Membrane Transport Mechanisms

Here are a number of questions from your classmates.  If you emailed me a question, it is answered here.  If this stirs up follow-up or other questions, please feel free to comment below and together we can answer your concerns and work towards a better understanding of Membrane Transport Mechanisms!

First a quick questions regarding logistics and studying:


1.  For some reason your study quiz on the G-drive is not opening up on my computer. I tried both files but I could not get it to work. Is there any way you can post the questions on a word document or PDF? Sorry for the trouble. Thank you

Please remember that these files were created in Articulate.  As I stated in the previous post:  Articulate is program that allows you to take quizzes (both the thought problems and study quizzes are presented in this format, see info below).  They are available on the ACTUAL G-drive (you MUST access it while on campus and through accessing the actual drive, not by 'online' means).  Regarding these quizzes the following are the 'directions':  

In the past, students have experienced problems when they try to run the Articulate quizzes from the “Students G-drive Online” connection, so please ONLY use the “Ross Net Drive [G drive]” available on campus.  In actuality, you do not need to run this quiz file from the G drive at all, and I recommend that you copy the entire folder onto your desktop or a USB drive and then run the quiz from any common browser (IE, Chrome, Firefox & Safari).  Note:  If you copy the folder off the G drive, be sure to copy the entire folder and not just the quiz – if you don’t have the contents of the entire folder (some application files are hidden) the quiz will not run.  Please email me if you experience any difficulties.  
In essence, go to the folder on the ACTUAL G-drive for the lecture (example: G:\Semester 1\Block 1. Fundamentals 1\Fundamentals.part 1\Membrane Transport Mechanisms Dr. Johannessen(PHYSIO)) and copy the folder within for either the thought questions or practice question onto your desktop or a USB drive (again, copy the ENTIRE folder as there are contents in the folder unseen that MUST BE present to operate the quiz), then open the quiz within a web browser by dragging the quiz file to an open web browser on your computer and the quiz itself should open automatically.  Either of the visible files within the folder should work within a web browser in this manner.  This needs to be taken from the G-drive ON CAMPUS (not the ‘online’ G-drive, but the actual drive on campus). You can do this on your personal computer while on campus or from a public campus computer.  

 

Alternatively, these are the SAME questions that are listed within the notes section of the last two slides of the powerpoint lecture.  The benefit of using the Articulate version is that each question provides you feedback after you answer it.  Therefore, it is designed to help your learning more than simply attempting the question and viewing the answers.

 

Now for some questions more pertaining to the lecture:

1.  As I was going through the slides I came across a question. Under physiological conditions, a maximal flux IS reached for carrier-mediated transporters, correct? 

Yes, for any type of carrier-mediated transport.  That includes facilitated transport (through transporters), primary active transport (through ATPases/pumps) and secondary active transport (through cotransporters and countertransporters).

2. In your lecture today you spoke of maximum flux. I googled flux and to my understanding it is referring to fluidity. When you stated that a maximum flux would not be reached physiologically did you mean that the movement of molecules in the body is constantly shifting so a maximum would not be reached? 

Yes, flux is defined as 'the action or process of flowing'.  Therefore, when defining it in respect to this lecture we are referring to the flow of molecules across the membrane through whatever pathway we are discussing (channels/pores, facilitated transport, primary active transport, secondary active transport).  Carrier-mediated transportation methods (defined above) all reach a maximal flux, meaning that the flow of molecules across the membrane will reach a maximal speed and value even while you increase the concentration gradient.  This is depicted in the graph on slides 21 and 25 of the lecture where as Extracellular Solute Concentration increases, the Flux into the cell levels off to a maximal level.  That can alternatively be representative of the molecules themselves (transporters and ATPase/pumps) becoming saturated and not being able to move the molecules across the membrane with increased speed, they have reached a maximal speed themselves and are all working to move molecules across the membrane.  It happens, though, that there are more molecules that need to also move across the membrane.

3. I understand pores are open at both sides of the membrane and transporters are only open to one side at a time, but seeing as channels are kinda of an intermediate between the two (passive but has a gate) are channels open to both sides or just one side at a time? 

Great question! Pores are indeed open to both sides of the membrane at all times.  Channels, when the gate is open are open to both sides of the membrane at once, but when the gate is closed it closes to both sides of the membrane as well.  Therefore, in contrast to transporters which open to one side at a time, channels are either opened (and pore-like) or closed.

4.  Pores and channels do not require ATP to move things across the membrane and do not exhibit saturation. Pumps, on the other hand, need ATP because they are moving things against their concentration gradient. Transporters may or may not use ATP depending on whether facilitated diffusion or secondary active transport occurs. Transporters can be saturated. Does saturation depend on whether ATP is used or not? Are transporters more likely to saturate when secondary active transport occur? Are transporters and pumps more susceptible to saturation because of their conformational changes when a solute bind?

Saturation is due to the fact that the flux (or movement) of the molecules across the membrane are dependent upon the kinetics of the proteins which move the molecules (both transporters and pumps).  Under physiological conditions (concentration gradients), there are not enough transporters and pumps to move molecules across the membrane as fast as the concentration gradient increases, therefore it reaches a maximal flux (as discussed in questions 1 and 2 above).

Sorry I just realized the answer to my question. Carrier mediated diffusion are likely to be saturated because there is a change in permeability whereas diffusion does not experience this. For general knowledge, is this the only factor that affects the saturation?  

Precisely.  Yes, therefore, the change in permeability (which can be affected by number of channels which is the case in carrier-mediated movement) causes saturation of the movement of molecules across the membrane.

5.  Can you please explain to me how vesicular transport is considered active movement? I know it isn't using ATP, so what am I missing here? Is it like secondary active transport where it uses other energy forms?

Good question.  I did not go into this in great detail, but as I mentioned in class this is a 'special' case.  The 'active' component of vesicular transport is not ATP precisely, but the fact that there are a number of proteins involved in the process (all of which will be covered as the semester continues, I believe endocytosis may have been covered today).  Therefore, as it is a complicated set of events, I include it in the 'active' category because it is moving molecules across the membrane both with and against their concentration gradient.

 

6.  You gave the example of voltage, ligand, and mechanical force-regulated ion CHANNELS, 

however there were no examples for pores-which I understand are always open.
So, I was wondering whether sodium or potassium leak CHANNELS were actually CHANNELS or PORES?

That's an interesting question.  Potassium leak channels do indeed seem to allow for potassium to 'leak' across the membrane and cause the movement of molecules that looks like a pore, but they are actually a channel.  They do have gates and are therefore qualified as a channel.  Aquaporin, however, is a great example of a pore that exists in just bout all cells.  It is a pore that allows for water (which will be discussed next week) to move easily across cell membranes thereby maintaining cell volumes.

As I mentioned above, these are just the questions that I have received so-far.  If there are further questions that you have, please feel free to leave them in the comments section below and/or email them to me and so as a group we can ensure that we all have a full understanding of Membrane Transport Mechanisms.