With organisms like animals and plants, it's somewhat easy to understand how they get their food. Everyone can see that a cow eats grass and hay, a lion attacks and eats a gazelle, and an oak tree's leaves convert the sun's light into glucose for energy, etc.
When it comes to small and microscopic organisms, it can be a bit more difficult to understand, especially when you have a diverse group of organisms like you do in the Kingdom Protista. Protista examples range from unicellular algae to sea kelp to mold to paramecium, which shows you just how diverse this kingdom is.
Protists are classified based on how they eat, how they move and how similar they are to the other eukaryotic kingdoms (plant, animal and fungi). Protists can be autotrophs, heterotrophs or mixotrophs.
Protist Definition and Information
Kingdom Protista describes eukaryotic organisms that are not fungi, plants or animals but have similar characteristics to some or all of those kingdoms. Most protists are microscopic and unicellular, but some organisms within this kingdom are multicellular. Sea kelp, for example, are large multicellular organisms within the Protista kingdom.
Many protists also form colonies. These colonies aren't technically multicellular organisms, though. They're simply large groups of single celled protists that form groups.
As eukaryotes, protists contain highly specialized membrane-bound organelles like the endoplasmic reticulum, Golgi body and mitochondria. Some protists also contain chloroplasts.
Three General Types of Protists
As previously stated, the most general protist definition is a eukaryotic organism that is not an animal, plant or fungi. You can further define and classify those protists based on which of those three kingdoms the organism is most similar to. This leads to three general classes of protist:
- Animal-like protists
- Plant-like protists
- Fungi-like protists
Each of these classifications can help shed light on how they get food and nutrition.
Protista Nutrition: Animal-Like Protists
Animal-like protists are heterotrophs. This means that in order to get food and nutrition, these protists must eat/ingest food from their environment. They can do this in a few ways.
Endocytosis, also called phagocytosis, is perhaps the most common method for heterotrophic protists. This is when animal-like protists physically engulf or "swallow" their prey. Amoebas, for example, are animal-like protists that engulf their prey and break them down inside their cell in order to get their nutrition. These types of protists are also called phagotrophs.
Other animal-like protists are filter feeders. They'll often use their flagellum to whip back and forth and create a flow or a current around them to filter through and absorb food from their environment. This type of heterotroph is also called an osmotroph, which means they absorb food to eat from the environment instead of engulfing it whole like a phagotroph.
Protista Nutrition: Plant-Like Protists
Plant-like protists are autotrophs. This means that they create their own food without having to eat or engulf other organisms/organic materials in the environment. Plant-like protists have chloroplasts in their cells in order to perform photosynthesis in order to convert sunlight into food (aka glucose).
Common plant-like photosynthetic protista examples include microscopic algae as well as huge multicellular seaweeds like kelp.
Protista Nutrition: Fungi-Like Protists
Fungi-like protists are also called mold. The two major types of fungi-like protists can be divided into water molds and slime molds.
These types of protists are heterotrophs, specifically osmotrophs. This means that they absorb their food (nutrients and organic materials) from the space, environment and organisms around them.
A Unique Case: Mixotrophs
Some rare protists can get food both autotrophically and heterotrophically. These protists are referred to as mixotrophs since they can both consume organisms/organic materials and perform photosynthesis in order to get food: It's a "mix" of both autotrophic and heterotrophic ability.