Why is Life Organized into Cells

What is Life Doing? (and why is it all so complicated?)

Living things must maintain a high level of organized activity. Let's work backwards from the cell's point of view:

The ultimate thing a cell must do is to produce copies of itself. All cells are mortal. Cell lines that do not reproduce will go extinct; therefore, logically, all existing cell lines must reproduce (simply because they are NOT extinct IF they are existing!).
To reproduce, a cell must be able to pass information to its daughter cells, so that "they know what it knew". This information is passed in the form of genes made of DNA.
To reproduce, a cell must also be able to grow in an orderly way by adding more to its own tissues. If cells could not grow, each generation of cells would be smaller than the last, until cells became too small to do all the necessary functions.
To grow, a cell must be able to synthesize the correct complex molecules, such as proteins, lipids, DNA, carbohydrates, etc. in the correct amounts, in the correct places, and at the correct times. Most of the synthesis in cells is directed by specialized proteins called enzymes, which in turn are synthesized under the direction of information in the genes. (To learn more about these complex molecules, click here.)
To synthesize new molecules, a cell must have the chemical raw materials in the correct forms that can be recognized and used by its enzymes. Sometimes, a cell must reprocess raw materials into the forms needed for some of its synthesis.
To get raw materials, a cell must have ways to acquire matter from its surroundings, i.e. a means of nutrition. It must selectively take the things it needs from a more or less chaotic and random assortment of substances in its environment.
Since the environment is never going to provide exactly what any cell needs in exactly the correct proportions, a cell must have ways to neutralize and/or get rid of waste generated during the processing of materials.
Since a cell must be so active in selecting, gathering, eliminating, rearranging, ordering and distributing matter, it must also harvest energy from its environment. The second law of thermodynamics (that's physics and we can't do anything about it!) tells us that energy will be degraded every time work is done, and cells are certainly doing a lot of work. Without an input of energy, the highly organized structure of a cell would rapidly disintegrate, and the cell would die, which brings us back to mortality, where this list started!

So Why is Life Organized into Cells?

In a living system, hundreds of specialized processes must be going on simultaneously, all at the right times and at the right rates. Dozens of different complex molecules must be present in the correct proportions and concentrations. There must be constant feedback within the system to keep everything in balance.

The environment tends to be chaotic. Order tends toward randomness. Destructive processes such as diffusion, oxidation, ionization, etc. will tend to rip apart the order of a living system.

The solution: Wrap all the important parts of the living system in a closed sac that isolates them from the environment and keeps them all close enough together to facilitate feedback. Voila! A cell. In fact the earliest thing on earth that we would have recognized as a living cell was probably nothing more than a membrane sac enclosing nucleic acids (genes), enzymes and their products. But what a potential that sac had, because once cells were isolated, they could evolve more and more efficient ways of doing the functions listed above. The modern cell is the result of over 3 billion years of improvements in design.

Send comments or suggestions to moffatt@hillstrath.on.ca