Cell Size and Diffusion Lab
Is Bigger Better? OR Is Smaller Smarter?
Introduction:
Diffusion is one of the very important processes by which substances such as nutrients, water, oxygen,
and cellular wasted are transported between living cells and their environment. Some substances,
such as water, carbon dioxide, and oxygen, move easily through the cell membrane. Other
substances need assistance moving into the cell, and others cannot pass through the membrane at
all. During diffusion, substances move from an area of high concentration to an area of low
concentration to maintain an equal concentration on both sides of the cell membrane. This process
does not require energy from the cell and will continue until there is no concentration gradient, or
unequal distribution of particles.
One of the core principles that governs the efficiency of diffusion is the ratio of surface area to volume.
Surface area is the amount of cell membrane available for diffusion. For a cell, surface area actually
represents how much diffusion that can happen at one time. Whereas volume is the amount of
cytoplasm contained within the cell membrane. So for a cell, volume is how long It takes to get from
the membrane to the center of the cell by diffusion.
The prime limitation to cell size is the limitation imposed by diffusion. Diffusion is a very slow
process. If a cell were 20 cm (~8 inches), it would take days for nutrients to reach its center or
for wastes to reach the cell membrane. The cell would quickly starve to death or poison itself
with its own wastes. So what’s the solution, if a cell approaches its maximum size? It’s time to
divide! If cells receive the proper signals, they will divide by mitosis before they become too big.
In this lab, you will create enlarged gelatin cell models of various sizes. The agar (gelatin) that you will
use in this lab has been prepared with phenolphthalein, an acid-bas indicator. Phenolphthalein is a
colorless substance that turns pink in the presence of a base (pH greater than 8.2), such as sodium
hydroxide (NaOH). As you soak your cubes in NaOH, you will be able to see diffusion taking place.
What determines the efficiency of diffusion throughout the model “cells”? Use this question to help
formulate a hypothesis.
Purpose: To explore the relationship between diffusion and cell size by experimenting with gelatin
model “cells.”
Hypothesis:
Materials: calculator, goggles, colored pencils, plastic knife, plastic spoon, metric ruler, tray, agar
section, electronic balance, 500 mL beaker, 100 mL graduated cylinder, 100mL 0.1 NaOH, 100mL
vinegar (part II), agar cubes with bromothymol blue
Procedures:
Part I: How Cell Size Affects Diffusion Time
1. Place the agar in your cardboard tray using the plastic knife. Cut the agar into l-cm, 2-cm,
and 4-cm cubes. Make sure to measure each side of the agar carefully (length, width, and
height) to ensure that each cube is accurate. Answer number 1 on your student data sheet.
2. Calculate the surface area and volume of each cube and record the data in Table 1 on
your student data sheet. Be sure to simplify, or reduce, any ratios. Answer numbers 3 -7 on
your student data sheet.
3. Place the three cubes into the beaker. Try to position them so that they are touching each
other as little as possible. Record the appropriate times in Table 2 on your student data
sheet. Pour 100mL of 0.1-M sodium hydroxide (NaOH) solution into the beaker until the
cubes are completely covered with the solution. Answer number 9 on your student data
sheet.
4. After 10 minutes, remove the cubes from the NaOH solution with the plastic spoon. Place
the cubes on your cardboard tray and carefully blot them dry with a paper towel.
5. Watch for any color change in the cubes. Cut each block in half and measure the pale
yellow area in the center. Calculate the volume of this yellow region by multiplying length x
width x height. Complete Table 3.
6. Answer the remaining questions on your data sheet.
7. Dispose of your agar blocks and the NaOH. Clean your area and return any materials to the
proper location.
Part II: Competitive Cell Diffusion Race!
Cells do come in many shapes and sizes in organisms. Natural selection has crafted them to do their
jobs better with their unique form. You will find that the relationship between structure and function is a
recurrent theme throughout biology. Each student will get an equal size block of agar and will have
the opportunity to design a cell to maximize mass but minimize diffusion time. The cell with the
greatest mass and the shortest diffusion time will be judged the winner.
RULES: No donut-like holes through the agar cell, no poking, prodding, touching beaker containing
agar cell in vinegar. Teacher will determine when 100% diffusion takes place. Students must mass agar
at conclusion of race. The cell must not break when handled. Disqualification if cell breaks upon
massing. Winner = highest ratio of mass divided by time.
1.
Create your own Data Table 4 for all necessary measurements and calculations for Part II.
2.
Take an ice cube block of the agar.
3.
Design a cell that maximizes volume and mass, but minimizes diffusion time.
4.
Take a picture of your “cell” and include it with your data.
5.
Record the mass of your "cell." (When using a weighing boat, zero your balance!)
6.
Fill each beaker 1/4 full with vinegar.
7.
Ready your timer.
8.
Wait until all teams are ready as we will be doing the race as a class.
9.
Immerse each block in common household white vinegar in small beakers.
10. Time until blue completely disappears. It helps to put beakers on white paper as a
background.
11. The winning team receives extra credit.
Data Collection and Processing:
1. At this point in the lab, which of the three "cells" do you think has the best chance of
survival? Why?
2. Complete Table 1.
3. How many 1-cm cubes would be needed to make a 4-cm cube? Use the data you
calculated in Table 1 to explain your answer.
4. Do the surface area and volume of a cell increase at the same rate? Explain your
answer.
5. List the cubes in order of their surface area to volume ratio, from largest to smallest.
6. What happens to the surface area to volume ratio as a cell increases in size?
7. Do you think the molecules will more easily pass in and out of a cell with a large surface
area to volume ratio or a small surface area to volume ratio? Explain your answer.
8. Complete Table 2 by entering the appropriate times.
9. What happened to the cubes as you poured the NaOH into the beaker? Why did this
change take place?
10. Complete Table 3.
11. Do the percentages that you calculated for Table 3 change your mind about your
answer to number 1? Explain.
12. On the graph below, use a colored pencil or marker to draw a cross section of each of
the cubes. Use your ruler and make the drawings to scale.
13. What is indicated by the width of the pink area of each cube?
14. What did you notice when you measured the width of the pink area of each cube?
Why do you think this is so?
15. For cellular respiration to occur, oxygen must diffuse into a cell and carbon dioxide must
diffuse out. Which cube accurately represents an efficiently respiring cell? Use the data
that you have calculated in this activity to support your choice.
16. Why do hamburgers cook faster than meatballs of the same weight?
17. If you use a batch of cake batter for cupcakes instead of cake and bake them for the
time recommended for a cake, what will be the result?
18. Why are the living cells in a whale about the same size as those of a mouse? Describe
different ways that cell shape can be modified so that diffusion rate will be decreased
to support life processes.
19. Give an example of a type of cell in a living organism (animal or plant) that is shaped
very differently than the classical round or boxy shape that you see drawn in
introductory textbook chapters on cells. Explain how that unique shape is tied to the
function that those cells perform.
20. Which cell design won the race? Offer an idea as to why.