This video, we're going to talk about secondary endosymbiosis and how it's believed to have led to several different eukaryotic lineages. Now whereas primary endosymbiosis is when a host cell engulfs a prokaryotic cell, secondary endosymbiosis is when a host cell engulfs a eukaryotic cell. And then that eukaryotic cell would live inside the host cell and develop a symbiotic relationship with it. And so it's called secondary endosymbiosis, not because it's the second endosymbiotic event, but because the eukaryotic cell that's being engulfed has organelles that it would have acquired via primary endosymbiosis. Now the ingested eukaryotic cell, eventually, over a very long period of time, would have evolved into an organelle with 3 or more membranes, which is indicative of secondary endosymbiosis.

We can't quite put a set number here because the membranes that are required via secondary endosymbiosis are sometimes retained, and sometimes they're lost. Now as an example, both red and green algae, which are eukaryotic cells themselves, are believed to have been engulfed by host cells in the process of secondary endosymbiosis leading to multiple different photosynthetic lineages. And down below in this image, we show you just a small example of that idea. So notice on the far left over here, we're showing you an ancestral photosynthetic eukaryote who would have acquired organelles such as the mitochondria and chloroplast via primary endosymbiosis. Now over a long period of time, it would have evolved into the red algae and the green algae that you can see here, which both have chloroplast organelles, allowing them to be photosynthetic.

Now here in this box, notice that the red and green algae are both being engulfed by a larger host cell. And because the red and green algae are eukaryotic cells themselves, this is going to be the process of secondary endosymbiosis. And over a long period of time, these engulfed red and green algae would have evolved into organelles with 3 or more membranes. As you can see here in these images leading to multiple different photosynthetic lineages here, we're showing you just two examples, the alveoliates and the chlorarachniophytes. Now the main takeaway of this video is for you to be able to distinguish between primary and secondary endosymbiosis.

Again, primary endosymbiosis is when a host cell engulfs a prokaryotic cell, and it's believed to have led to the very first eukaryotes, which would have been protists. And secondary endosymbiosis is when a host cell engulfs a eukaryotic cell, which is believed to have led to several different eukaryotic lineages. So that concludes this video. We'll be able to apply these concepts and problems moving forward, so I'll see you all in our next video.