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Diffusion (Joy Burton)


Joy Burton, Science Teacher, Sutter Middle School

Principle(s) Illustrated

  1. Diffusion of molecules move from areas of high concentration to areas of low concentration.
  2. Transport of substances across cellular boundaries are regulated.
  3. Diffusion cannot be observed when a cell is in a state of balance and equilibrium.
  4. Particles must be smaller than membrane openings in order for diffusion to occur.


  • 7.1 All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope.
  • 7.1a. Cells function similarly in all living organisms.
  • 7.5 The anatomy and physiology of plants and animals illustrate the complementary nature of structure and function.
  • 7.5a. Plants and animals have levels of organization for structure and function, including cells, tissues, organs, organ systems, and the whole organism.
  • 7.5b. Organ systems function because of the contribution of individual organs, tissues, and cells. The failure of any part can affect the entire system.
  • 7.6 Physical principles underlie biological structures and functions.
  • 7 Scientific progress is made by asking meaningful questions and conducting careful investigations.
  • 7c. Communicate the logical connection among hypothesis, science concepts, tests conducted, data collected, and conclusions drawn from the scientific evidence. (Diffusion lab)
  • 7e. Communicate the steps and results from an investigation in written reports and verbal presentations. (Diffusion lab)

Questioning Script

Prior knowledge & experience:

Middle school students know that they take in oxygen and get rid of carbon dioxide.  However, they do not know how this occurs. Students do not associate the word diffusion with the movement of particles, such as oxygen, across a cell membrane. Students know they excrete waste, solid and liquid waste, but do not know how this happens.

Root question:

How do particles in diffusion move?

How do vanilla particles move?

How do various substances diffuse through cell membranes?

What evidence is there that vanilla extract diffuses through the balloon's rubber membrane?

What is the relationship between the surface area and the volume of the balloon?

Target response:

Diffusion of the vanilla diffuses through the membrane of the balloons. Balloons are made of a rubber latex polymer of long chain-like molecules, or strands, that are all snarled together with bonds between the strands, called cross-links. Cross-linked molecule networks can be stretched and penetrated. Since the balloons's membrane is thin, semipermeable and there is a higher concentration of vanilla molecules inside the balloon vanilla will diffuse through the balloon's membrane. The vanilla molecules are small enough to diffuse out of the balloon where there is little or no concentration of vanilla. The pores of the balloon's rubber is large enough that the gases given off by the vanilla extract can move through them.

In the human body, capillary walls are one cell thick. Capillaries are the location where oxygen and glucose are exchanged between the blood and the body's cells. Carbon dioxide and cellular waste travel in the opposite direction of oxygen and glucose. This exchange between the blood and body cells is called diffusion.

Rates or velocities (u) of diffusion and effusion of gases are inversely proportional to the square roots of their molecular or atomic weights (M). According to Graham's law it states that the lighter the gas, the faster it will effuse (or diffuse). Mathematically speaking rates can be determined as follows: Rate1/Rate2=square root of (Mass2/Mass1).

Common Misconceptions:

A common misconception middle school student may have about this experiment is that since vanilla extract is a liquid it cannot diffuse through the rubber of the balloon. The vanilla cannot be detected. Students do not perceive that the balloon's rubber does not have pores, therefore the vanilla extract cannot diffuse through the rubber.