Cellular Hydration - Movement Across a Membrane Day 1

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SWBAT explain why water plays an important role in cells determining properties and facilitating function.

Big Idea

What can water do for cells? Water plays a role in moving substances in and out of our cells. We need to keep our cells hydrated or our ability to function starts to fail at the cellular level.

The Need for the Lesson

One common misconception of students is that cells do not need water. Just like our body, cells are approximately 70% water. Too much salt in out diet can cause even our cells to become dehydrated. In the context of a naval tragedy, this lesson explains the effects of dehydration at the cellular level.

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.

Investigation Preparation & Summary

10 minutes

Investigation Preparation & Summary

In this lesson students will learn about the effects of cellular dehydrate in the context of history by hearing about the symptoms experienced by the survivors of the USS Indianapolis as they waited for four days in the waters of the South Pacific for rescue. 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)

I engage students in learning about cellular hydration using a short read aloud from Left for Dead by Pete Nelson. Nelson describes what happened to the men of the USS Indianapolis at the physiological level as they awaited rescue in the South Pacific. As students are listening to the read aloud, they will hear well chosen descriptive language as the author retells the story of the men and their ordeal. The author's word choice brings the events to life. (RI.7.4 Determine the meaning of words and phrases as they are used in a text, including figurative, connotative, and technical meanings; analyze the impact of a specific word choice on meaning and tone.) As the reader, you will be modeling the reading and interpretation of non-fiction text for students. (RI.7.10 By the end of the year, read and comprehend literary nonfiction in the grades 6-8 text complexity band proficiently, with scaffolding as needed at the high end of the range.)

The USS Indianapolis was torpedoed by a Japanese submarine on July 30, 1945 and sunk in minutes. No one knew the ship was missing because it was on a top secret mission to deliver atomic bomb components to the Enola Gay, the plane that would later drop the bomb on Hiroshima. The crewmen spent four days in the ocean (not in boats) without food or water. 

Pete Nelson describes in detail what the men experienced as they struggled for survival, weaving science and history in this compelling story.

When you read this book, please take time to read the preface by Hunter Scott. You can summarize the preface for students. It offers a powerful connection for students given that Hunter Scott was a middle school student when he began researching the fate of the USS Indianapolis and its survivors.

Hunter was a 6th grader in Florida when he asked his father about the USS Indianapolis while watching the movie Jaws. He decided to make the USS Indianapolis the focus of his history fair project. Little information was readily available because the mission of the USS Indianapolis was top secret. So Hunter reached out to the survivors to learn more. Hunter's efforts are inspiring. Not only does he uncover stories of the survivors but at age 14, he testifies before Congress for the exoneration of Captain McVay who had been blamed for the the loss of his ship and his crew.

Hunter Scott is truly an inspiring role model. Pete Nelson takes the story of the survivors further by explaining what happens to the men at the cellular level as they experience dehydration over the four days in the ocean.

I always warn the librarian before I read this book. There is usually a rush to the library. Students love the reading from the book and boys especially are always looking for good non-fiction. 

There are a number of video interviews with survivors on youtube. Here is one that is especially moving.

A materials list for this lesson can be found in the resources section.

Students in Action

40 minutes

Students in Action

I begin by sharing these facts with students. Most of our body is made up of water - as much as 60% water. Our cells are also mostly water. In fact our cells are about 70% water.

Next I read the selected text aloud to students. (It is not possible to include the entire text from the book in this lesson. I have selected a few passages here that I read aloud to give you a feel for the narrative in the book.) Most of my students know someone who has served our country. When I read aloud from this book there is always silence. Students are deeply moved by the story. Most students know someone who has served our country. 

Pages I read - Beginning with the last paragraph on page 74 to end of first paragraph on page 82. I also read the Rescue of Harlan Tribble, the third paragraph on page 96.

"In a general sense, what the men in the water were experiencing was beyond their comprehension, a lesson of pain and suffering that made its impression as much on their minds as on their bodies."

"The pressure of the water alone can cause dehydration because water pressure increased as depth increases. The pressure a man treading water feels below his waist is greater than the pressure he feels above the water, that pressure squeezes the blood from the legs and forces it back into the torso where once again the brain senses excessive fluid levels in he core. It's the same reason some people feel the urge to urinate when they swim in a lake or a swimming pool."

"Without fresh water to drink, the survivors experience unimaginable thirst. First your mouth turns to cotton. Your saliva turns think and bitter, until it disappears altogether. You become aware of your tongue as a fat dry thing barricading your air passage."

"Severe dehydration feels like every cell in your body is crying out for water, largely because every cell in your body truly is."

"All the normal metabolic processes continue to function as best they can, each cell taking in oxygen and metabolizing it to produce waste products, which the body then has to get rid of by producing urine in the kidneys."

"The cellular craving for water affects the mind as well. The cells in the brain need water to function just lime any other cell in the body."

Nelson, Pete. Left for Dead. Delacorte Books for Young Readers, 2003.

I give students a few minutes to discuss the readings. They turn and talk for about 3 minutes, then we share out thoughts as a group. I am often emotional when I read the selections aloud, and so are my students. Many of us have connections to men and women who serve our country.

I compare a cell without water to a balloon without air. I ask students what happens to plants when they do not get enough water. Students explain that plants look droopy or wilted when they need water. What happens when you add water to these droopy, wilted plants? They absorb the water and stand straight again.

We take a look at an image of a plant cell and note the largest organelle is the central vacuole. I tell the students that the central vacuole contains water.

Plant Cell Image - Creative Commons 

What did you notice about the plant cells when you looked at the onion skins under the microscope? Were the cells separated or together? (Making a Wet Mount Slide) Students recall that the cells are next to each other. How does the location of the cells help the plant stand straight? The individual cells support each other helping the plant stand straight.

I often give book talks for my students. Stories are a great way for the brain to remember information. Some of the books I use to read aloud selected passages other I recommend for students because they are an extension of what we are learning in class. Still other books are recommended because they consider the possibilities - the main character in the Lunar Chronicles series is a cyborg. Are we really that far away from the technology that would allow us to replace body parts with electronics? 

I have included a video - Book Talks for the Science Classroom Video - in the resource section where I share some of my favorite books. There is also a document - Great Books for Science 2015. 

Connecting the Learning

3 minutes

I share with students a few additional properties of water that are great for our cells.

  • Water changes temperature very slowly. Because our cells are mostly water the temperature changes slowly in our cells as well. Quick temperature changes in our cells would be harmful. 


  • Water is necessary for transport of materials into our cells. Water also plays a role in removing waste products from the cell. 


Remember every time you take a drink of water, you are helping your cells!