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Today we are going to talk about an incredible process that uses light and gas to make something
awesome that we all love----sugar! Unfortunately, we cannot do this amazing, incredible process
---which is known as photosynthesis. Some types of protists can do photosynthesis. Some
types of bacteria can. And of course, plants can. Plants will be our focus for this video
clip. Animals and amoebas just sort of missed out on this ability. But we benefit from it
greatly as this process also produces oxygen---the very gas we need in order to breathe. So this
process is important for us to understand.First a little background. Plants and animals need
sugar. Specifically, glucose. Plants, as well as animals, use glucose to make ATP energy
in a process known as cellular respiration. ATP energy is critical for cells to be able
to carry out their cellular activities. But while we have to be in search of food to get
this glucose, plants instead can do photosynthesis to make their own. So that’s kind of nice,
because they don’t have to go anywhere to get it. Plants have adaptations to make them
able to carry out photosynthesis in a variety of environments. Here is the formula for photosynthesis.
On the left of the formula, you will find the reactants. That means, these are the inputs.
The plant has to have these in order to do photosynthesis. On the right of the formula,
you will find the products. That means, those are the items that are produced by the plant.
The outputs. Sometimes the formula is written a little differently. Technically, it needs
to be balanced and sometimes light is written on top of the arrow to show that it’s in
the presence of light so it may look like this. The C6H12O6 product is sugar, specifically,
glucose. Photosynthesis is not just a formula. Have you ever tried to capture light before?
It’s hard. Plants have light capturing molecules, called pigments, that help them do this. See,
visible light, has different wavelengths. Different wavelengths of light have different
colors. If you have ever played with a prism before, you may have been able to see how
light can be broken up into a rainbow of colors due to these different wavelengths. One pigment
that plants use for photosynthesis is called chlorophyll. Chlorophyll is an expert at absorbing
red and blue light, but not so much green light. Because it does not absorb very much
green light, it reflects green light. Therefore, many plants appear green to our eyes. There
are more pigments besides chlorophyll that work with different wavelengths of light and
this can explain why green is not the only color you see in plants. Chlorophyll is a
pigment found in the chloroplasts of plant cells. This amazing organelle is not found
in animal cells. There are two major reactions that occur in the chloroplast that, together,
make up photosynthesis. The light dependent reaction and the light independent reaction.
The light independent reaction is also called the Calvin Cycle or even...dark reaction.
The light dependent reaction happens in thylakoids. What are thylakoids? Well…they are little
compartments in the chloroplast and they contain the pigment. They tend to be nicely stacked.
As a collective stack, we call it a granum. The best way to describe a granum is that
it looks like…a stack of green pancakes. Yep. Very, very small green pancakes. In the
light dependent reaction, light is converted---by means of a complex process that involves multiple
“photosystems”---into chemical energy. By chemical energy, we mean ATP (we have mentioned
that before---that’s energy!) and NADPH. We will get to that later. During the light
dependent reaction, water (which is needed for this process to work) is “split.”
That means if you think of the chemical formula for water which is H20---it is split so that
you get electrons, protons, and oxygen. So oxygen is also a product of this reaction.
So let’s look at our formula. The light and water on the reactant side were used in
this light dependent reaction. And the oxygen produced was also from this light dependent
reaction. What about the other items in the formula? And why make this ATP and NADPH?
What do they do? Well…that’s the next step----the light independent reaction. The
light independent reaction—also occurs in the chloroplast. The name is misleading. It
doesn’t happen in the dark. It needs items from the light dependent reaction so they
happen at the same time. While it still happens in the chloroplast---we never get out of the
chloroplast with this, it does not happen in the thylakoids. Instead, it happens in
the stroma. The stroma is fluid outside of the thylakoid. In this reaction, carbon dioxide
must be fixed. What do I mean by fix? Nothing was wrong with it. By fixed, I mean that,
with the additional help of a major enzyme, the inorganic carbon dioxide is changed to
a more usable organic form. The ATP from the light dependent reaction helps supply energy.
The NADPH from the light dependent reaction supplies reducing power----by that, I mean
that it helps add high energy electrons to this process. So in a very complex series
of pathways, the fixed carbon dioxide, ATP, and NADPH are used to make a product that
ultimately can be converted into glucose. Let’s take a look at this formula. Last
time, I promise. So we have here the circled items from the light dependent reaction. Now,
notice the other items----the CO2 on the reactant side and the glucose on the product side---those
were from the light independent reaction. Together the light dependent cycle and the
light independent cycle---which happen simultaneously---create the glucose that plants need to survive. This
process of photosynthesis is necessary for plants to make sugar. Photosynthesis also
supplies organisms with the oxygen they need because remember, oxygen was also a product.
And, since plants are the producers in food webs, photosynthesis is important because
plants supply the energy for a lot of hungry consumers. That’s it for the amoeba sisters
and we remind you to stay curious!
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