Name: ________________________________ Date: ___________ Period: ____
Diffusion In Agar Cubes
Is Bigger Better? OR Is Smaller Smarter?
Introduction:
Diffusion is one of the very important processes by which substances such as nutrients, waste,
water, and oxygen are transported between living cells and their environment through the cell’s surface.
A cell needs a large surface in order to carry out metabolic functions and as the cell grows it
needs to carry out more and more reactions. As a cell grows in size (volume), the distance increases
between the cytoplasm at the center of the cell and the cell membrane. The rate of chemical exchange
with the surrounding environment may become too low to carry out all of the needed reactions. It may
not be able to excrete waste quickly enough or take in important nutrients.
The volume of a cell determines requirements while surface area determines supply. Because this
surface area to volume ratio is so important, its size of the cell is limited.
The rate of exchange of materials (nutrients/waste) and energy (heat) is a function of its surface area.
In this investigation, you will use agar cubes to which the indicator bromothymol blue has been
added. BTB is an acid/base indicator that turns yellow in the presence of an acid such as acetic acid
(vinegar). Thus the surface of the agar cubes will turn yellow immediately when put into an acid
solution. The acid will continue to diffuse through the cube and gradually turn the inside of the cube
yellow.
Materials:
• Agar
• ruler
• vinegar
• Knife/blade
• Beaker or cup
• Spoon/Tongs
Procedure:
1. ACCURATELY cut three agar cubes: A 3cm cube, a 2cm cube, and a 1cm cube.
2. Place your cubes in the cup or beaker.
3. Add enough vinegar to completely cover the cubes.
4. Let the cubes soak for 10 minutes, gently stir the solution and cubes periodically.
5. After 10 minutes, use a spoon or tongs to remove the cubes from the acetic acid (vinegar) solution
and blot them with paper toweling. Be careful not to squish or deform your cube.
6. Cut each cube in half and measure the width of the inner blue cube that was not penetrated by the
vinegar. This measurement is the Length, Width, and Height of your unpenetrated section.
Record the resulting volume in the data table below.
