earth’s crust (see Endolith) .
Microorganisms tend to have a relatively fast rate of evolution. Most microorganisms can reproduce rapidly, and bacteria are also able to freely exchange genes through conjugation, transformation and transduction, even between widely divergent species. This horizontal gene transfer, coupled with a high mutation rate and many other means of genetic variation, allows microorganisms to swiftly evolve (via natural selection) to survive in new environments and respond to environmental stresses. This rapid evolution is important in medicine, as it has led to the recent development of "super-bugs", pathogenic bacteria that are resistant to modern antibiotics.
And what is my purpose?
Microorganisms are vital to humans and the environment, as they participate in the carbon and nitrogen cycles, as well as fulfilling other vital roles in virtually all ecosystems, such as recycling other organisms’ dead remains and waste products through decomposition. Microorganisms also have an important place in most higher-order multicellular organisms as symbionts; some of which are mutually beneficial (mutualism), while others can be damaging to the host organism (parasitism). If microorganisms can cause diseasein a host they are known as pathogens and then they are sometimes referred to as microbes.
HOW I DO IT
There is only one thing that matters to me and every other living cell, and that is securing a readily available energy source to construct complex molecules that are required to maintain my integrity and to perform other biological functions, such as growth, replication and cell division. In most life forms carbohydrates, such as glucose, are the major energy source and here is how we do it.
Glucose is mainly metabolized by a very important ten-step pathway called glycolysis, the net result of which is to break down one molecule of glucose into two molecules of pyruvate. This also produces a net two molecules of ATP, the energy currency of cells, along with two reducing equivalents of converting NAD+ (nicotinamide adenine dinucleotide:oxidised form) to NADH (nicotinamide adenine dinucleotide:reduced form). This does not require oxygen; if no oxygen is available (or the cell cannot use oxygen), the NAD is restored by converting the pyruvate to lactate (lactic acid) (e.g., in humans) or to ethanol plus carbon dioxide (e.g., in yeast).
But it’s never that simple...
As discussed earlier, a cell is composed of four main classes of biomolecules produced from six primary chemical elements, these are; carbohydrates, lipids, proteins, and nucleic acids.  So, in order to survive, I must be able to breakdown each of these different types of biomolecules; actually lets get this into perspective, if I can’t breakdown every biomolecule all life on this planet will cease to exist eventually, as there will be a net transfer of the six primary elements to the one biomolecule that I can not breakdown, making them unavailable for the manufacture of other biomolecules!
Time is a real problem!
I need to breakdown a complex biomolecule, for example a protein, into a simple molecule, such as glucose, which will pass through my cell membrane and be available for metabolism. The hydrolysis of peptide bonds (i.e. the breakdown of proteins into smaller polypeptides or amino acids) is extremely slow, taking hundreds of years. Therefore I need to speed up the process in order to survive. Proteolysis (i.e. the breakdown of proteins) is typically catalysed by cellular enzymes called proteases. I make enzymes, which incidentally are proteins, as part of my metabolic pathways, so that is one problem solved, but enzymes are very specific in the reactions they catalyse; I can’t just absorb an enzyme, I must make it specifically for my needs and one protein molecule may require a number of different enzymes (i.e. one enzyme to break the protein chain into its’ constituent amino acids, another to remove the amino acid group, etc.). My objective is either to release energy from the protein molecule or to use the constituent parts to build a protein molecule I can use; ideally the best scenario is to obtain both!
Size cannot be conquered!
We know two things;
- every biomolecule must be degraded to release energy or produce another biomolecule,
- enzymes are used to reduce the amount of time taken to do this.
The third thing we need to know is that there must be a mechanism to release the six elements from the environment and create the initial biomolecules, otherwise eventually all life on this planet will cease to exist! The problem is I am only small (approx. 1 to 2 µm in diameter), I can’t possibly manufacture and store every enzyme for every biomolecule and every metabolic pathway; I must specialise, find my niche and perfect it.
In conclusion I am a microscopic living organism, I consume material to release energy so that I can sustain myself and build biomolecules to replicate myself. To achieve this in a timely manner I build enzymes which catalyse specific reactions in my metabolic pathways, but because of my small size I can only accommodate a few, essential enzymes, specific to my needs. Therefore in order to survive I specialise and attempt to find a niche food source that enables me to thrive. Our simple cell organisation and fast reproductive rate provides us with an evolutionary edge that enables us to diversify into every part of the biosphere; this diversification maintains and sustains every living organism, initially by releasing essential elements from the inert environment and making them available to other life forms, but also by metabolising complex biomolecules to release the energy to build other biomolecules required by myself and other cells.