Cells: Living Machines

NAME______PER______

LAB B: DIFFUSION AND CELL SIZE (25 POINTS)

PROBLEM How does the relationship between the surface area and the volume of a cell affect the rate of diffusion of materials in and out of the cell?

BACKGROUND Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration. Diffusion is an important process for living cells. Materials that can pass easily through the cell membrane, such as water, oxygen, and carbon dioxide, enter and leave cells by diffusion. The size of cells depends in part on how efficiently materials can move in and out of the cells by diffusion. When cells grow too large for diffusion to move enough materials in to maintain growth, cells stop growing. In this lab you will use cubes of agar that have an indicator, phenolphthalein, dissolved in them. In a basic solution, such as sodium hydroxide (NaOH), phenolphthalein turns pink or red. It remains colorless in an acidic solution. In this lab you will determine diffusion over a period of time by measuring how far the color changes in the agar cubes that were immersed in an acidic solution.

MATERIALS (PER PAIR)  Metric ruler  500-ml plastic beaker  Plastic knife  Phenolphthalein Agar Cubes (1, 2, and 3cm)  Plastic spoon  Paper towels  Vinegar

PROCEDURE 1) Obtain a large cube of agar from your teacher a) Place the cube on a paper towel and measure it using a metric ruler. b) Trim down the cubes with a plastic knife to make them exactly 1cm3, 2cm3 3cm3, c) It is best to start by preparing the 3cm3 first.

2) Place the three agar cubes in a 500ml beaker.

3) Pour 200mL of vinegar into the beaker. a) Record the time in on data table 2

4) Let the cubes soak for 10 minutes gently stirring and turning the whole time.

5) While you are waiting, calculate the surface area, volume, and the ratio of surface area to volume, for each of the three agar tubes.

6) Use the formulas below: FORMULAS: Surface Area = Length x Width x Number of Surfaces Volume = Length x Width x Height Ratio = Surface Area/Volume

7) After 10 minutes use a spoon to remove the agar cubes from the vinegar a) (record time in data table 2) b) Blot the cubes dry on the paper towel.

8) Cut each cube in half using the plastic knife 9) In millimeters, measure the depth that the vinegar diffused into each cube by measuring the distance that the color changed from pink to clear a) Record these measurements in Data Table 2.

DO NOT THROUGH AWAY ANY AGAR IT WILL BE RECYCLED!!!!!!

Name ______Period _____

LAB B: DIFFUSION AND CELL SIZE ANALYSIS (25 POINTS) Purpose: ______

DATA (CALCULATIONS) Data Table 1: Calculations of Agar Cubes CUBE DIMENSION SURFACE AREA VOLUME RATIO OF SA/V

3 CM/SIDE 2CM/SIDE 1 CM/SIDE Data Table 1A: Show work for calculations of SA and Volume of cubes CUBE SIZE CALCULATIONS ( SHOW ALL WORK)

3 CM/SIDE

2CM/SIDE

1 CM/SIDE

Data Table 2: Agar Cubes in Vinegar Solution

CUBE DIMENSION TIME IN TIME OUT DEPTH OF DIFFUSION 3CM/SIDE 3MM 2CM/SIDE 3MM 1CM/SIDE 3MM

1) Which agar cube did the vinegar diffuse closest to the center? ______2) Which agar cube did the vinegar remain farthest from the center? ______3) Which of the agar cubes has the largest ratio of surface area to volume? ______4) If an agar cubes measured 0.01cm on each side, what surface area-to-volume ratio would it have? (Show your work) ______5) Were the results consistent throughout the class? If not, explain sources of error that may have affected the results. ______6) Explain why color changes appeared in some areas of the agar cubes. ______7) What might you conclude if you observed changes in the appearance of the vinegar solution that was in the beaker? ______8) What relationship can you observe between the amount of diffusion and the surface are-to-volume ratio of a cell? ______9) Assume that the agar cubes were actually living cells; which one would be the most efficient at transporting vital nutrients around the cell? Explain your answer ______