Protist Life Cycles: Study with Video Lessons, Practice Problems & Examples

Eukaryotic life cycles can exhibit alternation of generations, involving both haploid (gametophyte) and diploid (sporophyte) multicellular stages. In this cycle, gametes fuse to form a zygote, which undergoes mitosis to develop into a sporophyte that produces spores through meiosis. Some organisms display heteromorphic forms, while others are isomorphic. Not all eukaryotes follow this pattern; haploid-dominant cycles have the zygote as the only diploid stage, while diploid-dominant cycles, like in humans, have gametes as the only haploid cells. Understanding these cycles is crucial for grasping reproductive strategies in various organisms.

Protist Life Cycles

This video, we're going to begin our lesson on protist life cycles. And, as we already know, protists exhibit huge diversity, and this is going to include their life cycles as well. Protist life cycles can be sexual, asexual, or both. They can include multiple hosts, just one single host or no hosts at all, and they can shift between haploid and diploid forms of the organism. Now, because protists exhibit so much diversity in their life cycles, we're not going to be able to cover all of that diversity here in this course.

Instead, in this lesson, we're going to cover just some of the life cycles that we expect your professors may want you to know about. This is going to include the Plasmodium life cycle, the Laminaria life cycle, and the Paramecium life cycle. We'll talk about each of these life cycles moving forward in their own separate videos, starting with the Plasmodium life cycle. So, I'll see you all there.

Plasmodium Life Cycle (Malaria)

This video, we're going to talk about the plasmodium life cycle and malaria. Now, malaria is actually one of the deadliest diseases in the world, killing about 600,000 people every year. And it's a common misconception that malaria is caused directly by mosquitoes, but that's not actually the case. Instead, malaria is caused by a protist called Plasmodium, which enters and exits humans via mosquito bites. And so mosquitoes get all the blame, but really, it's the plasmodium's fault.

Now, the plasmodium lifecycle actually requires multiple hosts in order to fully complete its lifecycle. It requires a mosquito host and a human host. Also, the plasmodium lifecycle contains both sexual and asexual stages, and we'll be able to see this in this diagram of the plasmodium lifecycle. So let's go ahead and enlarge this diagram so we can see it more clearly and notice that we've broken up this lifecycle into 7 steps that we have numbered throughout this diagram. So let's go ahead and wipe this diagram clean so we can approach this one step at a time.

Notice that the arrow over here on the left in orange represents events that occur inside of the mosquito host, whereas the arrow over here on the right in blue represents events that occur inside of the human host. And this is very important to pay attention to moving forward. Now let's go ahead and kick off this life cycle process with an infected mosquito biting a human. And, of course, the mosquito is going to be infected with the Plasmodium protist, more specifically, an infectious form of the Plasmodium protist called sporozoites, which are actually haploid in nature. And so we can consider this mosquito bite step number 1 of the life cycle.

Now, of course, upon biting the human, the sporozoites from the infected mosquito will be injected into the bloodstream of the human host, and the sporozoites will travel through the bloodstream to the human liver and infect the liver cells. Step number 2 is that the liver cells get infected by these sporozoites. Then, these sporozoites will develop into yet another form of the plasmodium protist called merozoites, and these merozoites will reproduce asexually via mitosis until they cause these liver cells to lyse, releasing a whole bunch of merozoites into the blood. Now in step number 3, these merozoites are going to infect red blood cells and reproduce asexually via mitosis inside of the red blood cells until they cause these red blood cells to lyse and burst open, destroying the red blood cells and releasing a whole bunch more merozoites that can continually go on to infect and destroy more and more red blood cells. And really, it's this step number 3, the infection and destruction of the red blood cells that causes the signs and symptoms associated with malaria, such as fever and chills.

Now in step number 4, some of these merozoites are going to develop into gametocytes, which are precursors to gametes. However, these gametocytes will not be able to develop into gametes inside of the human host. These gametocytes require very specific conditions in order to develop into gametes, and those conditions are only found inside of the mosquito host. And so notice that we're transitioning from the blue arrow over to the yellow arrow, which means that we're now transitioning over to the mosquito host. So let's say that an uninfected mosquito that does not have this plasmodium protist in it comes and bites this infected human.

Well, this mosquito bite is going to be step number 5, and that's going to allow the gametocytes from the human to be transferred over into the mosquito host. Now once inside the mosquito host, these gametocytes will develop into the gametes, the male form and female form, forming sperm and egg. And, of course, these gametes can fuse together in the process of fertilization, which is step number 6, to create a zygote. And the zygote is going to be diploid in nature, so we can put 2 n here in this blank. Then the zygote will undergo meiosis followed by several rounds of mitosis to produce a whole bunch of sporozoites that are haploid in nature.

So we can put an n here, and these sporozoites will travel to the salivary glands of the mosquito so that it can actually inject the sporozoites into the next human that it feeds on. And so this here completes the full life cycle of the Plasmodium protist, and moving forward, we'll be able to apply these concepts and problems and look at other protist life cycles as well. So I'll see you on our next video.

Protist Life Cycles Example 1

So here we have an example problem that wants us to number the following steps of the plasmodium life cycle so that they are in the correct order starting from when a mosquito bites and infects a human. Notice below, we've got these 7 bulleted steps that we need to properly order by filling in the numbers in these blanks. Given that the first step starts with an infected mosquito biting a human, infecting it with the sporozoite form of the plasmodium parasite, this is our starting point. Recall from our previous lesson videos that these sporozoites go on to infect the human liver cells.

The step below states that sporozoites infect human liver cells. Then, those sporozoites become merozoites and reproduce asexually until the merozoites are released from the liver into the blood. This is step number 2. Recall that these released merozoites in the blood go on to continuously infect and destroy red blood cells. Notice the step below says that merozoites continuously infect and destroy red blood cells as they reproduce asexually, forming more merozoites and causing further infection and destruction of red blood cells.

This process is what causes the symptoms associated with malaria, such as fever and chills, marking it as step number 3. Some merozoites will differentiate into plasmodium gametocytes, which is step number 4.

These gametocytes can form gametes, but only inside a mosquito. Therefore, the gametocytes need to make their way back to a mosquito, and the next step involves an uninfected mosquito biting this infected human and picking up the plasmodium gametocytes from step number 4, which is step number 5. From there, the gametocytes can produce gametes.

The step stating that gametocytes form gametes that then fertilize inside the mosquito's digestive tract, forming a diploid zygote, is step number 6. The final step, step number 7, involves the zygote undergoing meiosis then mitosis, releasing haploid sporozoites into the mosquito's salivary gland, thus completing the cycle so that step number 1 can occur again.

This concludes this example problem, and I'll see you all in our next video.

When an infected mosquito bites a human, it injects ___________ (the infective stage of plasmodium) into the blood. Once they have invaded liver cells they become ___________, which infect red blood cells. 

• A. Sporozoites; merozoites.
• B. Sporozoites; gametocytes.
• C. Merozoites; sporozoites.
• D. Merozoites; gametocytes.

Laminaria Life Cycle: Alternation of Generations

This video, we're going to talk about the Laminaria life cycle and alternation of generations. Laminaria is a type of brown algae that lives in aquatic ecosystems, commonly known as kelp. It is an example of a multicellular protist and plays very important roles in coastal ecosystems. It also exhibits alternation of generations, a life cycle that alternates between multicellular diploid and multicellular haploid forms of the organism.

The multicellular diploid form of the organism is called the sporophyte, which produces haploid spores via meiosis. The multicellular haploid form is called the gametophyte. It produces gametes, but via mitosis. Below, we have a diagram representing the Laminaria life cycle. Let's enlarge this diagram so we can see it more clearly, noting the 5 steps numbered throughout.

Let's wipe this diagram clean so we can approach this one step at a time. Before we get started, the bluish arrow on the left represents the diploid stage of the life cycle. Therefore, we put 2 in this blank, and the purplish arrows on the right represent the haploid stage. Let's place n in this blank. We'll start with the mature sporophyte, which contains structures called sporangia that produce spores, undergoing meiosis to produce specifically called zoospores. These zoospores are haploid.

These zoospores will undergo mitosis to produce multicellular haploid organisms called gametophytes. About half of the zoospores will produce male gametophytes, and the other half will produce female gametophytes. The male gametophyte produces sperm and releases them into the aqueous environment, allowing them free movement. In contrast, the female gametophyte produces eggs that remain attached to her. The sperm can merge with the egg in the process of fertilization to produce a zygote, which will be diploid. This zygote undergoes mitosis to produce a developing sporophyte and then a younger sporophyte. The main goal is to return to the mature sporophyte form. And that completes the Laminaria life cycle.

The last thing I want to do is draw your attention to the numbers for each of these steps in the diagram, shown in this table below the diagram on your worksheet. This table lists all steps, providing a brief description of what occurs in each, just as we have discussed. This concludes this video on the Laminaria life cycle and alternation of generations, applying these concepts in problems moving forward. I'll see you all in our next video.

Protist Life Cycles Example 2

So here we have a pretty straightforward example problem that asks, in an alteration of generations life cycle, which of the following is true? And we've got these 4 potential answer options down below. Now option a says, the organism alternates between multicellular diploid and multicellular haploid forms. And of course, this is going to be a true statement. This is really what the alterations of generations life cycle is all about.

Cycling or alternating between multicellular diploid sporophytes and multicellular haploid gametophytes. So option a is a true statement. Now option b says male gametophytes produce sperm while female gametophytes produce eggs. And once again, option b is a true statement. Now option c says organisms may look different during the two parts of their life cycle.

Now if the organisms look different during the two parts of their life cycle, meaning the sporophyte looks different than the gametophyte, then we call this heteromorphic. Now, if the sporophyte and gametophyte look identical to each other visually from the outside, then we call that isomorphic. But answer option c is a true statement, and so really what we're saying is that it's actually answer option d, all of the above that is the correct answer to this example problem. So that concludes this problem, and I'll see you all in our next video.

Which of the following represents the correct sequence of alternation of generations in laminaria?

• A. Sporophyte > spore fusion > gametophyte > gamete > sporophyte.
• B. Sporophyte > zygote > zoospore > gamete > gametophyte > sporophyte.
• C. Gametophyte > gamete > zygote > zoospore > sporophyte > gametophyte.
• D. Sporophyte > zoospore > gametophyte > gamete > zygote > sporophyte.

Select the answer option that correctly completes the diagram below:

• A. 1. meiosis, 2. meiosis.
• B. 1. mitosis, 2. mitosis.
• C. 1. mitosis, 2. meiosis.
• D. 1. meiosis, 2. mitosis.

Paramecium Life Cycle: Conjugation & Asexual Reproduction

This video, we're going to talk about yet another protist life cycle, the paramecium life cycle, which is not to be confused with the plasmodium life cycle that we talked about in previous lesson videos. Now paramecium is actually a unicellular ciliated protist, which means that it's single celled and contains cilia along its perimeter, which we call are microscopic hair-like structures that can be used for movement. Now paramecium can be found in many different aquatic ecosystems, including lakes, ponds, and rivers, for example. Now the protist paramecium exchanges DNA sexually via conjugation and reproduces via binary fission, a type of asexual reproduction that's usually associated with prokaryotes, but it can occur in some eukaryotes like this paramecium protist, for example. Now recall from our previous lesson videos that conjugation is the exchange of genetic information between two cells that are in direct contact with one another.

Now down below, we have a diagram representing the paramecium life cycle, but before we get to that, it's helpful for you to know that paramecia actually have two types of nuclei. They have a larger macronucleus and they have a smaller micronucleus. Now the larger macronucleus is responsible for day-to-day operations, while the smaller micronucleus is primarily active during reproduction, and we'll be able to see that throughout this entire diagram as we analyze it. So let's go ahead and enlarge this diagram so that we can see it more clearly and notice that we've broken up this life cycle into six total steps that we have numbered here. So let's go ahead and wipe this clean so that we can approach this one step at a time.

Now the blue arrow across the top represents the phase of this life cycle for conjugation, again, a type of sexual reproduction, while the pink arrow across the bottom represents the phase of this life cycle for asexual reproduction, which is going to occur via binary fission as we'll see here shortly. So let's go ahead and kick off this process with two compatible mates, two paramecia cells that are compatible with each other for the conjugation process to begin. And so these two compatible mates are going to pair up and come into direct contact with each other. And again, each of these cells is going to have two nuclei, a larger macronucleus and a smaller micronucleus. And all of the nuclei are going to be diploid, which is why we have 2n.

Now again, it's the micronucleus that's going to be functionally active throughout this life cycle here. And so it's no surprise that in step number two, the micronucleus in each of these cells is going to undergo meiosis in order to generate four micronuclei in each cell that are all haploid, since meiosis reduces the ploidy from diploid down to haploid. Now it's important to note that in this process of meiosis here, the cells themselves are not dividing. It's only the micronucleus in each of the cells that divides to produce four micronuclei that are haploid. Then three of the micronuclei in each of the cells are going to disintegrate, leaving only one functionally active micronucleus in each cell, which, then the one functionally active micronucleus in each cell is going to undergo mitosis to generate two functionally active micronuclei that are haploid in each of these cells.

And again, in this process of mitosis here, the cells themselves are not dividing. It's just going to be the functionally active micronuclei that divide. So now these cells are actually ready for the conjugation process to take place, so they're going to exchange genetic information in a process that we'll call the micronuclei swap here. So they swap micronuclei, exchanging genetic information so that each cell has one green and one yellow micronucleus. Then the conjugation process is complete, so these two cells can separate from one another.

Now this cell over here on the right, we're going to stop following, but it basically undergoes the same steps that this cell is going to undergo. So next, in step four, the micronuclei are going to fuse together in micronuclei fusion, and so when these two haploid micronuclei fuse together, it generates a single diploid micronucleus, which is why we have 2n here, and this diploid micronucleus that's generated is genetically different than either of these green and yellow micronuclei that we had, earlier in this process. Now from here, the micronucleus is going to undergo three rounds of mitosis to create eight micronuclei that are all diploid. From here, the original macronucleus is going to disintegrate, and four of the micronuclei that were just generated are going to develop into macronuclei, so that in the end we end up with a single cell with four macronuclei and four micronuclei, and all of these nuclei are going to be diploid at this point. So from here, we finally have binary fission that kicks into play, and binary fission is going to asexually reproduce the single cell into four cells, each with one macronucleus and one micronucleus, and all of the nuclei are diploid.

And this completes this process because then these cells can go on to find a compatible mate to repeat this entire life cycle once again. So now that this is complete, I want to draw your attention once again to each of these numbers for each of these six steps, and that's because the numbers here in this diagram correspond with the numbers that you can see here in this table for the paramecium life cycle. So notice here that we've got numbers and names for each of the steps and a little description of what's happening in each of the steps just for your convenience. So this here concludes our lesson on the paramecium life cycle. We'll be able to apply these concepts and problems moving forward.

So I'll see you in our next video.

Which of the following statements is true about conjugation in the Paramecium life cycle?

• A. It results in an increase in population size only.
• B. It results in genetic recombination only.
• C. It results in both an increase in population size & genetic recombination.
• D. It does not result in an increase in population size or genetic recombination.

Which protist life cycle has a distinct use of both mitosis & meiosis across haploid & diploid stages?

• A. Plasmodium; sporozoites (n) & merozoites (n) undergo mitosis, the zygote (2n) undergoes meiosis.
• B. Laminaria; the mature sporophyte (2n) undergoes meiosis, the gametophytes (n) undergo mitosis.
• C. Paramecium; meiosis occurs during conjugation, mitosis occurs during conjugation & before binary fission.
• D. All of the above.

Which of these statements about the plasmodium, laminaria, and paramecium life cycles is correct?

• A. Plasmodium requires 2 hosts, Laminaria requires 1 host, Paramecium does not require a host.
• B. Plasmodium only replicates asexually, Laminaria only has unicellular gametophytes, Paramecium undergoes conjugation.
• C. Plasmodium requires a vector (a mosquito), Laminaria may look different depending on which stage of its life cycle it’s in, Paramecium creates genetic variation during its asexual reproduction phase.
• D. Plasmodium requires 2 hosts, Laminaria may look different at different stages of its life cycle, Paramecium produces new cells during its asexual phase.

Here’s what students ask on this topic:

What is alternation of generations in protists?

Alternation of generations in protists refers to a life cycle that includes both haploid (gametophyte) and diploid (sporophyte) multicellular stages. The gametophyte produces gametes through mitosis, which fuse during fertilization to form a diploid zygote. This zygote undergoes mitosis to develop into a sporophyte, which then produces haploid spores through meiosis. These spores grow into new gametophytes, completing the cycle. This process can be heteromorphic, where the stages look different, or isomorphic, where they appear similar.

How do haploid dominant life cycles differ from diploid dominant life cycles?

In haploid dominant life cycles, the organism spends most of its life in the haploid stage, with the zygote being the only diploid phase. The zygote undergoes meiosis to produce haploid cells, which then reproduce asexually or form gametes. In contrast, diploid dominant life cycles, like in humans, have the organism primarily in the diploid stage. Here, only the gametes are haploid, formed by meiosis from mature diploid cells. The zygote undergoes mitosis to develop into the diploid organism.

What are the key differences between heteromorphic and isomorphic alternation of generations?

In heteromorphic alternation of generations, the sporophyte and gametophyte stages have different structures and appearances. For example, in some brown algae, the sporophyte and gametophyte look distinctly different. In isomorphic alternation of generations, both stages appear similar despite being genetically different, with the sporophyte being diploid and the gametophyte haploid. This structural similarity can make it challenging to distinguish between the two stages without genetic analysis.

Why is understanding protist life cycles important for studying plants and animals?

Understanding protist life cycles is crucial because they share fundamental life cycle patterns with plants and animals. For instance, the concept of alternation of generations seen in protists is also present in plants. Additionally, studying the variations in life cycles, such as haploid and diploid dominance, provides insights into the evolutionary adaptations and reproductive strategies of more complex organisms. This knowledge helps in comprehending the broader biological principles that govern life cycles across different eukaryotic lineages.

What role do spores play in the life cycle of protists?

In the life cycle of protists, spores are crucial for asexual reproduction and the continuation of the species. Produced by the diploid sporophyte through meiosis, spores are typically haploid and unicellular. They can germinate and undergo mitosis to form new haploid gametophytes. This process ensures genetic diversity and allows the organism to survive and proliferate in various environmental conditions. Spores are a key component in the alternation of generations, bridging the diploid and haploid stages.