1. Cell Growth, Division, and Reproduction
Read the lesson title aloud to students.

2. Learning Objectives Explain the problems that growth causes for cells.
Compare asexual and sexual reproduction.
Click to show each learning objective.
Have students consider why single-celled organisms are so small, mostly invisible to the naked eye. Ask them to consider why larger organisms like plants and animals are made up of trillions of little cells rather than a much smaller number of larger cells.
Ask students to give examples of organisms that reproduce asexually and those that reproduce sexually.
Ask: Is asexual reproduction associated with humans at all?
Answer: Yes! In fact, cells throughout a human body reproduce asexually all the time, even though humans as whole organisms reproduce sexually.
Point out that by the end of the presentation they should be able to explain why cells are limited in size, describe the relationship between cell division and reproduction, and describe ways that asexual and sexual reproduction differ.

3. Limits to Cell Size
Ask: Throughout your lives, how has your body gotten larger? How will this baby penguin grow? How do the cells making up its body compare to those making up the parent’s body?
Answer: The number of cells increases. (Individual cells do not simply grow larger to make the body larger.) The baby penguin has fewer cells in its body overall than the parent does.
Have students reflect on why larger organisms need to be made up of many tiny cells rather than just one or a small number of large cells. Elicit student ideas for why there are no truly “giant” cells.

4. A Visual Analogy Information overload Traffic problems
Explore the comparison of a cell to a town.
Ask: What do the library and books represent?
Answer: DNA and the nucleus
Ask: What part of town would be analogous to a cell membrane?
Answer: the town’s border
Ask: What happens to a town when its population grows? What problems do you see in the picture?
Answer: Traffic increases. A larger town means more natural resources and consumer products need to come in and more wastes need to go out.
Ask: What do the crowds and traffic suggest about the activity in this town?
Answer: Workers and materials are not getting to where they need to go.
Click to reveal the two major challenges to cell growth: information overload and traffic problems (exchanging materials).
Explain that an ever-larger cell would make ever-greater demands on its “library,” and eventually the library (DNA) would not be able to serve the entire town (cell). Like a growing town, a growing cell has a harder time moving needed materials in and wastes out. Some parts of the larger town (cell) will suffer if materials cannot get to them or away from them.

5. Surface-Area-to-Volume Ratio
Surface area = the amount of “covering” of the object
Volume = the amount of space inside the object; the amount of space the object takes up
SAcube = l × w × 6
1 cm × 1 cm × 6 = 6 cm2
Explain to students that one of the main problems of a cell’s size (the “traffic problem”) is a matter of surface-area-to-volume ratio. Tell students that to look more closely at this problem, they can think of a cell as a cube like the one here and consider what happens as the cube, or cell, gets larger and larger.
Ask: How do you find the surface area of a cube like this one?
Answer: Calculate the area of one side and then multiply by 6 to account for the 6 sides of the cube. Find the area of each sided by multiplying length times width.
Click to reveal the formula for surface area.
Ask: How do you find the volume of a cube?
Answer: Multiply length times width times height.
Click to reveal the formula for volume.
Ask for volunteers to come up to the board to carry out the calculations, with class input as necessary, in the space below each equation.
Click to reveal the answers.
Vcube = l × w × h
1 cm × 1 cm × 1 cm = 1 cm3

6. Surface Area to Volume in Growing CellsSA SA
Tell students: Let’s look at what happens when the cube, or cell, grows. Here are two larger cubes.
Click once to reveal two sets of boxes for surface area and volume under the cube images.
Ask a volunteer to come to the board and calculate surface area and volume for the two cubes in the boxes. Elicit class input as necessary.
Click twice to reveal the correct answers.
Ask: What do you notice about these numbers? Are both surface area and volume growing at the same rate when the cell gets bigger?
Answer: No, both surface area and volume get bigger with a bigger cell, but volume grows relatively more than surface area does.
24 cm2
54 cm2 V V
8 cm3
27 cm3

7. Ratio of Surface Area to Volume in Cells
Use the table to confirm what students observed through the calculations on the previous slide.
Ask: Where is the ratio of surface area to volume greatest?
Answer: the smallest cube
Click to reveal the circle and label for the ratio for the smallest cube.
Ask: Where is the ratio of surface area to volume smallest?
Answer: the largest cube
Click to reveal the circle and label for the ratio for the largest cube.
Ask: How would an even larger cube with sides of 4 cm compare?
Answer: The ratio of surface area to volume would be even smaller.
Largest ratio
Smallest ratio

8. Cell Growth Limitations
Information crisis: too many demands placed on DNA
Traffic problems: volume grows too fast relative to surface area, material exchange is insufficient
Ask: Considering the problem of surface-area-to volume ratio, how could cell growth create a problem that is similar to the traffic jam in the growing town?
Answer: As a cell grows, it needs more materials to cross its membrane. Traffic is comparable to the movement of materials such as nutrients, water, oxygen, and wastes across a cell’s membrane. As a cell grows larger (increasing volume), the number of traffic lanes (amount of surface area to cross) does not keep up, and materials cannot enter or leave the cell as quickly as necessary.
Click to reveal the problems caused by a decreasing surface-area-to-volume ratio for growing cells.
Reiterate that if a cell grew too large, it would not have enough relative surface area to get sufficient amounts of oxygen and nutrients into the cell and waste products out.

9. Cell Division Produces two daughter cells
Cell must replicate DNA before cell division.
Dividing to make more, smaller cells keeps SA to V ratio high.
Explain that to solve the surface-area-to-volume ratio faced by a growing cell, an individual cell divides. The division of a cell produces two daughter cells.
Click to reveal the first bullet point.
Challenge students to think about what a cell would have to do before it divides. Guide their thinking as necessary by asking them what major components a cell needs (e.g., DNA, cytoplasm).
Point out that before it can divide to produce daughter cells, there has to be enough DNA for two cells. Before cell division, the cell must replicate, or make a copy of its DNA. This avoids the problem of information overload.
Click to reveal the second bullet point.
Tell students: Cell division solves the problem of increasing cell size by decreasing cell volume. Making more cells instead of larger cells keeps the surface-area-to-volume ratio high.
Click to reveal the third bullet point.
Misconception alert: Students may think that cells get smaller and smaller with each successive cell division. Tell students that cells go through a period of growth after they divide. Remind students that cell division helps a cell avoid the problems of growing too large.

10. Asexual Reproduction
A single parent produces genetically identical offspring.
New individuals form from the separation of cells.
Distribute the lesson worksheet to the class.
Encourage students to use the worksheet to take notes describing the advantages and disadvantages of asexual and sexual reproduction strategies. Suggest that students create one column per strategy and mark each entry they make under a column with an “A” or “D” for “advantage” and “disadvantage.” For extra review, students should define the two strategies in the header row.
Remind students that reproduction is one of the most important characteristics of living things. Point out that for a single-celled organism like a bacterium, simple cell division makes the process of reproduction pretty straightforward, and it can allow a population to grow very quickly. This kind of cell division is a form of asexual reproduction, which is the production of genetically identical offspring from a single parent.
Click to reveal the bullet points.
Lead a discussion about how cell division relates to the process of asexual reproduction. To reinforce students’ knowledge, have them apply what they have learned to their everyday lives.
Ask: Why do bacterial infections spread so quickly through a school?
Answer: Bacteria can reproduce asexually, so they can reproduce quickly in the right environment, such as a crowded school.
Direct students to the photos of kalanchoe and hydra.
Ask: Both of these organisms are reproducing asexually. How are they different from the bacterium?
Answer: They are multicellular organisms.
Point out that tiny plantlets in the kalanchoe can break off and grow into full new plants. The bud on the hydra will eventually pinch off to become a separate individual.
Ask: What do the offspring off all three of these organisms have in common?
Answer: The offspring share the same genetic material as their parent.

11. Sexual Reproduction
Sexual reproduction involves the of two separate parent cells.
Offspring inherit some genetic information from each parent.
fusion
Remind students that at the start of the presentation you asked them about how asexual reproduction related to humans. Point out that humans, like other animals, grow and repair their bodies through cell division. But to reproduce themselves, humans use sexual reproduction.
Read the statement on the slide aloud.
Ask for a volunteer to identify verbally what term completes the statement.
Click to reveal the correct answer.
Tell students: Asexual reproduction involves separation of a cell. Sexual reproduction involves fusion, or joining, of cells.
Ask: How do offspring relate genetically to their parents?
Answer: They are not genetically identical to either parent. They inherit some genetic information from each parent.
Click to reveal the second bullet point.

12. Comparing Asexual and Sexual Reproduction
Produce many offspring in short period
Don’t need to find a mate
In stable environments, genetically identical offspring thrive.
If conditions change, offspring not well adapted.
Sexual
Relatively fewer offspring; growth takes more time
Need to find a mate
In changing environments, genetic diversity can be beneficial.
Offspring may be less well adapted to current conditions.
Lead a short discussion in which students consider how one form of reproduction might be more advantageous than another. Remind students that characteristics that make an organism well adapted to its environment are critical for survival.
Ask: Under what kind of conditions would it be helpful to be able to create a very large number of offspring in a short period of time?
Answer: when resources such as food and space are unlimited
Ask: Under what kind of conditions would it be useful to have offspring that are genetically identical to the parent?
Answer: when environmental conditions are stable
Ask: Under what conditions would it be helpful to need only one parent to reproduce?
Answer: when finding a suitable mate might be difficult or impossible
Ask: Under what kind of conditions would it be useful to have offspring that are not identical to either parent?
Answer: when environmental conditions are changing and new combinations of traits may promote survival
Then, ask students to identify, based on the discussion, what the advantages and disadvantages of asexual and sexual reproduction are.
After students have shared their thoughts, click to reveal the advantages and disadvantages.
Discuss each bullet point as it is revealed to ensure students understand why it is an advantage or disadvantage.
Be sure to point out that most organisms they are familiar with can use only one strategy to reproduce themselves. However, some organisms, such as yeast, can use both strategies. Typically yeast undergo cell division to reproduce asexually, but under certain conditions can enter a sexual phase. Point out that the ability to use both reproductive strategies can be an excellent adaptation for some organisms!

13. Student Worksheet Answers
Remind students that they should have been using their worksheets to take notes comparing advantages and disadvantages of asexual and sexual reproduction.
Have students work in pairs to compare their tables to make sure their information is complete.
Worksheet Answers:
Asexual Reproduction
A: Potential to produce many offspring rapidly
A: No need to find a mate
A: In stable environments, offspring will be as well adapted as their parents.
D: Lack of genetic variability limits adaptability of the population if the environmental conditions change.
Sexual Reproduction
D: Fewer offspring and longer time for growth and development
D: Use resources to find a mate
A: Genetic variability increases the odds of survival in a changing environment.
D: Offspring may be less well adapted than their parents to current conditions.