Introduction
The
movement of molecular materials within and between cells is accomplished by transport
mechanisms of two types. One group of mechanisms require an input of energy
by the system (cell or organism) and are known as ACTIVE TRANSPORT MECHANISMS.
In active transport molecules are moving against a concentration gradient
trying to find a place within an already crowded area. This is somewhat like
moving into a crowded bus or crowded room. Some active transport mechanisms
involve only the cell membrane while others require a movement by larger areas
of the cell.
The second
group of mechanisms are known as PASSIVE
TRANSPORT MECHANISMS. In these situations molecules are moving from areas
where they are in high concentration into areas where that same type of
molecule is in low concentration. No additional expenditure of energy by the
cell or organism is required to move the molecules. It is sometimes suggested
that they are moving down a concentration gradient. Examples of passive
transport are DIFFUSION, OSMOSIS and FACILITATED DIFFUSION. OSMOSIS
is a special case of diffusion involving movement of solvent molecules. Since
water is the solvent of the cells, osmosis is often thought of as diffusion of
water.
Cellular
membranes are selective of the molecules that
can move across them. Size, shape, mass, and charge of moving molecules are
all considerations as to whether the molecules may cross the membrane.
This
investigation will demonstrate osmosis and diffusion in living materials.
Materials:
Microscopes,
slides, cover slips, lens paper
Potatoes,
carrots, red onion, Elodea plants
50 ml
Beakers, forceps, triple beam balances, marking pens, cork borer
Distilled
water, tap water, salt solution (1%, 10%, 20%)
Sugar
solutions (1%, 10%, 20%)
Procedure:
Part I
1. Obtain 5
beakers and label individual beakers DI Water, Tap Water, 1% salt, 10%, salt
and 20% salt. Obtain a second set of 5 beakers and label them DI water, Tap
water, 1% sugar, 10% sugar and 20% sugar.
2. Place 25
ml of the appropriate solution in each beaker.
3. Obtain
pieces of potato (either by slicing or using the borer). Your instructor will
demonstrate. Weigh the individual potato pieces using the triple beam balance.
Record the weight of each piece and place a separate piece in each of the
solutions. Allow the potatoes to sit in the solutions for 30 minutes.
4. At the
same time that you are setting up solutions for the potatoes, set up a second
set of beakers and run a similar set of solutions for carrots.
5. At the
end of the 30 minutes, remove the potatoes and the carrots from the beakers,
gently pat them dry and weigh them. Record the weighs in your data table. Using
the formula below determine percent change in mass of the potatoes and the
carrots.
(final mass-initial
mass
% change
=
--------------------------------------- x 100
initial mass
Part II
6. While
waiting for the potatoes and carrots to interact with their solutions, prepare
3 wet mounts of Elodea and 3 of red onions. Place the first in tap
water, the second in a 10% salt solution and the 3rd in distilled
water. Observe under high power of the microscope after 5 minutes.
7. Record
pictures of each in the spaces below.
Data Tables
|
Treatment (Solution) Potato |
Initial Mass (grams) |
Final Mass (grams) |
|
Distilled
Water 0% |
|
|
|
Tap Water
0.1% |
|
|
|
1% salt |
|
|
|
10% salt |
|
|
|
20% salt |
|
|
|
Treatment (Solution) Carrot |
Initial Mass (grams) |
Final Mass (grams) |
|
Distilled
Water 0% |
|
|
|
Tap Water
0.1% |
|
|
|
1% salt |
|
|
|
10% salt |
|
|
|
20% salt |
|
|
|
Treatment (Solution) Potato |
Initial Mass (grams) |
Final Mass (grams) |
|
Distilled
Water 0% |
|
|
|
Tap Water
0.1% |
|
|
|
1% sugar |
|
|
|
10% sugar |
|
|
|
20% sugar |
|
|
|
Treatment (Solution) Carrot |
Initial Mass (grams) |
Final Mass (grams) |
|
Distilled
Water 0% |
|
|
|
Tap Water
0.1% |
|
|
|
1% sugar |
|
|
|
10% sugar |
|
|
|
20% sugar |
|
|
Elodea in
tap water
Onion in tap water
Elodea in
10% salt Onion in 10% salt
Elodea in
Distilled water
Onion in Distilled Water
Calculations
and Questions
1. Graph
the percent solution (x-axis) vs. percent change (y-axis) for each of the
situations. You may use Create a Graph
http://nces.ed.gov/nceskids/graphing/
2. Define
the following terms: solute, solvent, isotonic, hypertonic, hypotonic, tonicity.
3. Explain
in terms of tonicity why your skin doesn’t wrinkle when swimming in the ocean,
but instead you become very thirsty.