Download AP lab 1 Osmosis

AP lab 1 Osmosis
Laura Bushwitz and John Doe
September 26, 2008
AP Biology
East Ridge High School
Abstract
The purpose of this laboratory exercise is to demonstrate the process of osmosis using
dialysis bags (representing a semi permeable membrane) and solutions containing starch,
glucose, and known concentrations of sucrose. In exercise 1A a starch/glucose solution
was placed in a dialysis bag. The bag was placed in distilled water with IKI solution. The
exercise should show smaller particles diffusing down a concentration gradient while larger
particles are unable to do the same. The exercise showed IKI diffusing into the dialysis bag
after 30 minutes and glucose diffusing out of the dialysis bag after 30 minutes. This
demonstrated how pore size in a selectively permeable membrane allows specific
substances into/out of cells. In exercise 1B, different molar solutions of sucrose were
placed into dialysis bags and massed. The bags were placed into distilled water for 30
minutes and massed after the incubation time. The exercise should show an increased rate
of osmosis into the dialysis bags as the solute concentration in the bag increases. The
percent change in mass was calculated showing how an increase in molarity was
proportional to an increase in percent mass change. In exercise 1C potato cores were
massed and placed into different molar solutions of sucrose (by class groups) overnight.
After incubation, the potato cores were massed and the information was used to calculate
the percent change in mass (for each molarity). The exercise should demonstrate the
movement of water into/out of cells relative to the solute concentration in the solution vs.
the solute concentration in the cell. The percent change in mass was graphed and used to
calculate the equivalent molarity of the potato core. In exercise 1D, the calculated molarity
for the potato core was used to calculate the water potential for the potato core. Data was
also used to calculate water potential for zucchini. Inferences were used to determine how
water potential would affect RBCs in distilled water.
Introduction
In the natural world, molecules will always move form an area of high concentration
to an area of low concentration. This phenomenon is known as diffusion. In living things,
cell membranes prevent most molecules from simply diffusing down their concentration
gradient. This allows the living thing (the smallest of which is a cell) to maintain an internal
environment that is different and separate from the external environment. To maintain that
different and separate internal environment, living things need to be able to selectively
allow molecules into and out of their cells. Very small, uncharged molecules can move
in/out of cells via simple diffusion. Most molecules, because of size and charge, will move
in/out of cells through proteins found in the cell membrane. These proteins are specific to
the molecules they move making the cell membrane selectively permeable. Very small,
uncharged molecules can move in/out of cells through the cell membrane without the help
of these proteins.
Osmosis is the diffusion of water through a selectively permeable membrane. If
solutes cannot move down their concentration gradient due to a selectively permeable
membrane, water will diffuse down its concentration gradient (from an area where there is
more water to an area where there is less water). This movement of water through the
selective permeable membrane, such as a cell membrane, is called osmosis. While water
can simply osmosis through the phospholipids membrane in very small amounts, because it
is a slightly charged polar molecule, water moves quicker through the cell membrane via
proteins called aquaporins.
Cells naturally have a particular concentration of solutes they must maintain
internally. The concentration of solutes inside a cell can be affected by osmosis. As water
enters a cell, the concentration of solutes decreases and as water leaves a cell, the solute
concentration increases. Osmosis, therefore, is related to the concentration of solutes inside
a cell relative to the concentration of solutes outside of a cell. If there are more solutes
inside a cell, osmosis will move water in and if there are more solutes outside a cell,
osmosis will move water out. Solutions with a higher solute concentration than cells are
called hypertonic. Cells placed in hypertonic solutions will lose mass as the result of
osmosis, water moving out of the cell towards an area of lower water potential. Plant cells
in hypertonic solutions will become flaccid and eventually experience plasmolysis as their
cell membranes will pull away from their cell walls. Animal cells in hypertonic solutions
will shrivel up and experience crenation. Solutions with a lower concentration of solutes
than cells are called hypotonic. Cells placed in hypotonic solutions will gain mass as a
result of osmosis as water moves into the cell down its concentration gradient (from an area
of high water potential to an area of lower water potential). Plant cells placed in a
hypotonic solution will become turgid as their central vacuole fills to maximum with water.
Plant cell walls prevent plant cells from being able to ‘overfill’. Animal cells placed in
hypotonic solutions will gain water and eventually experience lysis before they are able to
reach equilibrium. The water potential (calculated as ψ = iCRT*) can be used to determine
which way water will move. Water will always move from the area of higher water
potential to the area of lower water potential.
*i = ionization constant (1.0 for sucrose)
C = molar concentration
R = pressure constant (R = 0.0831 liter bars / mole K
T = temperature (K = ºC + 273)
Methods and Materials
In exercise 1A 15% glucose/1% starch solution was placed into a bag made from dialysis
tubing. The glucose/starch solution represented particles of small/large size relative to the3
pores in the dialysis tubing. The dialysis tubing represented a semi permeable membrane
that selectively allows materials to move through it via diffusion. The dialysis bags were
placed into a beaker of distilled water with IKI solution. IKI wick react with starch to form
a color change. Prior to placing the full bag into the distilled water with IKI, the water and
the bag were tested for the presence of glucose with clinistix reagent. The colors of the
solution in the bag and the solution in the beaker were also recorded. The bag was allowed
to incubate at room temperature for 30 minutes. At the end of 30 minutes, the bag and water
in the beaker were tested for the presence of glucose. The color of the solution in the bag
and in the beaker were also recorded.
In exercise 1B, sucrose solutions of 0.0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1.0M were placed
in dialysis bags. The bags were massed and then placed into a beaker of distilled water.
They were allowed to incubate at room temperature for 30 minutes. At the end of 30
minutes the bags were massed again and the percent change in mass was calculated using
the formula final mass – initial mass / initial mass X 100.
In exercise 1C, potato cores were cut using a potato corer. The cores were massed and
placed into a sucrose solution of known molarity (either 0.0M, 0.2M 0.4M, 0.6M, 0.8M,
1.0M). *note different lab groups were assigned different molarities. The cores were left to
incubate at room temperature in the known solution overnight. The next day, the potato
cores were massed again and the percent change in mass was calculated (using the formula
above). The percent change in mass was graphed and used to determine the equivalent
molarity of the potato cores.
In exercise 1D, the calculated potato core molarity from 1C was used to determine the
water potential of the potato core using the formula solute potential = -iCRT with i = 1
(constant for non-ionizing sucrose) and C = the molar concentration and R = pressure
constant (0.0831 liter bars/mole K) and T = temperature K (K = 273 + C). Water potential
= pressure potential (0) + solutes potential. Data was provided to calculate the water
potential of zucchini using the same formula.
Results
Table 1.1: Dialysis bag results
bag
beaker
glucose/starch
solution
H2O IKI solution
color
initial
color
final
glucose
initial
glucose
final
clear
lt brown
dk blue
lt brown
yes
no
yes
yes
Table 1.2 : dialysis bag results – group data
contents in dialysis
bag
initial
mass
final
mass
difference
% change in
mass
0.0 M distilled water
0.2 M sucrose
0.4 M sucrose
0.6 M sucrose
0.8 M sucrose
1.0 M sucrose
14.1
14.7
15.1
15.3
15.5
15.6
14.2
15.1
15.7
16.1
16.5
16.7
.1
.4
.6
.8
1.0
1.2
.7
2.6
3.8
5.0
6.0
7.2
Table 1.3: Percent change in mass : Class data
dialysis bag
results
molarity of sucrose
group 1
group 2
group 3
group 4
group 5
group 6
group 7
group 8
group 9
0.0 M
1.60%
5.90%
0.10%
.7%
0.30%
6.22%
0.61%
2.70%
0.15%
0.2M
5.30%
7.90%
5.60%
2.6%
0.80%
5.74%
1.64%
7.70%
3.52%
0.4M
8.50%
11%
6%
3.8%
4.90%
9.20%
3.59%
9.20%
8.89%
0.6M
8%
9.40%
9.20%
5.0%
7.20%
17.65%
5.86%
18.11%
10.08%
0.8M
12.50%
38.10%
8.80%
6.0%
11.60%
11.85%
6.74%
16.82%
15.53%
1.0M
12.60%
22.80%
7.20%
7.2%
11.90%
22.85%
8.79%
19.54%
18.01%
Graph 1.1 Percent Change in Mass for Dialysis Bags in Different Sucrose Molar Solutions
Dialysis bag percent change in mass
8
% change in mass
7
6
5
4
3
2
1
0
0.00
0.20
0.40
0.60
0.80
1.00
sucrose molar concentration
Table 1.4 Percent change in mass Potato core: Class data
% change in mass potato core
results
group
2
group 1
0.0M
22.50%
1%
47%
12.40%
20.725
0.2M
11.90%
22%
14.70%
12.18%
0.4M
-8.40%
-3%
14.40%
0.73%
0.6M
-12%
15.00%
22.30%
0.12%
0.06%
0.11%
0.31%
-10%
27.00%
1%
-5%
13.80%
-7.13%
0.00%
-25%
10.50%
-9.20%
0.8M
1.0M
group 3
group 4
group 5
class
average
molarity of sucrose
Graph 1.2 Percent Change in Mass for Different sucrose Molarity
% change mass
Percent Change Mass Potato Core
20.00
15.00
10.00
5.00
0.00
-5.00
-10.00
-15.00
-20.00
1.0
0.8
0.6
0.4
0.2
0.0
sucrose molarity
Graph 1.3 Percent Change in Mass of Zucchini Cores
% change mass
Percent Change Mass Zucchini
20.00
15.00
10.00
5.00
0.00
-5.00
-10.00
-15.00
-20.00
-25.00
-30.00
0.0
0.2
0.4
0.6
sucrose molarity
0.8
1.0
Discussion
Data table 1.1 show glucose diffused out of the dialysis bag and IKI diffused into
the dialysis bag. This is evidenced by the presence of glucose outside of the bag after the
incubation period while no glucose appeared outside of the bag before the incubation
period. The dialysis bag represents the cell membrane. It is selectively permeable in that it
will only allow substances that can fit through the pores to diffuse. Because glucose
diffused out, the pores in the dialysis bag selectively allow glucose to move naturally down
its concentration gradient. In addition, IKI molecules diffuse down their concentration
gradient from the beaker into the dialysis bag. The pores in the bag are small enough to
allow IKI to move naturally down their concentration gradient. This is evidenced by the
change of the starch solution in the bag from clear to dark blue as the IKI reacts with the
starch. The color change due to the IKI/starch reaction occurs only inside the dialysis bag
indicating the pores in the dialysis bag are too small for the starch to diffuse through. The
dialysis bag, representing the cell membrane, is therefore selectively impermeable to the
starch molecules.
In exercise 1B the same selectively permeable membrane was used to demonstrate
the osmosis relative to solute concentration. Tables 1.2 and 1.3 show group data and class
data of the mass and percent change in mass of dialysis bags with varying molar solutions
of sucrose. The dialysis bags, representing the semi-permeable cell membrane, were filled
with sucrose molar solutions of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 and an initial mass taken.
Because the pores of the dialysis bag are impermeable to the large molecule sucrose, the
sucrose remained in the dialysis bag. This created a higher solute concentration inside the
dialysis bags causing water to osmoses into the bag through the semi-permeable membrane.
Dialysis bags filled with increasing concentrations of sucrose show an increase in mass
relative to the molar concentration. As the molar concentration increases, the percent mass
of the bag increases. This trend in increase can be seen in Graph 1.1. Percent mass was
calculated to account for any potential differences between initial volumes in each bag.
In exercise 1C the water potential for potato cores was calculated by placing potato
cores in different molar solutions of sucrose. Water potential is the tendency for water to
osmoses from an area where there is more water (relative to the solute) to an area where
there is less water (relative to the solute) because the solutes in the potato cells are unable
(too large or charged) to move through the selectively permeable cell membrane. If a
potato core is placed in a hypotonic solution in which the water potential outside of the
potato is greater than the water potential in the potato, the water will move into the potato
causing an increase in the potato mass. If a potato core is placed in a hypertonic in which
the water potential outside the potato is less than the water potential inside the potato, the
water will move out of the potato causing a decrease in potato mass. The data on table 1.4
indicates the percent change in mass of potato cores placed in varying molar concentrations
of sucrose. The data indicates water moved into the cells at lower molar concentrations and
out of the cells at higher molar concentrations.
By graphing the percent change in mass of potato cores placed in varying molar
concentrations of sucrose (graph 1.2), the water potential of the potato can be determined
by the point at which the data line crosses 0% change as that would be the point of
equilibrium where water is entering and leaving the cell at the same rate. Equilibrium can
only occur when the solute concentrations inside and outside the cell are the same,
representing the relative water potential of the potato cells. Living cells that are not in
isotonic solutions (solute concentrations are the same inside and outside of the cell) will
have water osmoses into or out of the cell causing animal cells to experience lysis or
crenation (respectively) and plant cells to become turgid or experience plasmolysis
(respectively).
Graph 1.3 shows data related to zucchini cores placed in the same molar
concentrations of sucrose. As with the potato core data, the point at which the data line
crosses 0% mass change represents the water potential of the zucchini. This water potential
can also be calculated using the formula for solute potential = -iCRT as the solute potential
is equal to the water potential.
i (ionization constant for sucrose =1.0, C = .35 (molar solution at which the data line
crosses 0% change), R = pressure constant 0.0831 liter bars/mole K, and T = K (273 + 22
C). This makes the water potential for the zucchini – 1 x .35 x 0.0831 x 295 (the units will
cancel out leaving bars) or –8.58 bars.