Membrane Transport Mechanism Questions

The following are questions that I have received from you regarding the lecture Membrane Transport Mechanisms.  I realize many of them are logistical, but as we are beginning the semester these are important questions to consider at this time.  These may not be all of your questions, but hopefully this can kick-start the process.  If anything else comes to mind regarding this topic, please feel free to either email me or write directly within the comments section on this page and we can all help each other become clearer on the topic of Membrane Transport Mechanisms.

I realize this is quite long, but hopefully you can pick out the questions that may interest you and ignore the rest as you see fit.  As I stated before, the questions on the exam come from the lectures and will not be asked on a specific question that someone else asks referring to that information.  This is for your benefit, hopefully it helps!

1.      I don't see your physio practice questions on the G-Drive. Have they been uploaded yet? 

Are you working within the file on the G-drive for this particular lecture? (G:\Semester 1\Block 1. Fundamentals 1\Fundamentals.part 1\Membrane Transport Mechanisms Dr. Johannessen(PHYSIO)) Within that folder there should be two folders: Membrane Transport Thought Questions and Membrane Transport Study Quiz.  The quizzes are within these files (along with hidden files that allow the quizzes to run).  All materials associated with each particular lecture will be contained within the file that is created on the G-drive for that particular lecture. 

To make the quizzes run, you need to utilize the quiz from the ACTUAL Server Network G-drive.  If you are within these files on the network G-drive (for most of you, you must be ON CAMPUS to do this), then you can drag them into an open web browser and the quiz will then open.

2.      I have been trying to print the membrane transport lec now for a few days, and I have not yet been able to print it off successfully. The file seems to be very large and I keep getting an error. Please advise me on how to print this lec. Thank you.

We are required to keep all lecture files under 5MB.  This lecture is about 4.3MB and therefore should be printable.  My advice would be to access the G-drive from the network G-drive on campus (not via the internet) and download the lecture from that file.  Then printing it should be no problem.  If this continues to be a problem for any of you, let me know and I will email you the file directly which should also work.  

3.      As I attempted to open your practice problems and thought questions for your first lecture using Google Chrome and Safari, I was unable to retrieve the files.  If you could help me get access to these aspects of the lecture I would be very grateful as I want to take advantage of every resource I can that will help in my studying.  The html would not open and the exe file lead me nowhere.

As I stated above, you need to utilize the quiz from the ACTUAL Server Network G-drive.  If you are within these files on the network G-drive (for most of you, you must be ON CAMPUS to do this), then you can drag them into an open web browser and the quiz will then open.  Many of you have been accessing the G-drive online to gather your lectures and material.  Through this method the articulate files do not function and therefore must be accessed from the actual network G-drive on campus.  It is for this reason that I encourage you to copy these files (while on campus) from the actual network G-drive to your own computer and then you can access them from wherever you happen to be.

4.      I was just going over the practice questions you provided on the powerpoint and I noticed that the corresponding answers at the bottom do not match the question numbers. Could you please let me know what the answers are so I can double check my choices?

I am unsure as to this problem.  In the powerpoint file for Membrane Transport Mechanisms, the final slide is that of study questions.  In the notes section of this slide there are 2 answers for the turningpoint questions followed by 10 practice questions (numbered 3-12 to follow the original 2 turningpoint questions within the lecture).  The MCQ practice questions are then followed by answers for those questions also numbered 3-12.  The answer choices 3-12, therefore correspond to the question numbers 3-12.  

5.      I was wondering if Ishould memorize/ know the content that is on the Powerpoint under the notes section, as some of the material we didn't go into as much depth during class, or should I focus more on what we did discuss in class?

The notes section under the powerpoint slides should accompany what is on the slide.  Sometimes it is difficult to say something with a picture and make it clear.  Therefore, additional text is required.  If it goes into more depth than I did in lecture, take it as additional clarifying information that may help you understand the concept to a greater degree.  Remember that I am writing the exam questions based on my knowledge of the subject as well and typically that information comes out of my mouth during lecture.  

A couple of additional points however.  Remember that physiology is not a subject of memorization…it is conceptual, so you should never be memorizing, but striving to understand!  Additionally, I typically do not answer questions of ‘what will be on the exam’, or ‘do we need to know that’.  This is because you are training to be a doctor here at RUSM.  Now, you need to pass some exams here in order to continue on that pathway, but you will ALSO need to pass exams PAST here in order to continue on that pathway and you will need to utilize this information in many different ways with patients along the way.  Therefore, I do not know all of what you ‘need’ to know and in what timeframe you will need to know it, this is information that nobody can accurately answer for you.  Remember that you are now a doctor in training, a professional field, and you will undoubtedly need all of this information in one capacity or another throughout the rest of your life, so committing it to your understanding will be of much greater value to you in your career!

6.      In the first learning objective, you asked to compare the relative concentrations of the different ions/solutes in the extracellular vs intracellular fluid.  Did you want us to separate the extracellular fluid into plasma and the interstitial fluid like you did in your notes?  Additionally, they state that we should know the "relative" concentrations for various ions. Does that mean we need to know their numerical concentrations, or just if that ion is more concentrated intracellularly/extracellularly? 

I apologize for this learning objective not being deleted.  I did not cover these concentrations and will do so in much greater detail when we get to membrane potentials.  Therefore, you do NOT need to know the relative concentrations of the different ions/solutes in the extracellular (ECF) vs. intracellular (ICF) fluid at this time.  When we do cover this in greater detail, however, you will need to know that Na+ is in higher concentration in the ECF while K+ is in higher concentration in the ICF and so-on, not the actual concentrations.  As you will learn, their ACTUAL concentrations change lots and lead to many changes that occur in membrane potentials.

7.      I had a quick question about carrier mediated diffusion, more specifically the maximum flux concept.
If I am understanding this, the carrier mediated diffusion has a Vmax that is determined by the concentration gradient of the molecules and the number of available integral proteins in the membrane that can transport the molecules.  Is approaching the Vmax the same as approaching the maximum flux?

Yes!

8.      Also, with this facilitated diffusion, I understand that the concentration gradient is the driving force and no ATP is hydrolyzed directly.  However, I also thought that energy is always required to do work, and the conformational change that the integral protein undergoes to facilitate the diffusion of these molecules appears to be "work."  

Indeed, there is work being done for the transporters involved in facilitated diffusion to undergo their conformational change.  This is the beauty of the second law of thermodynamics in this case, that IS providing the energy required for the transporter to undergo the conformational change.  In essence to have membrane transport you need a driving force and a pathway, in the case of facilitated diffusion the driving force is concentration gradient and the pathway is through a transporter, in the case of primary active transport the driving force is ATP hydrolysis and the pathway is through a specialized transporter called a pump.

9.      Do channels undergo conformational changes while they are signalled to open? I know they have a gate which opens, but does the channel itself change in any other way.

This ultimately depends on your definition of a conformational change.  In essence, a conformational change is any movement of a protein from one state to another, therefore the opening of a gate due to a signal is in essence a conformational change of that protein.  Each channel has specifics of these movements and some involve movements of other parts of the proteins as well in order to achieve this goal, while others do not, but the opening of the gate to allow for the movement of molecules across the membrane at this point is the important point.

10.  I am not sure what the policy on asking questions regarding practice MCQ you have at the end of your slides is, but I assume we can ask for clarification over e-mail. 

Absolutely you can ask clarification questions for my practice AND thought questions.  I may not give you as much guidance as you would like (sometimes I think it is more beneficial to tell you where you can FIND the information rather than giving it to you directly), but you can DEFINITELY always ask for clarification and help!

7.  Which statement is incorrect?

            A.  Diffusion of a solute through a membrane is considerably quicker than diffusion of the same solute through a water layer of equal     thickness.

            B.  A single ion, such as K+, can diffuse through more than one type of channel.

I thought B would be wrong, because K does not diffuse through channels, except for the leaky K channel. So I picked that as the wrong choice, because I was not even sure we touched upon option A. 

Indeed, K+ leaky channels may be the only type you know of at this time, but there ARE many other types of channels that K+ ions diffuse through (the movement or diffusion of ALL ions across a membrane will be through either a channel or a transporter because they are NOT membrane soluble molecules).  The statement in B is essentially saying that there are more than one type of channel for a given ion and as you will learn quickly there are LOTS of channel types for a given ion to move through.  Answer A, however is saying that it is A LOT quicker for a molecule to move through a membrane then it would be to move through water (remember a molecule that COULD move through a membrane would also be able to move within water).  Now, you know that a membrane contains phospholipids that the molecule will have to move around and therefore it would NOT be quicker than moving within water. 

11.  Which of the following characteristics is shared by simple and facilitated diffusion of glucose?

            A.  Occurs down an electrochemical gradient

            - was this something we should have already known, or did I miss this on a slide? As well, glucose is not a charged molecule, so why does the electrochemical gradient come into play? In general, I thought glucose is too big for simple diffusion.

Per slides 9-11, facilitated diffusion is a passive activity allowing for a molecule to move down its concentration gradient.  Now, just because glucose is not charged does not mean it would not have an electrochemical gradient, it would just mean that the electro portion of the gradient would be non-existent, therefore moving the molecule essentially down its chemical gradient.  Additionally, typically glucose IS too big for simple diffusion across a membrane, but if you were to move glucose within a water bath for example, that would be simple diffusion (moving away from areas of high concentration) and that would be passive as well. 

Remember that one of the caviats of MCQs is that you HAVE to choose an answer.  Sometimes you may feel as though the absolute best answer is not actually represented, or how you would answer it if you wrote the question, but one of the rules is that you need to pick the best answer from those given.  Sometimes this means you pick the least wrong answer and should utilize the ‘rule-out answers’ method, and sometimes this means you know exactly how it should be answered.    

12.  Which of the following transport processes is involved if transport from the intestinal lumen into a small intestinal cell is inhibited by abolishing the usual Na+ gradient across the cell membrane?

 I did not understand what exactly the question was asking. Where was I to begin deciphering what was being asked?

Similar to what I stated above, you have choices in a question of this sort.  You may not know much about the transport of molecules from the intestinal lumen to a small intestinal cell, but you DO know something about Na+ gradients…what do you know?  Na+ gradients are used A LOT in what type of membrane transport?  Is one of the answer choices this type of membrane transport?

11.  In slides 9 and 10 (the two conceptual overview slides) of the lecture, there are red asterisks under the pore/channel as well as vesicular types of membrane transport types. What is the significance of these asterisks? What are the similarities between these two?

Great question and great catch to identify these points.  The asterisks in this case are actually because I consider them special.  The pore/channels are a special case because, as I mentioned in class, under physiological conditions the kinetics of the movement of molecules through them adhere to those of diffusion (increase in concentration gradient leads to a direct increase in flux of the molecules across the membrane).  As I mentioned as well, under non-physiological conditions this is not always the case, but as we are working within the human body we are going to classify this transport mechanism under diffusion due to that.  Vesicular transport, also as I mentioned, is a special type of membrane transport that is not typically covered in other membrane transport chapters/lectures.  It is important to move molecules across a membrane by inclusion in another membrane, but it is the ‘different’ mechanism not involving any proteins and such, so I indicated it as special as well.

12.  I am confused about facilitated diffusion. On the channels/pores slides it states that these are forms of facilitated diffusion that use the same principles as simple diffusion (cannot be saturated), however on slide 22 it says that facilitated diffusion shares characteristics with carrier mediated transport and can be saturated. Could you please clarify?

I am unsure of the ‘channels/pores slides’ you are referring to, but I think your confusion is this:  Under physiological conditions channels and pores cannot be saturated and are therefore classified as diffusion (NOT FACILITATED DIFFUSION), but transporters that move molecules down their concentration gradient and can be saturated are classified as facilitated diffusion.  In biochemistry they may classify the movement of molecules through channels and pores to be facilitated, but for our purposes (due to their kinetics being identical to that of diffusion) we classify it as diffusion.

13.   I'm a bit confused over the secondary transport mechanism. Does it always happen right after a primary transport? As in, is it always coupled, or does it take place whenever required? I'm also guessing it happens in transporters adjacent to the Primary transport proteins, or does it happen in the same one? 

My understanding is that after a primary transport (which creates a concentration gradient, thus energy needed for a secondary transport) molecules move UP their concentration gradient in or out of the cell. Now the lecture says one substance is usually moving downhill while another is moving uphill. How and why does this happen? 

In the secondary active transport model diagram it says energy from one gradient (in this case, Na moving down its gradient) drives transport of another molecule against its gradient. Now isn't the movement of a particle DOWN its gradient diffusion? As in, it doesn't require energy does it? 

Secondary active transport can be confusing.  By definition, secondary active transport is the movement of one molecule against (uphill) its concentration gradient while moving another molecule with (downhill) its concentration gradient.  This movement is typically coupled (in a section of membrane close together) with a primary active transport that utilizes the hydrolysis of ATP (or GTP) to move a molecule against its concentration gradient.  Therefore, they say that secondary active transport indirectly utilizes the energy of ATP hydrolysis to move two molecules. 

Now, you could argue that the concentration gradient existed, therefore it is not actually indirectly using ATP, but because that concentration gradient must be MAINTAINED by the primary active transporter, a secondary active transporter is said to indirectly utilize the energy from ATP hydrolysis. 

14.  I had a couple of questions from today's lecture on Membrane Transport Mechanisms:

1.      Slide 7 -  Do I understand this correctly? When entropy is the driving force, diffusion is occuring, but the molecules are not moving down their concentration gradient, they are simply moving around via random thermal motion and will eventually diffuse equally throughout the container? Or is it implicit under the 2nd law of thermodynamics that the molecules will move by random thermal motion down their concentration gradient?

Indeed, it IS implicit that under the 2^nd law of thermodynamics that the molecules ARE moving by random thermal motion down their concentration gradient.  That is the part of the randomness always increasing, they are always trying to get as far away from each other as possible and increasing the randomness of their distribution.

2.      Slide 12 - same concept as the one mentioned above: "DIffusion refers to the random motion of molecules". What about down their concentration gradient?

Yes, diffusion does refer to the random motion of molecules, but DUE to the increase in entropy, this movement is always away from each other.

3.      Slide 19 - do we need to memorize Fick's Law and what the symbols mean? or will that info be given to us on an exam?

Yes, you do actually need to memorize the law and what the symbols mean.  Fortunately, this law will come back up again and again (in cardio and in respiratory for example) so if you spend the time to get to know it now, you will not have to learn it again then!