What makes a cell?
A cell is the smallest unit of life. Everything living is formed of cells, from single-celled organisms, familiar to us as bacteria, to the most complex of creatures such as ourselves, formed of mind boggling numbers of cells, but trivial in comparison to cell numbers in two hundred tons of blue whale. In its role as the basic building block of life, a cell might be considered a relatively simple collection of components, gently ‘ticking over’ to maintain itself and occasionally dividing to create a new cell. Nothing could be further from the truth. Each and every cell, from the simplest to the most complicated, is a self-contained molecular factory working frantically throughout every minute of its lifespan, whether this is the half hour of unique existence of most bacteria before they divide, or the self maintenance and day-to-day activity of our nerve cells, living for several decades. The analogy of a cell as a factory falls somewhat short because, to match cellular activity, the factory itself and much of its machinery would have to be dismantled and rebuilt on a daily basis, without any slowing of production levels. Both animal and plant cells are around a thousand-fold larger than bacteria with a much more complicated and intricate internal organization.
Just what sort of chemistry can support the extreme levels of synthesis that allow the simpler cells to double themselves in minutes, and more complicated cells within a day? At the fundamental level, life is based on the atoms of only six of the 117 known elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur. Hydrogen and oxygen, combined as molecules of water, make up 99 out of every 100 molecules in the cell. This might appear to make life a rather dilute affair, but some of this water is tightly bound into the structure of larger molecules, and does not occur as actual liquid. Life at the molecular level is based on a restricted set of small carbon-based molecules common for all cells, which include sugars (providing chemical energy), fatty acids (forming cell membranes), amino acids (the units of all proteins), and nucleotides (the subunits of informational molecules such as RNA and DNA). All proteins are formed from just 20 different amino acids, which are common to every living thing. This ‘alphabet’ of amino acids is combined in a variety of different ways similar to the use of letters to make words, forming a massive ‘vocabulary’ of proteins. Proteins exist in a remarkably diverse variety of forms, providing the structural materials, chemical catalysts, and molecular motors that support and drive the processes of life. The code for each unique protein is stored in another code, this time of four letters, which makes up the genes in our DNA and which is passed from mother cell to daughter cell at each division. Each of the 24,000 or so individual genes in our DNA is specific for a single protein, but our bodies may have many times this number of proteins, produced by modifying the original genetic message. Proteins are combined to form multi-protein complexes, the cogwheels and bearings that drive the motors of production and maintenance within the cell. This level of complexity works perfectly for the simpler cells such as bacteria, but in larger and more complex cells such as our own, specific tasks are undertaken in separate sites in the cell termed organelles, which are separated from other components within the cell by their own membranes. Adding yet a further layer of complexity, our own bodies contain 200 or so different cell types.