Question

Lab 2A:

Introduction

In this lab, you will be studying osmosis across the membrane of potato cells. You will create equal-sized pieces of potato and then place the pieces in solutions with differing concentrations. You will then, over time, measure any changes that take place in the size and weight of the potato pieces.

To measure changes in size, you will need either a gram scale (like a food scale) or a millimetre ruler. It is best if you have a scale that measures at least tenths of a gram. If not, measure the dimensions with a millimetre ruler.

Materials

• potatoes
• coarse salt or sea salt or pickling salt (use table salt only if you cannot obtain one of the other types)
• water (allow it to stand overnight)

Apparatus

• clear plastic millimetre ruler or food scale
• 5 containers (glasses) in which to make salt solutions
• round metal tube (about 1–2 cm or 0.5 inches in diameter) or a knife (if you have one, a French fry cutter may be a very good tool)

Method

1. Prepare your laboratory space and wear appropriate safety equipment.
2. Select 5 glass containers of the same size. Glasses will serve very nicely here. Prepare the following salt solutions:

In each container place 250 mL (½ cup) of water and label the containers 1 through 5.

1. Container 1—add nothing to it
2. Container 2—place about 1 mL (¼ level tsp) salt into the water
3. Container 3—place about 2 mL (2 x ¼ level tsp) salt into the water (twice as much salt)
4. Container 4—place about 3 mL (3 x ¼ level tsp) salt into the water (three times as much salt)
5. Container 5—place about 4 mL (1 level tsp) salt into the water (four times as much salt)

3. Stir the solutions to make sure all the salt is dissolved before going any further.
4. Select a potato that is at least 10 cm (4 inches) long. Peel the potato (you may have to use more than one potato).
5. The objective here is to make pieces of potato that are, as nearly as possible, the same size or weight.

1. If you have a hollow metal tube (like a piece of copper tubing) that you can use to make potato cores, make six potato cores that are at least 10 cm long. You may also find that a French fry cutter will work. If you do not have a tube or cutter, use a knife to make pieces of potato that are 1½ cm (¾ inch) by 1½ cm by at least 10 cm (4 inches) long.
2. If you are using a food scale, try to make the potato pieces about the same weight.

6. Cut the potato pieces into 5 cm (2 inch) lengths. Try to make sure that they are the same length and that they will fit in the glass container and be completely covered by the water or salt solutions.
7. Record either the weight (preferable) or the dimensions of the potato pieces (bear in mind as you do this that you want to be able to measure changes in the size of the potato pieces). Record the measurements in Table 2.1.1.
8. Place a potato piece in each of the labelled containers. You will make observations over the next several hours. Note the time at which the experiment was started.
9. After 30 minutes:

1. Carefully remove a potato piece from its container. If you are using a scale, remove excess solution from the surface of the potato piece before weighing it. If you are using a ruler, use the millimetre ruler to measure the dimensions of the potato core. Do the measurements as quickly as possible and place the piece back into its solution. Record the results in Table 2.1.1.
2. Repeat until all the pieces have been measured.

10. Repeat the measurements after one hour. If the measurements are the same as for the previous time measured, the experiment is finished. Go to step 12.
11. If the measurements are not the same, repeat the measurements again in one hour. You should not have to repeat this beyond two hours.
12. Remove the potato pieces from the solutions. Make observations about the “feel” of the potato pieces (compared to the original potato). Use words like turgid and flaccid, hard and soft, crisp and squishy. Also, note any change in colour. Record your observation in Table 2.1.2.
13. Clean up your laboratory space and put equipment away. You may discard the solutions down the drain.

Table 2.1.1. Changes observed in weight or length of potato pieces immersed in salt solutions of varying concentrations over time (one to two hours) Change in Measurement Measurements (Weight or Length) at Each of Three or Four Times Measured (Hours) Potatoes in Salt Solution (Weight/Length) 0,0 0.5 1,0 2.0 A (no salt) B (1 mL salt) C (2 mL salt) D (3 mL salt) E (4 mL salt) Table 2.1.2. Observations of potato pieces immersed in varying solutions of salt water Solution Strength Description Potato A (0 mL salt) Potato B (1 mL salt) Potato C (2 mL salt) Potato D (3 mL salt) Potato E (4 mL salt) A selectively permeable membrane is required for osmosis to occur. Where is the selectively permeable membrane in this experiment (consider each potato cell)? 1.

Answer 1

QUESTION 1

1. A selectively permeable membrane is required for osmosis to occur. Where is the selectively permeable membrane in this experiment?

First, we will define what osmosis is. This process refers to the movement of solvent molecule from a lower solute concentration to a higher solute concentration through a semipermeable membrane. Potatoes contain water and starch which means that If the potatoes is immersed in a water solution, the potato will gain water. If potatoes are immersed in an concentrated dilution, they will lose water. In this case the semipermeable membrane is the cell surface membrane of the cell potatoes