Modeling the Prokaryotic Cell (Part 1/2)
Lesson 1 of 4
Objective: The students will compare prokaryotic cell structure to eukaryotic cell structure.
By building a prokaryotic cell model, students learn the differences between eukaryotes and prokaryotes. They also explore why bacteria are a good model organism. Here is an overview of what students will learn today.
Project the Launch Lab presentation. In their lab notebooks, students should create a data table or use this data table. At the end of four minutes, ask the students what similarities they saw among the different cells by using these questions.
- Did they have any of the same structures?
- Were they able to tell if they were the same size?
Next, ask students what differences they saw among the different cells by using these questions.
- What structures were found in some of the cells that were absent in others?
- Which kingdom of organisms was not shown in the Launch Lab (Answer: Protista).
Remind the students that they will be returning to the Launch Lab at the end of the hour.
(Note: I have students download a copy of this presentation onto their laptops and view the presentation individually. Students can then proceed through the slides at their own pace. The slides will also appear with better resolution making it possible to see more of the specific details in the cells. Students may not get completely through with this Launch Lab as shown with this sample student work. However, it will be revisited at the end of the lesson.)
Using the presentation, walk students through constructing a bacterial cell. As each structure is being made, discuss how it models the structure in the electromicrograph. Also, discuss the limitations of each structure in the cell model as compare to the electromicrograph.
(Note: There are days when lecture cannot be avoided. To keep my students actively involved, I have my students build model of the bacterial cell being described. Each student should build a bacterial cell model. For large class, precutting the plasmids, ribosomes and the polypeptides is recommended.)
Here are some basic instructions for the construction of the model.
- The cell membrane is represented by a snack size or sandwich size ziploc bag.
- The cytoplasm is the air in the bag.
- To make the DNA, twist two white chenille sticks together and attach the ends to make a circle. Place the DNA in the ziploc bag.
- To make the plasmids, twist two more white chenille sticks together. Cut the twisted chenille sticks into five equal sized pieces. Fold the cut pieces into circles. (Note: I have shoulder buddies share plasmids. One model gets three plasmids and the other model gets two plasmids.)
- To make the ribosomes, use two red chenille sticks and two blue chenille sticks. Cut the red chenille sticks into 4 pieces of equal length. Fold the ends of the cut chenille sticks toward the center to create the 50S subunit of the ribosome. Cut the blue chenille sticks into 6 pieces of equal length. Fold the of the cut chenille sticks toward the center to create the 30S subunit of the ribosome. (Note: As there are hundreds of ribosomes in the cell, have each student make their own ribosomes.)
- Have students close the bag. Next, have them gently push their fingers into the closed bag. The plastic in the bag will distort to make the pili. Using a small plastic milk straw, tape it to the bag to create a sex pilus.
- Insert the bacteria model into the cloth drawstring bag to create the cell wall.
- Fold and wrap the end of the drawstring bag around the large piece of foam tubing to create the flagella. Tape into place with clear packing tape.
- The capsule is not shown in this model due to its messy nature.
Once the model is constructed have the students all place their models in one location. Label that location a bacterial colony. (Note: If you have more than one section of biology, students can continue to place their models in the same location to demonstrate growth of the bacterial colony.)
Once students return to their seats, they should summarize the function of each of the structure in their lab notebooks. Finally, explain the limitations of using electron microscopes to view bacterial structures.
Have students recall how multicellular organisms are organized.
(Answer: organelles-->cells-->tissues-->organs-->organ systems-->organism)
Return to the Launch Lab and ask students to compare the Animalia and Plantae slides with the Bacteria cell model that students created in the activity.
Have students identify as many structures as they can on the images.
(Note: the plant image was taken using a light microscope as magnification. Students should be able to see the cell membrane, cytoplasm, and DNA. They will not be able to see chloroplasts, mitochondria, and other organelles. The animal cell image was taken with an electron microscope. Students should be able to see many organelles in this image.)
Discuss with students how animals, plants, and fungi are able to specialize and have tissues because how because of organelles in the cells. Bacteria do not have organelles and therefore remain small.
Finally, in their lab notebooks, have students label these four images (electromicrograph of a bacterial cell, artist's depiction of a gram stained membrane, bacteria at high power magnification, and bacteria at low power magnification) to allow them to understand the differences between organisms seen under a light microscope as compared to an electron microscope. Encourage students to notice certain structure. Explain why the bacterial cell must be stained in order to view it under a light microscope. Then have students brainstorm why bacteria would make a good model organism to prepare for tomorrow's lesson. Have students turn in their lab notebooks at the end of the hour for evaluation.