The plasma membrane surrounds the cell and functions as an interface between the living interior of the cell and the nonliving exterior.

All cells have one.

It regulates the movement of molecules into and out of the cell.

The membranes of a cell are phospholipid bilayers that contain numerous proteins embedded within them. Some of the proteins extend all the way through the membrane; others do not.

Most of the lipids in a membrane are phospholipids.

Phospholipids contain glycerol, two fatty acids, and a phosphate group. The phosphate group is polar (hydrophilic), enabling it to interact with water. The fatty acid tails are nonpolar (hydrophobic) and do not interact with water.

Phospholipid Bilayers

Phospholipids spontaneously form a bilayer in a watery environment. They arrange themselves so that the polar heads are oriented toward the water and the fatty acid tails are oriented toward the inside of the bilayer (see the diagram below).

In general, nonpolar molecules do not interact with polar molecules. This can be seen when oil (nonpolar) is mixed with water (polar). Polar molecules interact with other polar molecules and ions. For example table salt (ionic) dissolves in water (polar).

The bilayer arrangement shown below enables the nonpolar fatty acid tails to remain together, avoiding the water. The polar phosphate groups are oriented toward the water.

Flexibility

The fatty acid tails are flexible, causing the lipid bilayer to be fluid. This makes the cells flexible. At body temperature, membranes are a liquid similar to cooking oil in consistency.

In animals, cholesterol is a major membrane lipid. It may be equal in amount to phospholipids.

It is similar to phospholipids in that it one end is hydrophilic, the other end is hydrophobic.

Cholesterol makes the membrane less permeable to most biological molecules.

Proteins are scattered throughout the membrane.

They may be attached to inner surface, embedded in the bilayer, or attached to the outer surface.

Hydrophilic (polar) regions of the proteins project from the inner or outer surface.   Hydrophobic (nonpolar) regions of the proteins are embedded within the membrane.

Proteins are capable of moving around in the bilayer.

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. The movement is due to molecular collisions, which occur more frequently in areas of higher concentration.

The dots on the diagram above represent molecules or ions.   After a period of time, the molecules or ions become dispersed (below). Overall, the movement is from the area of initial high concentration to areas that have a lower concentration.

Temperature and the Rate of Diffusion

Molecules, atoms, and ions normally move about in an irregular fashion called Brownian motion. As the particles move about, they collide with one another producing a random zig-zag movement as illustrated by the applet below.

http://www.aip.org/history/einstein/brownian.htm

Larger particles move slower, due to their larger mass and may be influenced by numerous collisions with many nearby smaller particles. Smaller particles move faster. 

The overall energy of movement is proportional to the square root of the temperature. Hotter particles move faster because they have more energy.

The rate of diffusion increases as temperature increases because the particles move faster. As temperature increases, the collisions among particles become more energetic, causing particles to move from areas of higher concentration to lower concentration at a faster rate.

Membranes are Differentially Permeable

The plasma membrane is differentially permeable because some particles can pass through, others cannot. It can control the extent to which certain substances pass through.

Nonpolar molecules pass through cell membranes more readily than polar molecules because the center of the lipid bilayer (the fatty acid tails) is nonpolar and does not readily interact with polar molecules.

The following substances can pass through the cell membrane:

Nonpolar molecules (example: lipids)

Small polar molecules such as water

The following substances cannot pass through the cell membrane:

Ions and charged molecules (example: salts dissolved in water)

Large polar molecules (example: glucose)

Macromolecules

Osmosis

Osmosis is the diffusion of water across a semipermeable membrane (see "Diffusion" above).

It occurs when a solute (example: salt, sugar, protein, etc.) cannot pass through a membrane but the solvent (water) can pass through. In areas where the solute concentration is high, the concentration of water molecules is low. In areas where the solute concentration is low, the concentration of water molecules is high. If there is no solute, the water is 100% water (high water concentration). Water moves areas where the concentration of water molecules is high (low solute concentration) to areas where the concentration of water molecules is low (high solute concentration).

In general, water moves toward the area with a higher solute concentration because it has a lower water concentration.

 

In the container on the left side of the diagram, water will enter the cell because it is more concentrated on the outside.  In the center drawing, water is more concentrated inside the cell, so it will move out.  If the solute concentration is the same inside as it is out, the amount of water that moves out will be approximately to the amount that moves in.

Osmotic pressure is the force of osmosis.   In the diagram above, the cell on the left will swell. The pressure within the cell is osmotic pressure.

Tonicity refers to the relative concentration of solute on either side of a membrane.

Isotonic

In an isotonic solution, the concentration of solute is the same on both sides of the membrane (inside the cell and outside). A cell placed in an isotonic solution neither gains or loses water. Most cells in the body are in an isotonic solution.

Hypotonic

A hypotonic solution is one that has less solute (more water). Cells in hypotonic solution tend to gain water.

Animal cells can lyse (rupture) in a hypotonic solution due to the osmotic pressure.

The cell wall of plant cells prevents the cell from rupturing.

Hypertonic solution

A hypertonic solution is one that has a high solute concentration. Cells in a hypertonic solution will lose water.

Plant cells placed in a hypertonic solution will undergo plasmolysis, a condition where the plasma membrane pulls away from the cell wall as the cell shrinks. The cell wall is rigid and does not shrink.

Left: These Elodea cells were placed in a 10% NaCl solution. The contents of the cells was reduced but the cell walls remained intact. Compare these cells to normal cells in the photograph below.  Click on the image to view an enlargement.

 

Left: Normal Elodea cells X 400 Click on the image to view an enlargement.

 

Functions of Membrane Proteins

Enzymes

Cell Identification Markers (Glycoproteins)

Lipids and proteins within the membrane may have a carbohydrate chain attached. Glycoproteins and glycolipids often function as cell identification markers.  This allows cells to identify other cells.  This is particularly important in the immune system where cells patrolling the body's tissues identify and destroy foreign invaders such as bacteria or viruses.

Cell Adhesion - Junctions

Proteins associated with the cell membranes of animal cells may bind to proteins of adjacent cells. These connections, called junctions may serve to bind cells together, to prevent the movement of material between the cells, or to allow cells to communicate with each other.

Attachment of the Cytoskeleton

Receptors

Hormones are molecules that cells use to communicate with one another. Receptors enable cells to detect hormones and a variety of other chemicals in their environment. For example, cells in the pancreas produce the hormone insulin when glucose levels in the blood become elevated. It stimulates liver and muscle cells to begin removing the glucose and storing it as glycogen.

Transport of Materials Across Cell Membranes

Facilitated Diffusion

Facilitated diffusion involves the use of a protein to facilitate the movement of molecules across the membrane.  In some cases, molecules pass through channels within the protein.

In other cases, the protein changes shape, allowing molecules to pass through.

As can be seen below, the protein changes shape and releases the molecule to the side of the membrane that has the lower concentration.

Additional energy is not required because the molecule is traveling down a concentration gradient (high concentration to low concentration). The energy of movement comes from the concentration gradient.

Active Transport

During active transport, carrier proteins within the cell membrane move ions or molecules against a concentration gradient (low concentration to high concentration).

Movement against a concentration gradient requires energy. The energy is supplied by ATP which is released by breaking a phosphate bond to produce ADP: 

ATP ®  ADP + P_i + energy

Active transport is like a water pump; it uses energy to pump water uphill where a siphon cannot. Facilitated diffusion (see above) is like a siphon in that additional energy is not required but it can only allow movement downhill.

Cells that use a lot of active transport have many mitochondria to produce the ATP needed.

Examples of Active Transport

The thyroid gland cells bring in iodine for use in producing hormones.

Cells in the vertebrate kidney reabsorb sodium ions from urine.

Endocytosis and Exocytosis

The process by which a cell engulfs material to bring it into the cell is called endocytosis. A vacuole is formed that contains the material that has been engulfed.

Exocytosis moves material to the outside.  A vesicle fuses with the plasma membrane and discharges its contents outside.  This allows cells to secrete molecules.