>>This will be the first video lecture in a series of lectures covering the cell membrane
transport. Keep in mind that the purpose of cell membrane transport is to move solutes
into or out of a cell. So this is the prior knowledge that you should know before proceeding
with this video lecture. If you’re unsure of any of these answers, be sure to go back
to the previous material and brush up on these concepts.
So here’s our chapter outline that will be discussed in this particular chapter on
membrane transport. This video lecture is going to be focusing on the factors that affect
the direction of transport. So before we begin, it’s important for us to review the driving
forces that act on molecules, and this is a very fundamental concept here. And so that
concept is random thermal motion, or Brownian movement. And so the question is: Do molecules
prefer to move from a high energy state to a low energy state, or do they prefer to move
from a low energy state to a high energy state? So take a moment to think about that, and
then answer this question. You could even pause the video if you’d like.
And so when you talk about random thermal motion, remember that that is synonymous with
Brownian movement. And what that means is that molecules are always moving. And since
molecules are always moving, that means then that they’re going to collide. And every
time they collide and they bounce off one another and collide into another molecule,
and bounce off and collide into another and so on, every time this happens they generate
energy. And it’s this energy that pushes molecules in one direction or another. And
so the direction then that these molecules prefer to move is from a high energy state
toward a lower energy.
Okay. So now with that in mind, let’s go ahead and review the driving forces that are
effecting the direction of movement or the membrane transport. So let’s begin with
the chemical driving force. A chemical driving force is the same thing as a concentration
gradient. So if you hear the term “chemical gradient” or you hear the term “concentration
gradient,” those are the same thing. So we’re going to abbreviate concentration
gradient with a change in C, because what we mean by a change in C is that there’s
a difference in concentration. We will always use brackets to abbreviate concentration.
So we have a difference in concentration across the membrane.
So this is just a really brief introduction to what a chemical gradient is; we’ll get
into more detail in just a moment on that. Then there’s the electrical driving force.
With an electrical driving force, now you need to think about having a difference in
the net charge across the membrane. So once again, we’ll be returning to the electrical
driving force here shortly. Lastly, there’s also an electrochemical driving force, and
so that’s a combination of both the chemical plus the electrical driving force. We’ll
return to electrochemical driving forces later on in the semester. And so first, let’s
go ahead and focus on a chemical driving force. And so remember when in a chemical driving
force, you always have to think about a concentration gradient. And so when you look at Figure A,
is there a concentration gradient illustrated in Figure A?
The red dots illustrate the solute. The blue background is your extracellular fluid, so
those red dots are the solute in the extracellular fluid. The tan color within the cell is representing
the solutes in the intracellular fluid. And so hopefully you responded, “Yes! There
is a concentration gradient.” There’s a lot more solutes, right, in the intracellular
fluid. So here we have a high-concentration in the intracellular fluid compared to the
extracellular fluid, where there’s a low concentration. So in this particular case,
you’re going to have more collisions or a high frequency of collisions, in other words,
in the intracellular fluid, which generates a lot of energy and pushes or provides the
energy to push those solutes out of the cell.
And then if you look at Figure B, we have a similar situation in the sense that yes,
there is a concentration gradient, but this time the gradient is into the cell. So solutes
want to move into the cell because there is a higher concentration…of solute in the
extracellular fluid, a lower concentration of solute in the intracellular fluid. You
generate more collision, so you have a high frequency of collisions in the extracellular
fluid, which then generates more energy. And then that pushes those solutes into the cell,
because there’s more energy there. So now let’s introduce an additional concept here.
And so, we want to think about the magnitude of the driving force. And what we want to
know—is the magnitude of the driving force proportional to the concentration gradient?
Okay. So to study this a little more closely, let’s take two elevators. So here’s going
to be Elevator A, and right next to it is going to be Elevator B. So you could imagine
that these are elevators in the parking garage. Elevator A has 20 people. Outside of Elevator
A there is one person. Elevator B has five people, and outside of Elevator B there’s
also one person. So which of these elevators is going to have a greater concentration gradient:
A or B? Elevator A has a higher concentration gradient. Remember a concentration gradient
is a concentration difference across the membrane. So in the Elevator A, there’s 20 people
inside and one person outside. So if you have 20 people crammed inside of that elevator,
they’re going to generate a lot of energy because they’re going to have a lot of collisions.
And so if you look at Figure B, there’s only five people in the elevator. So will
they have as many collisions as those that are in Elevator A? And the answer is no; there’s
fewer collisions, therefore there is less energy, so there’s not as strong of a gradient.
So Figure A has a larger driving force. So the magnitude of the driving force is definitely
proportional to the concentration gradient—the greater the magnitude, the greater the gradient.
Next then we turn to the electrical driving force. And so in the electrical driving force,
remember that we’re dealing with charges, solutes that have charges. So in the intracellular
fluid, there’s all sorts of ions, anions, cations and so on in the intracellular fluid.
But the idea here is that you have a net negative charge in the intracellular fluid, meaning
you have more negatively charged ions in the intracellular fluid than you do cations. And
then in the extracellular fluid, there’s a net positive, meaning there’s more cations
than there are anions in the extracellular fluid. So we’re going to be representing
this a little bit differently during—throughout the course.
And so to the right, here we have a cell. And this cell in the intracellular fluid,
we know, has a net negative charge. So we’ll be representing that with a negative along
the inside of this membrane. And then we know that the extracellular fluid is outside of
the cell, and it has a net positive charge. So we’ll be illustrating that with a positive
on the outside of this membrane. So once you see this then you can ask yourself, “Is
there an electrical gradient?” And you should say, “Yes! There is an electrical gradient,
because there’s a difference in charges across the membrane.” Another way to put
it is there’s a charge separation across the membrane.
Okay, so then when we look at these two images, let’s look at Figure A here and we want
to know why is the driving force into the cell? So if we have a net positive, right,
in the extracellular fluid and a net negative in the intracellular fluid, why do the positives
want to move in? Well remember, opposites attract. And so positives want to move into
the cell where it’s negative, right? They’re attracted to the opposite charge. And then
a similar situation in Figure B—now they, they have a situation where the positive—where
the negatives rather, want to move out because they’re attracted to the positives on the
outside of the cell. So that’s just showing you the direction of movement based on attraction
of these charged solutes. Okay, and then lastly is electrochemical driving force. Again, we’re
going to return to this concept later.