Photosynthesis evolution | Cellular energetics | AP Biology | Khan Academy

• [Instructor] In this video, we are going to talk about the evolution of photosynthesis on Earth because that's the only place that, at least so far, we're aware of photosynthesis occurring. I personally believe that it's occurring in many places in the universe, but we don't really know just yet.

But first, a reminder of what photosynthesis even is. It's the process where organisms are able to take carbon dioxide in the atmosphere in the presence of water. And then use the sun's energy, so sunlight, to then produce sugars, C6H12O6, and three oxygen, so O2.

Now, the way I've written it, this chemical equation right over here, it hasn't balanced. So see, I have six carbons here, so let me put a six out front there. But now, let's see, I have 12 hydrogens here. I only have two hydrogens here, so let me multiply this by six as well. And let's see, on the left-hand side, I have two oxygens times six is 12, plus another six oxygens, 18 oxygens, so I need 18 oxygens over here. I already have six right over here, so I'll need another 12 right over here, so I'll put a six out front. So, I have balanced that.

It's important for you to realize that this is a really big deal. You would not exist without photosynthesis, and I know what you're thinking. You don't photosynthesize things. But the things that you eat or the things that you eat, the things that they eat, they do some sort of photosynthesis.

At the end of the day, our energy is coming from the sun. Even if you're eating an animal, that animal might be eating another animal that eats a plant, and that plant is using the sun's energy, and it's storing it in the form of carbon-based molecules, oftentimes sugars.

And then when we eat these things, we are able to produce energy for them and do things like educational videos. But an interesting question is where does this come from? And a straightforward answer is we don't have all the answers, but scientists have a reasonably good idea of where it probably came from.

Today, we can observe cyanobacteria. This is what cyanobacteria looks like. It is like all bacteria prokaryotic, and it is able to conduct photosynthesis. And scientists believe that organisms not too different from cyanobacteria, probably an ancestor of cyanobacteria, existed on Earth two and half, three billion, maybe even older, maybe even further back in time, years ago, and that's early in Earth's history. Earth has only been around for about four and a half billion years.

And that ancestor of cyanobacteria, like cyanobacteria, was able to take carbon dioxide in the atmosphere in the presence of water and produce oxygen. And even though this is bacteria and each of these organisms are very small, in aggregate, they can have a pretty significant impact.

For example, this is a cyanobacteria plume near Fiji, and you can see that these are pretty significant things that can be a significant contributor to oxygen in the atmosphere. And scientists believe that it was these ancestors of cyanobacteria that, as they evolved on Earth and started growing and growing and multiplying, that they started to affect Earth's atmosphere.

So, what you see in this chart right over here is our best sense of what the oxygen content in the atmosphere was if we go back in time. And we can start to figure this out based by looking at rock samples that are very, very old, by looking at the fossil record, very, very old.

So, just so you get, you understand what's going on here, this horizontal axis right over here, this is billions of years ago. This is one billion years ago, two billion years ago, three billion years ago, 3.8 billion years ago, so it covers most of Earth's history. If you just wanna put things in context, humans, modern humans, have only been around for about two or 300,000 years, so we wouldn't even show up as a pixel on this diagram right over here.

So we're going back into deep time. And scientists believe that the first photosynthetic organisms might have evolved approaching three billion years ago, although we're not exactly sure. And those organisms might have been producing oxygen from the photosynthesis, but it might have been absorbed by things like the ocean.

But eventually, those organisms, probably these ancestors of cyanobacteria, became significant enough that the oxygen started pouring in the atmosphere, and we start to see that right over here. Before this point, the oxygen was pretty close to zero percent, and then right over here, at the Great Oxygenation Event, I'll just call it GOE, all of a sudden, oxygen becomes a larger and larger percentage of the atmosphere.

And these two lines represent two different estimates of what percentage of the atmosphere oxygen was at these various times, and it grew all the way to modern times where oxygen is roughly 20 or 21% of our atmosphere.

And the reason why this is sometimes called the Oxygen Catastrophe is that there were a lot of anaerobic organisms that found oxygen poisonous, and so there was a great extinction event from all of these oxygen in the atmosphere, but it was not a catastrophe for what eventually would be us because we are dependent, not just on the sugars from photosynthesis, but we're also dependent on the oxygen from photosynthesis.

We use this oxygen to conduct respiration. You can almost view respiration as a backwards process. You take your sugars and in the presence of oxygen, you are able to extract that energy from those sugars so that we can live.

Now, an interesting question is at what point did we go from these prokaryotic bacteria organisms to eukaryotic organisms that are able to perform photosynthesis? Most notably, plants. Well, this goes back to endosymbiosis theory.

We have a whole other video on that, but it's this idea that the ancestors of the cyanobacteria might have lived in symbiosis with another eukaryotic cell, where the cyanobacteria ancestor was able to harness light energy to produce sugars for the larger cell. And in return, the larger cell was able to give protection or nutrients.

And so, we believe, based on endosymbiosis theory that chloroplasts, and right over here, you see plant cells with visible chloroplasts in them, that chloroplasts are actually descendants of those ancestors of cyanobacteria. Cyanobacteria would be other descendants of them, but these are the ones that started to live in symbiosis with what would later become plant cells.

And good evidence that these might have ancestors that used to live independently is that they have DNA that is very similar to the DNA of cyanobacteria. They have ribosomes, their own ribosomes, that are very similar to the ribosomes of cyanobacteria.

And so, like mitochondria, we believe chloroplasts originated as independent bacteria-like organisms and eventually were engulfed into eukaryotic cells. So, I will leave you there. Photosynthesis is a big deal, and it's really interesting to think about where it came from.

It likely evolved on Earth many billions of years ago, probably close to three billion years ago, but around 2.3, 2.4 billion years ago, it fundamentally changed the planet where you had oxygen pouring into the atmosphere, killing a lot of organisms, but making it possible for many other organisms to live.

And those early photosynthetic organisms, some of them had descendants that ended up living in symbiosis with larger eukaryotic cells that would eventually evolve into things like plant cells.