One class period
Students will understand the following:
- how microscopes have contributed to our knowledge of life science.
- the basic differences between plant and animal cells.
- Review with students the basic differences between plant cells and animal cells. Students should be able to recognize that plant cells have cell walls, chloroplasts containing chlorophyll, other plastids that help store food, and a large vacuole. Animal cells do not contain cell walls and therefore their shape is less regular. Most animal cells have numerous small vacuoles that help in their storage of nutrients and waste products. Students should also be aware of the differences between prokaryotic cells and eukaryotic cells. Prokaryotic cells, such as bacteria cells, have no organized nucleus. Eukaryotic cells, such as plant and animal cells, have an organized nucleus.
- Tell students that in this activity they will be viewing images on the Internet of slide samples taken with a scanning electron microscope.
- Explain that the microscopes in your science lab are descendants of ones made in Holland around 1600. Some 400 years later, we still use instruments based on these compound microscopes, named because they used two or more lenses to magnify objects.
- Light microscopes, like the models in most schools, use compound lenses and light to magnify objects. The lenses bend or refract the light, which makes the object beneath them appear closer. If you examine a hand-held magnifying glass, you can see the convex light-bending shape of the lens.
- Explain that scanning electron microscopes (SEMs) allow scientists to view a universe too small to be seen with a light microscope. SEMs don’t use light waves; they use electrons (negatively charged electrical particles) to magnify objects up to two million times.
- Next, describe to the class how SEMs work:
- The specimen is specially prepared. It must be coated with something to make it conduct electricity—such as a thin layer of gold—to activate the electrons.
- The prepared object is placed in the bottom of a special column that creates a vacuum when all the air is pumped out. At the top of the column is the “gun” that shoots out electrons. Inside the column are lenses that focus the electrons on the specimen.
- Above the specimen, scanning coils move the electron beam back and forth across the entire object.
- When the beam moves across the specimen, it knocks other electrons off. The movements of these electrons are recorded and enhanced. The microscope is able to create a picture of the specimen based on the way the electrons bounce off it. The picture appears on a monitor. What you see when you look through a light microscope is a magnified image made from light reflecting off an object. In the case of a SEM, you do not see light but the activity of the electrons.
- Now invite your students to stimulate their brain cells by completing the Virtual Electron Microscope activity. Students will get a glimpse of several images created with an SEM. They will use clues and what they already know about cell structure to determine the identity of each image. Go to the The Virtual Electron Microscope.
- When students have completed the Virtual Electron Microscope activity, have them discuss what they observed. What images were easy to identify? Why? Were there cell parts that could be easily seen and identified? Compare the use of the compound microscope to the electron microscope. List advantages and disadvantages of each.
Tools and Technology High school students can do some research into high-tech devices that are revolutionizing the study of life science and medicine. In addition to SEMs, there are other types of microscopes that are bringing us closer than ever to a previously invisible world: transmission electron microscopes, acoustic microscopes, and scanning tunneling microscopes, which enable scientists to see individual atoms. Technology is also allowing us to see objects in space for the first time. High-powered telescopes, radio observatories, and space probes such as Galileo are bringing an otherwise invisible world to light. In the field of medicine, patients benefit from magnetic resonance imaging technology, computerized axial tomography, and ultrasound. Assign students a specific technology to investigate and have them prepare reports about it, including “demonstrations” of how it works and images, if available.
- What cell parts help plants survive in their habitats? What cell parts help animal cells survive?
- How do microscopes help scientists and doctors? Provide an example of how the microscope has greatly enhanced scientific knowledge.
- How might SEMs help diagnose and treat disease?
- Can you determine a cell’s function from its size and shape?
- Which images were most interesting and why?
- If you could look at something else under an SEM, what would it be? Why? What would you expect the magnified image to look like?
- What differences between plant and animal cells helped you to identify the cells in the specimens?
Provide students with some diagrams of cells or cell parts to label. This could be done for homework or as a class quiz. Students could also make a cell model and be graded based on their inclusion of all cell parts.
Join the Frog Force!
Students in grades 3 through 12 can help scientists monitor the conditions and populations of amphibians in your area. Some scientists believe amphibians are indicator species that may reveal the health of ecosystems. Since the mid 1990s, people have been finding frogs and other amphibians with deformities such as extra or missing limbs. As scientists try to find out why the animals are deformed, they could use students’ help.
Name that Slide
If you have access to compound microscopes in your school, set up a lab and let students sort several prepared slides similar to those in the Virtual Electron Microscope activity. Cover the labels on the prepared slides and have students identify each slide as a plant or animal cell or have them identify the object they observe from a list provided for them. Students could also diagram one of the specimens they observed. Be sure they draw only what they see under the microscope.
Access this resource at:
Virtual Electron Microscope
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Credit: Rhonda Lucas Donald, freelance writer specializing in science and natural history materials for students and teachers. Mary C. Cahill, middle school science coordinator, Potomac School, McLean, Virginia.