EFFECTS OF OSMOSIS ON LIVING TISSUE

Purpose

To measure the effect of osmosis on potato tissue and use this data to estimate the concentration of dissolved substances in the cytoplasm of a cell.

Background

The plasma (cell) membrane acts as a barrier between the external environment and the cytoplasm within. It controls which substances may enter or leave the cell, by a number of processes.

Water, the major component of cytoplasm and extra-cellular fluids, can diffuse back and forth through membranes with little difficulty. This process is known as osmosis. The direction of motion is regulated by the relative concentrations of dissolved substances inside and outside the cell.

Pre-lab Activities

Before proceeding with this investigation you must become familiar with the scientific terms used to describe activities or conditions associated with living membranes.

a) Clearly define each of the following terms: permeable; semi-permeable; non-permeable; diffusion; isotonic; hypertonic; hypotonic.

b) What will happen to the size of a cell if water enters it? Leaves it? Explain.

Materials6 Petri dishes/test tubes

·  Fresh potato sticks

·  Dissecting razor blade

·  Ruler

·  Distilled H20

0.4% NaCI solution

0.8% NaCl solution

1.2% NaCl solution

1.6% NaCl solution

2.0% NaCI solution

Procedure

Note: Work as a team of three. One member should do steps 1 and 2 while the others are doing steps 3 to 6.

1.  Obtain 6 Petri dishes or test tubes. Label the bases and tops as follows: distilled water; 0.4% NaCl; 0.8% NaCl; 1.2% NaCl; 1.6% NaCl; 2.0% NaCl.


2. Add enough of the appropriate liquids to nearly fill each of the Petri dishes.

c) Which solution has the highest percentage of water? The lowest? Explain.

3. Obtain six fresh potato strips. Using a razor blade, angle cut one end of each strip to a beveled point.

CAUTION: Use extreme care when cutting with a razor blade or scalpel.

4. Measuring from the tapered end, carefully and precisely angle cut each strip to a length of 70 mm so that both “points” are on the same side of the strip.

5. Place one measured potato tissue strip into each of the prepared Petri dishes. Cover the dishes and leave undisturbed for 24 hours.

d) In general terms, predict what might happen to the potato tissue in the different solutions. Explain.

e) Enter the test solutions (include the distilled water) and the initial length (li) of the potato strips under the appropriate heading. Measure in tenths of millimeters

Test Solutions (% salt) / Initial Length
li
(mm) / Final Length
lf
(mm) / Change in Length
Δl
(lf - li)
(mm) / Rigidity of the Strip
Distilled
0.4
0.8
1.2
1.6
2

6. After 24 hours, remove the strips of potato, one at a time, immediately measuring the length (mm) of each.

f) Record the final length (lf). Measure in tenths of millimeters.

g) Record any variations in the rigidity of the strips relative to the concentration of the solutions. Compare with the strips before they were placed in the solutions. Use “more,” “similar” and “less” to describe the condition.

h) Calculate and record the change in length (Δl) of each strip in millimeters. Indicate an increase in length with a positive (+) sign and a decrease with a negative (-) sign.

7. Along with each of the other lab groups, record your Δl data on the blackboard.

i) Record the group data, concentration then calculate the average Δl value for each

Group / Δl (mm) Distilled H20 / Δl (mm) 0.4% NaCl / Δl (mm) 0.8% NaCl / Δl (mm) 1.2% NaCl / Δl (mm) 1.6% NaCl / Δl (mm) 2.0% NaCl
1
2
3
4
5
6
7
8
9
10
11
12
Average Δl (mm)

8. Graph your own results by plotting the change in length against the concentration of the solution. It will be necessary to draw your “zero” line half-way up the y-axis in order to plot points indicating an increase (+) or decrease (-) in length

j) Which is the manipulated (independent) variable?

k) Which is the responding (dependent) variable?

9. Draw a straight line through the points (line of best fit).

l) Record the point (concentration) at which the line intersects the zero axis.

m) What information is provided by the intersection point in (mm)?

10. Use the class averages to prepare a second graph of the Δl data.

n) Using the techniques described in step 12, plot the points on the same graph as your own data. Use a different color when drawing in the straight line through the points.

o) Make a legend at the bottom of the graph which clearly indicates what each color represents.

p) Record the point where the line crosses the zero axis.

q) Explain what would probably happen to the length of a potato strip placed in a solution having a concentration equal to the value recorded in (q).

Questions for Analysis:

1. From your investigation, is there supporting evidence to suggest that water has entered or left the potato cells? Explain.

2. How can osmosis account for the changes in length of the potato tissue?

3. What term is given to a solution that has the same concentration as the cytoplasm of a cell?

4. From your graph, how can you determine when the water concentration inside and outside the cells is similar? Explain clearly.

5. Which would provide the more reliable information, the class average graph or your own data graph? Explain your reasoning.