Summer Research Program for Science Teachers

Harold Kucine

JHS 126, Brooklyn

2000

Why Are Cells So Small?

Goal: to help students to develop an understanding of the relationship between cell surface area, cell volume and the ability of materials to diffuse throughout a cell.

Materials: agar block infused with phenolpthalein , metric ruler , scalpel, mild base ( ammonium hydroxide-very dilute is acceptable), gloves, goggles, beakers, flat plates, lab notebooks, calculators, pencils. [Teaching Standard D- Make accessible science tools]

Overview: phenolpthalein is an indicator that turns pink in a basic environment. [5-8 Content Standard B- Properties of matter] Ammonium hydroxide is a basic solution that easily diffuses throughout a congealed agarose block.

The depth to which the solution diffuses is easily determined by simple observation of the areas inside of the block where a color change is visible. Through a series of simple calculations the student can see that the % of diffusion is clearly linked to the ratio of surface area to volume. [Content Standard Unifying Concepts- Change, constancy, and measurment] This offers students an opportunity to understand the need for multi-cellular organisms to maintain structural forms that are large numbers of very small cells as opposed to small numbers of large cells( the “blob” question!) [5-8 Content Standard C- Structure and function in living systems]

Procedure: prepare a solution of agar  which will sufficiently fill a baking pan to a depth of 3 cm. , mix into the solution 1 ml. of phenolpthalein. Allow the agar to  congeal into a hardened block. The block will have the consistency of gelatin. Have each group cut and remove a  piece of the block that measures (approx.) 10cm.  X  4cm.  X 3 cm.  . The responsibility of each of the lab teams will be to prepare cubes of the following dimensions: 1cm. X 1 cm. X 1cm. , 2 cm. X 2 cm. X 2 cm.  , and finally  3cm. X 3cm. X 3 cm.  Each group will calculate the volume and surface area of each cube, as follows:

volume =  s x s x s (cubic centimeters)

surface area =  6 x s x s  (square centimeters)

[5-8 Content Standard A- Use mathematics]

Each group will then calculate the s.a./vol. Ratio for each of the individual cubes. A  q&a session should follow this step to see if any of the groups have come to a conclusion about the ratio as the length of a the side of the cube increases. Each group will place each cube into the basic solution for approximately 1 minute. Immediately upon removal of each cube from the solution a group member will bisect (accuracy in this step is not essential- as long as the interior of the cube is exposed) each cube. Depth of penetration of the solution can easily be determined by observation of the depth to which a color change can be observed. We can assume that diffusion has occurred equally in all directions due to the cubic shape. Students will measure the depth of penetration

And  subtract that distance from the side of the original cube. This procedure will be repeated for each of the three original cubes. Accuracy in measurement and accuracy in the accumulation and recording of the data is essential and should be rotated among team members. Once the length of the un-penetrated side has been  calculated  the volume of the un-penetrated cube can be calculated for each of the three original samples. The volume of each of the penetrated  regions can be found by subtracting the volume of the un-penetrated region from the volume of the original cube. The final set of calculations will be to calculate the % of diffusion , as follows:

% of diffusion = volume of pink area/ original volume x 100

each group will be responsible for calculating the % of diffusion for each of the original cubes.

Follow-up: if all procedures have been followed correctly , the students should observe an inverse relationship between the length of the cube side and the % of diffusion  of the basic solution. The students should also be able to conclude that there is an inverse relationship between the length of the side of the cube and the s.a./vol. Ratio. [5-8 Content Standard A- Use evidence to explain]Students might be asked to draw conclusions about the design plan followed by most organisms. Follow-up assignments might center  about  the feasibility of designing an organism with the body plan of “the blob “ ( a very large unicellular organism) . [Content Standard Unifying Concepts- Form and function]