You ever wonder where the oxygen in the air you breathe comes from? In this
science snack I'm going to show you how to actually see and measure the oxygen
produced by a leaf during photosynthesis.
Here's what you'll need... spinach or ivy leaves, baking soda, also
known as sodium bicarbonate, a gram scale, water, liquid dish soap, a bottle,
a soda straw or a hole punch, a ten milliliter syringe without the needle, a clear
plastic cup or beaker that can hold about a cup of water, an incandescent or
100 watt equivalent light bulb in a fixture preferably with a clamp a ring
stand or something else to clamp your light on, a timer,
paper and pencil to record your results, foil.
First thing I'm going to do is make a buffer to carry out this reaction in this this buffer is going to be 0.1%
baking soda that's weight to volume. So I'm going to
measure out point five grams of baking soda using my scale and I'm gonna put it
in 500 milliliters it's about two cups of water and I'm gonna put the cap on
and dissolve the baking soda in the water.
This buffer needs one more thing which is a little bit of dish soap, the dish
soap helps the baking soda get past the water repellent surface of the leaf.
I'm going to mix that in but I'm going to be very gentle because I don't want
to create too many bubbles in my buffer. The next thing I'm going to do is make
my leaf disks so I'm going to use spinach leaves and I covered them with
tinfoil because it's a good idea to keep them in the dark until you're about
ready to do your experiment. I'm going to punch holes of spinach leaves and put
them inside this syringe so the first thing I'll do is remove the plunger from
the syringe and I'll punch about ten holes using the end of a straw out of
the spinach leaves if you're using ivy it's better to use a hole punch
since the leaves are a little bit tougher. To get these spinach leaf discs
into the syringe I'll just blow really gently. I'm going to tap the leaves down
to the bottom of the syringe so they're not stuck on the sides and replace the
plunger and push it almost all the way in being careful not to smush the leaves.
I'm going to add some buffer to this cup, the cup is clear which is important and
I'm going to pour carefully again trying to avoid too many bubbles in my cup.
Then I'm going to draw up a few milliliters of my buffer into my syringe with the
leaf disks. So my leaf disks are floating right at the top of the liquid in the
syringe, this is because there's actually air inside of leaves in the interstitial
spaces which means between the cells. I want my leaf disks to sink so I need
to remove the air I'm going to do that by applying a light vacuum so the first
thing I'll do is I'm going to depress the plunger just to remove most of the
air from the syringe and put my thumb over the tip of the syringe to make a
good seal and then I'm going to pull gently on the plunger and when I do that
I see little bubbles coming out of my leaf disks I'm gonna pull back a few
mils and then release. I'll tap a couple of times to dislodge any bubbles and the
least disk should start to sink they haven't sunk yet though so I'm
going to do this a couple more times again applying a gentle vacuum and then
releasing the plunger
and vacuum and release. So most of the leaf disks are settled to the bottom
I've sucked out enough air so that they sink to the bottom you might have to do
this a few times apply the vacuum and then release. So the
next thing I'm going to do is I'm going to put my leaf disks into this cup of
buffer, so to do that I want to make sure that I pull the plunger out slowly
directly over the cup so that they they just spill into the cup. So I've given
these leaf disks carbon in the water in the form of baking soda so now I'm going
to give them energy from this light bulb in the form of light.
We turn on the light and start this timer so that I can tell when my leaf discs start to do
something interesting and I notice that there are tiny bubbles accumulating on
the surface of the disks and around the edges, those bubbles are actually oxygen
that the leaf disks are producing as they photosynthesize. I'm keeping note of
when the leaf discs start floating. My first leaf discs started floating about
45 seconds into my experiment and now about a minute and a half later I have
three leaf disks floating. So by five minutes all of my disks are floating
this might take your disks longer or shorter depending on your leaves and
your light source. Once all my disks are floating I'm going
to do one more experiment, I'm going to cover them with foil you could just also
put them in a dark cabinet but basically we want to remove the light source
I'm going to leave these alone for a few minutes and come back and check on them later.
So I left my leaf disks in the dark for about half an hour. I'm going to
check on them now and most of them have sunk to the bottom of the cup.
They stopped floating in the dark .
So in this experiment you've given your leaf disks everything that they need to do photosynthesis
you've given them some carbon dissolved in the water in the form of baking soda you've given them
light from your light bulb and you've given them plenty of water. They're going
to use these materials to make glucose or sugar and as a byproduct they're
making the tiny bubbles of oxygen gas that you observe gathering around the
edges. You can actually use this experiment as an indirect measurement of
the net rate of oxygen production and thus of photosynthesis. The faster the
disks float to the top the more oxygen must be being produced and thus the more
photosynthesis must be happening. So notice that I said you're measuring the
net rate of oxygen production and photosynthesis. Plants make sugar and
oxygen but they also use it just like us, animals like us, do basically the reverse
reaction of photosynthesis we break down sugar to produce energy and as a
by-product carbon dioxide. In this reaction requires the oxygen that we
breathe this is called cellular respiration, plants also do cellular
respiration. They make sugar and they use sugar so as
they're producing sugar and oxygen they're also consuming it. So the bubbles
that you see represent the net oxygen - the oxygen that the leaves make minus
the oxygen that they're using. When you cover the cup with foil or put it in the
dark you're removing the light energy that the leaf uses for photosynthesis.
So it can't make sugar and oxygen anymore but it's still using sugar and oxygen
doing cellular respiration. That's why the leaves in the dark will eventually
sink, the leaves will use up the oxygen in the tiny bubbles that they made and
without the bubbles they'll sink to the bottom of the cup. It's very easy to
manipulate variables in this experiment to figure out what affects the rate of
photosynthesis, you can add more or less baking soda , use different light sources
or even use gel filters to change the wavelength of the light that's hitting
the leaves. If you keep a record of how quickly the leaves float under different
conditions you'll be able to figure out how these different conditions affect
the rate of net photosynthesis.