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I’ve daydreamed before about what it’d be like to have a special ability that other organisms
can do. Something that humans - or amoebas - cannot. For example, to be able to fly like
a peregrine falcon. Or to be able to walk up walls without any gear – like a Texas banded gecko.
But if I told you I really wish I could have a special ability of
a plant – you’d probably be confused. What can a plant do that’s so amazing?
There’s a lot of cool things about plants actually. But, in particular, I really wish
I could do photosynthesis. And it’s not just plants that can do this process. For example, some
protists and some bacteria can too. But plants will be our focus for this video clip.
Animals and amoebas may have missed out on this ability, but we benefit from it greatly as this
process also produces oxygen. A gas that we need. Any process that plants themselves need to survive
is important to us – because plants in general are major producers making them indispensable
in food webs. Many of our medications and foods come from plants. We need plants.
So understanding the nature of the process that plants use to make their own food is paramount.
So when I say make their own food, I’m talking about making a sugar that they need.
Specifically, glucose. You need glucose too,
but you get it from what you eat. Plants, however, get to make their own glucose in photosynthesis.
Here is the balanced overall equation for photosynthesis, similar to what you’ll find
in many introductory biology textbooks. As you will notice, it has some similarities to
aerobic cellular respiration. Recall that cellular respiration is used to make ATP,
which is an energy currency, and it’s done by plants as well as animals
and a lot of other organisms too. You can see how these reactants (inputs) of photosynthesis
are included in the products (outputs) in cellular respiration. And the products
(outputs) of photosynthesis are included in the reactants (inputs) of cellular respiration. While
this doesn’t mean that they’re simply reversed, it is interesting to see what they have in common.
So while both plants and animals need glucose for cellular respiration, plants don’t have to be in
search of glucose. Because they make it. Plants have adaptations to carry out photosynthesis in
a variety of environments. One thing plants have to do is capture light. Plants can use light
capturing molecules called pigments. Recall that visible light has different wavelengths and those
different wavelengths of light have different colors. If you’ve ever played with a prism before,
you can see how light can be separated into a rainbow of colors due to different wavelengths.
So, a pigment that plants commonly use to capture light is chlorophyll. Chlorophyll does a great
job at absorbing red and blue light – but not so much green light. Chlorophyll reflects green light
and this is one reason why many plants appear green to our eyes.
There are 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 that can be found in the chloroplasts of plants cells. There are two major
reactions that occur in the chloroplast that, together, make up photosynthesis.
They are: the light dependent reactions and the light independent reactions.
The light independent reactions can also be called the Calvin Cycle
or even…less commonly…the dark reaction. Sounds intriguing.
We’re going to talk about both of these briefly and please remember, like most of our videos,
this is pretty general. We’ve got some further reading links in the
video description where you can explore a lot more detail.
So light dependent reactions: happen in the thylakoids. Little compartments in the
chloroplasts that contain pigment. A collective stack would be a granum…multiple stacks would
be grana. In the light dependent reactions, light is captured and water (which is a reactant in the
photosynthesis equation) 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 the light dependent reactions.
The light dependent reactions also produce ATP and NADPH, which we’ll get to in a little bit.
Both the ATP and the NADPH will be needed for the next process: the light independent
reactions. Also known as the Calvin Cycle or Dark Reaction. The name is bit misleading. While,
yes, this process isn’t directly capturing light, it doesn’t require darkness either.
And, again, it will need items from the light dependent reactions like the ATP and NADPH. The
light independent reactions still happen in the chloroplast. But specifically, the light
independent reactions happen in the stroma. The stroma is a fluid outside of the thylakoids.
In the Light Independent Reactions -or Calvin Cycle- carbon dioxide enters. It is taken in
through pores – that are often but not always on the bottom of leaves – and those pores are called
stomata. Plants have the ability to open and close their stomata. The carbon dioxide gas enters
the stomata and will be fixed. 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 that had come from
the light dependent reactions will act as an energy currency for the Calvin Cycle. The NADPH
that had come from the light dependent reactions will supply 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. A sugar. Phew.
So, let’s take a look at this equation. Last time, I promise. So, we have here the circled items
from the light dependent reactions. And now, notice the other items----the CO2
on the reactant side and the glucose on the product side---those were from the Calvin Cycle.
Remember, there is so much more detail to explore in this amazing process.
You can learn about the photosystems that are in the light dependent reactions
or the detail of all the steps in the Calvin Cycle and how ATP and NADPH will be converted
to ADP and NADP+ which can then be used again by the light dependent reactions.
But before we end our short video, we do want to mention that plants have some amazing adaptations
that help them perform photosynthesis efficiently in different environments.
Many of these adaptations can involve the diversity of leaf shapes, coverings,
and pigments. This is definitely worthy of a completely separate video topic, but
to give a neat example of an adaptation involving photosynthesis: consider the cactus.
Cacti have a potential problem. They often live in a hot desert, and so if they open their
stomata during the hot day to get their carbon dioxide, they can easily lose more water than
would be ideal. The precious water can escape through the stomata if the stomata are open,
and that will happen at a faster rate in the hot desert sun. But cacti, and some other plants too,
can do something called CAM photosynthesis. In CAM photosynthesis, plants can open their stomata at
night – when it’s not so hot -and they can capture carbon dioxide and chemically store it. They can
then use this carbon dioxide the next day when the sun is shining and yet have their stomata closed,
allowing them to avoid having to open their stomata in the heat of the day. Well,
that’s it for the Amoeba Sisters, and we remind you to stay curious.