The cell membrane is a fluid mosaic of lipids, proteins, and carbohydrates. In this tutorial we will describe these three structures and how they function in the cell membrane. This topic provides another example of the relationship between structure and function.
The Structure of Lipids
Lipid molecules are sparingly to insoluble in water. Lipids
are hydrophobic because the molecules consist of long,
18-22 carbon, hydrocarbon backbones with only a small amount
of oxygen containing groups. Lipids serve many functions
in organisms. They are the major components of waxes, pigments,
steroid hormones, and cell membranes. Fats, steriods,
and phospholipids are very important to the functioning
of membranes in cells and will be the focus of this tutorial.
Fats (triacylglycerols)
Fats are synthesized from two different classes of molecules: fatty
acids attached to the alcohol glycerol. The
fatty acids are long, unbranched hydrocardons that terminate
with an acidic carboxyl group. The fatty acid can be of two
types: saturated and unsaturated. Saturated fatty
acids have no carbon-carbon double bonds (they are "saturated" with
hydrogen) while the unsaturated fatty acids have one
to four double bonds between adjacent carbon atoms. These
double bonds introduce "kinks" in
the carbon chain which has important consequences on the
fluid nature of lipid membranes.
To construct a fat, or triacylglycerol, three fatty acid molecules are attached to the glycerol through an ester bond between the carboxyl groupd of the fatty acid and the three alcohol groups of a glycerol molecule. This type of reaction is known as a condensation reaction because it releases a water molecule. A fat molecule can be composed of one, two, or three different types of fatty acids and can be saturated or unsaturated.
An unsaturated fat has at least one unsaturated fatty acid whereas a saturated fat has none. Becuase the double bond of the unsaturated fatty acid introduces kinks in the hydrocarbon backbone the fat will not pack into a regular structure and thus remain fluid at low temperatures. A saturated fat though will pack well and be a solid a low temperatures.
Fats are mainly energy storage and insulating molecules. Per gram, fats contain twice as much energy as carbohydrates. Layers of fat also surround the vital organs of animals to cushion them, and layers of fat under the skin of animals provide insulation
Phospholipids
Phospholipids contain only two fatty acid tails attached to a glycerol head. This occurs by a condensation reaction similar to the one discussed above. The third alcohol group of the glycerol is attached to a phoshpate molecule. The phoshate group is then attached to other small moluecules such as choline. The phosphate group along with the glycerol group make the head of the phospholipid hydrophillic, whereas the fatty acid tail is hydrophobic. Thus phospholipids are amphipatic: water loving and water hating. When phospholipids are in an aqueous solution they will self assemble into micelles or bilayers, structures that exclude water molecules from the hydrophobic tails while keeping the hydrophillic head in contact with the aqueous solution. View the animation that demonstrates the formation of micelles and bilayers.
Click on the lower right arrow to continue to the next scene in the animation.
Phospholipids serve a major function in the cells of all organisms: they form the phospholipid membranes that surrounds the cell and intracellular structures such as the mitochondria. The cell membrane is a fluid, semi-permeable bilayer that seperates the cell's contents of from the environment, see animation below. The membrane is fluid at physiological temperatures and allows cells to change shape due to physical constraints or changing cellular volumes. The phospholipid membrane allows free difusion of some small molecules such as oxygen, carbon dioxide, and small hydrocarbons, but not water, charged ions, or other larger molecules such as glucose. This semi-permeable nature of the membrane allows the cell to maintain the composition of the cytosol independent of the external environment.
Click on the upper right arrow to begin the animation.
Steroids
The steroids are a family of lipids based on a molecule with four fused carbon rings. This family includes many hormones of animals and cholesterol. Cholesterol is a component of the cell membrane in animals and functions to maintain membrane fluidity becuase it prevents packing of the fatty acid tails.
The Structures of the Cell membrane
The Fluid Quality of Membranes
The cell membrane must be a dynamic structure if the cell is to grow and respond to environmental changes. To keep the membrane fluid at physiological temperatures the cell alters the composition of the phospholipids. The right ratio of saturated to unsaturated fatty acids keeps the membrane fluid at any temperature conducive to life. For example winter wheat responds to decreasing temperatures by increasing the amount of unsaturated fatty acids in cell membranes. In animal cells cholesterol helps to prevent the packing of fatty acid tails and thus lowers the requiremnt of unsaturated fatty acids. This helps maintain the fluid nature of the cell membrane without it becomming too liquid at body temperature. The fluidity of the membrane is demonstrated in the following animation. The lipids in the membrane are in random bulk flow moving about 22 µm (micrometers) per second. Phospholipids freely move in the same layer of the membrane and rarely flip to the other layer. Flipping rarely occurs because flipping requires the hyrophillic head to enter the hydrophobic region of the bilayer.
Click on the upper right arrow to begin the animation. Click on the lower right arrow to continue to the next scene.
The Mosaic Quality of Membranes
Proteins
Because the cell membrane is only semi-permable the cell needs a way to communicate with other cells and exchange nutrients with the extracellular space. These roles are primarily filled by proteins. Proteins are a seperate class of molecules unrelated to the lipids and are composed of amino acids. Membrane proteins are classified into two major categories, integral proteins and peripheral proteins. Integral proteins are transmembrane proteins, with hydrophobic regions that completely span the hydrophobic interior of the membrane. The parts of the protein exposed to the interior and exterior of the cell are hydrophillic. Integral proteins can serve as pores that selectively allow ions or nutrients into the cell. They also transmit signals into and out of the cell. Unlike intergral proteins that span the membrane, peripheral proteins reside on only one side of the membrane and are often attached to integral proteins.Some peripheral proteins serve as anchor points for the cytoskeleton or extracellular fibers. Proteins are much larger than lipids and move more slowly, but some do move in seemingly directed manner while others drift.
Carbohydrates
The extracellular surface of the cell membrane is decorated with carbohydrate groups attached to lipids, glycolipids, or proteins, glycoproteins. These short carbohydrates, or oligosaccharides, are usually chains of 15 or fewer sugar molecules. Oligosaccharides give a cell identity (i.e., distinguishing self from non-self) and are the distinguishing factor in human blood types and transplant rejection.
Membranes are asymmetric
As discussed above and can be seen in the picture, the cell membrane is assymetric. The extracellular face of the membrane is in contact with the extracellular matrix. The extracellular side of the membrane contains oligosaccharides that distinguish the cell as self. It also contains the end of integral proteins that interact with signals from other cells and sense the extracellular environment. The inner membrane is in contact the contents of the cell. This side of the membrane anchors to the cytoskeleton and contains the end of integral proteins that relay signals received on the external side.
Summary: Membranes as Mosaics of Structure and Function
The biological membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer. The lipid bilayer is the main fabric of the membrane, and its structure creates a semi-permeable membrane. The hydrophobic core impedes the difusion of hydrophilic structures, such as ions and polar molecules but allows hydrophobic molecules, which can dissolve in the membrane, cross it with ease. 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.
