Download Microorganisms as Cells

Microorganisms as Cells
The cell is the fundamental unit of life. A single cell is an entity, isolated from other
cells by a cell membrane (and perhaps a cell wall) and containing within it a variety of
chemicals and subcellular structures. The cell membrane is the barrier that separates
the inside of the cell from the outside. Inside the cell membrane are the various
structures and chemicals that make it possible for the cell to function. Key structures
are the nucleus or nucleoid, where the genetic information, deoxyribonucleic acid
(DNA), needed to make more cells is stored, and the cytoplasm, where the machinery
for cell growth and function is present.
All cells are made up of four chemical components: proteins, nucleic acids,
lipids, and polysaccharides. Collectively, these are called macromolecules. It is the
chemical nature and arrangement of macromolecules in a cell of one organism that
makes it distinct from those of another. Although each kind of cell has a definite
structure and size, a cell is a dynamic unit, constantly undergoing change and
replacing its parts. Even when it is not growing, a cell may be acquiring materials
from its environment and incorporating them into its own fabric. At the same time, it
discards waste products into its environment. A cell is thus an open system, forever
changing yet generally remaining the same.
Where did the first cells come from? In some way the first cell must have
come from a noncell, something before the cell, a procellular structure. Although
evolution of the first cell over 3.8 billion years ago was an improbable event that may
have taken several hundred million years to occur, once the first cell arose, a series of
highly probable events followed, including growth and division to form populations
of cells from which evolution could select for improvements and diversification.
Then, through billions of years of evolutionary change, the tremendous diversity of
extant cell types that exist today arose. And, because all cells are constructed from the
four basic classes of macromolecules mentioned earlier and share many other traits in
common, it is hypothesized that all cells have descended from a common ancestor, the
universal ancestor of all life.
(Source: ‘Brock Biology of Microorganisms, Michael T. Madigan, John M. Martinko
and Jack Parker, Prentice Hall, Pearson Education International, 10nth Edition)