As you learned in our last unit, your cells are surrounded by a a flexible boundary called the cell membrane (plasma membrane).  This boundary is designed to (1) separate the cell's internal and external environments, (2) permit specific molecules to enter the cell, and (3) allow other specific molecule to exit the cell.   Molecular travel through the cell membrane is critically important to not only the cell's survival but to the overall well being of the entire body.  The following information details the types of "travel" that molecules can use in order to cross into or out of the cell's boundary.
 
 

Simple, Passive Diffusion -

This transportation package does not require any energy expenditure at all!  How can we offer such an economical deal?!?  Through the magic of group travel!  Here's how it works:  Simply gather alot of your closest friends (make sure they are all the same kind of molecule as you) and slide through the cell membrane's protein channels.  Some restrictions do apply however.  You must be a small, nonpolar and lipid soluble molecule to travel this way and you must travel from regions containing higher concentrations of molecules like yourself to regions of lower concentrations (in travel agency language this is called "with or down your concentration gradient").  We have had many satisfied customers choose this method of travel.  Some of our most notable clients include oxygen, carbon dioxide, alcohol and fat-soluble vitamins.  They travel across cells membranes using our simple diffusion package all of the time!

Facilitated, Passive Diffusion -

Are you a molecule that is too polar or too large to use the simple diffusion travel plan?  If so, don't dispair--we can get you to the cell's interior without any expenditure of energy!  How do we do it?!?  By using a unique set of cell membrane protein carrier molecules!  You will easily combine with these carriers who well, just like their name implies, slide you gently across the membranes width and into the cell's interior.  Our most popular protein ferryboat continually delivers big polar molecules into the cell's interior.  However, some restirctions do apply:  space on our carrier ferryboats is limited.  If the boats are full, you must wait until they have room for you or find an alternative mechanism.  In addition, this is a one-way travel opportunity.  You must travel with/down the concentration gradient (from an area of higher concentration of molecules to an area of lower concentration of molecules).  Book your reservation early!

Osmosis -

Our osmosis package is perfect for water molecules.  If you are one of these small, very polar molecules, this may be the transport method of choice for you because it is extremely efficient and, like the plans discussed above, requires not expenditure of cellular energy!  Water molecules have two options for their travel. Some molecules may choose to travel through special cell membrane proteins called aquaporins.  Others can sneak across the membrane border by actually passingbetween adjacent phospholipid molecules.  How is this possble?  As the membrane around the cell moves, small gaps open up between the phosphlipid molecules.  If you are at the right place at the right time, then you can sneak right on in, virtually unnoticed!  What a deal--two ways to enter, if your a water molecule!  Just be sure to follow your concentration gradient for a smooth trip.
 

Filtration -

This trip is offered only into a limited number of structures in the body--like the tubes that fill the kidneys.  Because of the shear fource of fluids trapped inside these (and other similar) tubes, some molecules can leak out of the tubes into the surrounding environment.  It works just like a leaky garden hose--the pressure of the water inside the hose forces a litttle bit of fluid out.  We call this force hydrostatic pressure and it forces fluids containing dissolved molecules out with/down their concentration gradients.
 

Active Processes - If you have lots of cash (cellular energy) to spare, try one of our active transport processes.  They are excellent travel services when a molecule needs to travel from a region of lower concentration to a region of higher concentration (called against the concentration gradient).  We offer two types of active transport to our clients.

Active Transport -

This method is similar to facilitated diffusion in that cell membrane carrier proteins are used to shuttle molecules across the membrane.  However, by spending energy (in the form of ATP or stored energy forms) the cell can send molecules from lower concentration to higher concentrations.  This is a great opportunity for the cell to acquire rare or limited molecules that might otherwise be missed if it had to wait until a normal concentration gradient was established.  The sodium-potassium pump system represents one of our biggest clients using this method.  As a result of our action, sodium and potassium ions are shuttled across the membrane so that your nervous system can function properly.  You'll learn more about this very important, award winning transport system in A & P II.

Vesicular Transport -

Are you a huge molecule--way too big to fit on any dinky protein ferryboat?  Do you still long for travel opportunities?  If you answered, yes, then keep on reading.  We have a plan for you!  In vesicular transport, a small piece of the membrane will actually surround you and pull you into or out of the cell.  If you enter is cell, we call this trip endocytosis.  If you exit the cell, it's called exocytosis.  Either way, it costs alot of energy but it can be done!

Regardless of the travel adventure you choose, we promise you'll have the time of your life.  So come down to our office and book your trip today.  So pack your bags.  Don't forget your ATP credit card, if you choose the active transport offerings!  And have a great trip!