Cellular Membranes 

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Reading: Campbell and Reece, 2002: Chapter 8 - Membrane Structure and Function

Student Objectives: As a result of this lecture and the assigned reading, you should understand the following:

1. Membranes confine the contents of cells.  




2. Membranes provide the structural basis for metabolic order in organisms.  They partition and compartmentalize functions, such as different enzyme systems.  Membranes also form the physical matrix on which enzymes may be positioned.
   
   


3. Membranes are selectively permeable (i.e., they allows certain things to pass through while excluding or blocking the passage of other substances). Control of exchanges necessary to: 1) protect the cell's integrity; 2) maintain the conditions at which optimal metabolic activities take place; and 3) coordinate the activities of different cells in a multicellular association.
   
   


4. The main structural components of membranes are lipids, mainly phospholipids containing a hydrophilic polar head region and a hydrophobic non-polar tail region, that spontaneously stable bilayers. The hydrophilic heads face toward the water interface, while the hydrophobic tails face each other to minimize exposure to water. This hydrophobic interior is one of the reasons that membranes are selectively permeable. Non-polar molecules, such as steroid hormones, are soluble in lipids and easily pass form one side of the membrane to the other. In contrast, polar molecules are not soluble in lipids, and the passage of these molecules across membranes depends upon the presence of proteins. Much of the selective permeability of membranes depends upon the proteins present.
   
   


5. Membrane proteins may be peripheral proteins or integral proteins depending upon the degree of interaction with the membrane. Integral membrane proteins often span the thickness of the bilayer.
   
   
   
6. The membrane is a fluid mosaic because at environmental temperatures the phospholipid molecules give the membrane a fluid nature and the components are distributed in an heterogeneous manner of a mosaic. The phospholipids move freely in the plane of the membrane (lateral diffusion), but they do not flip-flop readily between the membrane halves.



7. Mechanisms of crossing a cell membrane 
   
   
   a.    Passive mechanisms of transport: a) diffusion; b) diffusion of water (osmosis); c) facilitated diffusion
   
   b.    Active mechanisms of transport - all are characterized by the expenditure of energy and usually the movement of substances against their concentration gradient.