Science of Water : Concepts and Applications

126 The Science of Water: Concepts and Applications
Cells are the fundamental units of life. The cell retains a dual existence as distinct entity and a
building block in the construction of organisms. These conclusions about cells were observed and published
by Schleiden (1838). Later, Rudolph Virchow added the powerful dictum, “Omnis cellula e cellula”
… “All cells only arise from preexisting cells.” This important tenet, along with others, formed
the basis of what we call cell theory. The modern tenets of the cell theory include the following:
•
•
•
•
•
•
All known living things are made up of cells.
The cell is structural and functional unit of all living things.
All cells come from preexisting cells by division.
Cells contain hereditary information, which is passed from cell to cell during cell division.
All cells are basically the same in chemical composition.
All energy fl ow of life occurs within cells.
The modern tenets, of course, postdated Robert Hooke’s 1663 discovery of cells in a piece of
cork, which he examined under his primitive microscope. Hooke drew the cell (actually it was
the cell wall he observed) and coined the word cell. The word cell is derived from the Latin word
cellula, which means small compartment.
Thus, since the 19 th century, we have known that all living things, whether animal or plant, are
made up of cells. Again, the fundamental unit of all living things, no matter how complex, is the
cell. A typical cell is an entity isolated from other cells by a membrane or cell wall. The cell membrane
contains protoplasm (the living material found within them) and the nucleus.
In a typical mature plant cell (see Figure 5.1), the cell wall is rigid and is composed of nonliving
material, while in the typical animal cell (see Figure 5.2) the wall is an elastic living membrane.
Cell wall
Vacuole
FIGURE 5.1 Plant cell.
Cytoplasm
Chloroplast
Cell membrane
Nucleus