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Photosynthesis is a powerful process that is responsible for some of life’s most vital functions. In fact, consumers like us rely on photosynthetic producers to harness energy from the sun so that we can survive and thrive. In this episode of Crash Course Biology, we’ll explore photosynthesis in two acts, and learn how the process works inside and out.

How Can We Feed the World? 00:00
Producers & Photosynthesis 1:26
Chloroplasts 4:04
The Light-Dependent Reactions 5:01
The Calvin Cycle 7:30
Carbon Fixation 10:27
Review & Credits 11:33

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CC Kids:
The human population is growing.

Right now there are around eight billion of us, but by 2050, there could  be as many as ten billion. And while ants have us outnumbered by about 2.5 million to one, there’s still a lot of mouths to feed.

On average so far, agricultural production has risen to meet the growing demand year after year. But scientists have identified  that this average annual  increase in food production has begun to plateau. That means we’re not on  track to produce enough food  to feed everyone in an increasingly crowded world.

On top of that, rising temperatures due to climate change are predicted to continue worsening extreme weather and droughts, posing additional threats to our food sources. The good news? Scientists, farmers, engineers, and others are working hard to prevent this from happening.

And one way they’re doing that is by researching photosynthesis. You probably know that photosynthesis is the process plants use to make their own food — but the ability to photosynthesize efficiently can determine how successful a crop will be. So, learning more about it can literally help fight world hunger.

Hi! I'm Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology.

Oh, looks like the theme music I planted is blooming nicely! [THEME MUSIC] You and I are consumers, which means we have to eat plants and/or animals in order to get the energy  we need to live our lives. After you chow down on a huge slice of pizza,  your body uses the energy  in that food to function. At the smallest level, way down inside your cells, the specific process that unleashes that energy from food is called cellular respiration.

We’ve got more on that in episode 27 – a LOT more. But the whole world can’t  just be full of consumers. At some point, something has to be making food.

Enter producers: also known as the plant kingdom, the cyanobacteria in your local pond, and the phytoplankton in the ocean — and all the other photosynthetic organisms. Our photosynthetic friends can’t create energy — that would go against the laws of physics, like literally the first law — but they can transform the energy from something that’s not alive, like the Sun, into a form of energy that living things can use. And whether it’s happening inside a tiny plankton or a huge redwood, the fundamental process of photosynthesis works the same way.

Producers use energy from the Sun to combine molecules of carbon dioxide and water into a usable form producing a carbohydrate. In other words, they take a whole bunch of bouncing-around molecules and bond them together tightly to store chemical energy in sugars like glucose. So, photosynthesis is basically the opposite reaction of cellular respiration.

In cellular respiration, organisms take chemical energy stored in the molecules of food and convert it using reactions that release the stored energy. Molecules that are full of potential energy, like a coiled spring, get released to do work in our cells and our bodies more broadly. So, plants package energy in molecules through photosynthesis.

Then we eat those plants — or we eat animals that have eaten those plants— and the energy they have  stored in their molecules. And finally, our cells release that energy to power our bodies. And plant cells respire, too, to power their bodies.

They’re kind of an all-in-one deal. These processes are also the opposite in terms of inputs and outputs. Photosynthesis uses energy to convert CO2 and water into sugar and oxygen.

And then cellular respiration converts sugar and oxygen into CO2 and water, plus energy. Life is wildly symmetrical sometimes. The point is: these complementary processes are happening all the time, all over the world, in all kinds of organisms.

They’re the basic ways that energy moves through living things, from photons of sunlight into physical movement. But let’s get microscopic. Photosynthesis happens inside cells.

Even more specifically, it happens inside little structures called organelles within those cells. And the specific organelles  are called chloroplasts. If you zoom in on a chloroplast, you’ll see something that looks like a bean.

And if you open that bean up, you’ll see two main areas – the thylakoids and the stroma. Thylakoids look like little green coins, and they sit in stacks. Each thylakoid has a membrane, or barrier.

The stroma is the space surrounding the thylakoids, and it’s filled with fluid. Each of these areas houses a different step in photosynthesis. Ultimately, photosynthesis is an elaborate process  at the intersection of biology,  chemistry, and physics.

But at its heart, photosynthesis is like a play that comes to you in two acts – The Light-Dependent Reactions and the Light-Independent Reactions, or the Calvin Cycle. Act

One: the light-dependent reactions. Setting: Thylakoid. Now I probably don’t need to say this given the name, but the light-dependent reactions need light to work. And they take place in the thylakoid membranes encasing those little green coins.

See, those membranes are packed with chlorophyll, a pigment that gives green plants their color. But it’s not just about looks. When sunlight hits chlorophyll, it energizes the chlorophyll’s electrons, kicking them up to a higher energy level.

As the energy builds up, two electrons are separated from the chlorophyll and move on to prepare for the next act. Meanwhile, to replace what  it lost, the chlorophyll  steals a pair of electrons from a water molecule. The water can’t handle losing its hydrogen and has a breakdown of its own, releasing oxygen.

That’s right—one of the byproducts, or unintended products, of this first act, is oxygen. Stop and think about that for a second: a totally secondary, not-intentional result of the  process of photosynthesis  is that we get to breathe. Plants are just doing their plant thing, building their plant bodies — they ain’t thinking about us!

And yet, thanks to them and this process, here we are. Every time I see a plant, I give it a little quiet fist bump. You can fist bump a leaf, go ahead try it. [inaudible] Anyway, these electrons are  energized in the same way  that a toddler hyped up on  soda and birthday cake is.

They’re very unstable, and it’s easy for them to release their extra energy. Which the chloroplast is happy to take advantage of! The electrons are shuttled through a series of proteins inside the chloroplast, called the electron transport chain.

With every step in the chain, the electrons lose a bit of their extra energy, like if our sugar-hyped toddler was taking little five-minute naps. And at the end of the process, the chloroplast has used the energy released from the electrons to make two new molecules: the energy-storing molecule ATP, and NADPH, which is like an electron bank. But we’ll circle back to those two in a minute.

Now just to recap for those of you who weren’t with us before the intermission: The chloroplast has made oxygen, plus two molecules it’ll use later, and we’ve now moved outside the thylakoid membranes, and into the stroma. And at long last carbon dioxide shows up — probably with a lavish musical number, like Thomas Jefferson in Hamilton. What did you miss?

Only all the light-dependent reactions, buddy. Act

Two: The Light Independent Reactions. Setting: The Stroma. In this second half of photosynthesis, that CO2 goes through the Calvin Cycle, or the light-independent reactions. And this is where organisms really get to the work  of creating organic compounds  to fuel their growth.

Plants take in carbon dioxide  molecules from the air, and those molecules team up with another molecule inside the chloroplast called RuBP. The key thing to know about  this character is that,  like carbon dioxide, RuBP  also contains carbon atoms. When carbon dioxide and RuBP get together, the result is a group of new molecules with three carbon atoms each.

And they go on quite the adventure. As the Calvin cycle progresses, these three-carbon molecules are transformed. They’re infused with more and more energy, with various phosphate molecules and hydrogens getting tacked on along the way.

These transformations are  powered by the ATP battery  made way back in the light-dependent reactions, along with electrons from the NADPH electron bank. By the time this song and dance is over, the Calvin Cycle has synthesized carbon dioxide into something new: a basic sugar, called G3P. But don’t leave yet just because the curtains have closed on the Calvin Cycle!

There’s an encore! The chloroplast has to recharge so the plant can run the whole routine again. So, as the Calvin Cycle winds down, two of the G3P sugar molecules exit stage right, and the chloroplast converts  the remaining ten into  RuBP molecules, getting ready for the next go.

Once two of those G3P sugars leave the stage, the organism can use them for all kinds of things. Some, like glucose, are converted into more complex sugars that they can use to make  cellulose in a plant cell wall,  or store for use later in the form of starch. Others are used to make different

compounds the organism needs, like fatty compounds and amino acids, which can help with things like nutrition and stress responses.

From the light-dependent  reactions to the Calvin Cycle,  those are the basics of photosynthesis. With sunlight, water, and CO2 from the air, organisms like plants make… well, they make themselves. What's especially incredible  is that photosynthetic  organisms are basically building  their bodies from the air.

Think about it: using energy from the Sun, the chloroplast is turning carbon dioxide from the atmosphere into sugars that they can then use to make their bodies. So, that huge oak tree outside your window, or that hearty lentil salad I had for lunch — all that literally came out of thin air. Now that doesn’t mean that mass was created — again, we can’t break the laws of physics.

Most of the molecules that are here on Earth have been here for billions of years. We just move them around, and change them. That’s what’s happening when little Oakie grabs hold of molecules in the air and transforms them through photosynthesis, until many years later, it’s big Oakie.

When plants use the atoms  in carbon dioxide to build  compounds like sugars, it’s  called carbon fixation. The plants are turning carbon dioxide from something organisms can’t use to get energy, into something they can use: sugars. So, on the big picture level, carbon fixation is what allows plants to grow and function.

And since plants and other producers form the bottom of the food chain, and are responsible for feeding  everything else on Earth, carbon fixation is the process that keeps all of Earth’s ecosystems running smoothly. And I really do mean all of Earth's ecosystems. Like, in the 1970s, Filipino-American biologist Roseli Ocampo-Friedmann and her husband, the Hungarian-American biologist Imre Friedmann were amazed to find blue-green algae not only  living in Antarctica, but  living inside gaps in rocks.

These plants spend most of their time doing the plant version of hibernating. But sometimes, puddles form after snowfalls. And when this water seeps into the rocks and reaches the algae, they essentially wake up and  start photosynthesizing — producing food and fixing carbon in one of Earth’s least friendly environments.

And that brings us back to, well, us. Making sure that all of us  can get enough energy from  enough food to not only survive  but live a healthy life. The more we learn about how photosynthesis can happen even in extreme conditions, the more we can apply that knowledge when climate change creates even more extreme conditions.

And plant scientists are researching how to enhance photosynthesis — searching for crop species that require less water and considering how we could help plants capture more sunlight and carbon dioxide, to produce more food for a growing population. This work is just getting started, and with a human population approaching ten billion, we’re gonna need plenty of scientists working on this well into the future. In our next episode, we’re going to find out how each of our cells has its own little life cycle.

I’ll see you then! Peace! This series was produced in collaboration with HHMI BioInteractive.

If you’re an educator, visit for classroom resources and professional development related to the topics covered in this course. Thanks for watching this episode of Crash Course Biology which was filmed at our studio in Indianapolis, Indiana, and was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.