Download MOVEMENT OF MATERIALS THROUGH MEMBRANES

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MOVEMENT OF MATERIALS
THROUGH MEMBRANES
Name ____________________________ Pr____
All cells are surrounded by a plasma (cell) membrane. This membrane serves many functions.
One function is to control what goes into and out of a cell. Cells use this membrane to keep their
internal environment different from the outside environment. If they didn’t, there would be just a
chaotic mixture of chemicals inside the cell, and these would not be conducive to life.
Anything entering or leaving a cell must pass through the cell membrane. The cell membrane
has pores in it that are made out of protein channels. Some materials pass into a cell easily, others do
not. Of those that pass through, some pass through without any work on the part of the cell, others
are “pumped” in or out by special kinds of protein pumps in the cell membrane.
How will you know if sugar, starch, protein, or salt can go through a cell membrane when you
can’t see them? You can use indicators. As the teacher demonstrates how the following indicators
work, please fill in the chart below:
Molecule
Indicator
Original color
Color if molecule is
present
Sugar
Starch
Protein
NaCl
In this lab we will investigate the movement of materials through a cell membrane by using a
“fake” cell membrane called dialysis tubing. This tubing has microscopic holes in it that are about the
size of REAL cell membrane protein channels. Because the dialysis tubing is not real, we can assume
that the tubing is not “pumping” anything and that all movement of materials across the membrane is
passive (that is, takes no effort on the part of the tubing). Also, the dialysis tubing (our cheap dialysis
tubing is called “serpent skin”) is neither hydrophobic nor hydrophilic.
Hypothesis
Prediction:
Of the following compounds, which ones do you think will easily pass through a cell
membrane? Circle your predictions.
Glucose
Reason:
Starch
Protein
NaCl
WHY do you think the compounds you circled will easily pass though a cell membrane?
_____________________________________________________________________
Procedure
1.
2.
3.
4.
5.
6.
7.
Tie off one end of your “cell” as shown:
Squirt three pipette’s worth of each of the four
solutions into a small beaker. Stir it with the
pipette that is sitting in the beaker.
Pour 2/3 of this solution in to your “cell.”
Tie off the other end of your “cell”.
Rinse your “cell” very well.
Pat it dry.
Place your “cell” in the larger beaker and cover it with water.
8.
While your “cell” is sitting, test the contents of the rest of the solution in the small beaker to
make sure you really DID put sugar, starch, protein, and salt into your “cell.”
a. Squirt enough of the solution from the small beaker in to each of four test tubes to fill
them about 1/3 full.
i. Add about the same amount of benedicts to the first test tube (so it is now 2/3
full) and put it in a hot water bath.
ii. Add a drop or two of iodine to the second test tube. Do NOT heat it!
iii. Add about a pipette-full of Biuret A to the third test tube. Shake it. Then add
Biuret B (the same amount as Biuret A). Do NOT heat it!
iv. Add a few drops of silver nitrate to the fourth tube. DO NOT HEAT IT!
b. Record what was in the small cup, and thus originally in your “cell,” to begin with, in
your data chart.
c. Clean out your test tubes using the small test tube brush.
d. Go back to your desks.
e. At the end of the period, remove four samples of the water your “cell” was sitting in and
squirt each into a different test tube. Test one for sugar, one for starch, one for protein,
and one for salt. Record your results in the data chart.
Data
Indicator
Original
color
of the
indicator
Before Diffusion Occurs
Color the
indicator +
solution in
small beaker
turned
Kind(s) of
molecules that
must be
present in the
small beaker
(and thus in
the “cell”)
based on the
color
After Diffusion Occurs
Color the
indicator +
solution from
the large
beaker of
water turned.
Benedicts
Iodine
Biuret
Silver
Nitrate
Conclusions
1. Which of the molecules tested are smaller than the pores in a cell membrane?
A.
How do you know?
Kind(s) of
molecules
that must be
present in the
larger beaker
of water.
2. Which of the molecules tested are larger than the pores in a cell membrane?
A.
How do you know?
3. Based on this difference, which of the following molecules would you expect to be able to
diffuse readily through a cell membrane? Circle all that apply:
NaCl
fat
H2O
cellulose
KCl
glycogen
Why?
4. How is the “snake skin” similar to a real cell membrane?
A.
How is it different?
5. What do you think would have happened if you had placed the indicator iodine outside the
dialysis membrane, in the cup of water, from the START of the investigation? Why?
6. What must happen to molecules like starch and protein if they are to be used by cells?
7. What do you predict would happen if we put starch and amylase inside the dialysis tube and
iodine outside the dialysis tube? (NOTE: Amylase is an enzyme that digests starch.) Why?
8. What might be one reason glucose is a good food for “quick energy” in comparison to
starches?
9. Dialysis tubing is used in labs a LOT to separate chemicals. For instance, if I want to remove,
salts from a venom sample (venom is made of large proteins), I put the venom in a dialysis
tube, suspend the tube in a beaker of water, and let water from a faucet run into the beaker
and overflow into a drain for several hours. Why couldn’t I remove all the salts if I let the tube
sit in standing water instead of letting water run over it?
10. People whose kidneys are not functioning can be saved if they go on a dialysis machine. A
dialysis machine uses dialysis membranes to separate “bad” and toxic molecules from the
blood. Blood from the patient is run across the dialysis membranes in one direction and a
solution that is the correct concentration of chemicals runs in the opposite direction. Why don’t
the blood and solution run in the same direction? (Your answer will be similar to the answer to
question #9).