Theories about how they came into being, and became ever more complex, is a study in itself.
The earliest single-celled creatures were anaerobic. That is, they could break down complex organic chemicals to meet their needs, without the benefit of oxygen. They exist to this day in what seem the most inhospitable environments. They have been found in crude oil and in the boiling sulphurous thermal vents under the sea. However, they are important in the processes of decay, the recycling process of nature. Thus they appear in sewage and the black mess that build up in the bottom of your aquarium where the oxygen does not penetrate. In general, they do not survive in an oxygen-rich environment, in which the breakdown of waste material is performed by aerobic bacteria.
Some single celled creatures developed the ability to utilise the carbon dioxide-rich atmosphere of the time, producing oxygen which facilitated their internal biochemical processes, venting the surplus as a waste product. Some become able to utilise oxygen, and produced carbon dioxide as a waste product. They exist today, in their billions, the bacteria and yeasts.
For reasons that are still, to an extent, speculative, they began to form communal symbiotic colonies of cells. In time, each individual colony was far more complex as a whole than the sum of the cells that comprised it. The organisms that resulted, over the millions of years, became, from the first group, plants, from the second, animals.
Among such living organisms are viruses and the simpler single celled creatures known as prokaryotes. Our concern here are the cells which exist in humans, which have an internal structure, the nucleus, which encloses the DNA, called eukaryotes.
Such cells may have many other internal components, enclosed in a membrane with an internal supporting structure. This external membrane is itself complex. While protecting the cell against harmful materials, it has to allow nourishment to enter and waste products to exit. In addition it may transmit or receive molecules, such as hormones, in communication with other cells. Most important is the ionic concentration inside and outside the cell, notably sodium and potassium. Within the cell, complex chemical reactions occur, involving the exchange of energy.
Energy Input and Output.
The processes by which the cell processes energy is called metabolism. A range of nutrients, amino acids, fats and, especially, glucose enter the cell, along with oxygen from the bloodstream.
To obtain energy, the glucose must be oxidised in a precisely controlled manner. This involves four stages and some forty different biochemical reactions. Such processes which break down molecules to simpler ones are called catabolism, and, releasing heat, are referred to as exergonic.
This provides the energy for reactions which require heat, that is they are endogonic. In general, such processes are ones which synthesise more complex molecules, and are referred to as anabolic.
It follows then that the laws of thermodynamics are involved. For the moment, we will only consider the first, the law of conservation of energy. That is, in a given event, say a chemical reaction, the change in total energy is equal to the heat absorbed by the system from the surroundings, plus the work done on the system. In other words, energy cannot be created or destroyed, it flows from one system to another, or is converted between various forms. Within the cell, therefore, the energy required by endogonic reactions, such as building large molecules, must be balanced by exogonic reactions, by, for instance, the breakdown of glocose.
In order to use the first law, we have to consider only the system itself, and assume that there is nothing from outside the system influencing it, in other words a 'closed' system. One of the problems is that no system in 'real life' is entirely closed.
Amino Acids and Proteins.
The fundamental importance of the genes, that is, DNA, is that they are a template from which amino acids are assembled to form proteins, including enzymes.
There many thousand different proteins in the body, with a bewildering array of chemical and physical properties. Some are contractile, as in muscle fibres, other have specialised electrochemical properties as in the nerves, some are tough but elastic as in the skin and the many kinds of membrane in the body. They interact with each other in many diverse ways.
Most are made up from combinations of some twenty amino acids. All have a carboxyl group, -COOH, and an amino group, -H2N, joined to a carbon atom, with a side chain which determines its identity.
In a reaction where the carboxyl group is linked to the amino group, there is what is called a peptide bond. The resulting molecule is one of a group of dipeptides, and polypeptides. The difference between a protein and a polypeptide is somewhat vague. In general, proteins are molecules that have some biochemical function, while peptides are components of larger molecules.
Briefly, enzymes are proteins which affect reactions, though not necessarily being a part of the reaction. That is they catalyse the reaction.
The biochemical reactions that drive living processes take place in an aqueous environment, with a controlled temperature and acidity (pH). Nevertheless, most of the compounds involved are extremely stable, and require the input of a certain amount of energy to begin a reaction process. Enzymes decrease the energy required, either by binding the molecules of the reactants closer together, or by influencing their bonds.
In theory, all such reactions are reversible, and so reach a balance point, where there is a mixture of reactants, enzyme and product, thus it is necessary to remove the product to allow the reaction to proceed. Each enzyme is specific to a particular reaction, and its activity is sensitive to variations in temperature and pH.
Bland, J., (2003) The Cell http://www.gender.org.uk/about/03gene/31_cell.htm
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01.01.99 Last amended 24.11.03