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 2nd 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!
