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Lecture 3

BIO120H1 Lecture Notes - Lecture 3: Electrochemical Gradient, Atp Hydrolysis, Glucose Transporter


Department
Biology
Course Code
BIO120H1
Professor
Jennifer Harris
Lecture
3

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Tuesday, January 13, 2009
Slide 1
Slide 2
Slide 3 - Last lecture we covered different types of transporters & we covered
both passive & active transport looked at the glucose transporters as a
type of passive transport & we looked at symporters & antiporters that
carry out secondary active transport. And we also looked at ATP-driven
pumps the P-type ATP-driven pump (ATPase) as an example of
primary active transport.
Slide 4 - Today we’re also going to continue with primary active transport or
ATP-driven pumps & look at 2 additional types V-type ATPases & F-
type ATPases. We’ll also look at how the different transporter proteins
that we covered in the last lecture worked together in order to transport
small molecules across a cell. In particular, we’re going to look at how a
combination of transporters are used to transport glucose across the
intestinal epithelial cells.
Slide 5 - Last lecture we covered the glucose symporter that passively transports
glucose down its concentration gradient.
- We also covered the Na-glucose symporter an example of secondary
active transporter.
- And we also looked at Na-K pump which is an example of an ATP-
driven pump or transport ATPase that uses the energy from the hydrolysis
of ATP to transport both Na & K against their electrochemical gradients.
- Now these pumps don’t act on their own in a cell there are many
different types of transporters & they can act together in order to
transport a molecule across/into a cell.
Slide 6 - The example we’re going to go into is how these 3 pumps that we just
looked at are used to transport glucose from the lumen of the intestine
across the intestinal epithelial cell, into the extracellular fluid that will
eventually end up in the bloodstream.
- Diagram: This is the inside of the intestine (the cylinder) & we’re
looking at the lumen of the intestine here & what’s wrapping the intestine
would be these intestinal epithelial cells. So glucose needs to be
transported from the intestinal lumen to the extracellular fluid outside the
intestinal epithelial cells. On the right hand side: you can see what is the
relative concentration of glucose in each of these different compartments.
In the intestinal lumen, there is a relatively low glucose concentration – it
becomes concentrated inside the intestinal epithelial cells so you have a
relatively high glucose concentration. Then in the extracellular fluid,
again the glucose concentration is relatively low. Can see that to transport
1st from the intestinal lumen into the intestinal epithelial cells, you need to
go from low glucose concentration to high glucose concentration which is
against the glucose concentration gradient.
- Then if you look at transport of glucose from inside the intestinal
epithelial cells to the extracellular fluid, this is transport down the
concentration gradient of glucose.
Slide 7 Restricted by tight junctions
Na+/glucose symporter
GLUT2 uniporter & Na+/K+ pump
- What’s important is that the intestinal epithelial cells have different
domains of the membrane that are separated by what are called tight
junctions. So you have the apical plasma membrane as seen in the

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diagram which is distinct from the lateral & basal membranethese tight
junctions will occlude any proteins from traveling b/w these 2 membrane
spaces – so a protein that is found in the apical plasma membrane will not
localize down to the basolateral membrane & vice versa. So this allows
for the asymmetric distribution of proteins in these intestinal epithelial
cells.
- So the transport proteins, the movement of these is restricted by the
tight junctions so as we said proteins can be laterally in the membranes &
since there are these tight junctions there, the lateral movement is limited
to the region b/w these tight junctions here.
- So in an intestinal epithelial cell the apical membrane contains a Na-
glucose symporter. The basolateral membrane contains a glucose
uniporter & a Na-K pump.
- So if you recall the concentration gradient, glucose is very high inside
the cell relatively speaking so when the 1st transport by a symporter where
glucose will be transported against its concentration using the energy of
driving Na down its concentration gradient.
- So remember that Na is higher outside the cell than inside the cell. And
this is maintained by the Na-K pump which is continuously pumping Na
outside of the cell so this is carrying out primary active transport so any
Na found in the cell will be pumped out using the energy from the
hydrolysis of ATP to maintain the Na levels in the cell to a relatively low
level. So you’ll have a low Na concentration inside the cell, a relatively
high concentration of Na outside the cell & then this transport down the
concentration gradient, that energy can be used to drive glucose into the
cell against its concentration gradient through symport of Na & glucose.
- Then you’ll recall that glucose is relatively high inside the cell &
relatively low outside the cell so the glucose 2 uniporter can passively
transport glucose down its concentration gradient from the inside of the
intestinal epithelial cells to the extracellular fluid.
Slide 8 - This diagram is basically showing the glucose concentration on right
hand side.
- So 1st of all you have the Na-K pump which is pumping Na outside of
the cell against its concentration gradient & this is maintaining a low Na
concentration inside of the cell. And the Na-K pump also pumps K
against its concentration gradient so it’s pumping both of these against its
concentration gradient using the energy from the hydrolysis of ATP.
- So since there is a low Na concentration inside the cell relative to the
outside of the cell, glucose can be transported against its concentration
gradient by symporting Na down its concentration gradient so that energy
drives the movement of glucose into the cell against its concentration
gradient.
- Now once glucose gets inside the intestinal epithelial cells, it builds up
to relatively high concentrations relative to the extracellular fluid & then
the glucose transporter (the uniporter) can passively transport glucose
down its concentration gradient into the extracellular fluid.
- So you can see how these 3 transporters work in concert in order to
promote the transport of glucose from the intestinal lumen to the
extracellular fluid.
- What is the function of the Na/glucose symporter?
To transport glucose…
A. Into the epithelial cell down the [glucose] gradient
B. Into the epithelial cell against the [glucose] gradient
C. Out of the epithelial cell down the [glucose] gradient

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D. Out of the epithelial cell against the [glucose] gradient
Answer: B
What is the function of the GLUT uniporter?
To transport glucose…
A. Into the epithelial cell down the [glucose] gradient
B. Into the epithelial cell against the [glucose] gradient
C. Out of the epithelial cell down the [glucose] gradient
D. Out of the epithelial cell against the [glucose] gradient
- GLUT Uniporter carrying out passive transport.
- Answer: C
What is the function of the Na/K pump?
To transport Na…
A. Into the epithelial cell down the [glucose] gradient
B. Into the epithelial cell against the [glucose] gradient
C. Out of the epithelial cell down the [glucose] gradient
D. Out of the epithelial cell against the [glucose] gradient
- Answer: C
Slide 9 - That’s how the transporters that we’ve covered work together in the
intestinal epithelial cells.
- Now we’ll move onto 2 different types of ATP-driven pumps the V-
type & F-type ATPase.
Slide 10 - If you look at the structure of these 2 types of ATPases, you can see that
they look very similar see there is like a lollipop structure there is a
head of the lollipop & the stick is actually embedded in the membrane &
this green part on both of these ATPases is actually able to rotate in the
membrane & the orange part is actually static in the membrane & does
not move.
- What happens is the pumping of protons, in this case so both of these
are involved in moving protons, causes this rotor (the green part) to rotate
& when this rotor rotates, it actually induces conformational changes in
the head of the lollipop (A & B or alpha & beta subunits) & this energy
can be harnessed from the F-type ATPase so you get mechanical
movement of this rotor which induces a conformational change & that
energy can be harnessed in this case to synthesize ATP. So the movement
induced in this rotor by the movement of protons, in this case, it would be
down their electrochemical gradient, induces modifications in the protein
& that mechanical energy can be harnessed to drive the synthesis of ATP
- Now the V-type ATPase actually works in reverse in most cases, so it
hydrolyzes ATP & then that induces a rotation in this part of the molecule
which can then drive protons against their concentration gradient so one
key feature of these (although they look very similar) is that most cases,
they are working in opposite directions.
- These molecules can work in reverse depending on the situation & the
circumstances in the cell these molecules can actually work in the
opposite direction where this one would pump protons out or this one
would synthesize ATP. It depends on the cellular context but in most
cases, the V-type ATPases are pumping protons using the hydrolysis of
ATP as an energy source & the F-type ATPases are synthesizing ATP by
harnessing the energy of moving protons down their concentration
gradient. And they both have a similar overall protein structure.
Slide 11 Energy from ATP hydrolysis
No phosphorylation of the pump unlike the P-type ATPases which
are phosphorylated (P for phosphorylated) these ones (V-type) are not
phosphorylated.
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