Membrane Transport – what is the direction of the sodium gradient across

14 Jun Membrane Transport – what is the direction of the sodium gradient across

Question
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EXERCISE 16: Membrane Transport
Goal: To understand how membrane transport processes work together to move both water
and nutrients into the body.

Note: Refer to Appendix A5, Membrane Transport Clarification
Materials:
Large pieces of paper
Colored pens/pencils

Outer epithelium
2 layers

of

muscle tissue

Inner (absorptive)
epithelium

Figure 1. Cross-section of the small intestine. Note that this is a cartoon: not all components
are shown and the inner epithelium is highly convoluted due to circumferential folding and
finger-like projections called villi. As a result, when a section of small intestine is viewed
under a microscope, the lumen appears as narrow spaces between walls of tissue.
INTRODUCTION
ln this activity we will draw a picture showing the movement of glucose and water from the
lumen of the small intestine across the epithelial cells that line the intestine into the
bloodstream. First we will consider nutrients like glucose. These move through, not between,
the epithelial cells. This movement involves transport across two membranes: one at the
lumenal side of the cell (through the apical membrane), and the other at the basal side
(anchored to the vascularized connective tissue). Specific transport proteins embedded in
the membranes of these cells allow movement to occur in the right direction. We will next
consider the movement of water across the same membranes through water specific
transport proteins. We will see how the movement of nutrients influences the movement of
water.
For the study of nutrient uptake we will focus on the movement of glucose, but you should be
aware that the movement of other monosaccharides and amino acids would be similar; except
that they would move through their own transport proteins.

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Considerations:
The solution in the small intestine is very dilute compared to the solution in the
absorptive epithelial cells so glucose concentration will always be low in the lumen
compared to the cell.
Glucose can ONLY move from the lumen into the cell with sodium ions because of the
nature of the glucose channels’
Thus, in order to move glucose into the cell against its gradient, a sodium concentration
gradient must exist.

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.
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1. DRAW EPITHELIAL CELLS

Obtain a large piece of paper and draw at least 2large epithelial cells side by side. One side
of the epithelium faces ihe lumen of the small intestine, while the other side faces tissue rich in
blood vessels. Labetthe two sides. Keep your drawing simple -you do not have to draw
other intestinal cells or blood vessel cells. Make sure to include 1) tight-junction proteins
that provid e a barrier between the fluids in the lumen of the small intestine and the blood
stream on the apical end, and 2) desmosomes (anchoring junctions) that hold tissue cells
together. (see figure in your text)

2. NA+/K+ PUMP TRANSPORT PROTEIN
a. Draw Na+/K+ ATpase pump proteins in the basal cell membrane. These proteins require
the energy of ATP to pump solutes against their concentration gradient so this is a
primary active transport system. Add the Na+ and K+ ions in numbers that indicate
the correct concentration gradients. Recall that Na+ is always high outside the cells and
K+ is always high inside the cells.
Three Na+ are pumped out of the cell into the blood while two K+ are pumped into the cell
from the blood.'(Since two solutes are being transported through one carrier protein but in
opposite directions, this form of co-transport is an antiporf system.)
Also, since there is more Na+ leaving the cell than K* coming in, this makes the cell less
positive (or more neqative) than the lumen. The difference in charge and sodium
qradient. This
concentrafion Uetween tne lumen and the cell is called an electrochemical
gradient drives the secondary active transport of glucose from the lumen into the cell, which
pump sets up this gradient’
we explore in #3. below. First let’s see how the Na+/K+

As the Na+/K+ transport pump is working:
b. Question: What is the energy source tor tnis movement of Na* and K*?
Show representative Na* and K* ions moving across the basal membrane. lndicate on the
drawing where Na+ concentration becomes high and where it becomes low, and where K*
becomes high and where it becomes low’
d.

Question: whv does this process require an input of energy?

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3. SGLUT.I TRANSPORT PROTEIN

Now let’s move to the lumen (apical) side of the
epithelial cells. As stated above, a glucose
molecule can only
the ceiltogether with a il";.
1sin.e the tr"o *irt.s are transported
"n19f
in the same direct-ion,
this type oi"l+r"n"port is symp2rt-sysfem.)
Draw the transport
protein that transports glucose and sodium
ions intoL ceil from ilr" grillabel it sGlut-1).
This sodium/Glucose cotransporter allows
ftre’grucose to be tr"nspJ.t"d into the cell
aqainst
its concentration gradient (remember, the solutiJn
in the gut is dilute compared to the cell)l

i

Questions:

a’

what is the direction of the sodium gradient across
the lumenal membrane? That is,
on which side of the membrane is thJ concentialion
higher, and on which side is it
lower? lndicate this on your drawingti.. sn"* where sodium is higher and where it
is lower and add an arrow that followrTn. jrcoient
trom high to low – label it ,,sodium
gradient")

b’

what is the direction of the glucose gradient across
the lumenal membrane? lndicate
this on your drawing as well.

c’

while the sGlut-1 transport protein is operating, both
sodium and glucose enter the cell.
what is providing the energy to move the gluc6se against

L

(Exptain)

d’

e.

its concJntration gradient?

ls the movement of sodium through sGlut-1
during co{ransport simple diffusion,
facilitated diffusion, active transport, or secondary
active transport? ls the movement of
glucose through SGlut-1 during co-transport
simpte diffusion, rr.irlirt"o diffusion,
qrimary active transport, or secondary active transport?
EXPLAIN YOUR CHOICES.

As transport continues,
what happens to glucose concentration inside the cell?

i

ii

What does the SGlut-1 transporter do to the Na*
concentration inside the cell at
the Lumenal Membrane? overall sodium ion concentrrt’on
in the cell remains
relatively constant. Why doesn,t it increase?

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4.

GLUT-2 TRANSPORT PROTEIN
Now let’s explore how the glucose travels from the cell to the bloodstream. As SGlut-1 has
been operating, the concentration of glucose inside the cell has increased as compared to the
bloodstream. Draw the transport protein Glut-2 (which only transports glucose) in the basal
membrane of the epithelial cells.

Questions:
a. Does glucose move through the Glut-2 into the blood via simple diffusion, facilitated
diffusion, active transport, or secondary active transport? EXPLAIN YOUR CHOICE.

b. Why is a transport protein required

to move glucose through the membrane? (Think about
the characteristics of the plasma membrane and whether glucose is polar or non-polar.)

c. What is the energy source for the movement

d.

of glucose from the cell to the blood?

As glucose diffuses out of the cell, it moves into blood-filled capillaries. Why is it important
to move the glucose away from the basal membrane?

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z

5. AQUAPORIN TRANSPORT PROTEIN (see appendix AS and 83)
Water will always move, via osmosis, to an area that has a higher concentration of solutes
(and thus less water). Draw aquaporin proteins in the lumenal membrane and the basal
membrane. Study your diagram and note where the total solute concentration is higher (the
hypertonic area) and where is it lower (the hypotonic area). Consider the extracellular fluid at
the intestinal lumen (remember, it is very dilute), the cytosol of the cell, and the extracellular
fluid at the basal membrane (where a lot of sodium is being pumped into the blood adjacent to
sodium-potassium pumps). ls the lumen hypertonic, hypotonic or isotonic to the cell? ls the
extracellular fluid at the basal membrane hypertonic, hypotonic, or isotonic to the cell?

lndicate this on your drawing.

a. Why is a transport protein

required to move water through the membrane? (Think about
the characteristics of the plasma membrane and whether water is polar or non-polar.)

b.

lndicate the movement of water through the aquaporins with arrows. Consider both
the luminal and the basal membrane.
Does osmosis of water occur via simple diffusion, facilitated diffusion, active transport, or
secondary active transport? EXPLAIN YOUR CHOICE.

QUESTIONS

1. What is the source of energy for simple or facilitated diffusion?

2. What is the source of energy for primary active transport?

3. Describe how ATP is used to move glucose from the intestinal lumen into the epithelial cell.

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6. ln this exercise, we looked at one system that requires the transport of materials across cell
membranes. There are, of course many other such systems. ln fact all cells must move
chemicals in and out through their membranes. ln the table below, for each substance listed in
the first column, check all the transport mechanism(s) in the remaining columns that might be
used to transport the substance across the membrane. Refer to class notes, text and any
other resources.
Substance

Simple
diffusion

Facilitated
diffusion

Primary
active
transoort

Secondary
active
transoort

Co-transport

a. water
b. sodium
ions

c. oxygen
d. carbon
dioxide
e. glucose

f. potassium
ions

7. CLINICAL APPLICATION:
ln countries where lV therapy for dehydration is not readily accessible, ORT (oral rehydration
therapy) has saved the lives of millions of children suffering from severe dehydration due to
diarrhea caused by cholera and other intestinal pathogens. This therapy involves drinking a
solution of glucose and electrolytes (enriched in chlorides of sodium and potassium). This
therapy is also prescribed for children in this country suffering from diarrhea/vomiting (One
such drink you may have heard of is the product PedialyteTM;.
Explain why this simple sugar-salt drink works better to hydrate children than does
water alone.