Biology
Standard II, Objective 3
Title: Cell Size
Description: Students investigate the relationship between surface area (cell membrane) and volume of a cell.
Materials: Student sheets and cell cut-outs (see below), scissors, masking tape, sand, 25 ml and 100 ml graduated cylinders, calculators (optional)
Time Needed: 50 minutes
Safety Concerns: Proper use of scissors should be emphasized.
Procedures:
1. Read the introduction with students and discuss why cells are small. While there is some variation in cell size the upper limit seems to be the same for all organisms. See if students can generate ideas but do not provide answers.
2. Read the materials and procedures with students and assign groups if needed. 3-4 students per group is desirable.
3. Review the calculations needed to find the ratios. Assume that the cells have “lids” even though they do not.
4. The students should discover that a cubic centimeter is the same as a milliliter in this activity. This is why they measure volume 2 different ways. They should be able to use either volume, the calculated one or the one measured with sand because they should be similar numbers.
5. Have students post their final ratio of surface area to volume on the board. Compare their results and make sure groups made their calculations correctly.
6. Once students discover that smaller cells have more surface area to volume, discuss why that might be important to cells. (cells have no circulatory system, all materials must flow inside and out through cell membrane, osmosis occurs more easily when substances are near the membrane.
7. You may wish to see if students can apply this finding to raindrops.
8. Give students time to finish with analysis questions and conclusions.
9. Pair-share conclusions and encourage students to write complete thoughts that summarize their learning.
Title: Cell Size name______
Introduction: Why are cells small? Could they be larger and still get their jobs done? In this activity you will make some calculations and see what advantage there is to being small.
Materials: “boxes”, metric ruler, masking tape, 25 and 100 ml graduated cylinder,
Procedure:
1. Cut out the box outlines given to you.
2. Fold them up into "cells" that look like boxes. Tape the edges and corners. Assume there is a lid, even though there is not.
3. Measure the dimensions of the boxes in cm. and record. Calculate the surface area and volume.
4. Fill the box with sand. Pour sand into the graduate and record volume.
5. Calculate surface area and volume ratio.
Prediction: Which box will have the greatest surface area to volume ratio?
Data:
box / dimensions / surface area / volume in cc / volume in ml / s.a./volumeA
B
C
Analysis:
1. What do you notice about the volume in cc and in ml?
2. What does that tell you about cc's and ml's?
3. When would you use ml's? cc's?
4. Which model had the largest surface area?
5. Which had the largest volume?
6. Which had the highest ratio?
7. What do cells need, a large ratio or a small one? Why?
8. What is the relationship between size and surface area to volume ratio?
9. If you have a large, fat bear and a little, skinny one, which will get cold more easily?
10. Why?
Conclusion: 2 things you learned.