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This book attempts to provide an introduction to the massive diversity undertaken by cells in order to go about their business, and why any (cellular) shortcoming may result in disease.
Basic cell characteristics Everything that lives on the surface of the planet is cellular in nature. At this point we should exclude viruses, as they are unable to reproduce themselves without hi-jacking the synthetic processes of the cell they infect. Their non-vital nature is emphasized by the capacity to make crystals of purified viruses in solution. The cell is the basic unit of life, and as such must fulfil three requirements: (1) to be a separate entity, requiring a surface membrane; (2) to interact with the surrounding environment to extract energy in some way for maintenance and growth; and (3) to replicate itself. These parameters are the same for all living beings, from the smallest bacterium, to any one of the 200 different cell types that create a human being. Many organisms live as single cells, whereas a human has some 100 trillion cells in all. This number can be compared with the total number of people on Earth today (6–8 billion), or even the total number of people estimated to have ever been on the planet (106 billion). As an aid for the perception of these extremely large numbers, we can perhaps use an analogy based on time. One trillion seconds ago equates to approximately thirty thousand years, a time when the Neanderthals were roaming around Europe.