Buffers

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What is a buffer?

    A buffer is a molecule that tends to either bind or release hydrogen ions in order to maintain a particular pH.  You have seen that our blood, for example, needs to maintain the pH of 7.4.  Buffers help that occur.  In order to explain this, let me use the bicarbonate ion that you are already familiar with as an example.  You have already seen that the bicarbonate ion form reversibly from carbonic acid, and can reversibly form CO3-, the carbonate ion.   Here is that figure again...

bicarbonate.jpg (7395 bytes)

    You can see that if the reactions go to the right, hydrogen ions are released, making the solution more acidic, and if the reactions go to the left, hydrogen ions are sucked up, making the solution more basic.  The burning question that is probably eating away at you right now is:  "how do I know which way the reactions are going to go-- to the right or to the left?"

    Good question!  The bicarbonate ion isn't a very strong acid or base.  It doesn't have to go one way or the other all the time.   Instead, the direction it goes depends on the solution it is in.  You see, if it is in an acidic solution, there are lots of hydrogen ions floating around; in this situation, the presence of tons of hydrogen ions will force the reaction to go to the left.  Do you see why?  Look at the reaction again.  All you need for the reaction to be able to go to the left is available hydrogen ions (see the H+ in the middle that is needed for the reaction to go to the left?).  Hydrogen ions are readily available in acidic solutions.  So if bicarbonate ions find themselves in acidic solutions, they tend to act like the weak base they can be, and suck up the excess hydrogen ions.

    To finish answering the questions, consider what will happen if bicarbonate is put into a basic solution... A basic solution has very few hydrogen ions floating around.  In this condition, the bicarbonate reaction cannot proceed to the left, since no hydrogen ions are available.  Instead, it will go to the right, producing hydrogen ions.  In this manner, bicarbonate ions will tend to act like the weak acid they can be, and release hydrogen ions.

    Now back to the general question about what buffers are.  You see, a buffer can either accept or donate hydrogen ions, depending on the solution they are in.  Since the buffers will accept hydrogen ions in acids and donate hydrogen ions in bases, there must be some in-between-pH where they hit an equilibrium point and do not prefer to either accept or donate hydrogen ions.  Right?  That intermediate point, that equilibrium, is the pH that the buffer tends to maintain.  The bicarbonate buffer, so important in blood, has its equilibrium right at the pH of 7.4.   Pretty useful, huh?

    There are three important buffer systems in our bodies:

  1. bicarbonate buffer system
  2. phosphate buffer system
  3. protein buffer system

All three work similarly-- if they find themselves in a solution with a lot of free hydrogen ions floating around (an acid), they act as bases and suck up the excess hydrogen ions.  And if they find themselves in a solution lacking free hydrogen ions (a base), they donate their hydrogen ions to the solution.  Your book says the same thing, but uses different terminology.  So your book says that these three all turn strong acids into weak acids and turn strong bases into weak bases.  Let me try to explain...

    If a buffer finds itself in a solution with a strong acid, what happens?  The strong acid gives off its hydrogen ions.  The buffer grabs up many of the free hydrogen ions.  In this way, even if 100 molecules of strong acid were added to the solution, there will not be 100 hydrogen ions floating around.  Instead, there will fewer hydrogen ions floating around, maybe only 20.  A yield of only 20 hydrogen ions from 100 molecules of acid is a 20% yield, and that is what you would expect of a weak acid.  So, the buffer changed the strong acid into a weak acid.  Does that make sense?

    I find it more difficult to think about buffers the way your book does, but you might find it easier... so for that reason I have explained their description.  I hope it makes some sense to you!

    Now, it is time to go through the three buffer systems!

The bicarbonate buffer system

carbuffer.gif (3343 bytes)    We have basically finished this.  Quick overview:   carbon dioxide is converted into bicarbonate ions in the blood.  In this way, bicarbonate ions float around in the blood, and serve to maintain a pH of 7.4 in the blood.  I found this picture on another web site, and have included it here just to have a different image of this process from the one I have drawn.  Just ignore the stuff in blue.

 

The phosphate buffer system

phosbuffer.gif (1641 bytes)    You learned that nucleic acid and phosphoprotein break down yields phosphoric acid.  Phosphoric acid is H3PO4, and is shown at the far left edge of this figure (also taken from another web page).   Phosphoric acid changes pretty quickly into dihydrogen phosphate, or H2PO4-.   This dihydrogen phosphate is an excellent buffer, since it can either grab up a hydrogen ion and reform phosphoric acid, or it can give off another hydrogen ion and become monohydrogen phosphate, or HPO42-.  This figure shows that in extremely basic conditions, monohydrogen phosphate can even give up its remaining hydrogen ion, although your book doesn't show that.

    If the H2PO4- is in an acidic solution, the reactions above go to the left, and it if the H2PO4- is in a basic solution, the reactions above proceed to the right.  Therefore, the phosphate buffer system can accept or donate hydrogen ions depending on the solution it is in.

The protein buffer system

    Proteins themselves can act as buffers.  You know this, because I told you that in blood, when bicarbonate ions form, the hydrogen ions that are also products of bicarbonate production are absorbed by the blood proteins.  You also learned that this absorption of the hydrogen ions doesn't happen so well in cerebrospinal fluid, because there are fewer proteins in CSF.

    But how does this work?  To understand this, you have to think back to the structure of a protein.  Proteins are made up of amino acids.aminoac.jpg (21060 bytes)  And you will have to remember all about amino acids for this.  Amino acids have a central carbon with four groups off of it:

  1. a carboxyl group (COOH)
  2. an amino group (NH2)
  3. a hydrogen atom
  4. an R group

The carboxyl and amino groups are what enable proteins to act as buffers.  Let me explain.

The carboxyl group

    The carboxyl group is attached to the amino acid central carbon:   C - COOH.  In the figure above, you can see the carboxyl group off to the left.  You can see that the carboxyl group consists of a double bond to one of the oxygens and a single bond to the hydroxyl group.  The important part of the carboxyl group for our purposes here is the hydrogen atom within the hydroxyl group.

    At a near neutral pH, like the pH of blood, the carboxyl group is actually COO- instead of COOH.  Then, if a protein finds itself in a more acidic solution, the carboxyl group will be able to take on the extra hydrogen ions and return to the COOH configuration.

    Your book describes this by saying that if a protein is in a more basic solution, its carboxyl groups on its amino acids change from COOH to COO-.    This isn't exactly correct, because even at neutral pHs, the carboxyl group is COO-.  But it is true that if the carboxyl group is in the form of COOH, it will become COO- if it finds itself in a more basic solution.   OK?

The amino group

    The amino group is attached to the amino acid central carbon:   C - NH2.  The amino group is shown at the right hand side of the diagram of the amino acid above.  However, at a near neutral pH, like in blood, the amino group is actually NH3+ rather than just NH2.   It actually tends to carry an extra hydrogen ion on it at a normal pH.  Then, if a protein finds itself in a more basic environment, its amino groups on its amino acids can actually release their hydrogen ions and return to NH2.

    Again, your book describes this a little off... it says that when proteins are in acids, their amino groups tend to become NH3+.   In fact, the amino groups within protein amino acids are NH3+ even at neutral pHs.  But when they get into basic pH solutions, they return to NH2.

Diagramatic Overview of amino acids providing buffering functions:

protbuff.jpg (23396 bytes)

So, amino acids can accept or donate hydrogen ions, making them excellent buffers.  And any given protein typically has hundreds of amino acids.  So, proteins make superb buffers.  Remember, they are found in very high concentration in intracellular solutions and in blood.

2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.