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Biology
Why are Cells so Small?
Names____________________________________
Per_______________
Does the size of a cell matter? Cells come in different shapes and sizes in your body, but what is the best
shape and size? Rather than constantly creating billions of cells in our lifetime, why aren’t we composed
of larger cells? In this lab we will investigate the question “why are cells so small?”
Materials
Agar Cubes
Ruler
Tweezers
Timers
Knife
Vinegar
Graph Paper
Calculator
Beakers
Paper Towel
Triple-Beam Balance
Part 1: Cell Size
1. Obtain two agar cubes from Mr. Marshall, one that is 1cm on each side and another that is 3cm
on each side. Each cube has been laced with a chemical called bromothymol blue.
2. Place the two cubes into a cup or beaker. Fill this container with vinegar until the cubes are
covered.
3. Let the cubes sit in the container for 20 minutes, then take them out with the tweezers and
place on the paper towel.
4. With your knife, slice through the cubes along the very center of the side. Make sure your cut is
perpendicular to the edge and exactly in the center.
5. You should see that the liquid has diffused into the agar cube. When vinegar reacts with
bromothymol, it turns the chemical from blue to yellow. With your ruler, measure how far into
the agar cube the liquid has diffused. Record this number in millimeters in table 1.
6. Calculate and record in table 1 the volume of the inner cube of the agar (the portion of the cube
that liquid did not diffuse into) using this equation:
Volume of inner cube=[Length of outer edge, in mm – (2 x distance liquid has diffused)]3
7. Calculate and record the percent volume of the cube that the liquid has diffused to. Use this
formula:
(Volume of total cube – Volume of inner cube) x 100
Volume of total cube
Part 2: Design your own cell
1. For this lab you and your team will compete against the other teams in class to see who can
design the most efficiently sized cell. Your goal will be to have a cell that maximizes volume and
mass while, at the same time, minimizing the amount of time it takes a liquid to diffuse
throughout the cell.
2. Your cell will be carved out of a 3cm3 cube. Obtain a piece of graph paper and sketch a rough
draft of what your cell will look like. Include this sketch with your lab when you turn it in.
a. *Your cell may be any shape you like, with one exception. You cannot “donut-hole” your
cell. Whatever your structure is, it must be intact in the center of the cell.
3. Using a scalpel, carefully carve out your cell according to your design. You will not be given
“redo” cubes if you mess up.
4. Once your cube is carved, use a balance to mass your cell. Record this mass in Table 2.
5. Carefully place your cell in a beaker and have your timer ready. Fill your beaker with enough
vinegar to cover your cell. When the vinegar first touches the cell, start your timer.
6. Monitor your cell until you can visually see all bromothymol blue disappear in your cube. Once
the entire cube is yellow, stop the timer. Record the time in Table 2.
7. Calculate the ratio of the mass of the cubes (grams) divided by the total number of seconds to
diffuse. Record your ratio on the front board.
8. The team with the highest ratio in their class will win 10 points extra credit, and the team with
the highest ratio of all classes will win 30 points extra credit for this lab.
Table 1
Length of Cube
edge
Volume of
total cube (x3)
Cube #1
1 cm (10 mm)
1,000 mm3
Cube #2
3 cm (30 mm)
27,000 mm3
Distance liquid
has diffused
from edge (mm)
Volume of % volume of cube
inner cube that liquid diffused
(mm3)
into
Table 2
Mass of your Cube
Time to Diffuse Completely
Post Lab Discussion
1. What are the manipulated and responding variables for Part 1’s Lab
2. Which agar cube had the highest percentage of diffused liquid?
Ratio
(Mass of Cube)
(Total Seconds)
3. Why did this agar have the highest percentage?
4. If the agar cube represents a cell, what could the liquid represent?
5. Calculate the surface area of each cube by taking the area of one side of each cube and
multiplying by six.
a. Divide each surface area by the volume in the above chart for each cube to find that
cube’s surface-to-volume ratio.
b. What do surface-to-volume ratios show?
6. What was your rationale for your cell design?
7. Was your cell one of the more successful or less successful cells? Why was this so?
8. Sketch the design of the winning cell
9. What features did this cell have that helped it win?
10. Based on your results, why are cells so small?