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:
- Cells are the fundamental units of life, because a cell is the
simplest unit capable of independent existence.
- 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.
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.
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).
hyperbio@mit.edu