# Engage: The Ebb and Flow

Did you know you lose an average of 2.5 liters of water per day from normal bodily functions such as urinating and sweating? You replace it by drinking water and eating moist foods. It's like a cycle of ebb and flow— water in, water out. You maintain a balance of water in your body by replacing what you lose before you lose all 2.5 liters. Is this ebb and flow of water the same for the cells in your body?

Before you try to answer the question, get your science journal or notebook and something to write with so you can track what you learn. There are notes to take, data to record and analyze in a virtual lab, and sketches to make to help you remember the science vocabulary that goes with the concept of osmosis and cell homeostasis.

Are you ready? Start by watching the following video.

Note: Click "Related Items" at the bottom of the page to access the video transcript.

Did you make some notes during the video? What about the questions at the end? Does water flow into and out of cells? If you answered yes, how do you think it happens? Why would the water move at all? Write your thoughts down before you start the Cell Homeostasis Virtual Lab in Explore: Tug of War(ter).

# Explore: Tug of War(ter)

Keep your science journal handy. When you begin the lab, sketch the equipment used in the investigation. Make notes about the investigation procedures. Draw a data table and then graph the results so you can observe possible trends in water movement. Take charge of your learning.

Let's rethink the investigation. Look at the following chart to see how the experiment was set up. Beaker A represents the control group, and beakers B through E represent the experimental group. The change in concentration of the sugar solutions in the beakers is the independent variable. Can you identify the dependent variable? Do you remember which variables stay the same or are constant throughout the investigation?

Take a moment to answer a few questions in order to check your understanding of the investigation.

# Explain: Word Flow

Let's put some science words with what you've learned. Look at the images of the beaker and the dialysis tube. The percent concentration of both fluids is different. There is 0% sugar in the beaker and 10% sugar in the dialysis tube. Think of it from the water's point of view. There is 100% water in the beaker and 90% in the dialysis tube. So the beaker has a higher concentration of water than the dialysis tube. If you put the dialysis tube in the beaker, the water will flow across the tube's membrane into the tube. The term for the diffusion of water across a membrane from a higher concentration of water to a lower concentration of water is osmosis. Take a moment to make a sketch. Draw arrows from the beaker to the dialysis tube to show the flow of water. Write the definition in your own words.

These next three words are used to describe the comparative relation between the internal and external fluid environments of a cell. Examine the original meanings of the prefixes and root of the following words.

Put each prefix with the root and you get the words hypotonic, isotonic, and hypertonic. Now put the images with the words. Remember that these words describe the comparative relation between the two fluid environments.

During the Cell Homeostasis Virtual Lab, dialysis tube C is placed into a hypotonic solution in beaker C. The fluid in the dialysis tube is hypertonic compared to the fluid in the beaker. Consequently, the water will flow across the membrane and into the dialysis tube.

Next look at the comparative concentrations of dialysis tube D and beaker D. Is one higher or lower than the other? Would you say they are equal? Which word would you use to describe the relation between the two? If you chose isotonic, you are correct. Water molecules would still flow into and out of the dialysis tube through the membrane, but since there is equal tension, the movement is more random and not quite so directional. The word isotonic implies balance or homeostasis.

Last, examine and compare the concentrations in beaker E and dialysis tube E. In the Cell Homeostasis Virtual Lab, dialysis tube E is placed in the hypertonic solution in beaker E. What did you record in the chart you made during the lab? Did the dialysis tube gain water or lose water? If you said the tube lost water, you are correct!

Think of it from the water's point of view. Water flows across a cell's membrane from a higher concentration of water to a lower concentration of water. What is the process called? Osmosis. It might help to remember this phrase: "Higher to drier." Take the time to make some sketches and label them appropriately. Place arrows to indicate direction of water flow. Write the vocabulary words next to the sketches and reword the definitions to help you remember the terms.

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# Elaborate 1: A Closer Look

In this lesson, you have learned about the movement of water in and out of cells. You also have learned that the water molecules, not bound to a dissolved substance such as sugar or salt, move from a hypotonic solution toward a hypertonic solution until an isotonic balance is reached both in and out of the cell.

Did you know that some cells will not let water in at all? Did you know that an egg is a type of cell? Take frog eggs for example. In most cases, frogs lay their eggs in water. They are not protected by a shell. What keeps the water from moving into the egg until it lyses (bursts)? What keeps the water in the cell from leaving, making the cell shrivel and die?

 Frog Eggs

Watch this next video to figure it out!

Cite Source
Photo courtesy of the Forest Service, USDA
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Most plant and animal cells have aquaporins integrated into their cell membranes to facilitate the movement of water into and out of the cell. These protein pores are specialized to facilitate the transport of water and contribute to the selective permeability of plasma membranes.

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# Elaborate 2: Keeping It Real

Have you ever thought about what happens if too much water leaves a cell or too much enters? Think about the times you have eaten too many salty foods or had a soda with your food. Did you think about how the salt or sugar affected your cells? Check the information in the following image to see how much sugar is in different drinks and think about how your cells would react to the more hypertonic solutions.

Cite Source
http://wecan.nhlbi.nih.gov

Cells can maintain health by continuing the normal ebb and flow of water and other substances because of the characteristic of selective permeability of the cell membrane. Sometimes however, conditions around the cell are too extreme, causing the cell to either shrivel or explode.

There are two following videos that exemplify this behavior. The first video is of red onion cells initially exposed to a sugar solution and then exposed to water. The second video is of blood cells exposed to isotonic and hypertonic solutions and then to water. Make notes of your observations and answer the following question.

Why do the onion cells respond differently than the blood cells after being exposed to water?

If you are interested in finding out more about cell homeostasis and osmosis after viewing the videos, check Related Items at the bottom of the screen.

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# Teacher Notes

This resource is written in what we call a "5E lite" version. It doesn't include the probing questions that help a teacher check for understanding. There are no summative evaluations. There are no pre-lab investigations or additional assignments that bring in the critical component of collaboration in the learning environment. However, there are some components within the 5E lesson model that you, as an instructor, may use to spark understanding of the concepts of osmosis and cell homeostasis. Brief descriptions of each section and suggested (not mandatory) strategies follow.

Engage: The Ebb and Flow

This section introduces the idea of water flowing into and out of our bodies and our cells. It is an introduction to the virtual lab in the next section. You will notice that the video brings up additional concepts to which your students may or may not have been introduced within your content scope and sequence. One suggestion is to have students pair up to research information on these concepts to better inform the whole group.

Explore: Tug of War(ter)

The lesson supports and encourages students to be in charge of their own learning by taking notes, making sketches, recording data, and answering questions. In this section, students perform a virtual lab exercise exploring the concept of osmosis. There are a sample chart for recording data and a quick self-check quiz after the investigation. It is a good idea for students to either perform a pre-lab investigation or design additional investigations to further explore osmosis.

Explain: Word Flow

This section introduces the vocabulary words hypotonic, hypertonic, and isotonic. Students are encouraged to make or use sketches from the investigation to help them remember the word meanings. Students also may create vocabulary flash cards or a graphic organizer to support their learning. A "word flow" osmosis rap is included in this section to bring home the differences among the terms.

Elaborate 1: A Closer Look

This section introduces the "how" of water transport in cells by focusing on the aquaporin channels in the cell membrane. It starts with a picture of frog eggs, which are a type of cell that doesn't include any aquaporin channels in the membrane. A claymation video is included to explain how aquaporins work. The term selective permeability also is introduced. This may be an opportune moment to further explore the concept of selective permeability by having students research the structure and transport characteristics of the cell membrane.

Elaborate 2: Keeping It Real

The lesson closes with a look at sugar content in the drinks we include in our diet, the idea being that what we ingest has consequences, both positive and negative. There are two additional videos that compare plant cells to human cells and how they respond to hypertonic and isotonic solutions and then water. Students could do additional research or design other investigations correlating cell response to various types of solutions.

Related Items

Use the resources in related items to differentiate instruction for students.