Evolution of Protists: Study with Video Lessons, Practice Problems & Examples

Primary endosymbiosis occurs when a host cell engulfs a prokaryotic cell, leading to the evolution of organelles like mitochondria and chloroplasts in eukaryotes. In contrast, secondary endosymbiosis involves a host cell engulfing a eukaryotic cell, resulting in organelles with three or more membranes. This process has contributed to the diversity of eukaryotic lineages, such as red and green algae, which have evolved into distinct photosynthetic groups. Understanding these processes is crucial for grasping the evolution of complex life forms.

Primary Endosymbiosis Gave Rise to Eukaryotes

This video, we're going to talk about how primary endosymbiosis gave rise to the very first eukaryotes, which are believed to be protists. And so recall from way back in our previous lesson videos that we already covered endosymbiosis. So we already know that it is a symbiotic relationship where one organism lives inside of another. And so really the new piece of information here in this video is that it's specifically primary endosymbiosis that gave rise to the first eukaryotes. And so primary endosymbiosis is when a host cell engulfs a prokaryotic cell, which would then live inside of the host cell and develop a symbiotic relationship with it.

And so you can think that the "p" in primary endosymbiosis is for the "p" in a prokaryotic cell being engulfed. And so notice in this image down below, which should look very familiar to you, that primary endosymbiosis is believed to have occurred multiple times throughout biological history. So notice here that we're showing you 2 primary endosymbiotic events. In the 1st primary endosymbiotic event, notice that a prokaryotic aerobic bacterium is being engulfed by this host cell here and over a very long period of time eventually would have evolved into the mitochondria organelles that we can find in heterotrophic and photosynthetic eukaryotes today. Now in the second primary endosymbiotic event, once again, notice that a prokaryotic cell is being engulfed, this time a photosynthetic bacterium.

And over a very long period of time, eventually, it would have evolved into the chloroplast organelles that we can find in photosynthetic eukaryotes today. So in our next video, we're going to talk about yet another type of endosymbiosis, not to be confused with primary endosymbiosis. So I'll see you all there.

Secondary Endosymbiosis Led to Several Eukaryotic Lineages

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.

Evolution of Protists Example 1

So here we have an example problem that asks, why was it advantageous for the host cell to keep the engulfed aerobic bacterium alive during primary endosymbiosis rather than dissolve it for food? And we've got these four potential answer options down below. Now, of course, recall from way back in our previous lesson videos that this engulfed aerobic bacterium, over a very long period of time, 1,000,000 if not 1,000,000,000 years, would have eventually evolved and developed into the mitochondria organelles that we can find in eukaryotic cells today. And we know that these mitochondria organelles are very specialized for producing energy in the form of ATP, and so we might expect that the advantage that the host cell received for keeping the engulfed aerobic bacterium alive was that the aerobic bacterium provided the host cell with ATP. And so notice that matches with answer option b, which is going to be the correct answer to this example problem.

So we can go ahead and indicate that. Now, option c is not going to be correct because it says it allowed the host cell to photosynthesize, but this aerobic bacteria nor mitochondria allow for the process of photosynthesis. So that's why option c is not correct. Option a says it helped protect the host cell from pathogens, which is not going to be the best option since option b is much more likely. And then option d says, it provided an opportunity to swap genetic information with the host cell.

And again, this is not going to be as likely as answer option b. And so, that concludes this example problem, and I'll see you all in our next video.

Which of the following is a benefit that eukaryotes gained via endosymbiosis?

• A. Endosymbiosis allowed eukaryotes to have a structurally solid cell wall.
• B. Endosymbiosis allowed eukaryotes to survive in extreme environments.
• C. Endosymbiosis allowed eukaryotes to obtain energy via sunlight (photosynthesize).
• D. All of the above.

Which of the following is a characteristic that you could look for to determine if a cell has undergone secondary endosymbiosis?

• A. A rough endoplasmic reticulum.
• B. A photosynthetic organelle with 3 or more membranes.
• C. A complex nucleus & endomembrane system.
• D. A chloroplast.