Biol 1440, Principles of Biology I

Cellular Processes
Diffusion  across cell membranes, pt. 2 


Diffusion is a process for the passive transport of nutrients, water, and wastes into and out of cells.

Diffusion is the net movement of matter along a concentration or energy gradient.


The overall objectives are:

1. Investigate the factors that effect the rates on diffusion  on solutes (dialysis).

2. Investigate the role of membrane structure can play in permeability.

3. Learn to use a data acquisition system and couple it to data analysis.

Research Questions

  • What factors affect the rate of diffusion?

What would occur with regards to how a solute (ion) and water would move across the cell membrane, if:

  • the solute concentration is greater in the cytoplasm than in the environment surrounding the cell?
    How would the conductivity of the environment change?
  • the water concentration is less in the cytoplasm than in the environment surrounding the cell?
    How would the weight and size of the cell change?

Do the solutes and water diffuse in the same direction?




Ch. 8:136-140

Practice Quiz

Experimental Set-up using a Conductivity Probe

Creating graphs with computer assistance will be subject to your instructor's preferred software:
GraphicalAnalysis, Cricket Graph, or Excel.

Supplemental reading
     Diffusion Theory

Students need

  • a 3.5 in floppy disk suitable for your lab computer.
     Bring a disk that is;
    IBM_PC or unformatted for labs in LSC 144.
    Mac or unformatted for labs in LSC 140.
  • graph paper
  • metric ruler
  • calculator
Directed Investigation 1.
Changes in cytoplasm when salts diffuse into or out of the cell.
1. Investigate the responses of an artificial cell to changes in its salt status by creating different concentration gradients across a artificial cell membrane.

2. Learn to use  Loggerpro« a data acquisition system.

3. Use a conductivity probe to measure salts in solution with the data acquisition .

This investigation is designed to allow the student to make key observations about the processes of diffusion. From these observations, your lab group will design an experiment to test a hypothesis about what can happen to cells from changes in their cytosol or in their environment.

Cellular responses to solute (salt) changes

What will happen to conductivity of your simulated cell cytosol, if the concentration of a salt changes in the cell's environment?

Will equilibrium be reached or will the cell continue to take up salt?

What would you predict changing concentration will have on the rate of difffusion?

  • If you double the concentration, would the rate of diffusion double?  remain the same?  decrease by half?

Construct a hypothesis and design an experiment to test your hypothesis.

Experimental Design

1.  Measure the conductivity of the environmental solution (NaCl) & the simulated cytosol (distilled water).

2. Fill the each side of the chamber with the appropriate solution using the syringe.

3. Monitor the change in conductivity for 3-5 min (180-300sec) using LoggerPro.

4.  Analyze the rate of diffusion overtime in five 20sec blocks to the end of the collection period and find the maximum rate of diffusion.



Each concentration of salt is a treatment.  What would be a suitable control treatment?  Stock is 1M NaCl, 58g/mole

Replications = Instructor option.  Each lab group can do 3 or more, or each lab group will serve as a replication for combined class results.

Dependent variable = conductivity of the solution
The unit of conductivity = microSiemens/cm (ÁS/cm). 

Data Collection Table- 


Salt Conc., M

 Rate of Diffusion (conductivity),

Max Rate of Diffusion,
1st 20
2nd 20
3rd 20
4th 20
5th 20



1.  How much did the conductivity change?  Why did it change?
2.  Did the conductivity reach equilibrium?
3.  Is the rate of change constant?  Measure five 20sec increments and plot it.

Relevance to Biology Relevance of your investigation.
Diabetes mellitus results in a failure of the cells of the body to take up glucose even though the blood has high levels. The excess glucose is excreted in the urine, which is a diagnostic test for diabetes. High blood glucose leads to damage of kidneys and eyes among other organs. Could glucose in the blood be affecting diffusion and osmosis in certain organs like the kidneys or the eyes?

Cystic fibrosis and cholera both affect how salts and water move across cell membranes. Cystic fibrosis results in a mucous, composed of protein and other substances, lining the tissues of the lungs, salivary glands, pancreas, and small intestine.

Hypothermia and fever have adverse effects on the human body.  Thus, temperature could affect diffusion and osmosis, especially when you consider sweating from a fever.

Composition of the human body 
Ionic composition of bodily fluids