osmosis experiment case

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8.2: Lab - Osmosis and Types of Solutions

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Laboratory Preparation Guide

Equipment and materials.

Prepare 12 sets of equipment and materials for 24 students per class section. Include a few more for backup if needed. Each set should include

• Computers and internet access
• Potato cutter, knives
• Five 150 or 250 mL beakers
• Paper towels
• Glass stirring rods
• Potatoes should be provided by lab staff.
• Instructors will either pre-cut into disks of proper size or will instruct students in proper cutting technique.
• Each group will need 5 potato disks.
• Deionized water
• Can just prepare stocks of these. Students will need approximately 50 mL of each solution per group.
• Muddy water
• Orange juice with pulp

Waste Disposal

• Solutions, colloids, and suspensions provided should be kept in or returned to their containers for future use.
• \(\ce{NaCl}\) solutions can go down the drain.
• Used potatoes can go in the trash.

Learning Objectives

• Recognize a solution and its composition of a solute and solvent.
• Define the process of osmosis and express its importance to biological systems.
• Distinguish homogeneous colloidal solutions from heterogeneous suspension mixtures.
• Differentiate and identify isotonic, hypertonic, and hypotonic solutions.
• Apply osmosis to red blood cells and understand the impact that various substances have on the human body when introduced via intravenous (IV) solution delivery.

Laboratory Skills

• Measure mass quantities of affected substances.
• Use Excel to plot experimental data.
• Salt solutions: 0.5% (w/v), 2% (w/v), 5% (w/v) \(\ce{NaCl}\), and 10% (w/v) \(\ce{NaCl}\)
• 5 150-mL or 250-mL beakers
• Stations of labeled mixtures for identification

Safety and Hazard Information

Background information, osmosis and solutions.

In order to understand osmosis, you must first be able to define and understand the concepts of a solution, solvent, and solute. A solution is a homogeneous mixture of a solvent and one or more solutes. A solvent is the compound that dissolves or surrounds the solute, and is found in the greatest amount. A solute is the component(s) that is in the lesser quantity. Osmosis is the movement of solvent molecules from a region of low solute concentration to a region of higher solute concentration through a semi-permeable membrane. Semi-permeable means certain substances are allowed to go through, and certain substances are not. Within the human body, cells live within a fluid environment that can contain a combination of dissolved particles, such as sodium and potassium salts. Cells have a semi-permeable membrane that separates the intracellular fluid (inside the cell) from the extracellular fluid (outside the cell). Osmosis between intracellular and extracellular fluids depend on the number and types of dissolved particles inside and outside of the cell.

In the experiment you will do here, the cell walls of a potato will behave as a semi-permeable membrane. You can assume for this lab that the semi-permeable membrane on the potato will behave exactly like the semi-permeable membranes on the cell walls in your body.

At some point in time, the solvent will no longer freely travel only in one direction (towards the higher solute concentration side). This is due to osmotic pressure. Osmotic pressure is the pressure applied to a solution that prevents flow of solvent across a semi-permeable membrane. Osmotic pressure is the basis of filtering by reverse osmosis, a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure that is exerted by the water with solutes (“impurities”) dissolved in it. This allows the water molecules, but not the solute particles to pass through a membrane. Reverse osmosis works so well to purify water, it can produce fresh water from salty ocean water!

You will observe the effects of placing a potato (representing a biological cell) into solutions containing different solute concentrations. Since the concentration of salt (\(\ce{NaCl}\)) in a biological cell is 0.90%, solutions with both higher and lower \(\ce{NaCl}\) concentrations than this baseline will be utilized. These solutions are categorized as hypotonic, hypertonic, or isotonic. A hypotonic solution is a dilute solution that has a salt concentration lower than that of the cell. Since the solvent will travel towards the area that has the higher solute concentration, the cell will gain water through osmosis and will, therefore, swell up (hemolysis). A hypertonic solution is a concentrated solution that has a salt concentration higher than that of the cell. The cell placed in this solution will lose water through osmosis and the cell will shrivel (crenation). An isotonic solution is a solution that has exactly the same salt concentration as the cell. There will be no net movement of water across the cell membrane and the cell will experience no change in size. Figure \(\PageIndex{1}\) (below) depicts these various solutions as they relate to animal and plant cells.

You will measure the mass of the potatoes both before and after allowing them to soak in the solutions. After the experiment, you will evaluate the percent change in the mass, as in Equation \ref{1}.

\[ \% \text{ change} = \frac{\text{final mass } - \text{ original mass}}{\text{original mass}} \times 100 \label{1}\]

Please note that if the potato decreases in size, the percent change will be negative. Likewise, if the potato increases in size, the percent change will be positive.

Types of Mixtures

A mixture is a combination of two or more pure components. You are very used to one type of mixture, solutions, which was discussed in the previous section. In a solution , very small solute particles are dissolved in the solvent (usually water) to give a homogeneous solution. There are, however, many other mixtures that you encounter everyday where the solute and solvent are distinguishable. These are colloids (blood, Mayonnaise, and hairsprays) or suspensions (Kaopectate, antacids and liquid penicillin). Colloids are similar to solutions in that they are homogeneous and they do not separate or settle out. The difference is that the size of solutes in colloids is larger than that of particles in a solution. Suspensions , on the other hand, are heterogeneous mixtures with particles so large that gravity causes the particles to settle out in a solution. All of these types of mixtures have important applications in chemistry, industrial and clinical settings.

In this lab, you will observe five mixtures that you will identify as solutions, colloids, or suspensions. You will base your designation on observations about particle size and settling.

Special Instructions (if any)

All samples (solids and liquids) in this experiment should be retained for reuse, except water.

Part \(\PageIndex{A}\): Osmosis with Potato

1. Cut five potato disks using the tools provided for you. For best results, the disks should be circular, 2-3 cm in diameter, and 0.3-1 cm in height.

2. Place each potato disk on a paper towel labeled with the salt concentration (0, 0.5, 2, 5, or 10 %) to avoid mixing up the disks.

3. Record observations about the appearance and texture of each disk in Data Table \(\PageIndex{1}\).

4. Measure the mass of each disk and record in Data Table \(\PageIndex{1}\).

5. Place each disk in a beaker and add the desired concentration of \(\ce{NaCl}\) solution until the disk is completely covered. Note that one of the solutions is 0% \(\ce{NaCl}\), which is deionized water.

6. Allow the potato slices to soak for 45 minutes. Make sure the disks are always completely covered with the solution. If disks are floating, you may put a glass stirring rod on the disks to hold them under the liquid.

(While waiting, this is the time to start working on Part \(\PageIndex{B}\))

7. After soaking, record observations about appearance and texture in Data Table \(\PageIndex{2}\).

8. Blot each disk with a paper towel to remove excess liquid. Measure the mass of each disk and record in Data Table \(\PageIndex{2}\).

9. Copy masses before and after soaking into Data Table \(\PageIndex{3}\).

10. Calculate the percent change for each potato disk using Equation \ref{1}. Show your work.

11. Prepare a graph showing % change in mass as a function of % \(\ce{NaCl}\).

Part \(\PageIndex{B}\): Identifying Mixtures

1. There are five stations in the lab each with a set of mixtures labeled 1 to 5. Move to each station, observe each mixture and fill in Data Table \(\PageIndex{4}\) with the name, type of particles (small, medium, or large), and whether the particles settle (yes or no). Using these observations, classify each mixture as a suspension, colloid, or solution.

Please note that a good time to do this section is while you are waiting for your potatoes to soak in Part \(\PageIndex{A}\), Step 6.

Experimental Report

Data Table \(\PageIndex{1}\): Potato Masses BEFORE Soaking in Solutions

Data Table \(\PageIndex{2}\): Potato Masses AFTER Soaking in Solutions

*** Copy the measured masses from Data Tables \(\PageIndex{1}\) and \(\PageIndex{2}\) into Data Table \(\PageIndex{3}\). ***

Data Table \(\PageIndex{3}\): Percent change by MASS

Calculate the percent change by mass for each of the potato disks. Show all work. Then copy your results into Data Table \(\PageIndex{3}\) above.

Make an XY-scatter plot using Excel showing % change in mass (\(y\)-axis) as a function of % \(\ce{NaCl}\) (\(x\)-axis). Use the computer program to graph your data and calculate the line of best fit through your five data points. Where the line of best fit crosses the horizontal zero line is the point at which the potato is isotonic with its surroundings, and is therefore the estimated salt concentration of the potato. Draw or submit your plot in the space below, or submit your Excel file, as instructed by your professor.

Isotonic point of the potato: ___________________% w/v \(\ce{NaCl}\)

Data Table \(\PageIndex{4}\): Data Collection and Analysis

Follow-up questions

Exercise \(\pageindex{1}\).

Is deionized water a hypertonic, hypotonic, or isotonic solution when in the human body? _______________________

Exercise \(\PageIndex{2}\)

Describe what you would theoretically expect to happen to the potatoes in each of the three solutions, assuming potatoes mimic the salt concentration of red blood cells (0.90 % \(\ce{NaCl}\)).

a. 20% \(\ce{NaCl}\) _________________________________________________________________

b. 0.90% \(\ce{NaCl}\) ________________________________________________________

c. Deionized water ________________________________________________________

Exercise \(\PageIndex{3}\)

Describe the effects of each of the below solutions on the textures and masses of the potato disks. In each case, explain what happened to the cells in the potatoes and in which direction the water flowed. Feel free to include drawings to aid in your explanation.

a. Deionized water (0% w/v \(\ce{NaCl}\))

a. 10% w/v \(\ce{NaCl}\)

Write a summary of what you observed for this part of the experiment (Part B), and what you have learned about how to classify mixtures.

Practical Applications – Critical Thinking Questions

A patient has been admitted to the hospital with severe dehydration due to constant vomiting and diarrhea for the past 3 days. The first thing the hospital staff needs to do for this patient is administer fluids intravenously, but the patient is demanding he only be given “pure” water, and is refusing any other IV fluids. You are responsible for patient education at this hospital. In order to get the patient to agree to be administered the 0.9% saline IV fluids he needs to rectify his dehydration, you must first educate him on what will happen if he exposes his cells to “pure” water. Explain to this patient why he should not receive pure water intravenously, using the concepts from this lab.

Kidney dialysis machines use osmosis to take over the filtering function of the kidneys. Dialysis machines use a semi-permeable membrane through which some small molecules can pass (such as water, salts, metabolites, and some toxins) but through which larger objects (such as proteins and blood cells) cannot. Using the concepts learned in this lab, explain how dialysis could be used to get toxins out of the blood and replace the filtering function of the kidneys.

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AP®︎/College Biology

Course: ap®︎/college biology   >   unit 2.

• Diffusion and osmosis
• Hypotonic, isotonic, and hypertonic solutions (tonicity)
• Osmosis and tonicity
• Water potential example
• Mechanisms of transport: tonicity and osmoregulation

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1.4: Diffusion and Osmosis

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• Brad Basehore, Michelle A. Bucks, & Christine M. Mummert
• Harrisburg Area Community College

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The cell theory states that all living things are composed of cells and that cells only arise from other cells. Some cells are fairly simple in structure, while others are extremely complex. For example, some organisms are unicellular—they exist as a single cell, while multicellular organisms are composed of many cells that form tissues and organs. In either case, all cells share some common properties: the presence of DNA, intracellular proteins that enable the cell to perform its functions, and a plasma membrane. Some cells, known as eukaryotic cells, also contain membrane-bound organelles that allow a more complex level of functioning.

Homeostasis is defined as the maintenance of a stable internal environment. In order to maintain homeostasis, cells continually transport substances in and out across their cell (plasma) membrane.

The cell membrane serves as a “gatekeeper” and is the cellular structure that regulates the transport of materials into and out of the cell. The phospholipid bilayer architecture of the cell membrane allows certain molecules to pass through while keeping others out, therefore the cell membrane is selectively permeable (or semipermeable) . Things that need to enter a cell for it to function properly include ions, nucleotides, sugars, oxygen, amino acids, water, vitamins, and some hormones. Cells also allow certain molecules like water, ions, and secreted proteins to leave. Additionally, cells must eliminate waste products like urea and carbon dioxide

In the following exercises, you will examine the semipermeable nature of the cell membrane. You will also explore the concept of tonicity , which refers to the solute concentration of a solution, and its inherent ability to influence the rate and direction of osmosis.

PART 1: DIFFUSION & OSMOSIS

Diffusion is the movement of molecules from an area in which they are high in concentration to an area in which they are low in concentration. Molecules move down a concentration gradient until they are equally distributed, or equilibrium is reached (Fig. 1). At equilibrium, there is no concentration gradient. Molecules still move once equilibrium is reached, but there is no net movement in any one direction.

Osmosis is a specific type of diffusion : the diffusion of water molecules across a semipermeable membrane. Like other molecules, water molecules diffuse down a concentration gradient, from an area of higher free water concentration to an area of lower free water concentration. This means that water will move across a semipermeable membrane, like the cell membrane, in the direction of the higher solute concentration. (In solution, high solute concentration = low free water concentration; conversely, low solute concentration = high free water concentration.) You will observe this concept in Part 2: Osmosis & Tonicity .

In living organisms, most substances are transported as solutes , dissolved in water, a solvent . For example, if we dissolve salt (\(\ce{NaCl}\)) in a beaker of water, salt (\(\ce{NaCl}\)) is the solute and water is the solvent. Examples of solutes in the human body include glucose, small proteins, and electrolytes like calcium and sodium ions. Waste products, such as \(\ce{CO2}\) and urea are also transported as solutes. Solutes are carried by body fluids, such as blood plasma, and pass into and out of cells through passive and active transport . In either case, the cell membrane will either inhibit or facilitate the process of diffusion: some molecules can easily diffuse across a plasma membrane and some cannot. For example, small, nonpolar molecules (such as \(\ce{CO2}\) and \(\ce{O2}\)) can cross a membrane by simple diffusion. Large molecules or polar molecules, however, cannot easily diffuse across a membrane. Cells must have specialized membrane-bound proteins that function to transport such substances across the membrane.

In Part 1: Diffusion & Osmosis , you will learn about diffusion and osmosis using dialysis membrane, a selectively permeable sheet of cellulose that permits the passage of water and small solutes, but does not allow larger molecules to diffuse across. This is because the membrane has microscopic pores that only allow small molecules through; anything larger than the size of the pores is prevented from crossing. Some of the solutes in this experiment, sucrose (\(\ce{C12H22O11}\)) and starch (\(\ce{(C6H10O5})n}\) are too large to pass through the pores of the dialysis tubing, but the solvent molecules (\(\ce{H2O}\)) and glucose (\(\ce{C12H22O11}\)), are small enough to pass easily.

Exercise 1: Molecular Weight and Diffusion Rate

Molecular weight is an indication of the mass and size of a molecule. The purpose of this experiment is to determine the relationship between molecular weight and the rate of diffusion through a semisolid gel. You will investigate two dyes, methylene blue and potassium permanganate.

Employing Steps in the Scientific Method:

• Record the Question that is being investigated in this experiment. ________________________________________________________________
• Record a Hypothesis for the question stated above. ________________________________________________________________
• Predict the results of the experiment based on your hypothesis (if/then). ________________________________________________________________
• Perform the experiment below and collect your data.
• Petri dish of agar semi-solid gel (Mueller Hinton agar plates, 150 x 15 mm) - make sure the agar has been allowed to come to room temperature
• Methylene blue solution (0.2% in 25% EtOH)
• Potassium permanganate solution (0.1% KMnO4)
• Small straws
• Small plastic metric ruler

1. Obtain a petri dish of agar

2. Take the plastic straw and gently stick it down into one side of the agar. Lift up the straw, withdrawing a small plug of agar. Repeat on the other side of the dish.

3. Using a 1mL transfer pipet, place a single drop of each dye into the appropriate agar well. (Fig. 2).

4. After 20 minutes, place a small, clear metric ruler underneath the Petri dish to measure the distance (diameter) that the dye has moved. Enter the data in Table 1.

5. Repeat step 4 at 40, 60, and 80 minutes.

• What is the relationship between molecular weight and the rate of diffusion? Explain. ________________________________________________________________
• Go back and look at your initial hypothesis. Does your data support this hypothesis? Explain. _______________________________________________________________

Extension Activity: (Optional)

The results of this experiment can be presented graphically. The presentation of your data in a graph will assist you in interpreting your results. Based on your results, you can complete the final step of scientific investigation, in which you must be able to propose a logical argument that either allows you to support or reject your initial hypothesis.

• Graph your results using the data from Table 1.
• What is the dependent variable? Which axis is used to graph this data? ______________________________________________________________________
• What is your independent variable? Which axis is used to graph this data? ______________________________________________________________________

Exercise 2: Diffusion Across a Membrane

• Dialysis tubing
• Plastic clips or string
• 5 x 400 mL beakers (or plastic cups)
• Electronic balance
• Weigh boats
• 15% Sucrose solution (MW sucrose = 342 g/mol)
• 30% Sucrose solution (MW sucrose = 342 g/mol)
• 30% Glucose solution (MW glucose = 180 g/mol)
• Graduated cylinders (10 mL and 100 mL)
• Wax pencil or sharpie
• 15% starch solution (MW = variable)
• Iodine solution (MW = 166 g/mol)
• Benedict’s reagent
• Hot plate or heat block
• Cut 5 pieces of dialysis membrane approximately 10 cm long. Soak the pieces in tap water until they are soft and pliable (3-5 minutes). *This step may be done for you; check with your instructor.
• Obtain 5 beakers (plastic cups) and label them #1 - 5. Fill each beaker with 150 mL of a solution as follows:
• Beaker #1 – H2O
• Beaker #2 – H2O
• Beaker #3 – H2O
• Beaker #4 – 30% sucrose solution
• Beaker #5 - H2O and 1mL Iodine solution
• Set beakers aside.
• Remove one piece of dialysis membrane from the soaking water and open it, forming a tube. Close one end of the tube with a plastic clip, a piece of string, or simply tie it with a knot (Fig. 3)

• Fill the bag with 10 mL of H2O. Remove excess air, and close the other end of the bag with a plastic clip, a piece of string, or tie with a knot. Set aside on a paper towel.
• Repeat steps 4 and 5 for the 4 remaining dialysis tubes, filling them with 10 mL of a solution as follows:

• Bag #2 – 15% sucrose

• Bag #3 – 30% sucrose

• Bag #4 – H2O

• Bag #5 - 5 mL 30% glucose solution and 5 mL 15% starch solution

• Rinse off the outside of the bags with water and carefully blot dry.
• Weigh bags #1 - 4 to the nearest 0.5g. Record the weights in Table 1 below, in the column labeled “0 min.”
• Place each bag in the corresponding beaker (Bag #1 in Beaker #1, etc.). Make sure each bag is fully submerged in the solution.
• Set a timer for 5 minutes.
• At the end of 5 minutes, remove bags 1 - 4 from their beakers, blot excess fluid, and record the mass (in grams) in Table 1.
• Return the bags to the appropriate beaker, and wait another 5 minutes.
• Repeat steps 11 - 13 every 5 minutes and record the weights in Table 1.

• Calculate the total weight change (weight change = final weight – initial weight) for each bag. Record the values in Table 2. Calculate the rate (g/min) of osmosis for each bag by dividing the weight change by the time change. Since all 4 bags were recorded for a total of 20 minutes, the time change for all 4 bags is 20 minutes. Record the rate of osmosis for all 4 bags in Table 2.

• Make observations about bag #5 and beaker #5 in Table 3.
• Remove several mL of liquid from bag #5 and beaker #5 and add each to separate test tubes.
• Add several drops of Benedict's solution to each of the two test tubes and heat to 100 degrees Celsius in a boiling water bath or heat block for 2 - 5 minutes. Record the test results in Table 3.

• Did the weight of each bag (#1 - #4) change significantly over 20 minutes? Explain.
• In which bag(s) was there a net movement of water?
• Explain what is meant by “net movement”.
• Which carbohydrate molecules (glucose, sucrose, starch) were not able to move across the membrane? Explain.
• In terms of solvent (water) concentration, water moved from the area of _______________ concentration to the area of __________________ concentration across a selectively permeable membrane, which is defined as ________________________.
• What can you conclude about the movement of Iodine, glucose, and starch across the dialysis membrane based on your results in Table 3? Support your answers for each with the observation from bag #5 and beaker #5.
• We used the dialysis tubing to simulate a cell membrane. How is the dialysis tubing functionally the same as a cell membrane?
• We used the dialysis tubing to simulate a cell membrane. How is the dialysis tubing functionally different from a cell membrane?
• Prepare a line graph using the data from Table 2.

PART 2: OSMOSIS & TONICITY

Tonicity is the relative concentration of solute (particles), and therefore also a solvent (water), outside the cell compared with inside the cell.

• An isotonic solution has the same concentration of solute (and therefore of free water) as the cell. When cells are placed in an isotonic solution, there is no net movement of water.

• A hypertonic solution has a higher solute (therefore, lower free water) concentration than the cell. When cells are placed in a hypertonic solution, water moves out of the cell into the lower free water solution.

• A hypotonic solution has a lower solute (therefore, higher free water) concentration than the cell. When cells are placed in a hypotonic solution, water moves into the cell from the higher free water solution.

IMPORTANT NOTE: Notice that all of the above definitions have ‘solution’ as the noun. Sometimes the noun will refer to the cell instead of the solution. For example, a hypotonic cell will experience a net movement of water out of the cell. What this means is that if the ‘solution’ is hypotonic, the cell is hypertonic and vice versa.

Exercise 1: Observing Osmosis in Potato Strips

• Wax pencil or Sharpie
• Metric ruler
• 10% NaCl solution
• 0.9% NaCl solution
• Forceps and scalpel
• Obtain a potato and use a cork borer to prepare 3 cylinders of potato. Push the borer through the length of the potato. When the borer is filled, use the flat end of a wooden skewer to gently push out the potato cylinder into the petri dish. Use the scalpel to cut each potato cylinder into a length of 5 cm.
• With a wax pencil or Sharpie, label 3 test tubes (#1, #2, #3).
• Using the metric ruler, mark each tube at the 10 cm mark level from the bottom of the tube.
• Tube #1 - distilled water
• Tube #2 - 10% sodium chloride (NaCl)
• Tube #3 - 0.9% NaCl
• Place one potato cylinder into each test tube and allow them to soak for about 15 minutes in the solutions.
• You can now move on to Exercises 2 and 3 while your potatoes soak.
• After the elapsed time, observe each strip for limpness (water loss, flaccid) or stiffness (water gain,turgor).
• Which tube contained the limp (flaccid) potato strip? Explain.
• Which tube contained the stiff (turgid) potato strip? Explain.
• Which solution is isotonic to the inside of the potato cell?
• What happened to the potato strip in the isotonic solution?

Osmoregulation in Living Cells

Some organisms, known as osmoregulators , have special adaptations to keep tight control over their internal osmotic conditions while still others, known as osmoconformers , are able to live in a variety of osmotic conditions. Most living cells, however, are often at the mercy of their surrounding osmotic environment. Many freshwater plants live in an isotonic or hypotonic environment, so they have no adaptations to protect them from a hypertonic environment. Likewise, mammalian red blood cells live in the isotonic plasma inside your circulatory system so they have no protection from either a hypertonic nor a hypotonic environment.

A plant cell is surrounded by a rigid cell wall, so when the cell is placed in a hypotonic environment, the net flow of water is from the surrounding medium into the cell, and it simply expands to the cell wall and becomes turgid. When the same plant cell is placed in a hypertonic environment, water leaves the central vacuole and the cytoplasm shrinks. This causes the cell membrane to pull away from the cell wall. In this situation, the plant cell will undergo plasmolysis and die. Animal cells have no cell wall so when they are in a hypotonic environment they will expand and fill with water until they burst in a condition known as lysis. Figures 4 and 5 demonstrate these conditions.

Exercise 2: Observing Osmosis in Elodea Cells

• Blank slides and coverslips
• 10% salt solution
• Elodea leaf
• Place a drop of water onto a clean microscope slide.
• Using the forceps, gently tear off a small leaf from the Elodea plant.
• Prepare a wet mount by placing the Elodea leaf into the drop of water on your slide.
• Place a coverslip onto the slide.
• Use the scanning (4x) objective to bring the Elodea cells into focus. You may not be able to observe individual cells at this power.
• Switch to the low power (10x) objective. The Elodea cell walls should be visible. They will look like dark green grid lines. Use the fine focus adjustment to focus the specimen.
• Once you think you have located an Elodea cell, switch to the high power (40x) objective and refocus using the fine focus adjustment.
• Next, add several drops of 10% salt (NaCl) solution to the edge of the coverslip to allow the salt to diffuse under the coverslip. Observe what happens to the cells (this may require you to search around along the edges of the leaf). Look for cells that have been visibly altered.
• Record your observations in the following table. The cells in distilled water should look similar to the figure below.

• Which solution is hypertonic to an Elodea cell? Use your observations to support your answer.
• Would you expect pond water to be isotonic, hypotonic, or hypertonic to Elodea cells? Explain your answer.
• Explain what happens to a plant cell that undergoes plasmolysis.

Exercise 3: Observing Osmosis in Red Blood Cells (Erythrocytes)

• Sheep red blood cells
• Obtain 3 test tubes and label them (#1, #2, #3) with a wax pencil or Sharpie.
• Tube #1 - Distilled water
• Tube #2 - 10% NaCl
• Using a new transfer pipette, add 2 drops of sheep blood to each tube and swirl gently to mix the contents.
• Hold each test tube up to a sheet of paper with printed text. Attempt to read the print through each tube and record your results in the following table.

• Label 3 microscope slides (#1, #2, #3) with a wax pencil or Sharpie.
• Prepare wet mounts of each tube by placing a drop of the solution in each tube (#1 - 3) on the appropriate microscope slide (#1 -3). Add a coverslip to each slide.
• View slide #1 through the microscope using the scanning (4x) objective first. Focus the image using the coarse adjustment. Then view the blood cells under low power and then high power. Only use the fine focus adjustment to focus the specimen.
• Observe slide #2 and slide #3 in the same manner.
• Record the appearance of the red blood cells in each solution in the following table.

• Which solution allowed you to read print through the solution? Explain.
• Which solution is hypertonic to the RBCs? Use your observations to support your answer.
• Which solution is hypotonic to the RBCs? Use your observations to support your answer.
• Which solution is isotonic to the RBCs? Use your observations to support your answer.

Questions for Review

• Define diffusion. What is the energy source for diffusion? Is diffusion considered an active or passive process? Explain.
• Name a molecule that diffused through the artificial membrane (dialysis tubing) that we used in the laboratory. Can diffusion occur without a membrane? Give an example to support your answer.
• What is osmosis? Is it an active or a passive process? Explain.
• A solution that has a lower solute concentration than another solution is said to be ______________________ when compared with the second solution.
• A solution that has the same solute concentration as another solution is said to be ______________________ when compared with the second solution.
• A solution that has a higher solute concentration than another solution is said to be ______________________ when compared with the second solution.
• What does it mean when a membrane is selectively permeable?

Practical Challenge

• Isotonic solution –
• Hypertonic solution –
• Hypotonic solution -
• The concentration of glucose inside Elodea cells is 5 mM. What is the solution in moles (M)? What will happen to an Elodea cell if it is placed in a 1 M glucose solution? Explain.
• Apply what you learned in the lab to explain why it is said that marine organisms, which live in saline environments, literally live in a desert environment.

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6 Lab 5. Diffusion and Osmosis

Lab 5—diffusion & osmosis.

• Practice applying hypothesis testing, and further your understanding of the basic principles underlying the scientific method and experimental design.
• Identify independent and dependent variables in an experiment.
• Describe diffusion and osmosis.
• Practice graphing data obtained in the lab and designing useful data tables.
• Understand and be able to define the following terms: diffusion, osmosis, concentration gradient, tonicity, hypotonic, hypertonic, isotonic, turgor, selectively permeable, semipermeable, osmotic pressure, turgor pressure, plasmolysis, hypothesis, prediction, theory, control, independent variable, dependent variable, controlled variable, quantitative variable, qualitative variable.
• Draw a “flow chart” (in your lab notebook) to diagram what you will do (and in what order) in this Lab.

Exercise A – Plasmolysis in green leaf cells

• Predict what you think will happen and why, when the fresh water around the leaf is replaced by salt water. Predict what will happen with respect to osmosis and also with respect to what will happen to the cell itself.

Exercise B – Osmosis and dialysis tubing

• Predict what will happen inside the tubing comparing the three concentration treatments.

Exercise C – ‘Osmosis Egg as Model Cell’

• What hypothesis is being tested by this “experiment”/demonstration?
• What is the dependent variable?
• What is the independent variable?
• Are they quantitative or qualitative?

EXERCISE A – INTRODUCTION

In this lab you will explore the processes of diffusion and osmosis.  Diffusion can occur across a semipermeable membrane, however diffusion also occurs where no barrier (or membrane) is present.  A number of factors can affect the rate of diffusion, including temperature, molecular weight, concentration gradient, electrical charge, and distance.  Water can also move by the same mechanism.  This movement of water is called osmosis .

• Please work in pairs for Exercises A and B and work in groups of 4 for Exercise C.
• BEFORE YOU BEGIN make a group plan about what you will do and who will be doing what.  Your Flow Chart that you made for the Prelab Assignment will be useful for this.  Note that one of the exercises requires using a microscope and each student should make their own microscopic observations and drawing.
• All data/observations must be written in your lab notebook.

Exercise A: Plasmolysis – Osmosis in a Living System (plant leaf)

Plant cells rely on pressure exerted outwardly by the fluid in the their vacuole to help them maintain their shape, much as a tire must be pumped up with air to provide the outward pressure that maintains the tire’s shape.  The pressure exerted by the vacuole’s fluid is called turgor pressure .  That pressure is maintained by the cell by adjusting the concentration of solutes in the vacuole’s fluid.  In this exercise you will not be testing a hypothesis.  Instead, you will observe what happens when you replace the fresh water surrounding a specimen of plant cells with a salt solution.

1) Obtain a small section of a leaf from an aquatic plant and put it on a clean slide.  Place a drop of water on the specimen on the slide, cover it with a cover slip, and examine the leaf first at scanning (40X), the low power (100X) and then at high power (400X).  Locate a region of healthy cells where there are only one or two layers of cells.  Be sure that you can clearly see the individual cells.  Sketch several adjacent cells at 400x in your lab notebook.  Remember to include total magnification and a title that identifies the organism.   Label the structures that you can see such as cell walls, nuclei, vacuoles, and chloroplasts.   DO NOT move the slide while doing the next step; you will want to observe the same cells.

2) While touching one edge of the cover slip with a piece of Kimwipe to draw off the water, add a drop of 15% salt solution to the slide next to the opposite edge of the cover slip. Be sure that the salt solution moves under the cover slip.  Observe how the cell responds. After a few minutes, sketch the same cells you sketched in before (in step a).  Label the cell structures again, including the cell or plasma membrane. Be sure to label both sketches “before adding saltwater” and “after adding saltwater”.

Include in your lab notebook answers to the following questions:

• Describe your observations (i.e. what happened?) when the water in which the cells were mounted was replaced by the salt solution. Refer to the cell structures that you labeled.
• Assuming that the cells have not been killed, what should happen if the salt solution were to be replaced by water? Describe what you would likely observe (i.e., make a prediction), and explain.
• Can osmosis likely cause plant cells to burst? Explain, comparing plant and animal cells.

For photomicrographs of images similar to what you observed in lab, you can do a “Google” image search for ‘Elodea plasmolysis’.  These images will not be the same as your observations and drawings that you must do from your slides.

EXERCISE B – INTRODUCTION

Exercise b: demonstrating osmosis using semi-permeable tubing.

The water movement through a membrane is called Osmosis .

All living cells are surrounded by a selectively permeable membrane, which contains transmembrane proteins with hydrophilic interiors (channels) to allow smaller polar molecules to pass such as water, ions, sugars and amino acids. Especially favored is the movement of water molecules through many aquaporins or water channels.

In this exercise, you will use synthetic or human-made membranes, which were first developed in Seattle, and used in artificial kidneys or dialysis machines. We will use them to demonstrate the osmotic movement of water molecules into concentrated sugar solutions.

Materials:  (per group of 2)

• One large tray
• One large beaker (500mL)
• Three 16cm dialysis tubes
• Ring stand with clamps x3
• Three 10mL Falcon Tubes
• Three stretches of thread or cord
• Green tubing clamp (flat)

In this portion of the lab, please work in groups of 2 at your lab bench.

• Cut three 16-cm-long strips from the dialysis tube. It is dry and flat and needs to be soaked in warm water.
• Get three 10 ml-sized Falcon Tubes and remove the lower tip with a razor blade or scissors while leaving a cone-shaped opening.
• You now push one open end of the wet dialysis tubing over the cone-shaped end of the Falcon Tube. It will resist at first but finally fit snugly over the tube since both have similar diameters. Wrap some thread or cord around the tube fixing it firmly to the Falcon tube.
• At the other end of the dialysis tube you fold it twice and clamp it closed with a green tubing clamp. This should be done so that you have an exposed length of dialysis tube of about 10 cm. Attach the Falcon tube to a ring stand and a clamping fixture.

Fill it slowly with tap water to test for stability and potential leaks.

• Repeat this procedure to produce two more identical sets.
• You are now ready to fill the three tubes with different solutions. One tube will contain a Pancake syrup (fully or 1.0 x concentrated), another tube with diluted Pancake syrup (0.5 x concentrated), and the third tube with water (as a control). Fill all three tubes to a similar height and mark the upper meniscus with black marker line.
• You are set to start the experiment by lowering the filled dialysis tubes completely into a large (500 ml) beaker with water. Record time zero in the table below.

DATA ANALYSIS & INTERPRETATION

• Graph the results with 3 curves (placet the dependent variable on the y axis). Make sure to include a legend to tell the differences between treatments.
• Why does the water flow into the dialysis tubes rather than the sugar flowing out and into the water of the beaker?
• If the Falcon tubes would be closed, the liquid level could not rise. What would happen in this case? Then, compare what might happen if this was an animal cell.

EXERCISE C – OSMOSIS & TONICITY

Materials:  (per group of 4)

• 1 large tray                               •  3 – 400 ml beakers
• 3 weigh boats                          •  1 electronic balance
• 3 decalcified eggs                   •  paper towels

In this portion of the lab, please work in the same groups of four at your lab bench.

There will be three solutions available in lab. Your objective is to determine the tonicity of these solutions relative to the egg “model cells”.   These solutions may be isotonic, hypotonic or hypertonic relative to the egg.  Models or simulations are used extensively in science to study phenomenon that may otherwise be difficult to study.  We will model cells by using eggs.  The egg shells have been removed (decalcified) by soaking them in vinegar, and the remaining egg membrane is permeable to water but not sugar.

Work over the large tray when you are handling the eggs!

Your group should choose three solutions for your three beakers.  You will dry and weigh the three eggs before placing them in the beakers.  As the egg sits in the solution, it will either gain weight, lose weight, or remain the same weight.  By weighing the eggs, you will be able to determine the tonicity of each unknown solution relative to each egg.  Each egg should be in solution at total of 30 minutes.  Your group will need to determine how often to weigh your eggs. You will need at least 5 measurements for each egg in each solution, more may be better.  For your measurements, you will gently remove from its solution (work over the tray) and weigh every X minutes.

Include the following in your lab notebook:

• What hypothesis is being tested by this experiment?  Remember that this should mention both the independent and dependent variables and explain the phenomenon.
• Remember to use the “If … then …” format.  Remember that the “then…” part is stated with respect to the dependent variable (what you predict will happen to the dependent variable). Explain why you made your prediction based on your knowledge from lecture, readings, etc.
• What is the dependent variable? Is it a quantitative or qualitative variable?
• What is the independent variable? Is it a quantitative or qualitative variable?
• Record all of your experimental data in the table(s) that you created for your prelab.
• Using the wax pencil, label beakers, to identify the solution(s), for example solution “A”, solution “B” or solution “C”. Fill each beaker half full (approximately 200 ml) with the appropriate solutions (A, B or C). Also label the threeweigh boats (A, B, or C) with a wax pencil.
• Carefully dry off the eggs. Handle the eggs one at a time OVER the tray throughout the experiment, in case the egg breaks!
• Carefully weigh each egg (A, B or C) in its weigh boat.
• Record the initial weights of the egg in solution A, B and/or C in data tables in your lab notebook (remember to include units).
• Carefully place each egg in its corresponding beaker. Note the position of each egg in its respective solution. Record the time at which you placed each egg in its solution.
• After each egg has been in its solution for the required amount of time, carefully remove the egg from the beaker. Working over the tray (in case the egg breaks), dry off the egg with paper towels, and weigh the egg in its weigh boat.
• Collect and record your data. Record the times and weights in the data table in your lab notebook.
• Note the final weight of each egg compared to the initial weight. What is the difference for each egg.

DATA ANALYSIS

• Complete your data tables (in your lab notebook).
• Graph these data by hand in your lab notebook. Each student must produce their own graph. Include the weight of each egg (in grams), time or time elapsed (in hours:minutes or minutes), a key, and an informative caption.

INTERPRETATION

• What are the tonicities of your three solutions relative to the solutions inside the cells? Explain your conclusion.
• What can you conclude about the rates of osmosis, based on your data and your graph?
• Is the rate constant (over 30 minutes or more) for each egg?
• What do you think would happen if you allowed the egg to sit in each solution for hours?

ERROR ANALYSIS

• What mistakes occurred? If none occurred, what mistakes could have occurred?
• What other sources of error can you think of, besides any mistakes?

LWTech General Biology (BIOL&160) Lab Protocols Copyright © by Lake Washington Institute of Technology is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Osmosis Experiment: Dissolving Egg Shells With Vinegar

How does osmosis keep you healthy.

Right now, as you read this, there are millions of things happening throughout your body. The food you ate just a bit ago is making its way through a watery slurry inside your stomach and small intestines. Your kidneys are working hard to excrete waste and extra water. The lacrimal glands near your eyes are secreting tears, which allow your eyelids to close without damaging your eyeballs. What’s one thing that all of these processes have in common? They all rely on osmosis: the diffusion of water from one place to another.

Osmosis factors heavily in each of these processes and is an important force for keeping every single cell in your body healthy. Osmosis is hard to see without a microscope. But if we create our very own model of a cell, using a shell-less chicken egg, we can see what happens when we manipulate the osmotic balance in the “cell”!

• 3 glasses (large enough to fit the egg plus liquid)
• 3 butter knives
• White vinegar (about 3 cups)
• Distilled water (about 2 cups)
• Light corn syrup (about 1 ¼ cups)
• Slotted spoon
• Measuring cup (1 cup)
• Measuring spoons (1 tablespoon and ½ tablespoon)
• Sticky notes and marker
• Scale (optional)

Note : It’s okay to touch the eggs, but remember to wash your hands afterwards to avoid any nasty surprises!

1. Place one egg in each glass. Pour in enough vinegar to cover each egg. Bubbles will start to form around the egg, and it’ll float up. To keep it submerged, put a butter knife in the glass to hold it down.

2. Put the three glasses in the refrigerator and allow to sit for 24 hours.

3. Gently holding the egg in the glass, pour out the old vinegar. Replace with fresh vinegar, and let sit in the refrigerator for another 24 hours. Repeat this process until the shells are fully dissolved and only the membrane remains. This should take about 2-3 days.

4. Gently remove the eggs using the slotted spoon and rinse with tap water in the sink. Rinse out the empty glasses as well.

5. Gently put the shell-less eggs aside for a moment on a plate.

6. Prepare three different sugar-water solutions as follows, labeling with sticky notes:

Glass 1: Label “hypertonic”. Pour in one cup of corn syrup.

Glass 2: Label “isotonic”. Add 1 ½ tablespoons corn syrup to the one cup measuring cup, and fill the remainder with distilled water. Pour into glass (make sure you get all the corn syrup out!) and stir to dissolve.

Glass 3: Label “hypotonic”. Pour in one cup of distilled water. Gently put one shell-less egg in each of the glasses, and let sit in the refrigerator for another 24 hours.

7. Remove the glasses from the refrigerator, and gently put the eggs on a plate. If you weighed the eggs before putting them in each solution, weigh them again. What happened to each of the eggs?

How does osmosis work?

Osmosis is the scientific term that describes how water flows to different places depending on certain conditions. In this case, water moves around to different areas based on a concentration gradient , i.e. solutions which have different concentrations of dissolved particles ( solutes ) in them. Water always flows to the area with the most dissolved solutes, so that in the end both solutions have an equal concentration of solutes. Think about if you added a drop of food dye to a cup of water – even if you didn’t stir it, it would eventually dissolve on its own into the water.

In biological systems, the different solutions are usually separated by a semipermeable membrane , like cell membranes or kidney tubules . These act sort of like a net that keeps solutes trapped, but they still allow water to pass through freely. In this way, cells can keep all of their “guts” contained but still exchange water.

Now, think about the inside of an egg. There’s a lot of water inside of the egg, but a lot of other things (i.e. solutes) too, like protein and fat. When you placed the egg in the three solutions, how do you think the concentration of solutes differed between the inside of the egg and outside of the egg? The egg membrane acts as a semipermeable membrane and keeps all of the dissolved solutes separated but allows the water to pass through.

How did osmosis make the eggs change size (or not)?

If the steps above work out properly, the results should be as follows.

In the case of the hypertonic solution, there were more solutes in the corn syrup than there were in the egg. So, water flowed out of the egg and into the corn syrup, and as a result the egg shriveled up.

In the case of the isotonic solution, there was roughly an equal amount of solutes in the corn syrup/water solution than there was in the egg, so there was no net movement in or out of the egg. It stayed the same size.

In the case of the hypotonic solution, there were more solutes in the egg than in the pure water. So, water flowed into the egg, and as a result, it grew in size.

Osmosis and You

Every cell in your body needs the right amount of water inside of it to keep its shape, produce energy, get rid of wastes, and other functions that keep you healthy.

This is why medicines that are injected into patients need to be carefully designed so that the solution has the same concentration of solutes as their cells (i.e. isotonic). If you were sick and became dehydrated, for example, you would get a 0.90% saline IV drip. If it were too far off from this mark it wouldn’t be isotonic anymore, and your blood cells might shrivel up or even explode , depending on the concentration of dissolved solutes in the water.

Osmosis works just the same way in your cells as it does in our egg “cell” model. Thankfully, though, the semipermeable membrane of the egg is much stronger, so you don’t have to worry about the egg exploding as well!

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Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

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Demonstrate Osmosis Using Gummy Bears

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Osmosis is the diffusion of water across a semipermeable membrane. The water moves from an area of higher to lower solvent concentration (an area of lower to higher solute concentration). It's an important passive transport process in living organisms, with applications to chemistry and other sciences. You don't need fancy lab equipment to observe osmosis. You can experiment with the phenomenon using gummy bears and water. Here's what you do:

Osmosis Experiment Materials

Basically, all you need for this chemistry project are colored candies and water:

• Gummy bear candies (or other gummy candy)
• Plate or shallow bowl

The gelatin of the gummy candies acts as a semipermeable membrane . Water can enter the candy, but it's much harder for sugar and coloring to leave exit it.

What You Do

It's easy! Simply place one or more of the candies in the dish and pour in some water. Over time, water will enter the candies, swelling them. Compare the size and "squishiness" of these candies with how they looked before. Notice the colors of the gummy bears starts to appear lighter. This is because the pigment molecules (solute molecules) are being diluted by the water (solvent molecules) as the process progresses.

What do you think would happen if you used a different solvent, such as milk or honey, that already contains some solute molecules? Make a prediction, then try it and see.

How do you think osmosis in a gelatin dessert compares with osmosis in candy? Again, make a prediction and then test it!

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Education Through Engagement and Application

Osmosis Demonstration – Osmosis and the Mighty Potato

By Fergy on September 19, 2016 1

Osmosis and the Mighty Potato – Classroom Inquiry and Demonstration

Osmosis is simply the diffusion of water molecules across a semi-permeable membrane. As is the case with diffusion, water molecules like to go from an area of high concentration to an area of low concentration.

This demonstration shows quantitively and qualitatively, the effects of osmosis. At the bottom you’ll find the results of the experiment I did in my class and trust me, the results are significant.

Materials needed: • A potato (or multiple if you students will also be doing the experiment) • A knife • A balance • 2 plates with lips to avoid spills • Tap Water (fresh water) • Salt

The Activity: 1. Have your students draw this chart.

2. Ask your students what they think will happen when you place a cut potato to two different environments. One in fresh water (tap water) and the other in salt water. Have them write it down in the ‘My Opinion’ column including why they think so. 3. Have your students share their answers with the person beside them. Each student should then write the combined opinion in the ‘Group Opinion’ column as they did before. 4. Lastly, have each group share their results with another group and write the consensus opinion in the ‘Shared Opinion’ column. 5. Weigh both cut potatoes and record the masses.

7. Once the time is up, take the potatoes out of the water, dry them, and place them on the scale. Compare their masses before and after.

Explanation: Water moves from high to low concentration in order to dilute any solutes (substances in the water). Salt is a solute and when you place the potato in the salt solution, you are exposing the potato to a much lower water concentration (high solute concentration) due to the solutes present. Although water moves into and out of the potato, there’s a net movement of water from inside the potato into the salt-water solution. Less water in the potato means less mass. When the potato is placed into the fresh water dish, there’s no difference in water concentration between the potato and the surrounding water environment, therefore, there’s no net movement of water. This leaves the potato with an equal mass before and after the experiment.

10. Have your students complete the last row using the information you provide in the explanation.

My Experimental Results: Trial #1 – Fresh water Potato

Initial mass – 288 g

Final mass – 298 g

Difference = 10 g increase = 3.4% INCREASE in mass

Trial #2 – Salt Water Potato

Initial mass – 305 g

Final mass -272 g

Difference = 33 g decrease = 11% LOSS in mass

As shown above, there was a significant difference in the two trials. The fresh water potato gained some mass (small amount of water absorbed), while the salt water potato lost 11% of its mass. The loss in mass was due to water leaving the potato to balance the water concentrations as the environment was rich with salt.

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Reader Interactions

July 1, 2017 at 3:06 pm

I think this is a great activity and I think it will be easier to execute over doing the dialysis tubing. I also think the students will like it more because potatos are “real” to them, not dialysis tubing.

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Potato Osmosis Lab

Explore what happens to potatoes when you put them in a concentration of salt water and then pure water. Set up an osmosis potato lab and learn all about osmosis when you try this fun potato experiment with the kids. We are always searching for simple science experiments , which is perfect. Grab the free printable experiment below.

What is Osmosis ? Learn more about osmosis through a variety of experiments.

• 2 tall glasses of distilled water (or regular)

INSTRUCTIONS:

STEP 1: Peel and then cut your potato into four equal pieces about 4 inches long and 1 inch wide.

STEP 2: Fill your glasses half way with distilled water, or regular water if no distilled is available.

STEP 3: Now mix 3 tablespoons of salt into one of the glasses and stir.

STEP 4: Place two pieces of potato into each glass and wait. Compare the potatoes after 30 minutes and then again after 12 hours.

What happened to the potato pieces? Here you can see how a potato can demonstrate the process of osmosis. Make sure to go back and read all about osmosis!

If you thought the salt water would have a higher concentration of solutes than the potato, and the distilled water would have a lower concentration you would be correct. The potato in the salt water shrinks because water moves from the potato into the more concentrated salt water.

In contrast, water moves from the less concentrated distilled water into the potato causing it to expand.

What Happens to a Potato in Salt Water?

The process of moving water across a semi-permeable membrane from a low concentrated solution to a high concentrated solution is called osmosis . A semi-permeable membrane is a thin sheet of tissue or layer of cells acting as a wall that allows only some molecules to pass through.

In plants, water enters the roots by osmosis. The plants have a higher concentration of solutes in their roots than in the soil. This causes water to move into the roots. The water then travels up the roots to the rest of the plant.

ALSO CHECK OUT: How Water Travels Through A Plant

Osmosis works in both directions. If you put a plant into water with a higher salt concentration than the concentration inside its cells, water will move out of the plant. If this happens then the plant shrinks and will eventually die.

Potatoes are a great way to demonstrate the process of osmosis in our potato osmosis experiment below. Discuss whether you think the potato or the water in each glass will have the greatest concentration of solutes (salt).

Which potato pieces do you think will expand and which will shrink in size as the water moves from a low concentration to a high concentration?

CLICK HERE TO GET YOUR FREE POTATO OSMOSIS EXPERIMENT!

More Osmosis Experiment Ideas

don’t stop with a potato osmosis lab; try one of these osmosis experiments to extend the learning.

• Rubber Egg Science
• Glowing Spinach
• Growing Gummy Bears
• Colored Celery Science

MORE FUN EXPERIMENTS TO TRY

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Osmosis - Real-life applications

Cell behavior and salt water.

Two illustrations involving salt water demonstrate how osmosis can produce disastrous effects in living things. If you put a carrot in salty water, the salt water will "draw" the water from inside the carrot—which, like the human body and most other forms of life, is mostly made up of water. Within a few hours, the carrot will be limp, its cells shriveled.

Worse still is the process that occurs when a person drinks salt water. The body can handle a little bit, but if you were to consume nothing but salt water for a period of a few days, as in the case of being stranded on the proverbial desert island, the osmotic pressure would begin drawing water from other parts of your body. Since a human body ranges from 60% water (in an adult male) to 85% in a baby, there would be a great deal of water available—but just as clearly, water is the essential ingredient in the human body. If you continued to ingest salt water, you would eventually experience dehydration and die.

How, then, do fish and other forms of marine life survive in a salt-water environment? In most cases, a creature whose natural habitat is the ocean has a much higher solute concentration in its cells than does a land animal. Hence, for them, salt water is an isotonic solution, or one that has the same concentration of solute—and hence the same osmotic pressure—as in their own cells.

Osmosis in Plants

Plants depend on osmosis to move water from their roots to their leaves. The further toward the edge or the top of the plant, the greater the solute concentration, which creates a difference in osmotic pressure. This is known as osmotic potential, which draws water upward. In addition, osmosis protects leaves against losing water through evaporation.

Crucial to the operation of osmosis in plants are "guard cells," specialized cells dispersed along the surface of the leaves. Each pair of guard cells surrounds a stoma, or pore, controlling its ability to open and thus release moisture.

In some situations, external stimuli such as sunlight may cause the guard cells to draw in potassium from other cells. This leads to an increase in osmotic potential: the guard cell becomes like a person who has eaten a dry biscuit, and is now desperate for a drink of water to wash it down. As a result of its increased osmotic potential, the guard cell eventually takes on water through osmosis. The guard cells then swell with water, opening the stomata and increasing the rate of gas exchange through them. The outcome of this action is an increase in the rate of photosynthesis and plant growth.

When there is a water shortage, however, other cells transmit signals to the guard cells that cause them to release their potassium. This decreases their osmotic potential, and water passes out of the guard cells to the thirsty cells around them. At the same time, the resultant shrinkage in the guard cells closes the stomata, decreasing the rate at which water transpires through them and preventing the plant from wilting.

Osmosis and Medicine

Osmosis has several implications where medical care is concerned, particularly in the case of the storage of vitally important red blood cells. These are normally kept in a plasma solution which is isotonic to the cells when it contains specific proportions of salts and proteins. However, if red blood cells are placed in a hypotonic solution, or one with a lower solute concentration than in the cells themselves, this can be highly detrimental.

Hence water, a life-giving and life-preserving substance in most cases, is a killer in this context. If red blood cells were stored in pure water, osmosis would draw the water into the cells, causing them to swell and eventually burst. Similarly, if the cells were placed in a solution with a higher solute concentration, or hypertonic solution, osmosis would draw water out of the cells until they shriveled.

In fact, the plasma solution used by most hospitals for storing red blood cells is slightly hypertonic relative to the cells, to prevent them from drawing in water and bursting. Physicians use a similar solution when injecting a drug intravenously into a patient. The active ingredient of the drug has to be suspended in some kind of medium, but water would be detrimental for the reasons discussed above, so instead the doctor uses a saline solution that is slightly hypertonic to the patient's red blood cells.

One vital process closely linked to osmosis is dialysis, which is critical to the survival of many victims of kidney diseases. Dialysis is the process by which an artificial kidney machine removes waste products from a patients' blood—performing the role of a healthy, normally functioning kidney. The openings in the dialyzing membrane are such that not only water, but salts and other waste dissolved in the blood, pass through to a surrounding tank of distilled water. The red blood cells, on the other hand, are too large to enter the dialyzing membrane, so they return to the patient's body.

Preserving Fruits and Meats

Osmosis is also used for preserving fruits and meats, though the process is quite different for the two. In the case of fruit, osmosis is used to dehydrate it, whereas in the preservation of meat, osmosis draws salt into it, thus preventing the intrusion of bacteria.

Most fruits are about 75% water, and this makes them highly susceptible to spoilage. To preserve fruit, it must be dehydrated, which—as in the case of the salt in the meat—presents bacteria with a less-than-hospitable environment. Over the years, people have tried a variety of methods for drying fruit, but most of these have a tendency to shrink and harden the fruit. The reason for this is that most drying methods, such as heat from the Sun, are relatively quick and drastic; osmosis, on the other hand, is slower, more moderate—and closer to the behavior of nature.

Osmotic dehydration techniques, in fact, result in fruit that can be stored longer than fruit dehydrated by other methods. This in turn makes it possible to provide consumers with a wider variety of fruit throughout the year. Also, the fruit itself tends to maintain more of its flavor and nutritional qualities while keeping out microorganisms.

Because osmosis alone can only remove about 50% of the water in most ripe fruits, however, the dehydration process involves a secondary method as well. First the fruit is blanched, or placed briefly in scalding water to stop enzymatic action. Next it is subjected to osmotic dehydration by dipping it in, or spreading it with, a specially made variety of syrup whose sugar draws out the water in the fruit. After this, air drying or vacuum drying completes the process. The resulting product is ready to eat; can be preserved on a shelf under most climatic conditions; and may even be powdered for making confectionery items.

Whereas osmotic dehydration of fruit is currently used in many parts of the world, the salt-curing of meat in brine is largely a thing of the past, due to the introduction of refrigeration. Many poorer families, even in the industrialized world, however, remained without electricity long after it spread throughout most of Europe and North America. John Steinbeck's Grapes of Wrath (1939) offers a memorable scene in which a contemporary family, the Joads, kill and cure a pig before leaving Oklahoma for California. And a Web site for Walton Feed, an Idaho company specializing in dehydrated foods, offers reminiscences by Canadians whose families were still salt-curing meats in the middle of the twentieth century. Verla Cress of southern Alberta, for instance, offered a recipe from which the following details are drawn.

First a barrel is filled with a solution containing 2 gal (7.57 l) of hot water and 8 oz (.2268 kg) of salt, or 32 parts hot water to one part salt, as well as a small quantity of vinegar. The pig or cow, which would have just been slaughtered, should then be cut up into what Cress called "ham-sized pieces (about 10-15 lb [5-7 kg]) each." The pieces are then soaked in the brine barrel for six days, after which the meat is removed, dried, "and put… in flour or gunny sacks to keep the flies away. Then hang it up in a cool dry place to dry. It will keep like this for perhaps six weeks if stored in a cool place during the Summer. Of course, it will keep much longer in the Winter. If it goes bad, you'll know it!"

Cress offered another method, one still used on ham today. Instead of salt, sugar is used in a mixture of 32 oz (.94 l) to 3 gal (11.36 l) of water. After being removed, the meat is smoked—that is, exposed to smoke from a typically aromatic wood such as hickory, in an enclosed barn—for three days. Smoking the meat tends to make it last much longer: four months in the summer, according to Cress.

The Walton Feeds Web page included another brine-curing recipe, this one used by the women of the Stirling, Alberta, Church of the Latter-Day Saints in 1973. Also included were reminiscences by Glenn Adamson (born 1915): "…When we butchered a pig, Dad filled a wooden 45-gal (170.34 l) barrel with salt brine. We cut up the pig into maybe eight pieces and put it in the brine barrel. The pork soaked in the barrel for several days, then the meat was taken out, and the water was thrown away…. In the hot summer days after they [the pieces of meat] had dried, they were put in the root cellar to keep them cool. The meat was good for eating two or three months this way."

For thousands of years, people used salt to cure and preserve meat: for instance, the sailing ships that first came to the New World carried on board barrels full of cured meat, which fed sailors on the voyage over. Meat was not the only type of food preserved through the use of salt or brine, which is hypertonic—and thus lethal—to bacteria cells. Among other items packed in brine were fish, olives, and vegetables.

Even today, some types of canned fish come to the consumer still packed in brine, as do olives. Another method that survives is the use of sugar—which can be just as effective as salt for keeping out bacteria—to preserve fruit in jam.

Reverse Osmosis

Given the many ways osmosis is used for preserving food, not to mention its many interactions with water, it should not be surprising to discover that osmosis can also be used for desalination, or turning salt water into drinking water. Actually, it is not osmosis, strictly speaking, but rather reverse osmosis that turns salt water from the ocean—97% of Earth's water supply—into water that can be used for bathing, agriculture, and in some cases even drinking.

When you mix a teaspoon of sugar into a cup of coffee, as mentioned in an earlier illustration, this is a non-reversible process. Short of some highly complicated undertaking—for instance, using ultrasonic sound waves—it would be impossible to separate solute and solvent.

Osmosis, on the other hand, can be reversed. This is done by using a controlled external pressure of approximately 60 atmospheres, an atmosphere being equal to the air pressure at sea level—14.7 pounds-per-square-inch (1.013 × 10 5 Pa.) In reverse osmosis, this pressure is applied to the area of higher solute concentration—in this case, the seawater. As a result, the pressure in the seawater pushes water molecules into a reservoir of pure water.

If performed by someone with a few rudimentary tools and a knowledge of how to provide just the right amount of pressure, it is possible that reverse osmosis could save the life of a shipwreck victim stranded in a location without a fresh water supply. On the other hand, a person in such a situation may be able to absorb sufficient water from fruits and plant life, as Tom Hanks's character did in the 2001 film Cast Away.

Companies such as Reverse Osmosis Systems in Atlanta, Georgia, offer a small unit for home or business use, which actually performs the reverse-osmosis process on a small scale. The unit makes use of a process called crossflow, which continually cleans the semipermeable membrane of impurities that have been removed from the water. A small pump provides the pressure necessary to push the water through the membrane. In addition to an under-the-sink model, a reverse osmosis water cooler is also available.

Not only is reverse osmosis used for making water safe, it is also applied to metals in a variety of capacities, not least of which is its use in treating wastewater from electroplating. But there are other metallurgical methods of reverse osmosis that have little to do with water treatment: metal finishing, as well as recycling of metals and chemicals. These processes are highly complicated, but they involve the same principle of removing impurities that governs reverse osmosis.

WHERE TO LEARN MORE

Francis, Frederick J., editor-in-chief. Encyclopedia of Food Science and Technology. New York: Wiley, 2000.

Gardner, Robert. Science Project Ideas About Kitchen Chemistry. Berkeley, N.J.: Enslow Publishers, 2002.

Laschish, Uri. "Osmosis, Reverse Osmosis, and Osmotic Pressure: What They Are" (Web site). <http://members.tripod.com/~urila/> (February 20, 2001).

"Lesson 5: Osmosis" (Web site). <http://www.biologylessons.sdsu.edu/classes/lab5/semnet/> (February 20, 2001).

Rosenfeld, Sam. Science Experiments with Water. Illustrated by John J. Floherty, Jr. Irvington-on-Hudson, NY: Harvey House, 1965.

"Salt-Curing Meat in Brine." Walton Feed (Web site). <http://waltonfeed.com/old/brine.html> (February 20, 2001).

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Chapter 1 Osmosis - The Potato Lab

Learning objectives.

• Define a solution and the components of a solution.
• Describe why osmosis occurs and what is required for osmosis to occur.
• Describe osmotic pressure and how it can be used.
• Explain how osmosis is important in biological systems.
• Define isotonic, hypertonic and hypotonic solutions and the consequences in biological organisms.

Introduction

In order to understand osmosis, definitions for solvent , solute and solution need to be established. When considering a solution, first picture a glass of iced tea with sugar in it. The water that the iced tea was made with is the solvent; the tea flavoring and the sugar are solutes. Altogether they make up a solution. A solution is a homogeneous mixture of a solvent and one or more solutes. The solvent is the compound that dissolves or surrounds the solute. (The solute is said to be solvated). The solute is the component of the mixture that is usually in lesser quantity.

Osmosis is the movement of solvent molecules from a region of low solute concentration to an area of higher solute concentration through a semi-permeable membrane as shown in Figure OS.1 .

At some point in time, the liquid levels observed in the diagrams above will no longer change due to osmotic pressure. Osmotic pressure is the pressure applied by a solution to prevent the inward flow of water across a semi-permeable membrane.

In order to visualize this effect, refer to Figure OS.1 . As the water level in Side A increases, eventually the pressure exerted by the water in Side A becomes equal to the pressure exerted by the water in Side B which results in the rate at which water moves from Side A to Side B and from Side B to Side A to be equal. At this time, as the movement of water across the semi-permeable membrane is equal no further change in the heights of the water side and water plus solute side will occur.

Osmotic pressure is the basis of filtering (”reverse osmosis”), a process commonly used to purify water. The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it. Part of the chamber opens to a differentially permeable membrane that lets water molecules through, but not the solute particles. Reverse osmosis can produce fresh water from ocean salt water. Reverse osmosis is also used on a smaller scale in homes to further purify drinking water.

In this experiment, the cell walls of a potato will behave as semi-permeable membranes. The potato will be placed in pure water where the concentration of solute is lower than inside the potato cell walls. Since the solute concentration is lower in the water, the water is hypotonic relative to the solution inside the cell walls of the potato. The potato will also be placed in a very salty solution where the concentration of solute in the solution is greater than in the solution within the cell walls. The very salty solution is hypertonic relative to the solution inside the cell walls of the potato. Finally, the potato will be placed in a solution where the solute concentration is very similar to the solute concentration within the cell wall. This solution is isotonic and no osmotic flow should be observed.

As a further example of the effect of solutions on a biological cell, suppose an animal or a plant cell is placed in a solution of sugar or salt in water.

Essentially, this means that if a cell is put into a solution, which has a solute concentration higher than its own (hypertonic), then it will shrivel up, and if it is put into a solution with a lower solute concentration than its own (hypotonic), the cell will expand and burst. If the cell is put into a solution with equal solute to its own (isotonic), the cell will experience no change in size.

An example of a hypotonic situation could be a shipwrecked group. At the time of the wreck sailors are hydrated and cells full of water. Eventually, fresh water supply is depleted and the only source of water would be the ocean water. If ocean water was consumed, since the cells are hypotonic (full of water) to the ocean water (salty) the cells would begin to shrink (crenate) water flowing out of the cells.

An example of a hypertonic situation can be a dried and withered plant. The addition of water to the plant soil results in the rejuvenation of the plant to a green and upright state. Water will flow into the hypertonic plant cells causing the cells to enlarge and support the plant. The cells do not burst as plant cells contain not only a cell membrane as with animal cells but a cell wall which restricts the swelling of the cells.

Aqueous solutions of 20% w/v (weight per volume) and of 0.9% w/v NaCl will be prepared for you. A 20% w/v solution means that the solution is 20% by weight sodium chloride, assuming that the density of water is 1.00 g/mL.

If you need to make up 50.0 mL of the 20% w/v solution of NaCl, how much NaCl will you need to use? In order to determine the amount of sodium chloride and water to use, it is convenient to remember that the density of water is 1.00 g/mL, which means 20.0 grams of water has a volume of 20.0 mL and 100.0 grams of water has a volume of 100.0 mL, etc. (We will make the assumption that the density of a solution of water with a salt is the same as that of pure water.) It is also convenient to realize that if you have 100.0 grams (or 100.0 mL) of a 20% w/v sodium chloride solution, then there are 20.0 grams of NaCl in 100 mL of solution. Start with the assumption in Equation OS.1 :

\begin {equation} \label {assumption} \frac {20.0\: \text {g NaCl}}{100.0\: \text {mL of solution}} \times {50.0\: \text {mL of solution}} = 10.0\: \text {g of solution} \end {equation}

Therefore, to make 50.0 mL of a 20% w/v solution, add 10.0 grams of NaCl to a 50.0 mL volumetric flask and fill to the top with deionized water.

After the experiment, you will evaluate the percent change in the mass and in the volume of the potato slices. The change is referenced to the initial size Equation OS.2 .

\begin {equation} \label {change} \text {Percent Change} = \frac {\text {final size - initial size}}{\text {initial size}} \times 100 \end {equation}

Remember, the equation for the volume, V, of a cylinder is V= \(\pi \)r\(^2\)h where r is the radius and h is the height. The diameter is two times the radius. So, V= \(\pi (\frac {d}{2})^2\)h.

If the potato decreases in size, the percent change will be negative. If the potato increases in size, the percent change will be positive. (Remember that the number of significant figures in your final answer will be determined by the number of significant figures in the difference between the final and initial sizes.)

Chapter A CHAPTER 1.  OSMOSIS - THE POTATO LAB

    Section: ________________ Date: ______________________

Record the data in Report Table OS.1 and Report Table OS.2 .

1 pt for each blank.

Calculations

Calculate the percent change of mass for the potato disks and record in Report Table OS.3

1 point for each blank except 2 points for percent change.

Calculate the percent change of volume for the potato disks and record in Report Table OS.4

Show the calculation for the percent change in mass for Disk A. (4 pts)

Show the calculation for the volume of Disk A before soaking. (2 pts)

Show the calculation for the volume for Disk A after soaking. (2 pts)

Show the calculation for the percent volume change that occurred in Disk A after soaking. (4 pts)

A. Isotonic solution

B. Hypotonic solution

C. Hypertonic solution

A. isotonic relative to the potato?

B. hypotonic relative to the potato?

C. hypertonic relative to the potato?

A. 20% w/v NaCl solution

B. 0.9% w/v NaCl solution

C. Deionized water

a) Calculate the mass of NaCl needed to prepare 35.0 mL of a 0.90 % w/v NaCl solution.

b) Calculate the mass of NaCl needed to prepare 35.0 mL of a 20.0% w/v NaCl solution.

c) Describe how you would make the solution for 4a. Be brief, but quantitative, in your answer.

5 Engaging Ways to Teach Osmosis and Diffusion Without Lecturing

Life, at its most basic, is a series of complex chemical and physical exchanges, all harmoniously synchronized to ensure the seamless operation of biological systems. Osmosis and diffusion are central to these exchanges, representing two of nature’s most gracefully orchestrated principles.

Diffusion is like a dance of particles that move from a crowded place to a less crowded place until there’s balance. Osmosis is a special dance for water that goes through a thin barrier. These moves help keep us alive and let plants absorb water from the ground.

However, these fluid-movement processes can be hard to teach due to their abstract and intangible nature. Sometimes, the textbooks make them look boring too. That’s where this article helps. We will present five creative ways to teach osmosis and diffusion, making learning fun and memorable.

1. Engage Students With Interactive Models 

Teaching osmosis and diffusion can be a challenge, primarily because these processes are invisible to the naked eye. The chance to observe these phenomena directly can significantly enhance students’ understanding.

Interactive models provide an excellent solution. They offer students a visual journey into the minuscule world of particles, allowing them to witness these exchanges in real-life scenarios. This dynamic learning approach elevates the educational experience.

One such valuable resource is the virtual labs offered by Labster. For example, the Osmosis and Diffusion Lab simulation immerses students in a virtual environment where they can observe osmosis occurring in cell membranes. They can conduct experiments with various samples, studying these processes firsthand.

Discover Labster's Osmosis and Diffusion virtual lab today!

This engaging, virtual approach allows students to apply theoretical classroom knowledge in real-time, making the learning experience more impactful. 

2. Inject Fun With Games and Activities

Adding games and activities to the teaching process not only makes learning fun but also enhances understanding and retention of complex concepts.

Here are a few interesting activities for teaching osmosis and diffusion:

• Osmosis Egg-experiment: An engaging experiment where a de-shelled egg is placed in different solutions. Students can predict and see what the weight changes over time due to osmosis.
• Kahoot Quiz Game: Develop quizzes about osmosis and diffusion using Kahoot . This interactive game format will allow students to test their knowledge and compete with their peers.
• Role-Playing Game: Divide students among groups and have them play roles as molecules to demonstrate osmosis and diffusion physically. This can help to understand these processes in a fun and interactive way.

3. Infuse Technology into Lessons

In the digital era, technology is a powerful tool to transform teaching and learning. Especially when it comes to teaching osmosis and diffusion, infusing technology into lessons can create a more engaging, interactive, and immersive learning experience.

Using technology, educators can bring to life the otherwise invisible processes of osmosis and diffusion, fostering deeper understanding. For example, with multiple simulations and animated videos, you can take students on a microscopic journey into a cell to see how molecules move, bringing abstract concepts to life.

Online virtual simulations, like Labster's Osmosis and Diffusion simulation , allow students to experiment right from their tech gadgets. They closely observe these chemical exchange processes, which provides them with deeper insights into the subject. 

4. Inspire Students Through Career Exploration

When educators mention how abstract concepts will help them in the real professions in the future, it boosts their interest and curiosity. They begin to see the relevance and realize the importance of the concepts they are being taught. 

You can quote how multiple medical professions heavily rely on the understanding of osmosis and diffusion. Doctors need to understand these principles when administering intravenous fluids to balance electrolytes in a patient's body. Biomedical engineers apply these concepts in designing artificial organs or drug-delivery systems

By exploring careers related to osmosis and diffusion, students can see the practical and professional implications of these concepts. This sparks their interest and motivates them to explore it further.

5. Connect Topic to Real-World Applications

Beyond career references, you can extend the relevance to real-world applications. This allows students to understand these concepts are not just textbook theories, but vital processes that impact our everyday lives.

A classic example of osmosis in the real world is the process of water absorption by plant roots from the soil. Diffusion, on the other hand, is illustrated every time we spray perfume, and its scent spreads across the room.

Furthermore, osmosis knowledge is also crucial when designing saline solutions for IV drips - too much or too little salt can cause harmful fluid shifts in the body. 

Incorporating these real-world applications into lessons brings science to life, making it more relatable and meaningful for students. 

Final Thoughts

Although teaching osmosis and diffusion can be a challenging task, it can be transformed into an exciting and memorable journey with the right tools and approaches. Through virtual lab simulations, fun games, and real-world applications, educators can make these abstract concepts tangible and engaging for students.

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Osmosis Lab Setup

Materials :

• electronic balance (0.01 g range)
• metric ruler with mm scale
• metric measuring cups
• 6 cereal bowls or shallow pans

• single edged razor or knife
• paper towels
• watch or clock
• table salt, distilled or tap water
• 6 beakers (250 ml or larger) or cups
• Pre-mix 6 beakers of salt solutions (0%, 0.1%, 0.5%, 1%, 2.5%, 5%) in distilled water. You can use this solution calculator to help you make your solutions. Just enter the water volume of your container and the percentage of salt you want and it will tell you how many grams of salt to add. A 1% salt solution is 1 part salt to 100 parts water. To make a 1% salt solution, you could use a 100 ml bottle, add exactly 1 gram of salt (use your electronic balance) to your bottle, and bring the water volume up to 100 ml. To make a 0.1% solution, add 1 gram of salt to 1000 ml of water (or add 0.1 g salt to 100 ml of water). If you have more water than you need, just stir well and then discard the excess.
• Prepare six small potato cubes with no skin that are all about equal in size (approximately 5 millimeters in length, width and height) and blot them dry on a paper towel. (Blot means just gently remove the surface water; no need to squeeze them!)
• Mass (weigh) each to the nearest 0.01 grams, keeping them separate, and record each initial mass in Table 1. Don't wait too long before putting them into the solutions, as evaporation will occur.
• Fill each bowl with one of the 6 stock solutions, keeping track of which is which! Label them. You won't be able to tell the salinity just by looking. Note which potato piece went into which bowl.
• Leave one of the potato slices in each of the salt solutions for up to 24 hours so that they may gain (or lose) water by osmosis. (Keep them all in the salt water the same amount of time--leaving them overnight is likely to give the best results).
• Remove the slices, blot them dry on a paper towel, carefully re-weigh them and record in the data table as final mass.

Table 1: Changes in potato mass as a result of immersion in salt solutions.

• Why did some potato samples gain water and others lose water? Was there any pattern?
• When you drew the best fit line through your data and dropped the vertical line to the x-axis, what salt concentration did you obtain (Estimate if it is between numbers)? What does this mean for the potato?
• Why can't we use seawater to irrigate our crops?
• What happens when a thirsty person drinks salt water to try to quench their thirst?
• Why does salted popcorn dry your lips?
• What happens to a cell's water when the exterior liquid is saltier than its interior?
• What happens to water outside the cell when the interior is saltier than its surroundings?
• When a cell gains water, what happens to its size and weight?
• When a cell loses water, what happens to its size and weight?
• When you put limp celery stalks in water, they firm up. Why?
• Challenge question: Saltwater fish are hypotonic (less salty) to their surroundings while freshwater fish are hypertonic (more salty) to their surroundings. Assuming the salt can't move, what must each fish do with its fluids in order to compensate for the difference in salinity between the body and the surrounding environment?

Question 9 (i) Carry out the following osmosis experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now, (a) Keep cup A empty (b) Put one teaspoon of sugar in cup B (c) Put one teaspoon of salt in cup C (d) Put one teaspoon of sugar in the boiled potato cup D. Keep these for two hours. Then observe the four potato cups and answer the following: (i) Explain why water gathers in the hollowed portion of B and C.

Reason for same level of water in cup a: there is no solution/solute inside cup a. so, there will be absence of osmotic pressure in this case. hence, the water does not move from outside to inside of the cup. reason for increased water level in cup b: osmosis is the reason for increase of water level in cup b. osmosis is the movement of the solvent molecules through a semipermeable membrane from the region of low solute concentration to a region of higher solute concentration. here the cells of potato acts as the semi-permeable membrane through which water enters the potato cup. the medium inside the potato cup has a higher concentration of sugar (solute) as compared to outside the potato cups. hence, the water moves towards the internal part of the potato cup which is the movement of solvent from a region of low solute concentration to a region of higher solute concentration. reason for increased water level in cup c: osmosis is the reason for increase of water level in cup b. osmosis is the movement of the solvent molecules through a semipermeable membrane from the region of low solute concentration to a region of higher solute concentration. here the cells of potato acts as the semi-permeable membrane through which water enters the potato cup. the medium inside the potato cup has a higher concentration of salt (solute) as compared to outside the potato cups. hence, the water moves towards the internal part of the potato cup which is the movement of solvent from a region of low solute concentration to a region of higher solute concentration. reason for same level of water in cup d: cup d is made up of boiled potato. the cells of the boiled potato are dead including their cell membranes. therefore, there is no membrane across which the osmosis can take place. hence, there is no change in the level of water inside cup d. final answer: hence, water gathers in the hollowed portions of the potato cup b and c due to the occurrence of osmosis ..

Question 9 (ii) Carry out the following osmosis experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now, (a) Keep cup A empty (b) Put one teaspoon sugar in cup B (c) Put one teaspoon salt in cup C (d) Put one teaspoon sugar in the boiled potato cup D. Keep these for two hours. Then observe the four potato cups and answer the following: (ii) Why is potato A necessary for this experiment?