Help with the effect of Sodium Hydroxide and Germinating Seeds.

Ross Koning Koning at ECSUC.CTSTATEU.EDU
Fri Nov 15 09:20:08 EST 1996

At  1:14 AM 11/15/96 +0000, Paul Koch wrote:
>I have been investigating seed's cellular resperation by doing a lab that
>required putting germinating seeds in the bottom of a test tube.  Then a
>layer of cotton was put on top of that.  On top of this, I put a thin
>layer of Sodium Hydroxide and then another layer of cotton.  I put this
>test tube upside down in a beaker of water and let it sit over night.  The
>next day I found that the amount of air in the test tube had greatly
>decreased.  I also found that the layer of Sodium Hydroxide had
>disappeared.  I know that a germinating seed takes in oxygen to preform
>areobic resperation and as a byproduct of this, Carbon Dioxide is
>produced.  But, where did the carbon dioxide go and where did the sodium
>hydroxide go?  Why didn't the carbon dioxide take up the same amount of
>space as the air the seed took in?  I would appreciate it if you could
>answer these questions and maybe go further in depth into these processes.


You are correct about the seeds doing respiration, and the
fact that there should be a 1:1 relationship between oxygen
consumed and carbon dioxide released.  However, you made
the gases accessible to sodium hydroxide pellets...these are
very alkaline (caustic!) and hygroscopic (draw water).  Now
if you remember your chemistry well, about the equilibrium:
     carbon dioxide <-> bicarbonate <-> carbonic acid
then you can probably guess what happened to the carbon
dioxide.  It was trapped in an alkaline solution developing
on the surfaces of the sodium hydroxide pellets.  This is'
the purpose of those pellets.  If you had not put them in,
the oxygen tension would have dropped, the carbon dioxide
levels would have increased, and respiration would have
stopped.  Instead, the carbon dioxide was trapped and so
respiration could continue as the oxygen was being used up.
Indeed the use of that oxygen (forms water at the end of
ETS) and the loss of carbon dioxide will reduce the gas
volume in the container.  The pellets dissolved in the
humid atmosphere which is why you need the cotton plugs
to separate the living seeds from that caustic liquid!

You can use this to your advantage now, if you plug the test
tube with a soft! rubber stopper with one smoothly-drilled
hole fitted snugly with a piece of heavy-walled capillary
glass tubing.  You close the apparatus (handle as little
as possible and avoid temperature variations in the environment!),
allow a few minutes to equilibrate, then put a drop of 1%
eosin y (or other dye/ink) at the free end of the tubing.
The dye will move into the tubing at a certain rate.  This
will estimate the volumetric consumption of oxygen gas by
the seeds.  You can calibrate the volume consumed by weighing
the water held in a specific length of the tubing (that
assumes that the capillary tubing's ID is uniform...may not
be valid).  There are a number of assumptions made here about
pressures, volumes, and adsorption that may be, in part, invalid
but it is nevertheless an interesting project that can lead
students in an assessment of what they observe, what it means,
what the pitfalls are, etc.  Be sure to use a suitable control
with students...a separate supply of seeds that have been soaked
but then BOILED AND COOLED to ambient temperature must be
prepared in advance and kept separate.  I have used popcorn,
peas, and mung beans in this project.  The boiled seeds are
always a different color so they are easy to distinguish.  Each
group, then, needs to have two set-ups.  It is also instructive
to do a second run, and in mid-run HOLD the tube in your hot
little can guess the result...ask students to come
up with that on their own...maybe without the "hot little hint."
Maybe a more opaque hint is PV=nRT.  That kind of approach...


Ross Koning                 | Koning at
Biology Department          |
Eastern CT State University | Phone: 860-465-5327
Willimantic, CT  06226  USA | Fax: 860-465-5213

                Plant Physiology is Phun!

 /\|___/\     //\______COOH   NH-CH2-CH=C-CH2OH  \/OH
|  |  |  |    |  |  ||       //\___     \CH3     /\|/\\/\\COOH
 \/ \/|\/|    \\/ \ /       N  ||  N            |  |
 /\ | |__|=        NH       |  || ||           //\//\
  | COOH                    \\ /\ /            O
  COOH        H2C=CH2         N  NH

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