The Cell's Membranes

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Fluid Mosaic Model

The plasma membrane is essential for maintaining the cell's integrity and plays a critical role in homeostasis. The plasma membrane functions both as gateway and barrier -- regulating the exchange of material between biotic and abiotic worlds.

The plasma membrane is so thin that its existence and structure was originally credited to circumstantial evidence until the transmission electron microscope revealed the filmy nature of this lipid bilayer. One form of evidence for the cell membrane involved the observation that oils and hydrophobic solvents passed easily into cells. Knowing this researchers postulated that the membrane must have a lipid core.

Our understanding of the plasma membrane is based on the Fluid Mosaic Model which refers to the fluidlike qualities of the phospholipid sheets and the dynamic behavior of proteins that seem to float in or on a "sea" of phospholipids.

View a Chime rendition of a section of the plasma membrane

This is very large so be patient

The phospholipids have a polar head group and a nonpolar hydrocarbon chains that resembles a forked tail.

View a Chime 3D model of a phospholipid

The polar region is hydrophilic because of the positively charged variable group attached to the phosphate, whereas the nonpolar fatty acid tails are hydrophobic. The variable group is quite often choline illustrated below.


Cholesterol, illustrated below, is frequently found in the oily central core of the phospholipid bilayer.

Click here to view a full page interactive 3D model of cholesterol. The Chime plug-in must be installed in your browser.

Learn more about the role of lipids in biological membranes from the University of Kansas Medical Center.

The protein compliment of the plasma membrane includes three distinct varieties. Transmembranal (or integral) proteins with hydrophilic ends and hydrophobic midsection, peripheral proteins which are half hydrophilic and half hydrophobic, and surface proteins which are hydrophilic.

  For a more detailed illustration of the plasma membrane click on this link

The landscape of the external plasma membrane in animals is coated with an array of carbohydrates called the glycocalyx. The carbohydrates are part of the glycoproteins and glycolipids which stud the outer surface enabling cells to recognize each other and receive messages via hormones.

Glycocalyx: Extracellular polymeric material produced by some bacteria. Term initially applied to the polysaccharide matrix excreted by eukaryotic epithelial cells forming a coating on the surface of epithelial tissue. General term for polysaccharide compounds outside the bacterial cell wall. Also called slime layer, EPS, matrix polymer.



Water Potential

Water potential is directly proportional to the concentration of water molecules. Water molecules will move spontaneously from high to low water potential. For plants water potential is highest in the soil were water is relatively pure. It is lowest in the air surrounding a leaf because water is rare compared to other atoms of gas. A large amount of water concentrated in one place represents lots of free energy (delta G is very negative).

Hydrostatic potential

Hydrostatic potential is the potential needed to stop water from diffusing.


Passive Transport and Simple Diffusion


Diffusion is the movement of a substance (atoms, molecules, or ions) down its own concentration gradient.

The essential characteristics of diffusion are:

  1. that each molecule or ion move independently of the others and
  2. that these movements are random.

Diffusion is possible at any temperature above absolute zero (-273°C). The higher the temperature the more rapid the diffusion. The net result of diffusion is that substances become evenly distributed (dynamic equilibrium) and potential energy reaches zero. (There is still plenty of kinetic energy).

Efficient diffusion requires small distances and a high potential gradient.

For more information about cell membranes and diffusion click here.


Dialysis is a therapy which eliminates the toxic wastes from the body when the kidney fails

Dialysis is the diffusion of molecules of solute through a membrane.

Molecules that diffuse freely across membranes are small and nonpolar. Examples include CO2, O2, and N2 gases. Small, polar and uncharged molecules can move across the plasma membrane through hydrophilic apertures (openings). Examples include water, glycerol, ammonia, and urea. Larger molecules needed by the cell will diffuse so slowly that special processes are needed to allow them to pass through the membrane

(see active transport section).


The permeability of the plasma membrane varies inversely with the size of the molecule -- indicating that the pores are small and the membrane acts like a sieve. Polarity and electrical charge also determine the degree of permeability. Most ions cannot traverse the membrane without help.

Cell Size


A major factor limiting cell size is its dependence on diffusion for raw materials and waste removal. Diffusion is fastest when the concentration gradient is largest, but diffusion slows rapidly as the distance from the membrane barrier increases since concentration gradients decrease sharply.

"A typical small animal, say a microscopic worm or rotifer, has a smooth skin through which all the oxygen it requires can soak in, a straight gut with sufficient surface to absorb its food, and a simple kidney. Increase its dimensions tenfold in every direction, and its weight is increased a thousand times, so that if it is to use its muscles as efficiently as its miniature counterpart, it will need a thousand times as much food and oxygen per day and will excrete a thousand times as much of waste products.

Now if its shape is unaltered its surface will be increased only a hundredfold, and ten times as much oxygen must enter per minute through each square millimeter of skin, ten times as much food through each square millimeter of intestine. When a limit is reached to their absorptive powers their surface has to be increased by some special device. For example, a part of the skin may be drawn out into tufts to make gills or pushed in to make lungs, thus increasing the oxygen-absorbing surface in proportion to the animal's bulk. A man for example, has a hundred square yards of lung. Similarly, the gut, instead of being smooth and straight, becomes coiled and develops a velvety surface (brush border of microvilli), and other organs increase in complication. The higher animals are not larger than the lower because they are more complicated, They are more complicated because they are larger."

On Being the Right Size, J.B.S. Haldane.

Read about Why most cells are small  


Counter Current Exchange - Diffusion and Life 

In many organs the anatomical arrangement is such that a steep concentration gradient can be maintained thus maximizing diffusion rates.

An example of counter current exchange may be found in the gills of fish. This counter current arrangement maintains a constant concentration gradient between the bloodstream and the water. Water leaving the gills with low oxygen content will contact blood with an even lower oxygen concentration.



Osmosis is the process by which a liquid substance passes spontaneously through a semipermeable (or selectively permeable) membrane. The net movement of water from a hypotonic solution into a hypertonic plant cell is an example of osmosis. Osmosis is a special case of diffusion.

How osmosis is used by plants to circulate sugar sap is explored here!

Cells in a marine environment are generally isotonic with its salty water. However all single-celled organisms (such as paramecium) living in fresh water are hypertonic compared to their environment (which is hypotonic compared to the cell) Fresh water organisms would swell and burst if not for special organelles called contractile vacuoles which collect water and pump it out with rhythmic contractions.

Plants because of their cell walls and large central vacuoles can take advantage of steep water gradients to maintain the shape and rigidity of leaves and stems. The central vacuole is hypertonic to ground water. The subsequent intake of water by a plant cell creates great hydrostatic pressure (turgor or water pressure) against their cell walls.

Water is able to "climb" up the stems of plants by capillary action because of its adhesion to the cellulose. This effectively removes some water reducing the water gradient allowing more water to take its place higher up the stem.

Links to sites dealing with osmosis and membranes

Facilitated Diffusion


Like normal diffusion, facilitated diffusion, which transports materials across a membrane, is driven by potential energy of a concentration gradient. Ions and hydrophilic molecules require special transport proteins (carrier molecules) called permeases embedded within the membrane to allow passage of this type of molecule. Examples of diffusion are illustrated below.

  1. simple diffusion through the fluid lipid bilayer
  2. passage through the hydrophilic pores of a permease
  3. facilitated diffusion. The so-called "ping-pong" model suggests that the entry of a molecule triggers a shape change allowing entry to the opposite side of the membrane. Once the molecule is released the protein's shape reverts to the original or unoccupied state.

Review the details about facilitated diffusion by reading the Mini Essay from Kimballs Biology Pages.

Cell Junctions

Cells of plants and animals are frequently connected in order to communicate and share resources. In animals these connections are called gap junctions.

Materials move between certain animal cells through pores called gap junctions. Each pore is formed by the arrangement of 6 transmembranal proteins lined up back to back welding the membranes of adjacent cells together.


For plants their cytoplasm interconnects through plasmodesmata.


For a thorough but concise discussion of cell junctions you can link to Kimball's Biology Pages here.

Try MIT's site for a summary of Membrane Transport Mechanisms


 Links to other on-line lectures about cell membranes


Modified July 10, 2005