Protist Life Cycles - Online Tutor, Practice Problems & Exam Prep

Alteration of Generations

Hi. In this video, we're going to look at a few ways that eukaryotic life cycles can play out. We'll see these ideas come up again when we take a look at the various lineages of protists, and also when we look at plants and animals, as well as fungi, later on. Now, we've already talked about alternation of generations briefly when we were talking about sexual life cycles, and we mentioned alternation of generations in connection to plants, but it actually plays out in protists as well. And basically, the way that alternation of generations works is that, in the course of the life cycle of these organisms, there will be a haploid and diploid stage and both of these will be multicellular or will have a multicellular component, I should say. So, looking at our image here, you can see here we have our haploid phase, and down here we have our diploid phase. Now, the haploid multicellular stage is known as the gametophyte. And you can see we have our gametophytes right here, and these are going to produce our gametes. Our gametes will get together and fertilization occurs, and we form a zygote. Now, the zygote will undergo mitosis and form the sporophyte. Right? The sporophyte is the diploid multicellular stage, and this is what is going to undergo meiosis and produce spores. Spores are a unit of asexual reproduction. They're generally haploid and unicellular. And we'll see that sporophytes are going to produce spores. These spores will undergo mitosis to form those gametophytes that we started off talking about. And of course, those gametophytes will then produce gametes and the cycle continues and we continually alternate between generations, the gametophyte and the sporophyte generation. So, we'll see this again when we talk about various protists and also when we talk about plants. Now what's kind of cool about alternation of generations is in some species, the sporophyte and gametophyte are structurally different. They look different, you know, they have distinctly different structures and we call these heteromorphic, basically meaning different shape. Right? Different form. Now in some species, the sporophyte and gametophyte will actually look the same. Even though, of course, in the sporophyte, the cells are all diploid, and in the gametophyte, the cells are all haploid. Pretty crazy, I know. These organisms are called isomorphic or same form. Pretty cool and you can see here we have two images. Both of these are actually brown algae. So these are both protists, both types of brown algae. This one over here is isomorphic though. So both the sporophyte and gametophyte are going to look like that, and this one over here is heteromorphic. And, this is a different type of brown algae. You don't really need to worry about the names or anything about these types of brown algae. Just an example of how two organisms that are from sort of the same lineage. Right? These are both types of brown algae. How they can use these two different types of strategies. With that, let's flip the page.

Haploid and Diploid Dominant Life Cycles

Not all eukaryotes undergo an alternation of generations. Some have life cycles that are either dominated by a diploid or haploid phase. In the haploid dominant life cycle, what we'll see is that the zygote is the only part of the life cycle that is diploid. The zygote is going to form from the fusion of gametes like normal, and then it's going to undergo meiosis and reform haploid cells. These life cycles can be experienced by unicellular organisms or multicellular organisms. That's what's meant by "individuals or more cells". Now, these haploid cells will reproduce asexually through mitosis and cell division, and they can also form gametes through mitosis. Again, those gametes will fuse to form the zygote, and then the zygote will undergo meiosis. So, basically, the dominant part of the life cycle of these organisms is haploid.

In a diploid dominant life cycle, which is what humans have, the only haploid cells are the gametes. These are going to be formed by some type of mature cell that's going to undergo meiosis, like sperm and egg formation. Gametes are the only haploid part of this organism's life cycle, and they will fuse together to form a zygote, and then the individual, which is all going to be diploid. The difference between these two life cycles is that in the haploid dominant life cycle, it's the zygote itself that's going to undergo meiosis to produce the haploid cells. They are not actually the gametes, but the point is the zygote undergoes meiosis directly. Whereas in a diploid dominant life cycle, the zygote will undergo mitosis to form the organism, and then certain mature cells in that organism will be the ones that undergo meiosis to form gametes. So the zygote does not undergo meiosis. With that, let's conclude this video.

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.