Name ______Date ______

Surface to Volume Ratio Activity

Background Information

When an object, like a cell, is small it has a large surface area compared to its small volume. Surface area is the total area of the surface of a three-dimensional object. Volume is how much the object can hold inside of it.

Cells are limited to how large they can grow because the surface area and volume ratio does not stay the same as their size increases. Because of this, it is harder for a large cell to pass materials in and out of the membrane, and to move materials through the cell.

An amoeba is a small unicellular organism, but has a large surface area compared to its volume. This allows materials to easily move in and out of the cell.

An elephant is larger than an amoeba because it has more cells, not larger cells. It is multicellular, but its individual cells are about the same size as the single-celled amoeba.

Lab Activity:

Two potatoes have been cut to represent a 1x1cm cell and 3x3cm cell. The cells will be placed overnight into Lugol’s solution, which will diffuse into the cell.

Make a hypothesis about which cell (smaller or bigger cube) you think will allow the greatest amount of Lugol’s solution to penetrate into the cell (potato). ______

Procedure

Day 1

  1. With a ruler, draw a 1x1cm cube and a 3x3cm cube in the space below. Calculate the surface area and volume of each.

Draw 1x1cm cube / Draw 3x3cm cube

Comparing Cell Sizes

  1. Using the formula (Surface Area= Length x Width x # of sides) figure out the surface area for each of the cubes. Write the numbers in the data table.
  1. Next using the formula (Volume= Length x Width x Height) figure out the volume for each cube. Write the numbers in the data table.

Cell

/ Surface Area / Volume / Surface Area to Volume Ratio
Small 1x1cm
Large 3x3cm
  1. Your teacher will place each cube into Lugol’s solution overnight.

Day 2

  1. Your teacher will cut both cubes in half so that you can observe a cross section of the cubes and determine how far the Lugol’s was able to penetrate the cell. Draw your observations below and shade in the cell appropriately.

Draw 1x1cm cube cross-section / Draw 3x3cm cube cross-section

Questions:

1. Based on your cross section drawing, which cube (biggest or smallest) has the greatest fraction (portion) of its total volume penetrated by the Lugol’s solution? ______

2. Which cube (biggest or smallest) has the greatest fraction of its total volume NOT penetrated by the Lugol’s solution?

______

3. If the Lugol’s solution was a nutrient and the cubes were unicellular organisms, which cube (biggest or smallest) would be able to “feed” more of its volume? ______

4. Anything that the cell takes in, like oxygen and food, or lets out, such as carbon dioxide, must go through the cell membrane. Which measurement (surface area or volume) of the cells best represents how much cell membrane the models have?

______

5. The cell contents, nucleus and cytoplasm, use the oxygen and food while producing the waste. Which measurement (surface area or volume) best represents the cell content?

______

6. As the cell grows larger and gets more cell content, will it need more or less cell

membrane to survive? ______

7. Which size cell has the greatest chance of survival? ______

8. What can cells do to increase their surface area without increasing their volume?

______

______

9. Is it better to have four small cells or one large one? ______

______

10. Can unicellular organisms grow very large? Why or why not? ______

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