Case Study¹

NOTE: The case study and assignment questions were copied directly from Bonney, 2014 with minor revisions.

Part I – Introduction to Diffusion and Osmosis

Sue: “Hey Jude, what did we learn in Mr. Phillotson’s biology class today? I missed it because I wasn’t feeling well.”

Jude: “Today we mostly discussed diffusion and osmosis.”

Sue: “What are diffusion and osmosis?”

Jude: “Well, diffusion is the movement of molecules of a substance from an area of high concentration to an area of lower concentration. When molecules do that it’s called moving down a concentration gradient. Mr. Phillotson demonstrated diffusion in class by placing a few drops of green food coloring in a glass of water. At first most of the water was clear with a small amount of dark green food coloring concentrated in the center where he had placed the drops, but over a few minutes the molecules of food coloring spread out in the water until they were evenly distributed among the water molecules and the entire glass appeared a light green color. He said the same thing happens in the air when someone sprays perfume inside a room. At first the molecules of perfume are concentrated and strong-smelling in a small area, but over time they diffuse through the air in the room until they are evenly distributed and the entire room smells weakly of perfume.”

Sue: “Where does the energy come from to move the molecules during diffusion? Did Mr. Phillotson mention that?”

Jude: “Yes, actually, he did. He said that molecules move down their concentration gradient spontaneously, without any work being done, so no energy input is required. Diffusion of molecules can even occur through a membrane, as long as the membrane has holes or pores that will allow those molecules to pass through. Mr. Phillotson said that if molecules diffuse down their concentration gradient through a biological membrane, such as the plasma membrane of a cell, that is called passive transport. Passive transport does not require energy input from the cell.

Sue: “So, passive transport moves molecules across a cell membrane and does not require energy input from the cell because the necessary force is provided by the concentration gradient?”

Jude: “Correct. Molecules can also be moved across a cell membrane against their concentration gradient, but that would require energy input. Mr. Phillotson said that process is called active transport, and that we would learn more about it later. ”

Sue: “Ok. I think I understand diffusion now. So, what is osmosis?”

Jude: “Osmosis is the diffusion of water across a selectively permeable membrane. During osmosis water moves from an area of higher concentration of water molecules to an area of lower concentration of water molecules.

Sue: “If water molecules move down their concentration gradient during osmosis, that must happen spontaneously and not require energy input, right?”

Jude: “Correct.”

Sue: “I’m confused, though. I thought all water was the same. How can there be different concentrations of water?

Jude: “Mr. Phillotson explained that often, whether inside a living organism or in a laboratory experiment, water contains other molecules, or solutes. The higher the concentration of solutes, the lower the concentration of water. Do you understand, Sue?”

Sue: “I think so… but… did Mr. Phillotson happen to do a demonstration of osmosis?”

Jude: “Actually, he gave us a homework assignment to help us understand osmosis, and said we will go over another example in class next time, after we discuss the homework.”

Part II – Osmosis and Gummy Bears

You will need to take photos of your gummy bears before and after the experiment begins and a photo of them in their labeled glasses, with the proper contents added to each glass at the start of the experiment.  A piece of paper that includes your name, CHEM 1000, and the semester you are taking this class (for example, if you take this during the fall semester in 2023, write “Fall 2023” on the paper) should also appear in these photos.  This information must be handwritten, legible, and large enough to be clearly read on the piece of paper.  The assignment tied to this laboratory will not be graded unless all of these photos have been submitted to the assignment folder on Brightspace.

Sue: “Homework?! Ugh. What is the assignment?”

Jude: “Each of us was given three gummy bears and these instructions:

Step 1. Measure and record the starting mass of all three gummy bears. Record this data using the data table.

Step 2. Gather three glasses of the same size, and label them #1, #2, and #3.

Step 3. In glass #1, add one inch of water at room temperature and place one of the gummy bears inside.

Step 4. In glass #2, add 1/4 inch of salt and ¾ inch of water at room temperature, as well as one gummy bear.

Step 5. In glass #3 add nothing except the third gummy bear.

Step 6. Cover all three glasses with plastic and leave for at least 12 hours.

Step 7. After 12 hours, measure the final mass and record it on the data table.

Step 8. Determine each gummy bear’s mass change and record your results on the data table.

Part III – Osmosis in Animal Cells

The experiment that Mr. Phillotson’s class conducted simulates what happens when living cells, such as your own red blood cells (RBCs), are placed in solutions of different tonicities. The gelatin in gummy bears forms a structural matrix that acts somewhat like a selectively permeable membrane, allowing passage of certain molecules but not others. Water will move in or out of the gummy bear matrix when placed in solutions of different tonicities in order to obtain equilibrium. The plasma membrane that surrounds RBCs and other animal cells is similarly selectively permeable; water will move across the membrane more readily than will solutes such as sodium chloride (NaCl).

Like a gummy bear, an animal cell’s plasma membrane will shrivel if there is a net water loss from inside the cell. However, unlike a gummy bear, if there is a large net influx of water into the cell, the plasma membrane may break, and the cell will be destroyed. The bursting of the cell in that situation is called lysis.

Part IV – Osmosis in Plant Cells

Unlike gummy bears and animal cells, the cells of plants are surrounded by rigid cell walls. These cell walls will prevent cells from bursting if there is a large net water movement into the cell. However, when a plant cell swells due to a net influx of water, the cell wall can only expand so far before exerting pressure back on the cell, which is called turgor pressure. Many plants depend on turgor pressure and the firm, or turgid, state it creates to provide structural support. If you have ever seen a houseplant or a stalk of celery go limp after not receiving water for a while, you have observed plant cells that have lost turgor pressure and become flaccid. Conversely, a significant net water loss from inside a plant cell can cause its plasma membrane to pull away from the inside of the cell wall; this state is called plasmolysis.