This is a useful note I took in class:
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 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...