Characteristics and classification of Kingdom protists

 

KINGDOM PROTISTA

All the single celled (unicellular) eukaryotes are placed under protista. Members of protista are primarily aquatic. Living organisms included in protist are as follows :-

Dinoflagellates, Chrysophytes (Diatoms and Desmids), Euglenoids, Slime moulds, Protozoa.

All the living organisms included in protista are unicellular eukaryotes. i.e. the structure of all protists is similar to eukaryotic cell. The members of protista have characters of plants, animals and fungi also hence the boundary of this kingdom are not well defined. The protistan cell body contains a well defined nucleus and other membrane bound cell organelle.

Mode of nutrition

Mode of nutrition protista have different types

(1    (1)   HOLOPHYTIC OR PHOTOSYNTHETIC    :-

They prepare their own food by photosynthesis.

(2     (2)   HOLOZOIC  :-

Some protista have holozoic mode of nutrition, which is similar to animals i.e. food is first ingest then digest.

(3     (3)   OSMOTROPHIC OR ABSORPTIVE  :-

Food is first digest then ingest in this mode of nutrition.

(4     (4)   MIXOTROPHIC  :-

Some protists have more than one type of nutrition.

REPRODUCTION

Protista reproduce asexually and sexually by a process involving cell fusion and zygote formation.

(1)   Asexual reproduction :- It is following types

(a)   Binary fission – A cell divided to form to or more daughter cell e.g. Dinoflagellates, chrysophytes, Euglenoids

(b)   Spore formation – Some protists form special structure known as sporangium. Spores are formed in this sporangium. The sporangium burst after sometime and all the spores become free. Each spore forms a new cell of protists e.g. slime moulds

(2)   Sexual reproduction:- sexual reproduction was first of all seen in protists. In sexual reproduction two haploid gametes fuse to form a diploid zygote. This process is known as syngamy. It is a three types :-

(a)   Isogamy  :- Both gametes are morphologically  similar but physiologically may be similar or dissimilar.

(b)   Anisogamy  :- Both gametes are morphologically dissimilar but physiologically may be similar or dissimilar.

(c)   Oogamy  :- ( Advanced form of anisogamy) When male gamete motile / nonmotile and small in size but female gamete non motile and large in size.

LIFE CYCLE OF PROTISTs :-

(i)             Haplontic life cycle – In this type of life cycle, during sexual reproduction gametes are formed by mitosis. These gametes are haploid (n). these gametes fuse to diploid (2n) zygote. After that meiosis takes place in zygote (zygotic meiosis), as a result of which haploid cells or organisms are formed again. In this type of life cycle the zygotic phase is only diploid (2n) and remaining all the phases are haploid (n) so this type of life cycle is known as Haplontic life cycle.

(ii)            Diplontic life cycle – In this type of life cycle during sexual reproduction, meiosis takes place in diploid (2n) cell (Gametic meiosis), due to which haploid gametes are formed. Now haploid gametes fuse to form diploid zygote. And after that mitosis takes place in zygote, which again form diploid cells. In this type of life cycle only gametic phase is haploid and remaining all phase is diploid so this type of life cycle is known as Diplontic life cycle.

DIVISIONS :-

(A)  DINOFLAGELLATES :-

·       Protists with two flagella.

·       Mainly found in marine water and photosynthetic.

·       Cell wall present made up of cellulose.

·       Cell wall divided into stiff plate.

·       Dinoflagellate also called armoured algae and fire algae (Due to bioluminescence). E.G.- Noctiluca, Gonyaulax (RED) – Undergo rapid multiplication and spread on sea surface and cause red tide.

·       Holophytic mode of nutrition.

·       Pigments of dinoflagellates chlorophyll ‘a’, ‘c’ and xanthophylls (Dinoxanthin &Didinoxanthin) are found.

·       Histone protein is absent in chromosome of dinoflagellates.

·       They have starch as stored food.

·       Reproduction takes place by asexual reproduction (Binary fission).

·       Both Gymnodinium & Gonyaulax secrete toxins, which can cause paralysis in human beings. Humans acquire these toxins through food chain. These toxins release by dinoflagellates may even kill other marine animals such as fishes.

(B)  CHRYSOPHYTA (Diatoms) :-

·       Chief producers of ocean.

·       Diatoms and golden algae (Desmids) include.

·       Planktons – Passively float on sea surface.

·       Found in fresh water and as well as in marine water.

·       Cell wall present made up of cellulose divided into two halves.

·       Cell wall like soap box.

·       Silica particle embedded in cell wall thus the cell wall indestructible.

·       They have leucosin and oil as stored food.

·        Diatoms have pigments chlorophyll ‘a’, ‘c’, and xanthophyll (fucoxanthin) present.

·       Asexual reproduction (Binary fission) occurs.

·       Use of Keiselgurh / Diatomaceous earth :-

                                                                                              i.         Sound proofing

                                                                                            ii.         Filtration of oils and syrups

                                                                                           iii.         Stone polishing

                                                                                           iv.         Heat insulator

(C)  EUGLENOIDS :-  

·       Mostly found in fresh water

·       Cell wall absent

·       Instead of cell wall proteinaceous layer pellicle present.

·       Show mixotrophic mode of nutrition

·       Euglenoid have pigments chlorophyll ‘a’, ‘b’, and xanthophyll (zeaxanthin) present.

·       They have a contractile vacuole which helps in osmoregulation.

·       Euglenoid have haploid nucleus and chloroplasts.

·       Eye spot of euglenoid at present on anterior position.

·       Asexual reproduction takes place by longitudinal binary fission.

·       They have paramylum and fat as stored food.

(D)  SLIME MOULDS / MYXOMYCETES :-

·       They develop in slimy mass

·       It is also called false fungi

·       It is a saprophytic protists found on decaying twigs and leaves engulfing organic material.

·       Slime mould lack cell wall but spores possess true wall.

·       In favourable conditions they form an aggregation which several feet long structure called plasmodium.

·       In unfavourable conditions plasmodium differentiate and  form fruiting body called sporangium. Sporangium contain spores.

·       After sometime sporangium burst and spores release and dispersed through air current.