The Cell Membrane

All living cells and many of the tiny organelles internal to cells are bounded by thin membranes. These membranes are composed primarily of phospholipids and proteins and are typically described as phospholipid bi-layers.

In this sketch, the spheres represent the phosphate end, which is polar and water soluble (hydrophilic). The twin extensions represent the fatty acid components which are not water soluble (hydrophobic).

Cell membranes also contain cholesterol in the phospholipid bilayer. In some membranes there are only a few cholesterol molecules, but in others there are as many cholesterols as phospholipids according to Audesirk & Audesirk. Cholesterol makes the bilayer stronger, more flexible but less fluid, and less permeable to water-soluble substances such as ions and monosaccharides.

The above illustration of a cell membrane was modified from Wikipedia. It shows that the cell membrane contains much more than just the phospholipid bi-layer.

Transport across cell membranes
 Index

Reference
Audesirk & Audesirk
Ch 5
Enger & Ross
Ch 4
 
HyperPhysics***** Biology R Nave
 Go Back





Transport Across Cell Membranes

Essential and continuous parts of the life of a cell are the taking in of nutrients and the expelling of wastes. All of these must pass through the cell membrane.

Transport may occur by diffusion and osmosis across the membrane. It can also occur when a vescicle attaches to the cell membrane from the inside and then opens to form a pocket, expelling its contents to the outside. This may be called exocytosis. The cell membrane may also envelope something on the outside and surround it, taking it into the cell. This may be called endocytosis or phagocytosis.

There are also examples where molecules move across a membrane from a region of low concentration to an region of high concentration, and this requires a source of energy to "pump" the molecules uphill in concentration. Such processes are called active transport.

 Index

Reference
Audesirk & Audesirk
Ch 5
Enger & Ross
Ch 4
Karp
Ch 4
 
HyperPhysics***** Biology R Nave
 Go Back