In this lesson, you’ll learn about protists.
In particular, you’ll learn about their life cycles, including reproduction and stages of growth and development.
What do you call something that is alive, but is not an animal, plant, or fungus? Though we often do not see them, this type of organism is extremely widespread and diverse. In general, they fall under the large biological category kingdom Protista, which sets them apart from the animal, plant, and fungus kingdoms.
Protists, members of this category, are eukaryotes, which means they are made up of cells containing membrane-bound organelles, such as a nucleus. Some protists have multiple cells, and some are single-celled. Examples of protists include algae, seaweed such as kelp, and amoebas.
How Do Protists Reproduce?
Just like animals, plants, and fungi, protists have their own methods of reproduction. There are two main types: asexual and sexual.
Method 1: Asexual Reproduction
Many protists, especially single-celled organisms such as amoebas, reproduce asexually.
What this means is that they reproduce using cell division. The ‘parent’ cell splits in two, copying its DNA into a second nucleus as it divides. The end result is that the parent cell is gone, and has been replaced by two ‘daughter’ cells.
These two new organisms are genetically identical to each other, and to the parent cell they came from. In effect, the cell division method of reproduction is much like cloning.The cell division process can take anywhere from a few hours to a few days. The time frame depends on the organism, and the conditions under which it is reproducing.
That is, under ideal conditions, it will reproduce faster, and with poor conditions, it will take longer.
Method 2: Sexual Reproduction
The other main method of reproduction is sexual. In this process, gametes, or sex cells, are formed and merge together to create a new, genetically distinct generation. One example of a sexually reproducing protist is Plasmodium , a protist that causes malaria.
In general, though sexual reproduction is not rare, asexual is still the most common type of reproduction among protists. In fact, though Plasmodium reproduces sexually using mosquitoes as vectors (or carriers), they also reproduce asexually once they are within a host. So even within a protist species there can be variation in how they reproduce.
Growth and Development
Particularly in cases of sexual reproduction, the offspring has to go through a maturation period. This is sometimes true for asexually reproducing protists as well.
The time needed to mature varies between different species, just as it does with members of the other biological kingdoms. Let’s go through three prominent types of protist life cycles to see examples of this.
In some cases, the immature stages are noticeably different from the adult stages. For example, slime molds go through an amoeboid stage and a ‘feeding plasmodium’ stage before they reach their ‘mature plasmodium’ adult stage. (A note here: the term plasmodium for stages of a slime mold refers to their plasma-like state, and is different from the species Plasmodium that causes malaria).Slime molds are an example of sexually reproducing protists. They give off spores, which grow to the amoeboid form.
They are then fertilized and undergo mitosis to form the feeding plasmodium stage. Finally, they mature and eventually give off spores of their own. So then, the life cycle of slime mold protists is as follows:Spores (gametes) –> amoeboid form –> fertilized cell (zygote) –> feeding plasmodium –> mature plasmodium, eventually able to produce spores.
Algae, or algal protists, are another major type of protists. Like slime molds, they begin as spores.
The spores combine to form a multicellular organism, which then produces gametes that combine into a fertilized zygote. At this stage, the zygote might actually stop maturing and go through a dormant phase. This would be most likely to happen if resources in the area are scarce. In the dormant phase, the zygote would not need to consume resources, and so the species could survive through harsh times.
Once there are enough resources, the zygote continues maturing and eventually comes full circle to form spores once again. Here is a breakdown of the algal protist life cycle:Spores –> multicellular organism –> gametes –> fertilized zygote –> (dormancy phase, if one occurs) –> mature zygote –> spores
Finally, there are parasitic protists, such as the malaria-inducing Plasmodium. Some of these also go through multiple life stages, where the earlier stages require a different host than the later ones. For example, take the Giardia protist. The initial form lives in contaminated water, and the later form matures inside humans or other animals.
It takes up residence in the digestive tract and causes giardiasis. When the host vomits or has diarrhea, it helps the protist’s reproduction and sends immature forms back into the water system. The life cycle for the Giardia protist would look like this:Immature forms released into water systems –> consumed by a human or animal –> take up residence and mature in the digestive tract –> produce immature giardia –> immature forms released into the water by host vomiting or diarrheaAs you can see, there are several different types of protist life cycles, each with different methods of reproduction, and different stages of life.
Protists are a group made up of living eukaryotic organisms that cannot be classified as animal, plant, or fungus. Since they are a large and diverse group, they have multiple different life cycles, depending on the type of protist. Some reproduce asexually through cell division. Other produce gametes and undergo sexual reproduction. In most cases, the offspring have to go through a maturation period before they become adults. Some, like the slime mold, go through more than one.
Some algal protists even go through a dormant stage after fertilization, especially if they are born under poor conditions. In addition, parasitic protists often require different hosts for different stages of their lives. Essentially, the protist life cycle is a complex and varied one, and there are many different paths it might take, depending on the species.