What Are Protists And How To Identify Different Organisms?

Protists are usually peoples first image they see under a microscope, you can find them in pretty much any sample of dirty water.

What Are Protists And How To Identify Different Organisms?

As a classification, protists are very diverse so you can get a variety of characteristics from one sample.

This makes them ideal for an introduction to microscopy since they are a great way to see the structure of a cell.

All types of protists can be viewed under microscopes so it makes them a readily available learning tool. 

Despite all these benefits, it’s quite difficult to define what protists are exactly.

The simplest way to describe them is a collection of organisms that don’t have enough characteristics to be defined as an animal, plant, or fungus.

What Are Protists?

Even with this broad term, there is one thing all organisms in the protista kingdom have in common, they are all Eukaryotes. This means they have a nucleus.

Other than that one distinction they have no other shared characteristics. If talking in general terms, most of them are unicellular and are found in aqueous environments.

There are however, exceptions to these characteristics.

They can reproduce sexually (using gametes) and asexually (via binary fission).

As previously stated, protists are known to thrive in damp places and water, this includes the human body. A lot of parasites are classified as protists.

As the third proposed kingdom (after animals and plants), protoctista literally means “first established beings”.

Many people see the Protoctista Kingdom as the ‘junk drawer’ of classification as the groups in the Protista Kingdom aren’t related.

There are over 80 groups of more than 115,000 species defined as protists. The only distinction between them is if they act like an animal, plant or fungus.

Protists In The Scientific Community

In modern times, the scientific community generally doesn’t treat Protista as a formal taxon. However, the use of ‘protist’ is still used for convenience.

Since it can be used so broadly and can describe things without direct classification, it is commonly thrown around.

The taxonomy of protists is always changing, different branches are constantly being made.

While protists do have 3 distinctive sub-units (animal-like, plant-like, and fungus-like), the Protista Kingdom does have the inclusion of organisms that overlap between the three.

This type of overlap is exclusive to this kingdom.

The Six Kingdoms

In biology, everything is categorized under different taxonomic ranks, kingdom is the second highest just below domain. The six kingdoms are:

  • Animalia – Defined as multicellular and eukaryotic organisms. They generally can breathe oxygen, are able to move, can reproduce sexually, and consume organic material. Examples include the tiger, crabs and spiders.
  • Plantae – Defined as photosynthetic eukaryotic organisms. The scientific study of these is known as botany. Examples include roses, strawberries, and ferns.
  • Fungi – Defined as multicellular and eukaryotic organisms. Fungi do not photosynthesize. Examples include yeast and mushrooms.
  • Protista – What this article is about. Any eukaryotic organism that doesn’t fit in the previous 3 kingdoms.
  • Archaea – Defined as unicellular and prokaryotic organisms. Examples include thermoproteus tenax and sulfolobus tokodaii.
  • Bacteria –  Defined as unicellular and prokaryotic organisms. Examples include escherichia coli and lactobacillus acidophilus.
What Are Protists And How To Identify Different Organisms?

Archaea Vs Bacteria

While this article is about protists, the archaea and bacteria kingdoms used to be categorized under protista before becoming their own kingdoms.

So it is important to know what they are as it will help you understand protists.

Archaea and bacteria can seem like the same classification at first glance, but they do have some differences that require 2 separate kingdoms.

In fact it wasn’t until the late 1970s when they discovered there was a distinction between them.

Archaea are typically found in extreme environments, like the deep sea and hot springs.

Whereas bacteria are found in a variety of environments, including living and non-living beings. Some bacteria can photosynthesis, archaea cannot.

Introns (non coding sections of RNA transcript) are present in archaea, but are absent in bacteria.

While both of them have cell membranes, they each have a hydrophobic portion of it made out of different substances. Archaea has hydrocarbons and bacteria has fatty acids.

Similar to protists, archaea and bacteria have flagella but they vary in chemical structure. The flagella in an archaea (termed archaella) are synthesized at the base.

Whereas bacteria flagella are hollow on the inside allowing subunits to go up the inner tube and synthesize at the tip of the flagella.

The difference that is the main concern with most humans is that some bacteria are pathogenic, no archaea are.

Useful Terminology

Biology is filled with a lot of long and awkward words. Which is why we have curated a list for you. Here are some of the words you may see and a brief description of what they are.

  • Autotrophy – An organism that gains energy through photosynthesis or inorganic chemical reactions.
  • Chitin – A fibrous substance in the cell walls of fungi. It consists of polysaccharides, a carbohydrate whose molecules have bonded with sugar molecules.
  • Chlorophyll – A green pigment found in plants which helps with photosynthesis.
  • Cytoplasm – The solution in a cell. The cytoplasm is about 80% water and usually colorless.
  • Eukaryotes – Any cell or organism with a clearly defined nucleus.
  • Flagellums – A tendral-like structure that allows for cell mobility.
  • Heterotrophy – An organism that eats other animals or plants for energy. 
  • Multicellular – Organisms made up of many cells.
  • Nucleus – Membrane-bound organelle. Typically round in shape, it directs things such as growth and reproduction.
  • Organelle – Structures in a cell with a specialized function. Similar to how organs work in the human body.
  • Paraphyletic – Organisms descended from a common ancestor. This term doesn’t include all the descendant groups. 
  • Phagocytosis – A process of when a cell uses its plasma membrane to engulf a large particle. It is one of the main mechanisms used in the immune system.
  • Phylum – Direct line of descent within a group. It is a taxonomic term that ranks above class and below kingdom.
  • Plasmodium – When found in slime mold, it is essentially a ‘supercell’ with cytoplasm and thousands of nuclei.
  • Prokaryote – Any cell or organism with a lack of a nucleus and other organelle.
  • Unicellular – Organism made up of one cell.
  • Vacuole – A membrane-bound organelle typically used for storage.

Animal-Like Protists

Also known as Protozoan, this subunit includes the phylum:

  • Ciliates – Possess cilia, a hair-like organelle. Cilia are different to flagella as they are shorter and line the surface of the cell.
  • Sarcodines – Have the ability to produce pseudopodia, amoeba belong in this group.
  • Flagettes – Have flagella.
  • Sporozoans – Parasitic protozoans. Reproduces via spores.

These types of protists are defined by certain methods of movement:

  • Pseudopodia – Also called ‘false feet’, they move via projections of cytoplasm.
  • Flagella – Whip either one or two flagellums.
  • Cilia – Use of tiny hairs that line the outside of the cell.

Protozoans are heterotrophic and have organelles like a cell membrane and food vacuole. 


  • Amoeba – A type of cell or unicellular organism that can change its shape. They reproduce through mitosis (a cell replicates its chromosomes and then degregrate them, sort of like splitting themselves into two).
  • Paramecium – Are single celled protists defined by its oblong shape.
  • Giardia – A parasite which causes stomach bugs.

Plant-Like Protists

Also known as Diatoms or Protophyta, this subunit includes the phylum:

  • Chlorophyta – Comprising mostly green algae. Mainly unicellular.
  • Rhodophyta – Comprising mostly red algae and seaweed. Mainly multicellular.

Plant-like protists are classified into:

  • Dinoflagellates – They have two flagella. These are mostly marine plankton. These protists can contaminate shellfish causing shellfish poisoning if consumed. Some of them are also bioluminescent, primarily blue/green light.
  • Euglenophytes – They have a singular flagella and are one of the best known flagellates. They are fed by phagocytosis. These are a link between animal-like and plant-like protists. This exact type of conflict is why the use of the Protista Kingdom was adopted.
  • Chrysophytes – Various proportions of three different pigments. Those being chlorophyll (green), carotene (yellow), and xanthophyll (brown).


  • Laminaria – A type of seaweed, used as food in many Asian countries.
  • Ulva Lactuca – Also known as Sea Lettuce which appears in food like soup or salad.
  • Coralline Algae – Typically pink and commonly found in aquariums. 
What Are Protists And How To Identify Different Organisms?

Fungus-Like Protists

This subunit includes the phylum:

  • Decomposers – Gains nutrients through organic remains.
  • Mold – Absorptive feeders, found on organic remains.

Have cell walls similar to plants (contain chitin) but have the animal-like characteristic of heterotrophy.

The two types are:

  • Slime mold – These sustain on bacteria and microorganisms from decaying life. They come in a variety of colors and appear under wet conditions. There are also 2 families of slime mold: Cellular and Acellular. Cellular slime mold lives its life as an individual single celled organism. Acellular slime molds have a plasmodium stage in their life cycle.
  • Water mold – They can appear as a decomposer or a parasite. They are detrimental to plant life such as vegetables.


  • Downy mildew – A type of water mold, appears as yellow spots on leaves. It is harmful to plant life and can stop photosynthesis.
  • Stemonitis – Grow in clusters on rotting wood, they appear as tall brown stalks.
  • Physarum polycephalum – An acellular slime mold. It appears as a bright yellow network of interlaced tubes.

Importance Of Protists

Protists play a crucial ecological role despite their typically small size. Here are a few ways that protists are important or useful:

  • Plant-like protists are mainly water based, such as algae. These are responsible for over 40% of photosynthesis that takes place in salt and freshwater.
  • Most marine plankton are dinoflagellates, and are the lowest on the ocean food chain. This makes them crucial to marine ecology, especially to whales who mainly eat phytoplankton.
  • Decomposers (fungus-like protists) help with recycling nutrients back into the early. This is important for the general wildlife as well as food production.
  • Soil protists are great for fertilizer.
  • Some protists feed upon bacteria in various water environments, such as open water and sewage waste. This controls the general population of bacteria.
  • Many types of seaweed (which are Rhodophyta) are a popular food source in coastal regions. Seaweed is also high in nutrients and low on calories.
  • Some protists are used in medical research. Medicines made from protists can help in the treatment of arthritis, digestive problems and high blood pressure.
  • Slime mold is used in science to understand chemical signals in cells. They also help in analyzing the evolution of fungus.
  • It is important to note that most protists are harmless to humans. But, there are some that are the cause of a lot of human diseases, including malaria and dysentery. Malaria in particular is caused by Plasmodium Falciparum and is the deadliest species of plasmodium.
Jennifer Dawkins

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