3.1 Cell as a fundamental unit of life.
Did you know?
Up to 2% of an adult human cells
die every day. This amounts to
18 Million cells each second.
The cell is the fundamental unit of life. All organisms, whatever their type or size, are composed of cells.
Five important characteristics of all cells are:
self-feeding or nutrition (the "machine" function),
self-replication or growth (the "coding" function),
differentiation (form new cell structures such as spores as part of their life style),
chemical signaling (communication with other cells) and
evolution (change to show new biological properties).
Superficially, cells seem to disobey a law of physics - they are highly ordered structures in a world that generally becomes less ordered with time. How do they maintain order? By continuously generating energy, some of which is used to maintain cell structure. Energy generation is one important component of metabolism; other aspects of metabolism include the chemical reactions that synthesize the compounds and assembly reactions that make up the cell structure. Protein molecules called enzymes catalyze chemical reactions in cells. Enzymes must have a specific structure to function; therefore, there must be a set of information (a gene) that encodes the structure of each protein in the cell. The instruction set is encoded in DNA, the genetic material of all cells. There is also a translation system RNA, to convert the information coded in the DNA to proteins. Several types of RNA molecules (messenger RNA, ribosomal RNA, and transfer RNA) are important in this process.
The result of biosynthesis is cell growth. For a cell to replicate itself, it must synthesize more than 1000 different protein molecules. The cell has the genetic information to produce about 3000 distinct proteins; the genes that are expressed are those, which encode proteins that are most useful for growth or survival under the existing environmental conditions. The cell must also faithfully copy its genetic information, to pass onto the new cell. Mistakes in copying are made occasionally; these mutations are usually harmful and kill the cell. However, they do provide a mechanism for cells to acquire new properties.
3.2. The structure of prokaryotic cells
P
Bacterial cells share certain common structural features, but also show group-specific specializations.
E. coli is a usually harmless inhabitant of the intestinal tract of human beings and many other mammals. The E. coli cell is about 2 μm long and a little less than 1 μm in diameter. It has a protective outer membrane and an inner plasma membrane that encloses the cytoplasm and the nucleoid. Between the inner and outer membranes is a thin but strong layer of peptidoglycans (sugar polymers cross-linked by amino acids), which gives the cell its shape and rigidity.
The plasma membrane and the layers outside it constitute the cell envelope.
The plasma membrane contains proteins capable of transporting certain ions and compounds into the cell and carrying products and waste out.