Relation to topics / curriculum link:
y diffusion
y absorption of food substances in villi
y gas exchange in air sacs
Prior knowledge and skills needed:
y understanding of cell size and shape
y rate of diffusion
Concept:
The size and shape of a cell affect the rate of diffusion of substances.
Introduction
The rate of diffusion of substances can be determined easily by observing the change in
colour from blue to yellow in agar cubes. The agar cubes are blue in colour as they
contain the pH indicator bromothymol blue. When the blue agar cube is placed into
vinegar solution, acid will diffuse slowly into it and changes its colour to yellow. The
time taken for the acid to reach the centre of the cube can be recorded as the diffusion
time.
In this activity, students will apply what they learned about cell shapes and diffusion to
design a cell that is best suited for rapid diffusion. They will carve their own design from
an agar cube and test which cubes will change colour the fastest.
Materials
y
blue agar block (5 cm cubes)
containing bromothymol blue
y
vinegar solution
y
beaker
y
electronic balance
y
knife
y
stop-watch or timer
y
white tile
y
ruler
Fig. 1 Materials for the activity: A blue agar block,
vinegar, beaker, knife & ruler
CELL DESIGNER
Cell Designer
Activity I: Comparing the rate of diffusion in different agar cubes
1.
Obtain a blue agar block from your teacher. (CAUTION: Wear safety goggles, plastic
gloves, and laboratory coats while working with agar.)
Fig. 2 Three different-sized blue agar cubes
2.
3.
4.
5.
6.
7.
Fig. 3 Agar cubes turned yellow
Use a knife to cut three different-sized agar cubes (length of side: 0.5 cm, 1 cm and 1.5
cm) from the blue agar block.
Place one agar cube into a beaker.
Cover the agar cube completely with vinegar solution and start the stop-watch.
Watch your agar cube closely. When the cube has turned completely yellow, record the
end time. Repeat steps 3-5 for the other two agar cubes.
Find out the rate of diffusion for each cube.
Plot your result if necessary.
Questions for discussion:
1. Which cube changed colour first?
2. Which variable (size or shape) should be kept constant in the above activity?
3. From the results, which characteristic of cells do you think is the most important in
enabling cells to obtain nutrients and get rid of wastes efficiently?
4. From this activity, what is/are factor(s) that would limit cell size if the cell is to survive
under normal conditions?
5. What happens to the surface area to volume ratio if you increase the volume of a cell?
What is the effect of this change on the rate of diffusion? Explain you answer.
6. Besides a cube, what other shapes do you think might increase the surface area to
volume ratio? Explain your answer.
7. What is the disadvantage or limitation of using agar cube to represent a cell?
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Activity II: Making and testing a cell design
You will design a cell with the fastest diffusion rate using the blue agar block. Your goal is to
give your model-cell a shape that will allow it to change color quickly, while still having
significant mass (not less than 3 grams). To win, your cell design must be the one that has the
greatest mass and changes color the fastest, measured as the greatest value for mass per unit
time (g/min) or (g/sec). The winning cell must also be in one piece and should not have any
holes that reach from one side to the other. Give the design a name so you can identify it
during the activity. When you have tested your model-cells, complete the table below and
compare the mass/time data to find the winner.
Name of model-cell:
Mass of model-cell (g):
Time for complete colour change (min or sec):
Diffusion Index (g/min or g/sec):
mass
elapsed time
Questions for discussion:
1. How did your model-cell design differ from other designs of your classmates?
2. What were the characteristics of the model-cell with the highest mass/time ratio in the
class?
3. Most cells are microscopic, why do you think so? What is the limiting factor of cell size?
4. Some of the cells in your body are designed to allow rapid diffusion of nutrients and
gases. What are they and what characteristics do these cells have?
5. When a cell is very thin, flat or narrow, it can obtain nutrients quickly. What possible
disadvantages might there be to such a cell shape?
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Further Investigation
1.
Investigating the effect of temperature on diffusion rate
Temperature also affects diffusion rate. To determine how, design a simple experiment
with the following materials: agar cube stained with bromothymol blue, vinegar and hot
water bath. Discuss your hypothesis and experiment design with your classmates before
carrying out the investigation. Write a report to explain your conclusions.
2.
Investigating the effect of concentration gradient on diffusion rate
Concentration gradient also affects diffusion rate. To determine how, design a simple
experiment with the following materials: agar cube stained with bromothymol blue,
vinegar, and distilled water. Discuss your hypothesis and experiment design with your
classmates before carrying out the investigation. Write a report to explain your
conclusions.
Reference:
Sweeney, D., & Williamson, B. (2005). Design a cell. Biology exploring life – Laboratory
manual (pp.59-64). (Teacher’s edition). New Jersey: Pearson Prentice Hall.
Fig. 4 Agar cubes of various shapes
Fig. 5 Agar cubes at different stages of colour change
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NOTES
Laboratory Preparation
1.
2.
Agar cubes
y Mix 6 g agar in 1000 ml of water.
y Boil slowly in microwave or hot water bath until the agar melts completely to give a
clear solution. Watch for and avoid boil over.
y Remove from heat. Add 0.1 g of bromothymol blue solid or several drops of
bromothymol blue solution and mix.
y If the mixture is not deep blue, add more bromothymol blue. If the mixture is green
or yellow, add a few drops of dilute sodium hydroxide solution until it turns blue.
y Pour the agar solution into ice cube trays or a flat rectangular or square container.
Let agar hardens at room temperature or in a refrigerator.
Alternative material
y Instead of using bromothymol blue, phenol red which starts out red and turns yellow
in acid, can be used.
y Instead of agar, gelatin (diffusion time will be twice as long) can also be used.
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