Cellular Hydration - Movement Across a Membrane Day 2

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SWBAT explain the process of osmosis across cell membranes.

Big Idea

Using vegetables students will observe changes in crispness when vegetables are soaked overnight in salt water. The vegetables will be compared to vegetables soaked overnight in freshwater.

The Need for the Lesson

After a read aloud and short discussion on Day 1, students conduct an experiment on Day 2 to examine how the process of osmosis removes water from our cells and impairs the function of the cells.

Students make concrete observations in the changes of vegetables when osmosis causes the salt water outside the cell membrane to draw water from the cell. Students can apply that concrete example to understand what they cannot actually see about how cells move materials in and out of the cell.

Investigation Preparation & Summary

10 minutes

On day 1 of this lesson heard about the effects of cellular dehydration in the context of history by hearing about the symptoms experienced by the survivors of the USS Indianapolis as they waited in the South Pacific for 4 days for rescue. (Cellular Hydration - Movement Across a Membrane Day 1)

On day 2 of this lesson, students complete an osmosis experiment to see what how the cell membrane reacts when crisp vegetables are placed in salt water. (MS-LS1-2 Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function. Assessment Boundary: Assessment of organelle structure/function relationships is limited to the cell wall and cell membrane.) (SP2 Developing and Using Models)

A materials list for this experiment is in the resources section.

Students in Action

30 minutes

Students in Action

Students begin by reading independently a short article that explains why we feel thirsty after eating something salty - Why Does Eating Salty Foods Make Your Thirsty? and the vocabulary (osmosis) for this lesson. Our purpose for reading is to be able to answer the questions that is the title of the article.

Following the reading, I ask students key questions to check for understanding. This discussion will help students who struggled with the reading to gain an understanding of the reading.

  • What happens to your cells when there is an increase of salt in your blood? (When the salt level increases in your blood, water is pulled from your cells.) 
  • How do the cells communicate to your brain that they are loosing water and need help? (The cells in turn send a message to the brain making you thirsty in an attempt to dilute the salt content in your blood.) 
  • What is the definition of osmosis? (The process of water passing through the living cell wall is called osmosis.)

Student groups are given either carrot or celery pieces for the experiment. We want to make sure our observations are not specific to a single vegetable by only testing one type of vegetable.

Students begin the procedure section for this lesson and I walk around the classroom to check their observations.

 Students should observe that the vegetables are crisp and snap when you bend them.

The lesson asks students to identify their vegetable and record the percentage of water.

What is your vegetable? ___________________

What is the percent of water in your vegetable? _______

Students will use the Water Content of Vegetable Resource Document to find water content. A copy is in the resources section.

Students should find the carrots are 87% water and celery is 95% water.

I tell the students the water content is slightly more than the average water content of our cells. Based on the definition of osmosis and the short article they read, what do you think will happen to the vegetables after soaking in the fresh water overnight? Nothing or maybe even a bit more crisp. What do you think will happen to the vegetables after soaking in the salt water overnight? The vegetables will wilt.

The next day we check and verify the student predictions.

Students are curious if the process will reverse itself if they place the wilted vegetables back in the fresh water. The next day we find that they are not as wilted as before but do not seem to be as crispy as they were on day 1. 

In this short video, I explain some of the key elements of this lesson - reading for purpose and quoting text to support predictions and observations.

Connecting the Learning

5 minutes

Connecting the Learning

I ask students how does this experiment relate to the experience of the men from the USS Indianapolis in the reading from Cellular Hydration - Movement Across a Membrane Day 1? Students make the connection to the salt water in the ocean. There is a part in the reading when men believe that they can hold water in their hands and drink it after some of the water has evaporated. I ask students to use the term osmosis to explain why this is the worst possible thing the men could do? Students explain that there is less water and more salt, the water is saltier and will draw even more water out of their cells.

Students ask if osmosis is the same process that causes our fingers to wrinkle when we are in the water too long. I share that scientists do not think this is the case. The natural oil on our skin is washed away be the water. Our nervous system responds by wrinkling our fingers. Scientists think this allows us to grip items better is wet conditions.