My Science Box

Summary
These experiments use a bell jar (or any other very large, clear, glass jar) to determine the identity of the gas produced by plants. It mirrors the famous experiments of Joseph Priestley and Jan Ingenhousz from the 1700’s that first demonstrated the existence of oxygen and its importance to plants and animals.

In 1771 and 1772, Priestly conducted a series of experiments using a bell jar. It was known that a candle placed in a sealed bell jar would eventually burn out and could not be relighted while still in the jar. Priestly discovered that a plant can survive indefinitely within a jar. Thus, he tried placing a plant into the jar with the burning candle. The candle went out as before and could not be relit right away. Priestly waited several days and tried again. The candle could be relit! The plant had restored the air inside the jar! (Do not try the next series of experiments since it harms animals!) Next priestly investigated what would happen to animals. He found that a mouse placed inside a sealed jar will eventually collapse. However, a mouse can survive in a sealed jar with a plant since the plant restores the air. Priestly was the first to demonstrate that oxygen is necessary for fire and animals but that given time, plants can create oxygen, allowing fires to burn and animals to breathe.

Elodea nuttallii: Image courtesy of Christopher Fischer.Summary
Students often believe that only animals “breathe”, but all things exchange gases with their environment. It’s just that the process is not so obvious in plants. Elodea is a very common water plant that can be found in aquarium stores. As photosynthesis occurs, oxygen is produced as a by-product. Elodea releases bubbles of oxygen as it photosynthesizes. In fact, the number or volume of bubbles in a certain amount of time can be used as a rough measure of photosynthetic rate.

Chlorophyll extractionSummary
Chlorophyll is the pigment in plants that captures sunlight energy and uses it to drive photosynthesis. While chlorophyll does give plants their characteristic green color, chlorophyll actually comes in many colors and subtypes ranging from green to yellow to orange to red. In this experiment, students use paper chromatography to separate the many pigments from one another. First the pigments are extracted from the plants by simply crushing the plant cells open on the filter paper with the edge of a penny. When the filter paper is then immersed in rubbing alcohol, the pigments are carried upwards through capillary action. The smallest pigments travel more quickly and thus separate from the larger pigments that remain closer to the origin line.

Summary
Here you will find a toolbox full of inquiry investigations on photosynthesis and respiration. Rather than the detailed lesson plans provided elsewhere at My Science Box, each experiment only contains a short background, materials, procedure, and going further section. It is up to you to decide which of the many experiments you wish to try with your students and how to sequence them. You will find everything from descriptions for how to extract chlorophyll, discover that plants “breathe”, recreate the experiments of Priestly and Ingenhousz, detect carbon dioxide production, and measure the rate of yeast respiration. None of these experiments require expensive equipment such as metabolism chambers or oxygen meters although those are great tools if you can afford them.

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.

Assessment

1. 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.
2. Provide other example conversion and ratio problems for students to solve.
3. 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.

Lesson Plan

1. Begin class with a review of the parts of the cell.
2. 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.
3. Discuss as much of the chemistry behind the making of slime as you wish.
4. 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.
5. 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?)
6. 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).
7. 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.
8. 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.
9. 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!)
10. 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
11. 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:
    

Getting Ready

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

To make PVA solution:

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

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 (C₂H₄O)_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.

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 (Na₂B₄O₇*10H₂O), 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