Title: Diffusion in Potato Slices
Materials: two potatoes
cut into slices, eight digital scales, water, salt water
Student prior knowledge: Students need to understand the concept of diffusion, the conceptual meaning of "concentration," and the function of the cytoplasm and cell membrane of a cell.
Explanation: When doing this activity for a 7th grade life science classroom, the focus of the lesson is not to calculate percent changes in mass. Rather, the focus is to have the students understand that the cell membrane allows materials to move both in and out, and that this movement of molecules is not random. Whether water will move into or out of a cell depends on the concentration of water molecules both in the cytoplasm of the cell and the surrounding solution. Discussing this concept at a cellular level can be too abstract for middle schoolers, therefore it is useful to use potatoes to model the process.
The activity really is as simple as it sounds, procedure wise. The key is to use the 30 minute wait time as an opportunity to introduce the idea that the movement of molecules into or out of a cell is not random, but rather that it occurs when there is a concentration gradient. During that wait time, present the concept that molecules move from an area of higher concentration to an area of lower concentration. Then ask the students to predict the movement of water molecules into or out of the potato cells. For my class, I provided the information that for Potato Slice 1, there is a higher concentration of water molecules in the surrounding liquid than their is in the cytoplasm of the potato cells. I also provided that for Potato Slice 2, there was a higher concentration of water molecules in the potato cells than there was in the surrounding salt water.
After the students have collected their data when re-weighing the potato slices, have the student analyze their data. At a seventh grade level, this could mean simply stating whether their data matches their predictions or not. Ask the students to interpret why the potato cells got bigger or smaller, using the vocabulary words from that days lesson (concentration, osmosis, cytoplasm, cell membrane, etc.)
1. Suppose a carrot is soaking in pure water. There is a higher concentration of water molecules in the pure water than there is in the cytoplasm of the carrot cells. Do you think the carrot will get bigger, smaller, or stay the same size? Why?
The carrot will get bigger. Molecules move from areas of higher concentration to areas of lower concentration. If there is a higher concentration of water molecules outside of the cell than inside of the cell, the water molecules will move inside the cell in an effort to create equilibrium. The movement of water into the carrot's cells will cause the carrot to increase in size.
2. A cucumber and a carrot are both soaking in the same bucket of pure water for the same amount of time. The cucumber gains one gram of water, while the carrot gains two. Why didn't they gain the same amount of water?
There are two possible reasons why the fruit and the vegetable did not gain the same amount of water. The cucumber may have gained less water due to it's thicker skin. If the skin or membrane of a material is harder to permeate, it is less likely that water or other molecules will be able to get through. If the cucumber was peeled, there is a different explanation. It could be that there was a higher concentration of water molecules in the cytoplasm of the cucumber cells than there was in the carrot cells. This would mean that it would take less movement of water molecules into the cucumber cells to have the cucumber be in equilibrium with the surrounding fluid, compared to the carrot.
3. Why do your hands get prune-y when you're sitting in a bathtub for too long? Explain using scientific vocabulary.
A bathtub is filled with water molecules. The appearance of "prunes" on your hands suggests that water molecules have diffused out of your skin, and into the surrounding bathwater. This occurs because the cytoplasm of your body's cells have a lower concentration of water molecules than the surrounding bathwater. In an attempt to put your body in equilibrium with the bathwater, water diffuses out of your cells and into the bath.
Applications to Everyday Life:
1. As stated in the question above, the process of diffusion explains why people's hands get "prune-y" if they swim for too long. Water molecules diffuse out of the skin and into the surrounding water, leaving the skin on your hands looking shriveled.
2. The process of osmosis explains why freshwater fish die when placed in a saltwater environment (whether it be a saltwater tank or the ocean). The water concentration inside the cell's of the fish is greater than the concentration of water molecules in the salt water, which causes water to move from the fish into the surrounding liquid due to the concentration gradient. The fish can desiccate/dehydrate and die.
3. The process of brining vegetables, such as brining cucumbers to make pickles, is explained by osmosis. The water concentration inside the cytoplasm of the vegetable's cells is greater than the water concentration of the brine. Therefore water moves from the vegetable into the surrounding brine, intensifying the flavor of the vegetable and giving it a satisfying crunch.
Videos: This is an example of a more advanced experiment, but it involves the same materials and concepts.