Lab: Cell Size and Material Movement

Introduction

Virtually all cells are dependent on the processes of diffusion and osmosis. Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration. Cells need to obtain certain essential substances, like oxygen, for their survival. Cells also need to be able to get rid of wastes. In this lab you will create models of cells of three different sizes using agar blocks. Agar is made from seaweed proteins that give it a firm gelatin like texture. The agar that you are using for this lab was mixed with an indicator that changes color when placed in a basic (alkaline) solution. You will measure the amount of diffusion that occurs in the agar blocks to determine the effect of cell size on the cell’s ability to obtain substances by diffusion.

Equations

Surface area = 6 (length x width)

Volume = length x width x height

Surface area to volume ratio = surface area : volume, (example, 3:1)

Diffusion rate = divide the diffusion depth (measured in millimeters) by the time in minutes that your cell models soaked in sodium hydroxide. The unit will be mm/min.

Materials

  • Block of agar
  • Metric ruler
  • Sodium hydroxide solution, 0.1 M NaOH
  • Cutting utensil
  • Beaker

Safety Precautions

  • Be careful when using the razor or knife to cut your blocks of agar. Keep the agar on a cutting surface and cut on the surface, not in midair. Never make a cut toward your body or other hand when using a knife or razor. Make your cuts to the right or left, then turn your block to make each cut.
  • Sodium hydroxide can cause skin burns and damage eyes. Wear safety goggles at all times when performing this lab. Wear gloves at all times during this lab until your solution has been discarded and you have washed your beaker. Wash your hands when the lab is complete. If you get sodium hydroxide on your skin, rinse your skin with water immediately.

Procedure

  1. Cut your block of agar into three cubes to simulate three different size cells: 3 cm x 3 cm x 3 cm, 2 cm x 2 cm 2 cm and 1 cm x 1 cm x 1 cm.
  2. Pour 200 mL of 0.1 sodium hydroxide solution into a 400 mL beaker.
  3. Carefully immerse your blocks of agar using a plastic spoon into the solution and immediately note the time or start a stopwatch.
  4. Let the blocks soak for 10 minutes. Gently stir the blocks periodically.
  5. Fill out the first three columns of the data table while you are waiting and predict which cell model will be the most efficient at moving in materials. Write your prediction here: ______
  6. After 10 minutes, remove the blocks with a spoon and place on a paper towel. Blot the dry with another paper towel.
  7. Promptly cut each block in half and measure the depth of the color change in millimeters as accurately as possible.
  8. Complete the following data table:

Cube / Surface Area (cm2) / Volume (cm3) / Surface Area to Volume Ratio (cm2:cm3) / Diffusion Depth (mm) / Diffusion Rate (mm/min)
1 cm
2 cm
3 cm

Answer the following questions:

  1. In which cell model did most of the volume change color? ______
  2. Which cell model has most of its original color at its center? ______
  3. Which cell model was most efficient at obtaining the diffused sodium hydroxide? ______
  4. Based on the results of your lab, explain why most cells are very small? ______
  5. Diffusion occurs because all molecules have kinetic energy. What do you think would happen to the rate of diffusion in your agar cell models if the sodium hydroxide was heated by 10 °C? Explain your answer.

______