How Do Living Cells Deal With Changes in pH?

One of My Favorite Labs .... 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 organelles.  Many of the biochemical reactions that occur produce end products that affect the pH of the cell.  Every cell has an optimum pH at which the cell will best function.  If the pH of the cell environment varies from the optimal pH too much, the cell could die. It is vital that cells be able to manipulate their environment to maintain a pH of optimal functioning. Thankfully, cells can produce buffers which help to maintain a stable pH inside living cells.  Buffers are substances that are either weak acids or weak bases that are produced in response to the cell becoming too acidic or too basic.

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 have to 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.  This serves as a control, showing what happens to the pH when no buffers are being produced.
• Now it is time to test living cells.  Students are given a liver homogenate (liver ground up in blender) 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.  It is immediately obvious to the students that the cells are producing buffers to control their pH.  
• Repeating the process with raw potato proves that plant cells also produce buffers.
• Results are graphed and followed by final analysis questions.

I use the pH meter that you see in this 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 is one of my favorite labs that I do with my Biology students.  It clearly shows the concept of buffers, it requires the students to use good lab techniques, the students must table large amounts of data, and the data must be graphed.  Much is accomplished by this lab! This lab is suitable for grade 8 and up.  I hope that you will give this lab a try.  You will not be disappointed.

Want to view this lab in my TpT store? Click the image below.