The protist kingdom is a classification that includes a diverse group of eukaryotic organisms. Typically, protists reproduce asexually via mitosis and range from unicellular to multicellular organisms. In the protist kingdom, there are two main groups: protozoa (which are generally heterotrophic) and algae (which are generally autotrophic). Organisms in the protozoa group include things like amoebas, slime molds and paramecium while common organisms in the algae group include green algae, brown algae, diatoms and euglena.
When studying classification, it's kind of annoying to teach it because when life started, it forgot to make a textbook, to make things nice and simple and easy for us and instead of just having maybe one kingdom or two kingdoms or three kingdoms, it winds up having this mess and when people started studying they said, "Okay, there's plants and animals" and other people came along and said, "What about things like fungi? They're not really plants 'cause they don't make their own food" and then people eventually came up with a five kingdom system. Well, unfortunately, that is falling by the wayside too but we're left by this mess called the protist. They used to be all gathered together into one big happy kingdom called the protist kingdom but now everybody's saying, "But they're not one big kingdom, they're actually this whole mess of lots of other kingdoms but we're still like to talk about them," so I'm going to go through some of the basics of the protist. The protist can be kind of subdivided into two major groups but with those groups all share is that they're eukaryotes and they reproduce mainly asexually using mitosis. That doesn't mean that they can't do meiosis, some of them do but they usually reserve meiosis which is the correct pronunciation.
They use that primarily for extremely for extreme circumstances because that's sexual reproduction and they figure, "Hey, if I'm reproducing asexually life is good for me so if I call myself asexually, life will be good for my descendants." It's only when life starts to get bad they say, 'My genes aren't good enough, maybe I can try a new combination. It's time to have some sex.' They range in size from unicellular, individual single-celled organisms to multicellular, things like kelp is lumped together originally into the protist or kingdom and they're huge, they can be over 150 feet long.
Now, the two major groups are the protozoa and their name means early animals and they're in general are heterotrophic and then there's the algae which are generally autotrophic. Heterotrophic means that you hunt down other things and then you consume them for their food. Autotrophic means that you generally make your own food but what you'll find and this is one of the reasons why this grouping hasn't really worked out so well is that some of the protozoa, yeah, they'll eat other things but they may also do some photosynthesis and sometimes some of them will eat some of the algae and then use them as reserve photosynthetic organisms inside of themselves while continuing to eat other things. It's kind of freaky weird, so let's take a look at the protozoa.
They are very diverse organization, they include things ranging from amoeba to slime molds to things called paramecium that use little cilia to move around, while within the algae, wide range of things ranging from the green algae which are the ancestors to our modern land plants to the glass cell-walled diatoms which are a major photosynthetic organism in our oceans to things as big and huge and multicellular as the brown algae.
So as you can see the protist kingdom is huge and that's why nobody really uses it anymore except as a matter of convenience.