Membrane Lipids
The cell membrane is a dynamic structure composed of lipids, proteins, and
carbohydrates. It protects the cell by preventing materials from leaking out,
controls what can enter or leave through the membrane, provides a binding
site for hormones and other chemicals, and serves as an identification card
for the immune system to distinguish between “self” and “non-self” cells.
We will first investigate the anatomy of the cell membrane and then
continue on to study the physiology of membrane transport.
The phospholipid bilayer is the main fabric of the membrane. The bilayer's
structure causes the membrane to be semi-permeable. Remember that
phospholipid molecules are amphiphilic (amphipathic), which means that
they contain both a nonpolar and polar region. Phospholipids have a polar
head (it contains a charged phosphate group) with two nonpolar hydrophobic
fatty acid tails. The tails of different phospholipids face each other in the
core of the membrane while the polar heads face either the exterior of
interior of the cell. Having the polar heads oriented toward the external and
internal sides of the membrane allows them to interact with the polar water
molecules found on each side of the membrane. The hydrophobic core
blocks the diffusion of hydrophilic ions and polar molecules.
Small hydrophobic molecules and gases, which can dissolve in the
membrane's core, cross it with ease.
Other molecules require proteins to transport them across the
membrane. Proteins determine most of the membrane's specific functions.
The plasma membrane and the membranes of the various organelles each
have unique collections of proteins. For example, to date more than 50 kinds
of proteins have been found in the plasma membrane of red blood cells.
Importance of Phospholipid Membrane Structure
What is important about the structure of a phospholipid membrane?
First, it is fluid. This allows cells to change shape, permitting growth
and movement. The types of phospholipids and the presence of
cholesterol regulate the fluidity of the membrane. Second, the
phospholipid membrane is selectively permeable.
The ability of a molecule to pass through the membrane depends on
its polarity and to some extent its size. Many non-polar molecules
such as oxygen, carbon dioxide, and small hydrocarbons can flow
easily through cell membranes. This feature of membranes is very
important because hemoglobin, the protein that carries oxygen in our
blood, is contained within red blood cells. Oxygen must be able to
freely cross the membrane so that hemoglobin can get fully loaded
with oxygen in our lungs, and deliver it effectively to our tissues. Most
polar substances are stopped by a cell membrane, except perhaps for
small polar compounds like the one carbon alcohol, methanol.
Glucose is too large to pass through the membrane unassisted and a
special transporter protein ferries it across. One type of diabetes is
caused by misregulation of the glucose transporter. This decreases the
ability of glucose to enter the cell and results in high blood glucose
levels. Charged ions, such as sodium (Na+) or potassium (K+) ions
seldom go through a membrane, consequently they also need special
transporter molecules to pass through the membrane. The inability of
Na+ and K+ to pass through the membrane allows the cell to regulate the
concentrations of these ions on the inside or outside of the cell. The
conduction of electrical signals in your neurons is based on the ability
of cells to control Na+ and K+ levels.
Selectively permeable membranes allow cells to keep the chemistry of
the cytoplasm different from that of the external environment. It also
allows them to maintain chemically unique conditions inside their
organelles.