1. Ask students to report their research on the largest and smallest living organisms. Then ask:

• What do you think the largest and smallest living organisms have in common?
• What is the smallest functioning structural unit of life?

2. Show the Single-Celled Organisms video and discuss the following:

• What characteristics do unicellular (single-celled) organisms have in common? How do they differ from one another?
• What characteristics do unicellular organisms such as the ones you have just seen share with multicellular organisms such as insects, birds, dogs, or humans?
• How do these unicellular organisms get nourishment, water, and oxygen and get rid of wastes?
• How do unicellular organisms move?
• How do unicellular organisms reproduce?

3. Show the Mitosis video. Explain that cells divide during a process called mitosis. Single-celled organisms can reproduce by this method to produce two identical cells. Multicellular organisms use mitosis to grow and to replace worn-out cells.

4. Have students complete the mitosis section of the How Cells Divide: Mitosis vs. Meiosis Web activity

5. Explain that multicellular organisms are more complex than unicellular ones. They have tissues and organs made of specialized cells that perform various functions such as digesting and circulating nutrients and oxygen and getting rid of wastes. Ask:

• How do multicellular organisms get nourishment, water, and oxygen and get rid of wastes?
• If all multicellular organisms start from a single cell -- the fertilized egg -- how do they develop specialized tissues, organs, and systems like the heart and circulatory system, the stomach and digestive system, and the kidneys and excretory system?

6. Show the Cell Differentiation video followed by the The Embryo Takes Shape video. Discuss the following:

• Why do you think the heart forms early in the embryo's development?
• How would you explain the similarities in the embryos of birds, mammals, and reptiles at early stages of development?
• What directs the sequence of events that turn a blob of cells into an embryo with specialized tissues and organs?
• How do cells in different parts of the embryo become different kinds of cells and organs?
• What role does DNA play in the development of the embryo?
• What kinds of proteins are found in different cells and organs and how are they produced by cells?

7. Have students explore the Gallery of Cells for similarities and differences in cell structure and function. Ask them to draw at least three types of specialized cells, one on each index card, and to label the cell type and any cell structures. Then have students describe the similarities and differences among the different cells and how their structure is related to their function.

8. Attach the life-size outline drawing of the human on the board and ask students to tape their cell drawings to the place in the body where these cells would be found. Afterwards, ask:

• Are all these cells in the correct location?
• Where else might these cells be found?
• How do the cell drawings compare to the actual size of cells in a human body?
• Where in the body would you expect to find the greatest number of each kind of cell?
• What other kinds of cells are in the body?

Have students research and draw these other cells and then add them to the body outline.

9. Show the Immune Cells in Action video. Discuss how white blood cells are specialized to fight viruses. Ask students where they would expect to find white blood cells in the body and if there might be more of these cells in one area of the body than in another. Have students explore the action of immune cells with the Fighting Back Web activity. Ask:

• What disease-fighting cells are always present in the body?
• What cells or parts of cells are produced after the disease-causing virus enters the body?
• What are some ways in which the body develops immunity to a disease?
• Must a person have had mumps to be immune to the mumps virus?
• If the human body can successfully fight off the mumps virus, why can't it fight off the HIV virus that causes AIDS?