There is, however, at least one other feature of cells that is universal: each one
is bounded by a membrane the plasma membrane. This container acts as a
selective barrier that enables the cell to concentrate nutrients gathered from its
environment and retain the products it synthesizes for its own use, while
excreting its waste products. Without a plasma membrane, the cell could not
maintain its integrity as a coordinated chemical system.
This membrane is formed of a set of molecules that have the simple physicochemical
property of being
hydrophobic (water-insoluble) and another part that is hydrophilic (watersoluble).
When such molecules are placed in water, they aggregate
spontaneously, arranging their hydrophobic portions to be as much in contact
with one another as possible to hide them from the water, while keeping their
hydrophilic portions exposed. Amphipathic molecules of appropriate shape,
such as the phospholipid molecules that comprise most of the plasma
membrane, spontaneously aggregate in water to form a
small closed vesicles (Figure 1-12). The phenomenon can be demonstrated in atest tube by simply mixing phospholipids and water together; under
appropriate conditions, small vesicles form whose aqueous contents are
isolated from the external medium.
Although the chemical details vary, the hydrophobic tails of the predominant
membrane molecules in all cells are hydrocarbon polymers (-CH
and their spontaneous assembly into a bilayered vesicle is but one of many
examples of an important general principle: cells produce molecules whose
chemical properties cause them to
needs.
The boundary of the cell cannot be totally impermeable. If a cell is to grow and
reproduce, it must be able to import raw materials and export waste across its
plasma membrane. All cells therefore have specialized proteins embedded in
their membrane that serve to transport specific molecules from one side to the
other (Figure 1-13). Some of these
the proteins that catalyze the fundamental small-molecule reactions inside the
cell, have been so well preserved over the course of evolution that one can
recognize the family resemblances between them in comparisons of even the
most distantly related groups of living organisms.
The transport proteins in the membrane largely determine which molecules
enter the cell, and the catalytic proteins inside the cell determine the reactions
that those molecules undergo. Thus, by specifying the set of proteins that the
cell is to manufacture, the genetic information recorded in the DNA sequence
dictates the entire chemistry of the cell; and not only its chemistry, but also its
form and its behavior, for these too are chiefly constructed and controlled by
the cell's proteins.2-CH2-CH2-),self-assemble into the structures that a cellmembrane transport proteins, like some ofamphipathic that is, consisting of one part that isbilayer that creates
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