What are Protists?
Let's begin with answering "What are protists?". The biological kingdom Protista includes a group of unique life forms composed of eukaryotic cells which can be unicellular as well as multicellular. Besides Animalia and Plantae, Protista is a significant biological kingdom of organisms that exhibit features, functions and characteristics that greatly differ from organisms belonging to either of the other two kingdoms.
The various different organisms or life forms that are grouped under the kingdom protista do not have much in common, except for the fact that they have a very basic, simple structure. Even the multicellular protists have a very elementary structure insomuch as they lack specialized tissues. Broadly, there are three types of protists and these three types are further classified under five distinct groups. Let's proceed towards taking a look at the various kinds of protists that exist in the Earth's biosphere.
Different Types of Protists
As mentioned above, the three broad types of protists are those that are plant-like, those that are animal-like and those that are fungus like. Now, these three broad categories can be scientifically sub classified under the following five super categories, each of which are divided into several subcategories :-
Archaeplastida consists of eukaryotic life forms that have chloroplasts which are enclosed by two membranes. It is proposed that this kind of structure is a hint at the endosymbiosis of cyanobacteria.
Subcategories of Archaeplastida
This category may be further divided into the following two subcategories:-
- Rhodophyta - Eukaryotes that lack flagella a good example of which is red alga.
- Glaucophyte - For instance, freshwater microscopic alga which is characterized by the presence of a murein layer which is believed to be a sign of endosymbiosis of cyanobacterial plasmids.
Chromalveolata are eukaryotes that are single-celled organisms with morphologies of most members resembling those of terrestrial plants and having cell walls. These types of protists are also equipped with the plant like ability to perform photosynthesis.
Subcategories of Chromalveolates
This super group is divided into the following five sub groups:-
- Alveolate - Having mitochondria, cortical alveoli, flattened vesicles and distinctly structured flagella.
- Cryptomonad - Mostly chloroplast containing algae.
- Haptophyte - Eukaryotes having pigmented chloroplasts an example of which is the coccolithophore alga.
- Heterokont - Diatomic organisms, mostly algae such as kelp, that are characterized by their chloroplast content and motile cells.
Excavates consists of single-celled eukaryotic organisms that may be free-living as well as symbiotic. Most of these life forms often lack a typical mitochondria. Those which have mitochondria have it in the form of discoidal, laminar or tubular cristae. Most excavates have two or more flagella.
Subcategories of Excavates
This super category is further divided into the following three subcategories:-
- Euglenozoa - These are protozoan life forms having flagella and these are unicellular with some being free-living while others being parasitic.
- Metamonads - These are anaerobic flagellate protozoa which are mostly in symbiotic relationship or parasitic.
- Percolozoa - These are protozoan organisms that lack color pigmentation and some of have the ability to transform into amoeboid, flagellate as well as encysted forms.
Rhizaria consists solely of single-celled eukaryotic organisms that have mitochondria with tubular cristae. Most of these organisms are amoeba like, having pseudopodia (false feet).
Subcategories of Rhizaria
This super group is further classified into the following three subcategories:-
- Cercozoa - Amoeboids and flagellates that can form pseudopod in absence of a well-defined mouth for feeding.
- Foraminifera - These are amoeboids that possess very fine cytoplasmic strands branching out and merging to give the protist the appearance of a microscopic net with a nucleus.
- Radiolaria - These amoeboid protists possess complex skeletons made of minerals, instances of which may be seen as marine zooplankton.
Unikont have a genetic structure wherein three genes are fused with each other. The cellular structure is eukaryotic and most of these organisms are either amoeboid without flagella or are organisms having just one protruding flagellum.
Subcategories of Unikont
This super category is further divided into the following two subcategories:-
- Amoebozoa - These are the generic amoeboids whose movements are dependent upon their internal cytoplasmic flow.
- Choanozoa - These are animal like protists most of which are parasitic.
Thus a precise overview of the different kinds of protists complete with a three-tier classification is given. Since all protists have a eukaryotic cell structure, they all undergo the typical eukaryotic cell cycle which includes the three stages of resting phase, inter phase and mitosis. Being mostly of microscopic magnitude, a lot of protists have potential pathogenic capabilities. Such protist pathogens may take up animals as well as plants as hosts and, as a result, make the latter diseased.
The malaria-causing protist, Plasmodium falciparum is a prominent example of pathogenic protists. As we can see from the above varieties of protists, this biological kingdom consists of a vast diversity and each group and subgroup of protists differ from others, sometimes slightly and sometimes significantly, in terms of morphology, characteristics and a lot of other aspects.