Chapter 3.  How Cells are Put Together, part two

Cell Membranes

Membranes not only surround the cell (e.g. the plasma membrane), but they also extend throughout the cytoplasm and form the structure of many cellular organelles including the membranes of chloroplasts, mitochondria, central vacuoles, vesicles, golgi bodies, and endoplasmic reticulum.  For chemicals to get into or out of the various organelles within a cell they must pass across a membrane and membranes to a large extent control what chemicals go in and what chemicals go out of the organelles.

I.          Membrane Structure - Know the structure of a single phospholipid molecule and know its hydrophobic & hydrophilic parts; know that some fatty acid tails are unsaturated and thus kinked at the double bond and this helps impart the fluid nature to the membrane; know that membrane structure is composed of billions of phospholipid molecules arranged in two layers (a bilayer) and that a mosaic of embedded proteins completes the structure of the membrane (see figure 3.7).  Know that the name given to this knowledge or understanding of membrane structure is the Fluid Mosaic Model.  

Some Proteins Found in Cell Membranes: See Fig. 3.7, p. 42-43

 1.        Adhesion proteins - found on the membranes of cells forming intact tissues.  Adhesion proteins help adjacent cells within a tissue stick together.

2.         Transport proteins (passive transporters & active transporters) - transport some molecules and ions in/out of cell. Therefore, transport proteins control the flow of substances (molecules + ions) into and out of cell.  Some transport proteins are gated and require chemical signals to open.  We can think of the hormone insulin as a chemical signal that opens a transport protein for glucose such that plasma membranes in contact with insulin in body fluid will open glucose transporters for transporting glucose out of body fluid (blood) and into cytoplasm for eventual storage as glycogen.

  3.         Enzymes  - Membranes are sites of many chemical reactions and embedded enzymes (proteins) are chemical catalysts (speed up chemical reactions) that occur on the surface of membranes.  [Enzymes are not illustrated in Fig. 3.7]

  4.         Recognition proteins enable a cell identification system, i.e., immune system - recognize self from nonself.  Ex. ABO blood groups, Type "A" has “type A” self recognition protein.

  5.       Receptor proteins - Membranes, or more specifically the proteins protruding from the membranes, receive chemical messages.  For example, consider the hormone insulin (which incidentally is a protein itself that is release from the pancreas).   Insulin is the chemical signal that operates gated transport proteins found on the surface of muscle and liver cells.  These gated transport proteins allow for the inward passage of sugar from the blood and body fluids into a cell's cytoplasm.  The gated transport proteins are open for sugar transport only when signaled by the presence of insulin.  Many other types of receptor proteins exist for receiving other hormonal signals.