Lab: Importance of Cell Size

Introduction: All organisms are composed of cells. The size and shape of a cell determines how well it can deliver nutrients to its interior. Since all cells and organisms depend upon the efficient delivery of gases, nutrients, and other important molecules, the relationship between a cell's surface area and its volume is an important regulating concept.

Purpose: This investigation illustrates why cells stop growing when they reach a certain size, why virtually all cells are about the same size, and finally, how the ratio of surface area to volume affects the way organisms have adapted to their environments!

Materials Plastic spoon 400 mL beaker Paper towel(s) Agar with 1% phenolphthalein 100 mL graduated cylinder Plastic knife indicator solution 0.1 M NaOH Metric ruler

*Remember safe laboratory techniques and to keep the lab area as neat and clean as possible.

PART 1: SURFACE AREA TO VOLUME RATIO 1. Each group will cut three agar cubes: A 3cm cube, a 2cm cube, and a 1cm cube. CUT AS ACCURATELY AS POSSIBLE. (This may be already completed for you.) 2. Measure the length, the width, and the height of each cube and calculate the volume using the following equation and record below: V = Length x Width x Height

3. Calculate the Surface are of each cube using the following equation and record below:

SA = Length x Width x (# of sides)

4. Calculate surface area to volume ratio by using the following equation and record below:

Surface area to volume ratio= Surface Area/Volume

5. Record all your answers in the chart below.

Data Table 1: Surface Area to Volume Ratio Cube I cm Cube 2 cm Cube 3 cm Cube

Volume (cm3)

Surface Area

Surface Area to Volume Ratio

PART 2: PERCENT DIFFUSION 1. Place all 3 cubes in a 400 mL beaker. 2. Pour enough 0.1M sodium hydroxide solution into your 400mL beaker so all cubes are fully submerged. 3. Let the cubes soak for approximately 10 minutes. Start Graphing your Surface Area to Volume Ratio Graph. 4. Periodically, gently stir the solution, or turn the cubes over. 5. After 10 minutes, use a spoon to remove the cubes from the sodium hydroxide solution. 6. Blot the with a paper towel. 7. Promptly cut each cube in half and measure the depth to which the pink color has penetrated. Sketch each block’s cross-section. 8. Record your measurements and sketch each cube in table 1. 9. Do the following calculations for each cube and complete the following data table:

Calculating % Diffusion  On one side, measure the distance the pink solution diffused to the center. Record this measurement below.  Use the following equation to determine the % Diffusion: o % Diffusion= (Length to center of cube/ Distance Diffused) X 100%

Data Table 1: Diffusion Cube 1 cm Cube 2 cm Cube 3 cm Cube

Sketch of cube (Include measurements)

Length to .5 cm 1 cm 1.5 cm Center of Cube

Distance pink solution diffused % Diffusion (use equation above)

Post-Lab Questions: Answer in complete sentences 1. What type of cellular transport did this investigation demonstrate? How do you know?

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2. What prediction can you make about the efficiency of a very small cell in: getting oxygen, getting rid of wastes, keeping water in a dry environment, and keeping heat in a cold environment? ______

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______3. What prediction can you make about the efficiency of a very large cell in: getting oxygen, getting rid of wastes, keeping water in a dry environment, and keeping heat in a cold environment? ______

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4. Using what you observed, explain why cells stop growing when they reach a certain size and why all cells are about the same size? Be sure to cite data from the lab to support your answer.

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5. Compare the appearance of the Northern Arctic hare (right) that lives in a cold climate and the Desert Jackrabbit that lives in a hot climate. Explain their appearance using the ratio of surface area to volume. Explain the evolutionary advantage for each.

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______6. Based on your data and observations, give a reason as to why many cell organelles have folded membranes as opposed to flat membranes.

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7. All of the cells shown below have the same volume. Circle the one that has the largest surface area.

8. Which of the cells in number 7 would completely change color in the experiment you just performed. Explain.

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______9. Some of the cells in your body (such as the walls of small blood vessels and the lining of air sacs in your lungs) are designed to allow the quick passage of nutrients and gases. Which of the shapes in number 8 would you expect them to be? Explain.

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