Return to Biology IndexCell Size and Division or How Big Would You Want To Be If You Were A Cell

Nicholas DiGiovanni Naperville Central High School

440 W. Aurora Rd

Naperville IL 60540

708-420-6417Objectives:

1. For all grades: to illustrate the usefulness of models to represent things

which are too small (cells or molecules) or too large in science.

2. For primary grades: to learn to measure with a ruler, to cut a cube, and

determine smaller particles react faster than larger particles.

3. For middle grades: to determine surface area and volume of a cube in addition

to the above.

4. For upper grades: to determine the surface area to volume ratio and relate

this to cell size, to determine why cells divide and 1-3 above.Materials:

2500 ml 2% agar solution (sufficient for 15-20 set-ups or pairs)

a cake pan

phenolphthalein powder

1 250 ml beaker or cup

50 ml .4% NaOH solution

a metric ruler, stirrer or spoon, plastic knife, and paper towelsStrategy:

1.Advance Preparation:

Mix enough agar powder in boiling water to make a 2% agar solution. Use enough

water to fill a cake pan to a depth of 3 cm (approximately 2500 ml). Stir until

all the powder is dissolved. As the agar cools, add 1 g of phenolphthalein (if

solid is unavailable, add several ml of liquid phenolphthalein indicator) per

liter of solution and stir thoroughly. If the color is pink, add dilute acid

drop by drop until the solution turns colorless. Pour the mixture into the cake

pan to solidify. This will provide the agar for the model of the cells. If

agar is unavailable, substitute potatoes, but then razor blades must be used and

a dye found which will penetrate the potato in a short time.

2. Discuss models and their importance with the class. In this activity we will

use agar blocks to represent cells.

3. Give the students a 6x3x3 cm block of agar cut from the cake pan, a plastic

knife, and metric ruler. Ask them to cut three separate cubes 1x1x1, 2x2x2, and

3x3x3 cm from the block.

4. Ask the students, "If you were a cell which cell would you rather be (small,

medium, or large) and why?" Write this down.

5. Ask the students to place the cubes into the beaker. Then the teacher pours

the NaOH into the beaker to just cover the cubes. (CAUTION: Sodium hydroxide

is caustic and can burn the skin and eyes.)

6. The cubes should remain in the solution for 10 minutes. They should be

stirred occasionally with the spoon. When the NaOH comes into contact with the

agar blocks, the blocks and perhaps the solution will turn a pink color. The

students enjoy this.

7. Depending on the grade level, students should be given a task to do while the

cubes are "soaking". Primary grades may be asked if this were a cell, what type

of things might move into it. Older students may be asked the same as well as

to explain diffusion since this is what is happening. They should also be asked

to set up a data table in which they determine the surface area, volume, and

surface area to volume ratio for each cube.

8. After 10 minutes the cubes are taken out of the beaker with a spoon and dried

off with a paper towel. The students should cut the cubes in half and measure

the distance from the outer edge inward that has turned pink and record this.

9. Students will discover that the distance that the solution travelled in each

cube is the same (5 mm). There is a pink border around the 2x2x2 cm and 3x3x3

cm cube, but the 1x1x1 cm cube is pink throughout. Ask if the pink represented

food, water or something else needed by the cell to survive, which "cell" got

the needed substance distributed to all its parts. They should see that the

smallest cell is most efficient since it is pink throughout.

10. Mathematically, students should observe that the smallest cube has the

largest surface area to volume ratio (SA:VOL). Therefore this illustrates that

a large SA:VOL promotes better efficiency in moving things into and out of cells

and thus survival. This can also be related to smaller particles reacting

faster than larger particles in chemical reactions (i.e. Granular sugar

dissolves easier than sugar cubes.)Assessment:

Students can be asked which type cell they think would have a better chance

for survival, one which is 1x1x1 cm or one which is .1x.1x.1 cm. They need to

justify their response. 5 points for a proper mathematical as well as written

explanation. 4 points for an explanation which is a little unclear. 3 points

for a proper explanation but improper or no math. 2 points for an unclear

explanation but shows thought. 1 point for an honest attempt.Sources:

Adapted fromBiological Sciences: An Ecological Approach. Kendall Hunt. 1987