Cell Membrane Reference Sheet
There may be a dozen different types of materials passing through the
membrane of a cell at any one time. The job of the cell membrane is to
regulate this movement in order to maintain the proper balance of ions,
water, oxygen, carbon dioxide, nutrients, and other molecules. This interactive
feature illustrates the movement of some of these materials and describes the
structures that make it possible.
The cell membrane is critical to the survival of a cell. It acts as a boundary
between the cell and its environment, keeping the cytoplasm and organelles
inside and harmful organisms and particles out. It is not an impenetrable wall.
If it were, the cell would quickly use up any nutrients & oxygen it had to start
with and, having no way to acquire more, become depleted & die. It is semi-permeable or selectively
permeable.
The cell membrane, also called the plasma membrane, functions
more like a bag of tightly woven fabric than like a wall. The cell
membrane is called a phospholipid bilayer because it is mostly
made up of a double layer of phospholipids with some proteins mixed
in. The membrane allows some molecules, including gases like
oxygen and carbon dioxide, to pass readily through its surface. Water
and other small molecules also move into and out of the cell with
relative ease. Transport through the cell membrane that does not
require energy is called passive transport. Simple diffusion is a
form of passive transport where small molecules move across the
phospholipids in the membrane from areas of high concentration to
low concentration. Simple diffusion does not require energy.
The openings in the cell membrane allow the cell to take in and release
fairly large molecules like glucose, as well as ions (atoms with a
positive or negative electrical charge) that are unable to pass through
the main, lipid-bilayer portion of the membrane. This second form of
passive transport is called facilitated diffusion. Molecules are still
diffusing from areas of high concentration to low concentration without
energy. However, these molecules are too large to move through the
phospholipids. Large molecules like glucose need a protein “escort” to
help them through the cell membrane.
Depending on the type of molecule or particle involved and the environment inside and outside the
membrane, the cell may have to actively pump molecules or ions along the channel in a process called active
transport. Active transport moves molecules across the membrane from low concentration to high
concentration and requires ATP energy.