I Read Your Article, Is Inquiry the Answer

POTATO LAB

Introduction: The cell membrane is semi-permeable. This means that some substances can go in and out of the cell freely, and other substances cannot. Water is one substance that can go freely through the cell membrane. Salt is a substance that cannot go through the cell membrane. The cell membrane is permeable to water, but impermeable to salt. Substances tend to go from areas of high concentration to low concentration. In other words, substances spread out. If we pushed everyone in class into a corner, but allowed them to move if they wanted to, the classmates would tend to spread out around the room rather than remained crammed in a corner. The classmates moved from an area of high concentration in the crammed corner to low concentration, the rest of the room. Water and salt molecules do the same thing. Water molecules move to areas with a LOWER CONCENTRATION of water molecules, and salt molecules move to areas with a LOWER CONCENTRATION of salt molecules.

In this diagram, you can see that the cell has a higher concentration of water than the beaker of saltwater. Since only water can pass through the cell membrane we only need to look at the concentration of water inside and outside of the cell. Water will from the area of area of high concentration inside the cell to the area of low concentration outside of the cell.

Procedure: 1. Half fill a beaker with pure water. 2. Half fill another beaker with a concentrated salt solution. (Add enough salt to the water so that some salt remains on the bottom of the bowl.) 3. Cut 2 of potato, about 2 mm thick. They should look like a potato chip. 4. Place one potato slice in the beaker of pure water. 5. Place the other potato slice in the beaker of salt water. 6. Let the potatoes sit for 10-15 minutes. 7. Observe any differences between the potatoes. (Try bending them. Do you notice a difference in "bend-ability"? Prediction Based on the introduction and the procedure, what do you predict will happen to the water inside of the potato. How will this change make the potato feel?

Results Write down your observations in your notebook.

What Happens to the potato cells. The introduction explains what happens in terms of the permeability of the potato cell membranes and the concentration of water inside and outside of the cell.

Extensions Explain why celery in fresh water become stiff and crispy, while celery in salt water become flimsy and rubbery. Explain why lettuce left in the refrigerator without a cover becomes limp.

Now its your turn to investigate

Ask a question that could be answered by experimentation using the same procedure used in the potato lab. Questions may begin with Will ..., What will happen if I ..., or What are the effects of ... on ... or any other question that will lead to an experiment. Be sure you use materials you can bring from home or that we have in schools. (I must approve your question before you begin. I reserve the right to disqualify experiments for safety or cost reasons.) Once I approve your question send yourself a message to remind you of the things you need to bring tomorrow in order to answer your question.

Sample questions you might investigate, “Will the potato do the same thing if I use sugar water instead of salt?” “What will happen if I use apple instead of potato?” “What will happen if I use a closed plastic bottle instead of a potato?”

You will present your results to the class. Be sure you can answer the following questions during your presentations. Where was water in the highest concentration in your experiment? Where was water in the lowest concentration? Were your results like the ones with salt (potato)? Why do you think the results were the similar or different? What was your control? To the Teacher The students complete the Potato Lab be sure to have different students read their answers the extension questions. Draw what is happening in each situation. Be sure students understand that substances move from higher concentration to lower concentration and that only water can move out of the potato cells (note the potato cells contain lots of other substances in addition to salt which means the concentration of water inside the cell is very low compared to pure water.)

Allow students to write down their own questions that can be answered by experimentation. Sort the questions that are the same and put students with the same question or similar questions in the same group.

When students give their presentation be sure they have answered each question, if they have not ask the questions directly. Students may present in groups.

Have extra materials on hand for students who forget to bring materials. Materials might include sugar, corn starch, more potatoes, apples, carrots, celery, lettuce, beets, any other types of fruit or vegetables. You might be able to get some damaged fruit and vegetables from the grocery for free if you tell the manager what you need them for.

You might give students extra credit for remembering to bring their materials.

They might use their cell phones, computers, or notebooks to remind them to bring their materials.

See Inquiry with Anything for more details on how to guide students through their own experiment. http://www.curriki.org/xwiki/bin/view/Coll_VirginiaMaloneet/InquirywithAnything