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Cell Basics

Cell Cell Basics


Cell hierarchy
Relative Sizes of Biological Objects
Cell Elemental Composition

How can one define life? The simplest definition is that any living thing must have three general properties:

For example, a rock isn't alive because it does not reproduce or metabolize, though it can grow by the addition of deposits to its surface. The physical structure of MIT exhibits both growth and metabolism (taking in baccaleaureates and money and spitting out PhD's and theses), but it cannot reproduce. For a more extensive definition read chapter 1 of Purves or discuss these properties with your tutor.

The cell is the fundamental unit of life. The cell theory, put forth in the middle of the 19th century, states that:

  1. Cells are the fundamental units of life, because a cell is the simplest unit capable of independent existence.
  2. All living things are made of cells.
This theory still holds true, with the minor caveat that viruses are only alive while infecting a cell.

Life's Hierarchy

Life on Earth is incredibly extensive and, to make it easier to study, biologists have broken living systems up into generalized hierarchical levels:

The focus of this course is on the fundamentals of life; that is, the properties that are held in common among all living things. We will concentrate almost exclusively on the molecular through the cellular level.

Relative Sizes of Biological Objects

It is important to have some grasp of the relative sizes of the things we will be talking about through this course. Look at Box 4.A in Purves (page 62) showing the differences in scale of various biological objects.

Cell Elemental Composition

Cells are 90% water. Of the remaining molecules present, the dry weight is approximately:

Total approximate composition by element: Note that these four elements make up almost the entire composition of all living organisms. The only other notable elements that are significant constituents of biological molecules are P, phosphorus, and S, sulphur. In addition, living things use traces of sodium, magnesium, chlorine, potassium, calcium, and iron, and even less of certain other metals (see Purves page 20).


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