The Cell Membrane
The cells of all organisms, prokaryotic and eukaryotic
alike, are surrounded by a thin sheet called the cell membrane.
This barrier keeps cellular materials in and foreign objects out. The
membrane is key to the life of the cell. By regulating what gets
into and out of the cell, the membrane maintains the proper chemical
composition, which is crucial to the life processes the cell carries
Structure of the Cell Membrane
The cell membrane is made up of two sheets of special
fat molecules called phospholipids, placed on top of
This arrangement is known as a phospholipid bilayer.
Phospholipid molecules naturally arrange in bilayers because they
have a unique structure. The long chains of carbon and hydrogen
that form the tail of this molecule do not dissolve in water; they
are said to be hydrophobic or “water fearing.” The hydrophilic phosphorous
heads are attracted to water. Forming a bilayer satisfies the water
preferences of both the “heads” and “tails” of phospholipids: the
hydrophilic heads face the watery regions inside and outside the
cell, and the hydrophobic tails face each other in a water-free
junction. The bilayer forms spontaneously because this situation
is so favorable.
The Fluid Mosaic Model
Phospholipids form the fundamental structure of the cell
membrane, but they are not the only substance found there. According
to the fluid-mosaic model of the cell membrane, special proteins
called membrane proteins float in the phospholipid bilayer like
icebergs in a sea.
The sea of phospholipid molecules and gatekeeper
membrane proteins is in constant motion. The membrane’s fluidity
keeps the cell from fracturing when placed under strain.
Transport Through the Cell Membrane
The most important property of the cell membrane is its
selective permeability: some substances can pass through it freely,
but others cannot. Small and nonpolar (hydrophobic) molecules can
freely pass through the membrane, but charged ions and large molecules such
as proteins and sugars are barred passage. The selective permeability
of the cell membrane allows a cell to maintain its internal composition
at necessary levels.
Molecules that can pass freely through the membrane follow
concentration gradients, moving from the higher concentration area
to the region of lower concentration. These processes take no energy
and are called passive transport. The molecules that
cannot pass freely across the phospholipid bilayer can be carried
across the membrane in various processes that require energy and
are therefore called active transport.
There are three main types of passive transport: diffusion,
facilitated diffusion, and osmosis. In fact, osmosis is simply the
term given to the diffusion of water.
In the absence of other forces, substances dissolved in
water move naturally from areas where they are abundant to areas
where they are scarce—a process known as diffusion. If there is
a higher concentration of carbon dioxide gas dissolved in the water
inside the cell than in the water outside the cell, carbon dioxide
will naturally flow out from the cell until its distribution is
balanced, without any energy required from the cell.
Nonpolar and small polar molecules can pass through the
cell membrane, so they diffuse across it in response to concentration
gradients. Carbon dioxide and oxygen are two molecules that undergo
this simple diffusion through the membrane.
The simple diffusion of water is known as osmosis.
Because water is a small polar molecule, it undergoes simple diffusion.
SAT II Biology problems on osmosis can be tricky: water moves from
areas where it is in high concentration to areas where it is in
low concentration. Remember, however, that water is found in low
concentrations in places where there are many dissolved substances,
called solutes. Therefore, water moves from places where there are
few dissolved substances (known as hypotonic solutions)
to places where there are many dissolved substances (hypertonic solutions).
An isotonic solution is one in which the concentration
is the same as that found inside a cell, meaning osmotic pressure
in both sides is equal.
Immersing cells in unusually hypotonic or hypertonic solutions
can be disastrous: water can rush into cells in hypotonic conditions,
causing them to fill up so fast that they burst. To combat this
possibility, many cells that need to survive in freshwater environments
possess contractile vacuoles to pump out excess water.
Certain compounds important to the functioning of the
cell, such as ions, cannot enter the cell through simple diffusion
because they cannot pass through the cell membrane. As with water,
these substances “want” to enter the cell if the concentration gradient
demands it. For that reason, cells have developed a way for such
compounds to bypass the cell membrane and flow into the cell on
the basis of concentration. The cell has protein channels through
the phospholipid membrane. The channels can open and close based
on protein membranes. When closed, nothing can get through. When
open, the protein channels allow compounds to pass through along
the concentration gradient, which is diffusion.
Quite often, cells have to transport a substance across
the cell membrane against the normal concentration
gradient. In these cases, cells use another class of membrane proteins. Instead
of relying on diffusion, these proteins actively pump compounds
in the direction the cell wants them to go, a process that requires
energy. Cells can turn active transport on and off as needed.
Endocytosis and Exocytosis
Cells use yet another type of transport to move large
particles through the cell membrane. In exocytosis, waste products
that need to be removed from the cell are placed in vesicles that
then fuse with the cell membrane, releasing their contents into
the space outside the cell. Endocytosis is the opposite of exocytosis:
the cell membrane engulfs a substance the cell needs to import and
then pinches off into a vesicle that is inside the cell.
There are two kinds of endocytosis: in phagocytosis the
cell takes in large solid food particles that it then digests. In pinocytosis,
the cell takes in drops of cellular fluid containing dissolved nutrients.