The biological classification system is widely used for grouping the huge bio-diversity into different taxa for the ease of study and recognition.
And the present day, the classification system that is widely used is the Phylogenetic Classification System.
The present day Phylogenetic Classification System is the system of biological classification in which all the living and extinct organisms in the world are classified based on evolutionary relationships among the organisms.
Protozoa and Protista are somewhat the same things but with a bit difference. In fact, Kingdom Protista is a broad kingdom that includes all the single-celled eukaryotes like slime molds, and protozoans with well-defined cell organelles that are neither animals, nor plants, nor fungi.
All unicellular eukaryotes that include protozoans, protophyta, protofungus, etc. and others that can’t fit into any other taxa are placed under Kingdom Protista of the Phylogenetic Classification System introduced by R.H. Whittaker in 1969.
There are a diverse collection of organisms in this Kingdom Protista but, the boundaries of this kingdom are not well-defined.
Now, what’s the catch?
As already said that Kingdom Protista is a very diverse kingdom that includes all eukaryotic organisms that are neither animals, nor plants, nor fungi.
So, for the proper classification, study, and systemic positioning, the protists are divided into three groups: Animal-like protists, Plant-like protists, and Fungi-like protists.
The Protozoans (singular: Protozoa) are Animal-like protists that were understood to be animals and so often termed as animalcules and so they are studied in the Departments of Zoology.
However, still, there are some protozoans that are both animal-like and plant-like like Euglena. So they are studied in both the Departments of Zoology & Botany.
That’s why it’s said that the boundaries of Kingdom Protista are not well-defined and very confusing as well considering the majority of the characteristics.
What is Protista?
Protista is one of the broad Kingdom of the 5 Kingdom Classification system that includes all unicellular, eukaryotic organisms that can be neither considered animals, nor plants, nor fungi.
Protists are only considered as animal-like, plant-like, or fungi-like but not true animals, true plants, or true fungi either.
The 5 Kingdom Classification system includes Kingdom Monera (includes all prokaryotic organisms), Kingdom Protista (includes all unicellular eukaryotic organisms), Kingdom Fungi (includes all eukaryotes without chlorophyll), Kingdom Animalia (includes all true animals), Kingdom Plantae (includes all true plants).
It is very easy to classify a plant from an animal. It’s obvious that you will notice cell walls in plant cells and no cell wall in animal cells. You will also notice that a plant is photosynthetic and an animal is heterotrophic.
Another easy feature is that plants can’t move and animals can. These are all usual reasons to differentiate a plant from an animal. There are more advanced molecular biological reasons as well to better differentiate both.
But in the case of a majority of the unique, unicellular, and eukaryotic organisms, it was very difficult to fit them in neither plants, nor animals, and not even fungi.
So in 1969, Kingdom Protista was introduced for the first time to make systemic studies a lot easier by including all these no true-plants, no true-animals, no true-fungi organisms into one single kingdom.
Protozoans (animal-like organisms), Protophyta (plant-like organisms), and Protofungus (fungi-like organisms) are included in Kingdom Protista.
What is Protozoa?
Protozoa is a phylum that falls under the Kingdom Protista. Protozoans are a bit complex than the prokaryotes and are composed of unicellular, eukaryotic, microscopic animals. They do not have any type of tissue.
They come in many different shapes and sizes ranging from an Amoeba which can change its shape to Paramecium with its fixed shape and complex structure.
They are animal-like so studied in Zoological studies whereas there are some protozoans that are both animal-like and plant-like for example Euglena.
With only one cell (they can also be referred to as acellular with some exceptions), the protozoan body is a complete animal performing all the vital functions of life such as nutrition, excretion, respiration, reproduction, locomotion, etc.
These are the first and the most primitive animals included in the 5 Kingdom Classification by R.H. Wittaker under the Kingdom Protista.
The cell organelles are well specialized with membrane-bound organelles like mitochondria, Golgi bodies, chloroplast, Endoplasmic Reticulum, Nucleus, lysozyme. There can be single or many nuclei in each organism depending on the species type.
Nutrition is holozoic, holophytic, and saprozoic and they can be free-living, commensal, symbiotic, or parasitic in nature. They can be seen in Freshwater, Marine, or Moist Terrestrial habitat.
How do Protozoa and Protista differ?
1. Protista is a Kingdom whereas, Protozoa is a Phylum under the Kingdom Protista.
2. Protista includes animal-like, plant-like, and fungi-like organisms. Whereas, Protozoa includes all animal-like organisms.
3. Most Protists are microscopic and unicellular, but some true multicellular forms exist. For example, Kelp is a multicellular protist. Whereas, all Protozoans are multicellular. But a few Protozoans may form colonies with little division of labour, but that can’t be counted as a multicellular trait.
4. Protists are both autotrophs and heterotrophs. Protozoa are heterotrophs, which means they cannot make their own food, but instead must ingest other organisms for energy.
5. Many Protozoans have a special form of locomotion that is not found in other types of Protists because they need to chase down their food source.
6. Protists may or may not move depending on their Phylum group. Whereas, Protozoans are those Protists that are motile and generate movement with cilia, flagella, or pseudopodia.
7. Some Protozoans are parasites, meaning that they live in or on an organism of another species (host) and benefits by deriving nutrients at the host’s expense. Some protists and other protozoans may also be free-living or parasitic in nature.
8. Most of the Protists have cell walls such as in Algae and Slime Moulds, whereas Protozoans do not have cell walls.
9. Motility is essential for the survival of Protozoa, whereas Protista does not need motility for their survival.
10. Some of the Protista have different steps in their life cycle from sexual to asexual as for example Algae. Whereas, Protozoa have vegetative form, which is called trophozoite and dormant form called cyst.
How are Protozoa and Protista similar?
1. Both Protozoa and Protista includes unicellular eukaryotic organisms.
2. Both Protozoa and Protista includes all those organisms that are neither true-animals, nor true-plants, nor true-fungi.
3. According to the present-day Phylogenetically based Classification System, they are the evolutionary bridge between bacteria and multi-celled organisms.
4. Like all protists, protozoans are single-celled organisms with a cell nucleus. Some have more than one nucleus.
5. They both can show solitary or can form loose colonies in which the individuals remain alike and independent.
6. Their single cell can perform all the essential and vital activities, hence only sub-cellular physiological division of labour can be seen.
7. The boundaries between the various species under Kingdom Protista are not well-defined. Therefore, you will find various relationships between the various species of Protozoans, Protophytons, and Protofungus. For example, Euglena shares both animal-like and plant-like characteristics.
8. Because they are so tiny, protists and protozoans do not need any specialized organelle, such as red blood cells, or even some plant-like species lack chlorophyll to meet their oxygen demand. In fact, many can live in water with very low concentrations of oxygen.
9. They both can benefit from each other and can stay in a mutual association. Some protozoa ciliates have specially adapted green algae living inside them. In proper sunlight conditions, these algae convert the carbon dioxide produced by the ciliate into oxygen, ensuring an abundant internal supply of oxygen for the ciliate. For example the algae Zoochlorella is an endoplasm-specific symbiont of the ciliate Paramecium bursaria.
10. Protists use contractile vacuoles to remove excess water from their cells. If the contractile vacuole’s function of a cell is compromised, the cell swells until it ruptures. The same will also happen to a marine protozoan when placed in fresh water. Marine members have no contractile vacuoles, but ciliates are those protozoans that have permanent contractile vacuoles throughout their lifetime.
Living organisms are classified into 5 kingdoms: Monera, Protista, Fungi, Animalia, and Plantae. Kingdom Protista is a special kingdom among these.
All living eukaryotes, which cannot be included in other kingdoms such as Plantae or Animalia, are grouped altogether into one group called Protista.
Protozoa is a Phylum of the Kingdom Protista. And, the taxonomic Kingdom Protista is a collection of single-celled organisms that do not fit into any other category.
All the individuals under Kingdom Protista are the most primitive and simple living organisms.
They are in fact the evolutionary bridge between bacteria and multi-celled organisms. They can find their applications in both Zoological and Botanical studies.