To solidify students’ conceptualization of cells, students build a model of a cell in a ziplock bag using polyvinyl alcohol slime as cytoplasm. So far, students’ experience with cells has ...

**Summary**

To solidify students’ conceptualization of cells, students build a model of a cell in a ziplock bag using polyvinyl alcohol slime as cytoplasm. So far, students’ experience with cells has been 2 dimensional – diagrams and microscopic slides. The 3 dimensional nature of cells comes to life as students use everyday objects to represent the many parts of a cell. In addition, students can use this activity to develop a sense of scale, calculating how big a human would be if the ziplock bag cell model were really the size of a cheek cell.

**Objectives**

Can build a three dimensional scale model of a cell.

Can name and describe the function of certain plant and animal cell organelles.

Can draw and label a picture of plant and animal cells.

Can recognize that all living things are made of cells.

Can use proportions and ratios to calculate the size of a person made of ziplock bag sized cells.

Can begin to use the metric system of measurement.

**Vocabulary**

Polyvinyl alcohol (PVA)

Borax

Polymer

Cell

Organelle

Cell membrane

Cell wall

Cytoplasm or cytosol

Nucleus

DNA

Chloroplast

Mitochondria

Ribosome

Golgi apparatus

Endoplasmic reticulum

Vacuole

Lysosome

Scientific notation

Metric system

Attachment | Size |
---|---|

5slimy_cells.doc | 67 KB |

cell_model-handout.doc | 44 KB |

**Time**

20-25 min introduction and make PVA slime

30-45 min assemble cell models

10-15 min create cell model keys

20 min discuss the metric system of measurement and use ratios to calculate the relative size of a person made of ziplock bag sized cells

**Grouping**

Individual

**Materials**

Each student needs a copy of the Slimy Cell Models handout.

For enough PVA slime for a class of 30 students:

- 240 g PVA (order from Flinn Scientific catalog # P0154, $19 for 500 g)
- 75 g Borax (Sodium Tetraborate Decahydrate (Na
_{2}B_{4}O_{7}*10H_{2}O), marketed as a “laundry booster” and found in most grocery stores and pharmacies among the laundry detergents for around $10 for a lifetime supply, 76 oz box) - water
- several colors of food coloring
- 30 ziplock sandwich bags
- 9-12 empty water bottles with pop-top lids
- 3-4 empty two liter soda bottles
- 8-10 100 ml graduated cylinders
- 8-10 50 ml beakers with gradations on the side
- 3-4 trays or plastic bins for placing materials for making slime
- large pot and stove (PVA requires heat to fully dissolve in water)

For cell walls, gather as many pint-sized strawberry baskets as you can – two baskets, one inverted over the other, readily enclose a filled ziplock bag, demonstrate the structural support provided by the cell wall, and illustrate the permeability of the cell wall.

For organelles, assemble a wide assortment of small, inexpensive items that students can select from. It is best to avoid food items like beans and candy since they will decompose and grow mold inside slime, creating a disgusting mess. An alternative is for students to bring objects from home for their models. Some items you may want to consider include:

- large confetti
- pony beads
- pom-pom balls
- wooden beads
- Styrofoam peanuts
- paper clips
- tin foil
- saran wrap
- bubble wrap
- aquarium gravel
- plastic drinking straws
- aquarium tubing
- yarn
- ribbon
- Christmas tinsel
- plastic Easter eggs

**Setting**

Classroom

**Teacher Background**

Making models of cells is a fun, meaningful activity for students to help them visualize the 3 dimensional nature of cells. See the background section of the Seeing Cells activity for a description of cell parts and their functions. This puts a slightly different twist on the standard shoebox cell model by using PVA slime to suspend the various organelles much like a real cell's cytoplasm does. It is also much less messy than the often-used jello cell models since everything stays contained within a ziplock bag (and is not sticky if spilled on the floor - though avoid getting slime on carpet).

The PVA slime recipe used in this activity is:

- 180 ml 4% PVA solution
- 30-35 ml 5% Borax solution

This makes a wonderful, viscous, oozing slime that is wet to the touch but holds together well even if removed from the ziplock bag. Polyvinyl alcohol exists in water as a long polymer of (C_{2}H_{4}O)_{n} units. Each chain is up to 2,000 units long. When Borax is combined with the PVA solution, the PVA chains crosslink, forming a highly viscous gel. Since the crosslinks are weak, they continually break and reform as the slime is handled.

PVA slime is quite safe to touch and handle, although you don't want to eat any since the Borax is toxic in large doses. It is easy to clean up with soap and water. Unadulterated slime can be stored for several weeks in a ziplock bag.

I also use this activity to introduce students to the metric system of measurement and the use of ratios to see the relative size of things. Although students realize that cells are tiny, especially after looking through the microscope at them, it is often hard for them to imagine just how tiny cells really are. By going through all the steps of calculating how big a human would be if one of their ziplock bag cell models was really a cell, they are better able to recognize just how tiny a cell is.

For your reference, below is a table showing standard versus scientific notation as well as the common metric prefixes for each.

Standard notation | Scientific notation | Common prefix | Common symbol | Example |

1000 | 1 x 10^{3} |
kilo- | k | kilometer (km) |

100 | 1 x 10^{2} |
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10 | 1 x 10^{1} |
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1 | 1 x 10^{0} |
none | none | meter (m) |

0.1 | 1 x 10^{-1} |
deci- | d | decimeter (dm) |

0.01 | 1 x 10^{-2} |
centi- | c | centimeter (cm) |

0.001 | 1 x 10^{-3} |
milli- | m | millimeter (mm) |

0.0001 | 1 x 10^{-4} |
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0.00001 | 1 x 10^{-5} |
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0.000001 | 1 x 10^{-6} |
micro- | u | micrometer (um) |

0.0000001 | 1 x 10^{-7} |
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0.00000001 | 1 x 10^{-8} |
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0.000000001 | 1 x 10^{-9} |
nano- | n | nanometer (nm) |

0.0000000001 | 1 x 10^{-10} |

A human cheek cell is approximately 58 micrometers (um) or 0.000058 meters (m) wide. A typical seventh grader is approximately 1.6 meters (m) tall. A standard ziplock sandwich bag is approximately 16 centimeters (cm) or 0.16 meters (m) wide. Thus you can set up a proportion to figure out how big a human being would be (x) if the ziplock bag represented a cheek cell:

____x____ = ____0.16 m____

1.6 m 0.000058 m

Solving for x you get 4414 meters or 4.4 kilometers. Thus, a human made of cells as big as a ziplock bag would be 4.4 kilometers tall (over 2.7 miles)! Just imagine how many slimy cell models it would take to fill a statue over 4 kilometers tall (around 10 trillion, that's 1 x 1013). A blood cell takes around 30 seconds to circulate around the human body - in our enlarged model, that's comparable to a ziplock bag blood cell completing a 3 mile round-trip journey in 30 seconds, at 360 miles an hour!

**Student Prerequisites **

Students need a good background in cell structure, parts of a cell, and their functions before undertaking this activity. It is helpful if students have experience with ratios and proportions in math class and if they have had some exposure to the metric system of measurement though not required.

**Getting Ready**

- Make copies of the Slimy Cell Models handout (or make a single transparency copy for the teacher).
- Set out assorted materials for the cell parts in a sort of buffet line.
- Set out graduated cylinders, 50 ml beakers, ziplock bags,

__To make PVA solution:__

- Bring 5.76 liters of water to a boil in a very large pot.
- Measure out 240 g PVA.
- Mix PVA into boiling water. Simmer on medium-low heat, stirring regularly, for 10-15 minutes or until all PVA has been dissolved.
- Allow PVA solution time to cool slightly in the pot.
- Pour into 3 or 4 two liter bottles.

__To make Borax solution:__

- In a large container, combine 1.5 liters of water with 75 g Borax.
- Stir until fully dissolved.
- Distribute the Borax solution among 9-12 pop-top water bottles.
- Add 3-4 drops of food coloring to each of the bottles. I made my stock solutions the standard red, yellow and blue so that my students could mix the colors themselves to achieve other colors of the rainbow.

**Lesson Plan**

- Begin class with a review of the parts of the cell.
- Describe the activity to students – they will be making 3 dimensional cell models. A ziplock bag will represent the cell membrane. Slime will represent the cytoplasm. If they wish to make a plant cell, strawberry baskets will represent the cell walls. Students can choose the rest of the “parts” to make up all the organelles.
- Discuss as much of the chemistry behind the making of slime as you wish.
- Pass out the slime making materials and lead students through the creation of slime.
- First, each student will need a ziplock bag.
- Using the graduated cylinders, measure out 180 ml PVA and add that to the bag.
- Using the small beakers, measure out 30-35 ml colored Borax – mix and match colors as you wish to get the final color you want – and add that to the bag.
- Zip the bag closed and gently massage the contents until the colored Borax is evenly distributed throughout the PVA and the slime coalesces.
- The slime may now be touched and/or carefully taken out of the bag.
- Pass out the handout and describe the assignment – in addition to adding organelles to the cell, students should create a key describing what was to represent each part of the cell and why that object was chosen. (Is it similar in size? shape? function? design?)
- Answer any questions. Have students set up the key on a piece of paper before they go to the cell parts “buffet” (or else some may never get around to creating a key at all or understanding the purpose of the activity).
- Once students have their key outlined and decided whether to make an animal or plant cell, allow them to browse the cell parts “buffet” and add objects to their cell. Make sure that they record what they chose for each organelle and why on their key.
- When all the cells are complete, go through each part of the cell and survey what different students chose to represent that part and why. This helps reinforce the vocabulary and the functions of each organelle.
- Ask students how many times larger this ziplock bag cell is compared to the cheek cells they observed under the microscope. (Almost 3,000 times larger!)
- To show students how this is calculated, present and describe the metric system of measurement.
- A ziplock bag is around 16 centimeters wide. A centimeter is one hundreth of a meter (0.01 meters). This a ziplock bag is 0.16 meters wide.
- A cheek cell is 58 micrometers wide. A micrometer is one millionth of a meter (0.000001 meters). Thus, the cheek cell is 0.000058 meters wide.
- To see how much larger a ziplock bag is compared to a cheek cell, divide the size of a ziplock bag (in meters) by the size of a cheek cell (in meters): 0.16 m/0.000058 m = 2,759
- Since all living things are made of cells, how tall would a person made of ziplock bag sized cells be? A typical middle school student is around 1.6 meters tall. To calculate this, you need to set up a ratio:

____x____ = ____0.16 m____

1.6 m 0.000058 m

Solving this ratio, you find that x is 4414 meters (4.4 kilometers or 2.7 miles)! Thus a person made of ziplock bag sized cells would be 2.7 miles tall! See some of the other fun facts in the teacher background.

**Assessment**

- Collect the ziplock bag cells and the keys. They can be displayed around the room for some time although students tend to like to take the models home and play with them.
- Provide other example conversion and ratio problems for students to solve.
- Revisit the characteristics of life list from the Is It Alive? activity. Revise the criteria as necessary to include that all living things are made of cells.

**Standards**__Grade 7__

Cell Biology

1. All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept:

a. Students know cells function similarly in all living organisms.

b. Students know the characteristics that distinguish plant cells from animal cells, including chloroplasts and cell walls.

c. Students know the nucleus is the repository for genetic information in plant and animal cells.

d. Students know that mitochondria liberate energy for the work that cells do and that chloroplasts capture sunlight energy for photosynthesis.

e. Students know cells divide to increase their numbers through a process of mitosis, which results in two daughter cells with identical sets of chromosomes.

f. Students know that as multicellular organisms develop, their cells differentiate.

Structure and Function in Living Systems

5. The anatomy and physiology of plants and animals illustrate the complementary nature of structure and function. As a basis for understanding this concept:

a. Students know plants and animals have levels of organization for structure and function, including cells, tissues, organs, organ systems, and the whole organism.

b. Students know organ systems function because of the contributions of individual organs, tissues, and cells. The failure of any part can affect the entire system.

Investigation and Experimentation

7. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:

d. Construct scale models, maps, and appropriately labeled diagrams to communicate scientific knowledge (e.g., motion of Earth's plates and cell structure).

__Grades 9-12 Biology__

Cell Biology

1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism's cells. As a basis for understanding this concept:

a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.

c. Students know how prokaryotic cells, eukaryotic cells (including those from plants and animals), and viruses differ in complexity and general structure.

e. Students know the role of the endoplasmic reticulum and Golgi apparatus in the secretion of proteins.

f. Students know usable energy is captured from sunlight by chloroplasts and is stored through the synthesis of sugar from carbon dioxide.

g. Students know the role of the mitochondria in making stored chemical-bond energy available to cells by completing the breakdown of glucose to carbon dioxide.

j. * Students know how eukaryotic cells are given shape and internal organization by a cytoskeleton or cell wall or both.