Kingdom Protista | Characteristics and Classification of Protists

All unicellular eukaryotes , irrespective of their mode of nutrition, are included in the kingdom Protista in Whittaker’s system. The protista term was coined by Ernst Haeckel . This kingdom forms a link between kingdom Monera on one hand and other three kingdom i.e., plantae, fungi and Animalia on the other hand. Protistans are ancestors of all multicellular eukaryotes ( plants, fungi and animals).

Classfication of Kingdom Protista

General Characteristics of Kingdom Protista :

Unicellular, eukaryotic organisms. Some are colonial without much cellular differentiation. Organisation at tissue level is absent.
Mostly aquatic organisms.
Cell structure is eukaryotic type having all kinds of membrane bound organelles and 80 S cytoplasmic ribosomes and cells may possess cellulosic cell wall.
Flagella and cilia have ( 9+2 ) pattern of microtubule organization consisting of tubulin protein.
Movement by pseudopodia, flagella or cilia where ciliary mode is fastest.
Mode of nutrition may be photosynthetic ( holophytic), Holozoic ( ingestive), saprobic or parasitic ( absorptive). Some have mixotrophic nutrition ( photosynthetic and saprobic) as in Euglena.
Reproduction occurs by asexual and sexual means.
Life cycle is of two types – (i) showing zygote meiosis (ii) showing gametic meiosis.
These are decomposers, photosyntheticor parasites. Parasitic protistsmay cause diseases like dysentery, malaria, sleeping sickness etc.

Photosynthetic Protists and Slime moulds are described below:

(1) Photsynthetic Protists

A. Diatoms

Diatoms are gold brown photosynthetic protistsand are called chrysophytes (Including both diatons and desmids). They are both aquatic and terrestial. Some are marine. They support much of marine life. Their important characters:

These are microscopic organisms possessing varying colours.
They are basically unicellular, but may form pseudofilamen and colonies, lacking flagella except in the reproductive stage. They may be free floating (phytoplanktonic), remaining afloat on surface of water due to presence of light weight lipids.
The cell wall is impregnated with silica to transparent siliceous shell, known as frustule. Depending upon the symmetry, diatoms may be centric type, having radial symmetry (e.g., melosira) and pennate type, having bilateral symmetry (e.g., Navicula).
The cell wall is characteristic, made up of two halves ; one half covering the other ( epitheca over hypotheca ) resembling a soap box.
The cell wall encloses the peripheral layer of cytoplasm ( primordial utricle ) surrounding a large central vacuole.

B. Dinoflagellates :

Dinoflagellates are golden brown photosynthetic protists belonging to class Dinophyceae ( Pyrrophyta ). They are mainly marine, though few are fresh water forms . They may appear red, yellow, green, brown or blue depending upon the main pigment present cell.

General characters of dinoflagellates are as follows:

Unicellular, motile, biflagellate, golden brown photosynthetic protists (some are non – motile, amoeboid, palmelloid or filamentous).
They are mostly marine, some are found in fresh water.
The body is encclosed by a rigid coat called theca or lorica consisting of 2 to many articulated or sculptured plates of cellulose and pectin, hence are also called armoured dinoflagellates.
Theca has generally two grooves i.e., longitudinal called sulcus and transverse called cingulum or annulus or girdle.

C. Euglenoid (Euglena – like )

It is group of chorophyllous and non chlorophyllosprotists. Largest genera being Euglena amongst them.

Euglenoids are unicellular, flagellate protists found in water or damp soil. There are many fresh water organisms found in stagnant water .
Body is spindle shaped with blunt anterior end and pointed posterior end .
Cell well is absent but a covering periplast or pellicle is present which is proteinaceous (elastic) in structure.

(2) slime moulds or consumer – decomposer protists

They were included in class myxomycetes of fungi in two – kingdom classification. They were called mycetozoa by De Bary as they are closely related to animals. Mycologists include them in gymnomycota. Because of their nature they are are called protistan fungi.

General characteristics of the slime moulds are.

They are usually free- living, creeping over debris like fallen leaves and rotting logs of wood.
They have naked protoplast, not covered by any cell well in vegetative stage.
They lack chlorophyll and have saprobic or phagotrophic mode of nutrition.
The body moves along decaying twigs and leaves engulfing organic material. Under favourable conditions, they from an aggregation called plasmodium which may grow and spread over several feet.

A. Acellular or Plasmodial Slime Moulds

General characters:

Slimy masses found on decaying leaves and lumber.
Somatic body is free living, multinucleate, naked diploid mass called Plasmodium. Movement occurs by means of pseudopodia.
During unfavourable conditions, entire plasmodium forms many fructifications / fruting bodies (polycentric). The fruiting body is called sporocarp which contains a stalk having a sporangium at its tip. The wall of sporangium is called peridium.
Sporangium has an intricate network of cytoplasmic threads called capillitium.
Diploid protoplast forms haploid spores by meiosis.

B. Cellular slime moulds or communal slime moulds