Cell Size: Surface Area to Volume Ratio
Biology Lab
Introduction:
All organisms depend upon their outside surface (cells in direct contact with the environment) for diffusion of essential gases and food. The volume of an organism (mass—muscles, organs) depends on a constant flow of nutrients and gases so the organism can meet the energy requirements of all bodily function (metabolism). All organisms are composed of cells, the size and shape of which determines how efficiently a cell can carry out all of the life processes.
Purpose:
This demonstration illustrates why cells stop growing when they reach a certain size, why virtually all cells are about the same size and finally why some animals have long limbs, big ears and tails, and other animals have no more appendages than a torpedo!
Materials:
Plastic spoon
Potato
Beaker
Iodine/food color
Paper towel
Scalpel (or plastic knife)
Metric ruler
Procedure:
1. Obtain three potato cubes; a 1 cm cube, a 2 cm cube and a 3 cm cube.
2. Place the 3 cubes into the beaker. Pour enough iodine solution in the beaker to cover the cubes. Allow the blocks to remain in the solution undisturbed for 5 minutes.
3. After 5 minutes remove the blocks with the plastic spoon. Place them on the paper towel and blot them dry.
4. Cut each block in half with the scalpel. Using the metric ruler, measure the distance in millimeters that the purple/black color has diffused into each block. Then measure the distance from the end of the purple/black color to the center of the cube. Record these measurements in the data table.
Some helpful information:
Surface area of a cube= length x width x number of sides
Volume of a cube = length x width x height
Ratio of surface area to volume = surface area/volume
Example: A cube 5 cm on each side
Surface Area = 5 cm x 5 cm x 6 = 150cm2
Volume = .001 cm x .001 cm x .001 cm = .000000001 cm3
Ratio = .0000006 / .000000001 = 6,000:1
Data:
Data Table 1:
Cube Size / Surface Area(cm2) / Volume
(cm3) / Surface Area to
Volume Ratio
1cm
2cm
3 cm
Data Table 2:
Cube Size / Distance Brown or Blue Color has diffused into block in mm / Distance form End of Brown or Blue color to block’s center in mm
1 cm
2 cm
3 cm
Analysis:
1. Compare your calculations of surface area and volume. Does either calculation explain what you observed in the cell models? Why or why not?
2. How does the ratio of surface area to volume explain the efficiency of the “cells”?
3. How do your observations and calculations relate to the question of why cells stop growing when they reach a certain size and why are all cells about the same size?
4. What prediction can you make about the efficiency of a very small cell in: getting oxygen, getting rid of wastes, keeping water in a dry environment, keeping heat in a cold environment?
5. What predictions can you make the efficiency of a very large organism in the same four categories?