Re: active and passive transport of molecules

Hi Kerry, Great questions! I'm going to tell you some basic information on passive and active transport and then provide a good website that will allow you to research the rest of your questions (I know you didn't want me to do your assignment for you). First things first. Almost any molecule, lipid, protein, carbohydrate will be able to diffuse across a protein-free lipid bilayer given enough time. This diffusion depends on the size of the molecule, how well it mixes with lipids (cell membrane) and its charge (polar/nonpolar). Example, small nonpolar molecules diffuse well in a lipid bilayer and cross the membrane quickly. Polar (charged) molecules are highly impermeable and require some form of help (in the form of energy)to cross the lipid membrane. As you already know, passive transport is the diffusion of water and small nonpolar molecules across the membrane. Water for example can readily diffuse into and out of a cell. However, polar molecules, such as, amino acids , sugars, ions, and nucleotides have trouble crossing the membrane and require a process called Facilitated diffusion. In Facilitated diffusion, membrane-bound transport proteins are utilized to transport these charged molecules across the membrane. These transport proteins belong to two general classes; Carrier proteins, that transport bound molecules across the membrane by changing their conformation; and Channel proteins that form a charged pore and allow nonbound molecules to pass. The transport proteins are imbedded within the lipid bilayer and pass completely from the outside of the cell to the inside. The composition is such that their nonpolar amino acids face toward the lipids in the membrane and the polar amino acids face into the pore created by the transport protein. The diffusion of these polar molecules requires an electrochemical gradient. Think of it this way, if there are more negative charges on the outside of the cell then inside, then the flow of negatively charged molecules would follow the electrochemical gradient and enter the cell (providing the proper transport molecule was present). Active transport is the transfer of molecules into or out of a cell against the electrochemical gradient and requires carrier proteins and, most importantly, energy. Active transport is very costly to a cell when it comes to using energy. In fact, it uses more energy overall than any other cellular process. The energy to transport a molecule can take the form of ATP, or the use of an ion gradient. If an ion gradient is used, the energy released by movement of a charged molecule flowing with the electrochemical gradient (ex: high negative charge toward low negative charge)is used to transport a different molecule against the electrochemical gradient. Now, I think I have provided you enough information to get you started in answering the remaining parts of your question. The following virtual Cell Biology website should provide all the help you need: www.bwh.harvard.edu Once at the site, click on the cytoskeleton link and explore! Good luck. Jeffrey Stiefel, Ph.D.