Photosynthesis | Energy and matter in biological systems | High school biology | Khan Academy

Hey everybody! Dr. Sammy here, your friendly neighborhood entomologist.

Today, we're going to talk about photosynthesis. There's very little life on this planet that could exist without photosynthesis. It is the prerequisite for pretty much everything you see around you. It's how you get from the intangible light of the sun to physical bodies like those of humans or hungry, hungry caterpillars.

But what does photosynthesis actually mean? I hear people say all the time that photosynthesis is the process by which plants make sugar from light, and it almost seems like magic. Light is not a substance. It is not made up of the molecular building blocks that compose all matter, and thus it doesn't have mass. You can fill a room with light and never run out of space!

So how could you possibly make something physical out of it? Well, you can't. So instead, light is a form of energy—energy being the capacity to do work.

This is where it's helpful to know word origin. The word "photosynthesis" is made up of two Greek words. It literally means light and to put together. That's right! In addition to being an entomologist, I dabble a little bit in etymology, just to make sure I'm maximally confusing to people.

Anyway, you're literally using light to drive reactions that combine ingredients into new products—a form of work. So you're not turning light itself into material sugar; you're taking matter that already exists in the form of six molecules of carbon dioxide, six molecules of liquid water, and using the energy of the sun to power a reaction that combines them into a new substance with molecular oxygen as a byproduct.

Think of it this way: when you bake a cake, you don't say that you made cake from heat. It would be more accurate to say that you took flour, eggs, sugar, and butter and used heat to combine them into something new.

So, sticking with our analogy, the ingredients for photosynthesis are just carbon dioxide from the atmosphere and water from the ground, with light to do the heavy lifting. Carbon dioxide and water are put together to produce a carbohydrate, which literally means "watered carbon." As you can see with this typical carbohydrate molecule glucose, your carbon is attached to the same atoms that compose water: two hydrogens and one oxygen.

This carbohydrate has more chemical energy, or bond energy, than the molecules of water or carbon dioxide that served as ingredients. Thus, the energy at the end of the process is much greater than the energy that the ingredients had at the outset. This means that photosynthesis is a sort of useful reaction that stores energy like a big biochemical solar cell. We call these endergonic reactions, and you might be able to see already why they would be so valuable in biological systems.

You take molecules that don't have much energy and use them to produce something that can do work. The kitchen where all this work happens is the chloroplast—literally the green maker. The chloroplast is a little organ or organelle present in some plant cells, and it's what makes plant cells and ultimately plants green. This is because the chloroplasts bear the green pigment chlorophyll.

Now, the same way that the word "photosynthesis" can be broken down neatly into its two base words, the process itself can be separated into two neat little segments. I like to think of them as a charging step, where energy from light is converted into chemical energy, and a synthesis step, where that energy is used to do the work of actually synthesizing the end product—typically a carbohydrate.

So the first segment requires the direct input of light and, as such, is referred to as the light-dependent reactions. These happen in a section of the chloroplast called the thylakoid. They form these neat little pouches, the inside of which is called the lumen and the outside called the stroma.

When a photon of light ends its eight-minute journey from the surface of the sun to the surface of a leaf, its energy is absorbed by the chlorophyll embedded in the thylakoid membrane. This energy powers a pump which literally charges the inside of the thylakoid like a battery by moving the ions inside. As the charge builds up, the energy can be used to do work.

But the next segment of this process happens outside of the thylakoid, in the stroma. So to get that energy where it's needed, the thylakoid transfers the energy to a molecule called ADP (adenosine diphosphate) by adding another phosphate bond and making it ATP (adenosine triphosphate).

Now, you might have heard of this one. It's often called the energy currency of the cell. Pretty much wherever energy is needed for a cell to do work, ATP is involved. You might have also heard that the energy is stored inside of the phosphate bonds and that breaking them releases the energy.

But try to remember that these bonds, depicted by the lines in these diagrams, are just a convention. It represents an adherence of these atoms together via attraction, and it shows you where they adhere. But the energy isn't actually in the bond. The attraction between the atoms builds up potential energy, like a rubber band that's being pulled really tightly.

When you let it go, it could hit a paper cup and do the work of displacing that cup. The rubber band then falls to the ground in a low-energy state because its energy has been released. So when you put the energy into a system great enough to overcome the attraction between the atoms and force them apart, thereby breaking the bonds, atoms—or in this case, a whole phosphate group—can go spiraling off, taking what was potential energy with it.

If it hits something that it can bond with, that energy is released as the bond is formed and can be used to do work, such as the magic of ATP. It's described as energetic because it's easy to break the bond between it and the last phosphate group, meaning you don't have to put much energy in, but you get a ton of energy out.

So back to the thylakoid. Those photons of light were able to net us a highly energetic ATP molecule. But the next segment of photosynthesis is going to need some electrons too. That light energy is used to do the work of loading up a mobile electron carrier with electrons and a proton. This carrier is called NADP+, and when it’s got a full load to take to the next set of reactions, it’s called NADPH.

And that's pretty much the light-dependent reactions in a nutshell. The only other thing you should probably remember is that, well, this is where all the oxygen in your lungs comes from. So, you know, no big deal.

No big deal! When chlorophyll gets excited by that photon of light, it turns into a real bully. The work it's doing creates such a powerful electrochemical imbalance, and the chlorophyll balances the equation by just stealing an electron from water. This causes water to fall apart, releasing its oxygen, which the plant just lets go of.

So now that we've taken care of the section that's dependent on light, let's discuss the section that isn't: the light-independent reactions or the Calvin cycle. These occur in the stroma of the chloroplast, and this is where the earth-shattering chemical reaction occurs that allows for all life on this planet: the fixation of gaseous carbon, or inorganic carbon, into carbon chains, or organic carbon.

And this is so important because fixation doesn't just happen on its own. CO2 in the atmosphere doesn't form organic chains or sugars when it bumps into more CO2 in the atmosphere. It requires the help of enzymes and energetic molecules made by living organisms. It's why, when we first landed a rover on Mars, we immediately started looking for the evidence of organic molecules. It would be evidence that something is or was living there.

In the light-independent reactions, a plant enzyme fixes carbon dioxide from the air into a chain of carbon. So, ATP and NADPH, which were produced in the light-dependent reactions, provide the energy and the electrons to create two energetic reactive molecules that can be combined to make glucose or other useful molecules.

And the beauty of it is that the byproducts of the light-independent reactions, ADP and NADP+, are shuttled off from the stroma back to the thylakoid for more light-dependent reactions, where they can be recharged and recycled for use again later.

And voila! Photosynthesis! The takeaway here is that photosynthesis allows you to go from the intangible energy of the sun to the stored chemical energy that life on this planet is based on. The sun's energy is converted to the chemical energy of a carbohydrate molecule in the chloroplast.

That molecule can later be broken down, and most of that energy reclaimed, either by the plant or by creatures that eat that plant. All life on Earth is carbon-based, and every single molecule of that carbon once existed in the atmosphere in gaseous form as carbon dioxide until some enterprising plant or microorganism synthesized it into something you can use.

And while they were at it, many of them filled the atmosphere with the oxygen that we all need to breathe. So the next time you see a plant, shake its leaf and say thank you!