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Sunday, September 2, 2012

How Do Living Cells Deal With Changes in pH? Buffers!



A Biology Lab Favorite for Me....and My Students Love It, Too!!

The chemical reactions that occur within living cells are required for life, but often result in creating a hazardous work place for the cell.  The internal pH of most living cells is close to 7, and even a small change in pH can be harmful.  Many of the reactions of the cell cause changes in pH that can place the life of the cell in jeopardy. Thankfully, cells can produce buffers which help to maintain a stable pH inside of living cells.

The technical definition of a buffer is:  "A substance that consists of acid and base forms in a solution and that minimizes changes in pH when extraneous acids or bases are added to the solution."   A great example of buffers in living systems is seen in human blood.  The pH of human blood is close to 7.4.  A person cannot survive for more than a few minutes if the blood pH drops to 7 or rises to 7.8.  It is critical that the pH of 7.4 is maintained.  The presence of buffers in the blood allows for a relatively constant pH despite the addition of acids and bases.  Buffers minimize changes in the concentrations of H+ and OH- ions.  They do so by accepting hydrogen ions when they are in excess and by donating hydrogen ions to the solution when they have been depleted.

This principle can be demonstrated to your students by a very simple, (but pretty amazing!) lab experiment.  I admit that each school year I really look forward to doing this lab.  It is very easy to set up, doesn't require fancy pieces of lab equipment, works every time, and most importantly, my students are usually very impressed by the results.

Here are the key point to the lab:

  • Students begin by placing tap water in a beaker. They add drops of dilute acid one drop at a time and record the pH after each drop.  They repeat a second time, but add a dilute base. Predictably, the pH drops significantly with the addition of the acid and rises significantly with the addition of the base.
  • Now it is time to test living cells.  Students are given a liver solution to which they add drops of acid and base.  There is very, very little change in pH.  Students will often call me over and say that their pH meter is not working because there is so little change in the pH readings.  
  • Students repeat by using a potato solution to show that plant cells produce buffers just as animal cells do.
  • Results are graphed and followed by final analysis questions.
I use the pH meter that you see in the above photo.  These are relatively inexpensive and are pretty durable.  The set that I use has been in use for years, and they are still going strong.  Batteries are easily replaceable, but rarely need to be replaced.  If pH meters are unavailable, this lab can easily be conducted using pH paper.  This lab is suitable for grade 8 and up.  I hope that you will give this lab a try.  You will not be disappointed.

Have fun teaching!




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