3cm cube:
Welcome to the fourth On-Line Lab. In this lab we will try to understand why cells remain so microscopically small by examining the relationship between cell size and the diffusion of materials. The movement of molecules into and out of the cell and throughout the cell is important for the cells survival. Waste products must be removed. Raw materials must be moved to wherever they are needed. Molecules manufactured by the cell must be relocated.
Many atoms and molecules move into and throughout the cell
by diffusion. Diffusion is the movement of substances from where
they are abundant to where they are scarce. In this lab you will
use agar cubes to simulate a cell. The agar has been prepared
with an indicator- phenolphthalein, which in the presence of a
basic compound turns pink. In this lab the basic compound will
be NaOH (Sodium Hydroxide). You will be able to determine how
much diffusion takes place by seeing how far the pink color penetrates
the cube.
1. Obtain a block of agar. Carefully cut out with a scalpel three cubes- one with 3 cm sides, one which has 1.5 cm sides, and the third with 0.5 cm sides. Try to be as exact as possible. Cut the largest one first to insure you have enough!
2. Place all three cubes in an empty beaker with them neither touching each other nor the sides of the beaker.
3. Cover the cubes with NaOH solution.
4. Every two minutes, for a total of six minutes, turn the cubes over carefully, try not to injure the cubes.
5. After six minutes, carefully remove the cubes and place them on a piece of paper towel.
6. Cut the cubes in half with the scalpel.
7. You will see how far the NaOH traveled with in the cube.
Examine the animations below to see what the cubes looked like:
3cm cube:
3cm cube:
3cm cube:
In order to determine the percentage of each cube into which the NaOH diffused you must first determine the volume of each cube. To do this you multiply the width, times the height, times the length. In a cube, all three are the same. Volume is given in units of cm3.
1. Calculate the total volume for the cubes. The smallest has already been done. Put your answers in Chart 1 below. Assume all six sides to be equal.
2. Assuming that the NaOH diffused evenly from all six sides, measure the length of on side of the uncolored portion of each cube and determine the volume of the uncolored region.
3. Determine the volume of the colored area by subtracting the uncolored area from the total area.
4. Determine the percent of diffusion of NaOH by dividing the colored area by the total area and multiplying by 100. Place all of these figures in the chart.
5. Plot the percentages on Graph 1 and draw the appropriate curve.
In this section, you will determine the relationship between the amount of surface area a cell has and the volume of the cell
1. To determine the surface area, the area of all surfaces, you multiply the area of one surface (length x width) times the number of sides. Do this for the two largest cubes. The smallest has been done as a guide.
2. Determine the ratio of surface area to volume by dividing the surface area by the volume. This figure, the INDEX, is the measure of surface area per unit of volume. Do this for all cubes and write all figures in Chart 2 below.
Graph the indexes on the Graph 2 and draw the proper curve.