In this activity, students explore the structure and function of cells. They learn that all organisms are made up of one or more cells and that single-celled organisms do many of the things multicellular organisms do: grow, eat, excrete, and reproduce. Students observe that all the things cells need to take in or get rid of pass through the cell membrane. Using balloons as models, they explore the relationship between the surface area of a cell and the cell's volume, and discover that the ratio of surface area to volume constrains cell size. Finally, students learn about cell replication, by which a cell makes a copy of its genetic material and then divides into two "daughter" cells, each identical to the parent cell.
- Recognize that all organisms are made up of one or more cells
- Observe how the cell membrane serves as gatekeeper for all substances that enter and leave the cell
- Understand that as a cell grows larger, its surface-area-to-volume ratio decreases
- Learn that, prior to dividing, cells must replicate their DNA
- Discover that cell division is the means by which single-celled organisms reproduce and multicellular organisms grow
- Observe that mitosis results in two daughter cells that are identical to the parent cell
- Understand the role of oncogenes in the development of cancer
- Two to three class periods
- Single-Celled Organisms QuickTime Video
- Cell Membrane: Just Passing Through Flash Interactive
- Mitosis QuickTime Video
- How Cancer Cells Grow and Divide QuickTime Video
- How Cancer Grows Flash Interactive
- Round balloons (enough for every student in the class)
Before the Lesson
- Review the concepts of cell structure and function, using the lesson App Exception: tdc02.sci.life.cell.lp_strufx Make sure students understand the gatekeeper role of the cell membrane.
- Read the background essay that accompanies each resource.
After the Lesson
Do an experiment with students to demonstrate how a cell's surface-area-to-volume ratio affects the rate of diffusion of substances into and out of the cell. (See BSCS Biological Science: An Ecological Approach for a possible lab.)
Part I: The Cell and Its Limitations
1. Show the Single-Celled Organisms video and discuss the following:
- How are single-celled organisms different from multicellular organisms? How are they similar?
- What are the two basic types of cells, and how do they differ from each other?
- Describe some of the structures and methods single-celled organisms use to move.
- What must all species be able to do in order to survive?
2. Lead students in an exploration of the Cell Membrane: Just Passing Through Web activity and discuss the following:
- What are some of the things that cells must take in to survive?
- What are some of the waste products that cells must eliminate to survive?
- Name some of the systems in a multicellular organism that serve to take in and distribute substances and get rid of wastes.
- Do single-celled organisms have transport systems similar to those of multicellular organisms?
3. Explain to students that single-celled organisms take in and release substances by means of diffusion across the cell membrane, and that this process limits cell size. Ask students to consider why this might be.
4. Blow up a balloon and tie it off. Tell students that the balloon represents a cell. Then blow up a second balloon larger than the first and tie it off. Hold up the balloons and ask students:
- As a cell grows, what happens to its surface area?
- As a cell grows, what happens to its volume (the amount of space the cell takes up)?
- What is the relationship between the surface area and the volume of a cell? (As one increases [or decreases], so does the other.)
- If the surface area of a cell doubles, does the volume double also? (No. For example, a balloon with a surface area of 300 square inches has a volume of about 490 cubic inches. If the surface area doubles to 600 square inches, the volume increases to nearly 1400 cubic inches -- almost three times the original volume. The surface area and volume change disproportionately to one another.
5. Now have students work in pairs to demonstrate this for themselves. Provide each pair with two balloons and ask them to blow up and tie off the balloons. One balloon should be noticeably larger than the other. Have pairs of students measure the radii of the two balloons and calculate the surface area and volume of each. (Note: Tell students to assume that each balloon is a perfect sphere. The formula for calculating the surface area of a sphere is A = 4 π r 2. The formula for calculating the volume of a sphere is V = 4/3 π r3.)
Now remind students that cells rely on diffusion to take in what they need and to get rid of wastes. This means that it is best to have all cell material as close to the cell membrane as possible. Ask students:
- What happens to the surface-area-to-volume ratio as a cell gets larger?
- How would a smaller ratio (i.e., a smaller surface area per volume) affect the cell's ability to function?
- Is it easier for a large cell or for a small cell to take in and get rid of substances by diffusion? Why?
Part II: DNA Replication and Cell Division
6. Show the Mitosis video and discuss the following:
- What is the function of mitosis in a cell that is about to divide?
- What is the difference, if any, between the two daughter cells that result from cell division following mitosis?
7. Explain that single-celled organisms reproduce via DNA replication and cell division. When single-celled organisms divide, the size of their population increases and they pass their genetic material on to the next generation. This is called asexual reproduction, and it produces daughter cells that are identical to the parent cell.
8. Explain that multicellular organisms use DNA replication and cell division to grow by increasing the number of cells they have and to repair their tissues by replacing damaged cells. Tell students that in the time it took them to watch the Mitosis video each of them lost approximately 40,000 skin cells. Ask:
- How does the body compensate for this high rate of skin cell loss? (The body regenerates cells at a rate equal to the rate of skin loss, thus our skin is maintained.)
- What would happen if cell replication within a multicellular organism proceeded in an uncontrolled manner?
- What do you think regulates cell division in organisms?
10. Show the How Cancer Cells Grow and Divide video. Discuss the following:
- What other kinds of signals do you think cells respond to that make them grow and divide appropriately?
- Why do you think the oncogene acts as though it is stuck in the "on" position?
11. Have students explore the How Cancer Grows Web activity to learn more about cancer growth and treatments for cancer. Discuss the following:
- What are some ways in which cancer cells and cancerous tissue develop abnormally?
- How do cancer cells differ from normal cells of the same kind of tissue?
- How does the body respond to cancer cells?
Check for Understanding
Discuss the following:
- What limits cell size?
- How do some single-celled organisms increase their surface-area-to-volume ratio and surpass the size limits that constrain spherical cells?
- How do multicellular organisms like humans grow larger despite the constraints on cell size?
- What are the similarities and differences between cell replication in single-celled organisms and cell replication in a multicellular organism?