Membrane Transport Mechanisms

Hello All,

As I mentioned within lecture, I like to utilize this blog to clear up any confusions you may have regarding my lectures and to discuss any relevant or current topics surrounding this subject.  I received a few questions from the membrane transport mechanisms lecture and they are answered below.  If you wrote me an email with a question, it will be answered below.  Questions are in bold and answers given in blue.  If you have any follow-up questions please feel free to leave a 'comment' or write me an email and I will be happy to help you further!

1.  I was studying your lecture, and I am a little confused about the small red stars that you have below pores & channels as well as vesicular transport.  If I understood correctly, you mentioned that those stars mean that those mechanisms never reach maximal flux, and that they don't saturate.  Why doesn't simple diffusion also have a small red star?  If molecules are simply moving through the membrane, I do not understand why or how they would have a maximal flux. 

 I apologize, it may not have been clear what the small red 'stars' meant on that slide.  Now, the big red star indicates that I believe that figure to be a 'target figure', meaning that I believe that you can explain most all of what I was conveying in the lecture n that single figure.   

 The small red 'stars' are to notify to you that, although those mechanisms (channels/pores and vesicular transport respectively) are categorized how I documented them, they could be categorized differently under biochemical or cell biology definitions.  For example, the movement of molecules through pores and channels is classified as diffusion under the physiological definition because under the concentration differences that occur physiologically the movement of molecules through pores or channels never reaches a maximal flux (see slide 21 for a graphical representation of this).  As you increase the concentration difference, within physiological ranges, flux through pores or channels will increase continuously never reaching a maximal flux.

Vesicular transport, in contrast, has a small red 'star' because it falls into the 'active movement' category, but is not as simple as the hydrolysis of ATP, but is a more complete set of steps to cause either endocytosis or exocytosis.

2.  I was looking at the slides, and it seems that sodium is high outside the cell, but to move down it's concentration gradient into the cell where it's low it requires ATP. Is this because it has a charge ?

The movement of Na+ will always require some type of a protein as a charged molecule cannot simply move through the membrane without a pathway.  Now, there are a variety of pathways that the Na+ can take.  If Na+ moves through a channel (voltage-gated, ligand-gated, or mechanically-gated), it will move down its concentration gradient and not require ATP to do so (ie. the driving force will be passive diffusion).  If Na+ is moving through a secondary-active transporter as the molecule that is moving down its concentration gradient (remember, it is the most common + to utilize for secondary active transport), the movement of the Na+ is actually passive, but it is allowing for the movement of another molecule in an active-movement direction against its concentration gradient (example:  Na+/glucose co-transporter moves Na+ down its concentration gradient and the energy from that move allows glucose to move against its concentration gradient).  The gradient of Na+ that allows for it to move down its concentration gradient in each of the above examples, however, is maintained by a primary active transporter, typically the Na/K ATPase (pump) which directly utilizes ATP to move Na+ back against its concentration gradient to the extracellular side of the membrane.

3.  Regarding secondary active transport, are both steps uphill?  or Is primary creates the uphill energy difference, so that the secondary could use the energy to bring something downhill?  or does the secondary one use the energy to bring something UPhill, against its gradient?  The oval/star example illustrates both are uphill and against their gradient, but in one of your definitions for secondary, it says the secondary part is downhill.  So, just wanted some clarification on that. 

In secondary active transport, the primary creates the uphill energy (the movement of the ovals alone on slide 27) and the secondary active transport utilizes that energy to move something downhill (the ovals) and another molecule uphill (the stars).  Therefore, secondary active transport ALWAYS moves at least two molecules, one which moves downhill and one which moves uphill.  These can be in the same direction (co-transport) or in opposite directions (counter-transport) and the downhill gradient is ALWAYS maintained by a primary active transport of some type. 

4.  I am currently reviewing the lecture slides from "Membrane Transport Mechanisms" and i have a question regarding primary and secondary active transport. I just want to confirm that primary transport derives energy directly from ATP, while in secondary active transport, the energy is derived indirectly from ATP? Is this correct?

If asked whether secondary active transport requires ATP, would you say yes? I know it relies on ATP from primary active transport to establish a reversed gradient, ie not directly required, but if specifically asked, what would you say?

Yes, primary active transport directly derives energy from ATP while secondary active transport indirectly derives energy from ATP.  Now, what would I say about 'does secondary active transport require ATP?', I would say yes.  However, you need to remember any context that a question of that sort is given.  In my written exam questions it will be clear and I will indicate 'directly OR indirectly' if I mean both primary active and secondary active transport mechanisms.  However, in your future you may have this type of question not written by yours-truly...therefore, remember that you need to ALWAYS pick the BEST ANSWER CHOICE.  So, look at your contextual clues and if secondary active transport is truly the best answer choice, even though it is an indirect use of ATP, that may be the answer that should be best chosen.  

5.  For the Fick's equations, what is the formula for P? Is it P= DB/w? since in the 2nd equation we are getting rid of DB and w and using P and P takes thickness into account along w solubility into membrane and diffusion?

For our purposes now yes.  However, mathamatically speaking the 'permeability coefficient' actually takes into account additional information such as molecule size, fluid density, pathway for movement, etc.  However, remember that will be a constant given to us to utilize this equation.  You need to know that there IS a way to calculate the flux of a molecule and how it would change given the different mechanisms of movement across a plasma membrane.