BioEd Online

Teacher Background

Every living thing is composed of cells, the microscopic building blocks of life. In fact, most life forms exist as single cells that carry out all functions needed for their own independent existence. Examples of common single-celled organisms are bacteria (tiny organisms found in almost every habitat on Earth), diatoms (algae that are common components of phytoplankton), and yeast (a kind of fungus). Multicellular organisms consist of several to many cells. Single-celled and small multicellular organisms, which must be magnified to be observed, are called microbes or microorganisms.

Plants and animals are examples of multicellular organisms visible to the naked eye. These macroscopic multicellular organisms can have up to trillions of cells that carry out specialized functions.

This activity uses plant cells, because many of these are relatively easy to see. Students will observe onion cells (in the thin membrane around each onion “ring”) and a leaf from Elodea. With these examples, students will be able to see basic parts of cells, including the nucleus (structure in the center of the cell that holds hereditary information), cytoplasm (gel that fills the cell), cell wall (rigid outer boundary of plant and other kinds of cells), and chloroplasts (large green structures in which photosynthesis occurs).

Objectives and Standards

Inquiry

• Communicate scientific procedures and explanations.

• Use mathematics in all aspects of scientific inquiry.

• Develop descriptions, explanations, predictions, and models using evidence.

• Use appropriate tools and techniques to gather, analyze, and interpret data.

Life Science

• Living systems at all levels of organization demonstrate the complementary nature of structure and function.

• All organisms are composed of cells—the fundamental unit of life. Most organisms are single cells; other organisms, including humans, are multicellular.

• Cells carry on many functions needed to sustain life.

Materials and Setup

Teacher Materials (see Setup)

• 6 sheets of card stock

• overhead projector

• pair of scissors or a paper cutter

• safety goggles

• sharp knife

• transparency of "Magnifying Cells" sheet

Materials per Group of Students

• 4 pairs of safety goggles

• 2 plastic cover slips

• 2 plastic microscope slides

• 2 pairs of forceps

• 1/6 of an onion, vertical slice

• small stalk of Elodea leaves

• iodine solution and pipette (dropper)

• water and pipette (dropper)

• microscope (one or more per group)

• science notebooks or drawing paper, or clean copy of "Magnification Observations" sheet (see Lesson Media tab, above).

• Prepared set of "Preparing & Viewing Slides" cards on cardstock (see Lesson pdf)

• Group concept map (ongoing)

SETUP

1. Make copies of the "Preparing and Viewing Slides" page on cardstock, and cut out one set of cards per group. Make a transparency of "Magnifying Cells" page.

2. Prepare a tray for each group with all materials listed above and place trays in a central location.

3. Have students work in groups of two or four, depending on resources.

4. Optional: If Elodea is not available, new growth celery leaves may be substituted.

Procedure and Extensions

1. Begin by showing students a transparency of the "Magnifying Cells" sheet. Explain that the structures visible in each frame cannot be seen without magnification. Let students ask questions about what is visible in the images. Tell students that they will be making their own slides to observe the tiny structures, called “cells.”

2. Point out the labeled parts of the cells on the transparency. Help students understand that they will look for similar structures in their specimens.

3. If necessary, review microscope use with all students. If available, use a micro projector or video attachment on a microscope to demonstrate how to view cells, change magnifications, and make observations.

4. Have students work in groups. Tell them to follow the instructions on their "Preparing and Viewing Slides" cards to prepare their slides.

5. After each group has created both slides, have students take turns observing and drawing their specimens (noting the magnification being used). Have students first examine the cells using low power and then refocus using a higher power objective. Instruct students to make detailed drawings and to label any cell parts that are recognizable. Tell students that some parts of a cell may not be visible when viewed under a microscope. Allow 10–20 minutes for this step.
   
   Note: Have students determine the total magnification by multiplying the power stamped on the eyepiece (for example, 10x) by the power of the objective (lens).

6. Students usually will be able to observe the cell nuclei in the stained onion skin cells. They also should be able to observe cell walls and cytoplasm in both kinds of cells and identify chloroplasts in the Elodea cells.

7. Display the "Magnifying Cells" transparency for students. Encourage groups to discuss among themselves what they observed. Ask, Are all the cells about the same size? Could you see a dot (nucleus) inside all the cells? If not, why?

8. Explain the names and functions of the cell structures that students observed and drew.

9. As an assessment, ask students, What are the major parts of the cells you observed? (Structures most likely to be identified include cell wall, nucleus, chloroplasts, and cytoplasm.) You also might ask, What similarities and differences did you observe between the two kinds of cells? Students can record responses in their science notebooks or turn in their answers as assignments. You also may question each group individually.

10. Allow students time to add information to their concept maps. Explain that while the class has examined some cell structures of multicelled organisms, many organisms consist of only one cell. Students will have opportunities to learn more about single-celled microorganisms in later activities.

Handouts and Downloads