71 The Benefits of Being Multicellular
Key Question:
What are some disadvantages of being a multicellular organism?
Objectives:
Describe the function of different organ systems and how collectively they enable complex multicellular organisms to survive.
Bell Ringer:
Why can’t you use your teeth to breathe? Why can’t you use your arm muscles to digest food?
Materials:
 calculator (optional)
 cubic cell patterns
 heavy paper or poster board
 fine sand
 scale or balance
 scissors
 transparent tape
Note: Some students may find it difficult to work with a nonspecific unit of measurement. If so, the cube models easily convert to centimeters. You may want to add some small items, such as peas, beans, popcorn, or peppercorns to the sand to represent organelles floating in the cytoplasm.
Some students may need to review what a ratio is and how ratios are used.
Prepare four patterns for students to use to make their cubes. Make one cube 1 unit wide, one cube 2 units wide, one cube 3 units wide and one cube 4 units wide. The unit can be the size of your choosing.
Engage:
Ask students to think of ways that they use their muscles. Responses will probably include walking, riding a bike, swimming, and throwing or kicking a ball. Lead students to understand that muscles are also involved in swallowing food (tongue and esophagus), digestion (stomach and intestines), and blinking eyes (eyelids). Also, help students understand that sometimes muscles act voluntarily (jumping, writing), and sometimes they act involuntarily (heart beating).
Explore:
 Use heavy paper or poster board to make four cubeshaped cell models from the patterns supplied by your teacher. Cut out each cell model, fold the sides to make a cube, and tape the tabs on the sides. The smallest cell model has sides that are each one unit long. The next larger cell has sides of two units. The next cell has sides of three units, and the largest cell has sides of four units. These paper models represent the cell membrane, the part of a cell’s exterior through which food and waste pass.
 Copy the data table shown below. Use each formula to calculate the data about your cell models. Record your calculations in the table. Calculations for the smallest cell have been done for you.
 Data Table for Measurements

Length of Side Area of One Side
(A = S x S)Total Surface area of Cube Cell
(TA = A x 6)Volume of Cube Cell
(V = S x S x S)Mass of Filled Cube Cell 1 unit 1 unit^{2} 6 unit^{2} 1 unit^{3} 2 units 3 units 4 units  Key to Formula Symbols: A = area, S = the length of one side, TA = total area, 6 = number of sides, V = volume
 Carefully fill each model with fine sand until the sand is level with the top edge of the model. Find the mass of the filled models by using a scale or a balance. What does the sand in your model represent? (cytoplasm) Add some small items, such as peas, beans, popcorn, or peppercorns, to the sand to represent organelles floating in the cytoplasm.
 Record the mass of each filled cell model in your Data Table for Measurements. (Always remember to use the appropriate mass unit.)
Explain:
Data Table for Ratios
 Make a data table like the one shown below.

Length of Side Ratio of Total Surface Area to Volume Ratio of Total Surface Area to mas 1 unit 2 units 3 units 4 units  Use the data from your Data Table Measurements to find the ratios for each of your cell models. For each of the cell models, fill in the Data Table for Ratios.
 As a cell grows larger, does the ratio of total surface area to volume increase, decrease, or stay the same? (decreases)
 As a cell grows larger, does the total surface areatomass ratio increase, decrease, or stay the same? (decreases)
 Which is better able to supply food to all the cytoplasm of the cell: the cell membrane of a small cell or the cell membrane of a large cell? Explain your answer. (the cell membrane of a small cell; a small cell has a higher surface areatovolume ration than a large cell has, so more nutrients per cubic unit of volume can enter a small cell)
 In the experiment, which is better able to feed all of the cytoplasm of the cell: the cell membrane of a cell that has high mass or the cell membrane of a cell that has low mass? You may explain your answer in a verbal presentation to the class, or you may choose to write a report and illustrate it with drawing of your models. (the cell membrane of a cell that has a low mass)
Extend:
Imagine that you have a tiny boxshaped Protist for a pet. To care for your pet Protist properly, you have to figure out how much to feed it. The dimensions of your Protist are roughly 25 µm x 20 µm x 2 µm. If seven food particles per second can enter through each square micrometer of surface area, how many particles can your Protist eat in a minute?
Evaluate:
 Paper models representing the cell membrane to illustrate the concept of surface areatovolume ratio
 Teacher Observation
 Calculations, measurements and data tables
Checks for Understanding:
Explain how different organ systems interact to enable complex multicellular organisms to survive.
CrossCurricular Extension:
A great way to learn something is to teach it to someone else. Have students write a letter to a friend explaining how cells, tissues, organs, and organ systems are related.
**Copied from Holt Science & Technology Tennessee Grade 7, page 6869, Holt McDougal, Houghton Mifflin Harcourt 2010