Unit A: The Life Processes

Why aren't any single-celled organisms as big as you?

1. Get Set to Explore

Vocabulary

• diffusion: The movement of particles from an area of higher concentration to an area of lower concentration.
• organelle: Small structures inside a cell that perform specific functions.
• ratio: The relationship between two quantities, which can be expressed as a fraction.
• surface area: The area of the surface of an object; the surface area of a cell can be expressed as 4πr2, where r is the radius of the cell.
• volume: The amount of space something occupies; the volume of a cell can be expressed by the formula ()πr3, where r is the radius of the cell.

Building Background

• Review what a cell is and draw a circle on the board to represent a cell. Referring students to Student Edition pages A8–A10, call on volunteers to come up and draw different organelles inside the cell. Have them label each with its name and function.
• Go over the idea that some organisms are made of one cell and some are made of many cells. Ask students to name some single-celled organisms. Discuss the size of these organisms. Elicit the idea that single-celled organisms tend to be small, in comparison to multi-cellular organisms. Write the Discover! question on the board and give students a chance to discuss answers. Record their ideas on the board.
• Present the vocabulary words, reinforcing the math concepts related to ratio, surface area, and volume. Define π for students. Explain that the vocabulary words give hints to help them answer the Discover! question. Encourage students to try to formulate an answer to the question. Let them revise the answers already written on the board.

2. Guide the Exploration

• Have students launch the Discover! Simulation. Tell them to listen closely to the directions. Point out the scroll bar for changing the size of the cell. Explain that moving the cursor over the controls gives definitions of key terms.
• Students should try changing the cell to several different sizes, taking notes on what they observe. They should pay attention to how the bar graphs comparing surface area and volume change as the size of the cell changes. They should watch the animation and listen to the narration at the conclusion of each animation.
• Before students do Step 3 of the simulation, return to the Discover! question and let students propose answers. Encourage them to revise or add to the ideas already up on the board. Then, direct them to complete the simulation.

3. Review/Assess

• Ask students to summarize Step 3's Wrap-Up text in their own words. Encourage them to explain what happened in the simulation when the cell got too large. Make sure students understand the relationship between a cell's size and its need and ability to take in nutrients. Allow students to revise their previous answers to the Discover! question, which are recorded on the board. To tie concepts together, have a volunteer read the final paragraph of the Wrap-Up text aloud to the class.
• Refer students to the drawing of the cell on the board the class made before doing the simulation. Students can return to the simulation to do the Extension activity. Challenge them to look for all of the organelles shown in the drawing on the board. Students should be able to identify these following organelles in the computer cell: cell membrane, nucleus, mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus.

If time permits, present students with the following question and activity:

• Critical Thinking Classify Study the structure of and organelles in the cell pictured in the simulation. What kind of cell is shown? Answer: An animal cell is shown. Students can infer this is an animal cell because of the lack of cell wall and chloroplasts.
• Inquiry Skill Use Models Use a rubber band or another type of thin, stretchy material, to make a model of a cell membrane. Use beads of different colors to represent nutrients and wastes. Show how the cell membrane can change to aid a single-celled organism in taking food into the cell. Answer: Students can use the rubber band to make arms like an amoeba for surrounding and ingesting food. They may be able to modify a rubber band to represent cilia and model the process a paramecium uses to take in food. If necessary, refer students to Student Edition page A16.

4. Reaching All Learners

English Language Learners

Group English Language Learners with native English speakers who are good at explaining things and are strong in science and math. The groups should go over the vocabulary terms. Let students remain in their groups to do the simulation. Check in with the native English speakers working with the groups from time to time to help clarify concepts for groups.