Eukaryotes (Protists)

Eukaryotes and Protists

In contrast to prokaryotes, eukaryotes contain a membrane-enclosed nucleus and depending on the organism, several other organelles. For example, mitochondria are nearly universal among eukaryotic

cells, while pigmented chloroplasts are found only in phototrophic cells. Other internal structures typically include the Golgi complex, peroxisomes, lysosomes, endoplasmic reticula and microtubules and microfilaments. These internal structures compartmentalize activities of the cell for efficient function. Some eukaryotic cells have flagella and cilia (organelles of motility) while others do not. Eukaryotic cells also may have extracellular components, such as a cell wall in fungi, algae or plant cells (cell walls are not found in animal cells or most protists), or an extracellular matrix in animal cells. Also, many eukaryotes, even many eukaryotic microorganisms, form multicellular structures.

A protist is any eukaryotic organism that is not an animal, plant or fungus. The protists do not form a natural group, or clade, but are often grouped together for convenience, like algae or invertebrates. In some systems of biological classification, such as the popular five-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues”. Protists include three major groups of organisms including,

• Protozoa the unicellular "animal-like" (heterotrophic/parasitic) protozoa which was further sub-divided based on motility such as (flagellated) Flagellata, (ciliated) Ciliophora, (phagocytic) amoeba and spore-forming Sporozoans
• Protophyta the "plant-like" (autotrophic) protophyta (mostly unicellular algae)
• Slime Molds the "fungus-like" (saprophytic) slime molds and water molds.

(Proto – first; Zoon – animal) Protozoa are single celled eukaryotic organisms, which lack cell wall and ability to move at some stage of their life cycle. They are referred as the first animals. There are about 65000 described species grouped into 7 phyla. Among them, 50 per cent are not available as living forms (as fossils) and out of remaining 50%, 22,000 species are free-living and 10,000 species are parasitic forms.

Occurrence of Protozoa: Protozoa present in all moist habitat; Sea, soil and fresh water are common habitat; Also present as parasites in higher animals.

Ecological groups : Protozoa may be divided into three groups based on their ecological conditions namely,

1. Free-living – Occur in salt water, fresh water, sand, soil and decaying organic matter
2. Symbiotic forms - Occur in guts of termite and help in the digestion of woody materials. Ex. Trychonympha protozoa present in the guts of termites help to digest the cellulose of woods.
3. Parasitic forms - Occur in the body of higher animals and human and cause diseases

Morphology: Protozoa exhibit a wide variety of morphologies. There is no definite shape or morphology which would include a majority of protozoa. Shapes range from amorphous and ever-changing forms of amoeba to relatively rigid forms. Size and shape of these organisms show diversity. Shape varies from amoeboid to all possible shapes. Size ranges from 200 to 600 mm diameter protozoa are available. Plasmodium – 2-5 mm; Paramecium – 200 - 500mm; Spirostomum – 3 mm;

Like eukaryotic cell, the protozoan cells consist of cytoplasm, nucleus, endoplasmic reticulum and all regular cell organelles. Several protozoa express photosynthetic pigments thus coloured. (Ex. Euglena) Many protozoa exhibit complex life cycles with multiple stages (Ex. Plasmodium).

Cyst: Many protozoa form resistant cysts at certain times of their life cycle. They are able to survive in adverse environmental conditions such as desiccation, low nutrient supply and even anaerobiosis. In parasitic protozoa, the cyst stage is often used to transmit from one host to another.

Locomotive organs:

The motility is the key character of protozoa and the mode of motility varies with organisms. The following are the possible ways by which the protozoa motility occurs.

1. Pseudopodia (False foot) – Temporary projection of part of cytoplasm of those protozoa which don’t have rigid cell are referred as pseudopodia. Ex. Amoeba. The pseudopodia also helpful to capture food through engulfing.
2. Flagella – Fine filamentous extension of cell similar to that of bacteria. The number varies from 1 to 8. Ex. Euglena
3. Cilia – Fine short thread like extension from cell used for locomotion and ingestion of food. Ex. Paramecium
4. Gliding motility Ex. Sporozoa

Feeding: The ingestion is the primary mode of feeding habit of protozoa.

1. Pseudopodia of amoeba used to feed the prey by phagocytosis

2. Cytosome or oral groove – An opening of ciliate protozoa through which food is ingested. It ranges from simple round opening to slit like structures.

Algae are a large and diverse group of eukaryotic organisms that contain chlorophyll and carry out the oxygenic photosynthesis.Study of algae is referred as phycology or algalogy. They vary from cyanobacteria or blue green algae by their eukaryotic nature.

Occurrence: The algae are mainly present in aquatic ecosystems. They are present in (i) Oceans, sea, salt lakes, fresh water lakes, ponds and streams; (ii) Damp soils, rocks, stones, tree barks etc. (iii) Small aquatic free floating microscopic algae in water are referred as planktons or phytoplanktons; (iv) Some are present as parasites to plants

Morphology: Wide range of size and shapes are occurred in algae.

• Unicellular: single cells, motile or nonmotile Ex. Chlamydomonas, Chlorella
• Colonies: Assemblage of individual cells with variable or constant number of cells that remain constant throughout the colony life Ex. Diatoms, Volvox
• Filaments: daughter cells remain attached after cell division and form a cell chain; adjacent cells share cell wall (distinguish them from linear colonies) Ex. Spirogyra
• Coenocytic forms: one large, multinucleate cell without crosswalls Ex. Vaucheria
• Parenchymatous and pseudoparenchymatous algae: mostly macroscopic algae with tissue of undifferentiated cells and growth originating from a meristem with cell division in three dimensions; pseudoparenchymatous superficially resemble parenchyma but are composed of appressed filaments Ex. Porphyra
• ­Eukaryotic nature
• Presence of thin and rigid cell wall
• Presence of chloroplasts

Cell wall: The algal cell wall is primarily made up of cellulose fibres. But, the cellulose fibres were modified by additional polysaccharides like xylans, mannoses, alginic acids etc. Some algal cell walls deposited with calcium carbonate, referred as calcareous or coralline algae. Some algae, the cell wall is absent. Ex. Euglena. Silica deposited cell wall also found in some algae Ex. Diatoms. They are extreme resistance to decomposition.

Slime mold is a nonphototrophic protist that lacks cell walls and that aggregates to form fruiting structures (cellular slime molds) or masses of protoplasm (acellular slime molds)

The slime molds were previously grouped with fungi since they undergo a similar life cycle and produce fruiting bodies with spores for dispersal. As protists, however, slime molds are motile and can move across a solid surface fairly quickly. There are two groups of slime molds:

1. Plasmodial slime mold (also known as acellular slime mold): vegetative forms are masses of protoplasm of indefinite size and shape called plasmodia.
2. Cellular slime mold: The vegetative forms are single amoebae.

Slime molds live primarily on decaying plant matter, such as leaf litter, logs, and soil. Their food consists mainly of other microorganisms, especially bacteria, which they ingest by phagocytosis. Slime molds can maintain themselves in a vegetative state for long periods but eventually form differentiated spore-like structures that can remain dormant and then germinate later to once again generate the active amoeboid state.

Plasmodial slime molds, such as Physarum, exist in the vegetative phase as an expanding single mass of protoplasm called the plasmodium that contains many diploid nuclei. The plasmodium is actively motile by amoeboid movement, the plasmodium flowing over the surface of the substratum, engulfing food particles as it moves. From the plasmodium, a sporangium containing haploid spores can be produced, and when conditions are favorable, the spores germinate to yield haploid flagellated swarm cells. The fusion of two swarm cells then regenerates a diploid plasmodium.

Cellular slime molds are haploid and form diploids only under certain conditions. In addition, instead of a single mass of protoplasm, cellular slime molds are individual, independent amoeboid cells, and when the available food is consumed, the cells aggregate to form a fruiting body. The cellular slime mold Dictyostelium discoideum has been used as a model cellular slime mold.