For most individuals, the first images seen through the lens of a microscope are protists -- unicellular organisms that don’t possess enough characteristics to be defined as purely plant or animal.
The organisms within the Kingdom Protista contain a nucleus, like all Eukaryotes, and are categorized as plant-like, animal-like or fungus-like.
Protists are abundant in the world around us, usually thriving in aqueous environments; they survive in bodies of water as well as the human body.
A sample of pond water or its moist surrounding area placed on a slide under a compound microscope yields images of living organisms such as paramecium and amoeba – inexpensive and easy, this is often a student’s introduction to microscopy.
Samples are live, often moving and differ with each drop of water; these seemingly simple images provide the basis for identifying structures within a cell – an invaluable foundation for the study of prokaryote and eukaryote specimens.
Protists possess characteristics that make them “like” multi-cellular organisms, yet they lack certain properties to be classified as animal, plant or fungus. The presence of a nucleus in all protist organisms means they are all eukaryotic.
The three primary classifications in the Protista Kingdom and subsequent phylum include:
Animal-like or Protozoan
Plant-like or Diatoms
Many protists overlap in classification, such as lichen – considered a type of algae and fungus and some, like red algae, are amongst the few multi-cellular protista organisms.
Literally defined as “the very first,” Protista are believed to be the first known organisms. However, it is important to note the miscellany nature of this Kingdom, specifically that groups of protista are not related to one another in the same manner as Eukaryotes and Prokaryotes.
Containing over 80 groups and over 115,000 species, the Kingdom Protista represents a wide range of Eukaryotic organisms bound only by the fact that they act like animals, plants or fungi.
Eukaryotic organisms possess a nucleus in each cell. Whether an organism contains one or multiple cells, the nucleus – the first and largest organelle discovered – is essential to cell life. Protected in a double envelope, cytoplasm crosses through pores in the outer membrane; this is unlike prokaryotes, where cytoplasm directly crosses the cell wall.
Appearing like a darkened area inside the nucleus under a light microscope, the nucleolus is surrounded by chromatin, which contains DNA and RNA – necessary for cell division.
Serving as a conduit between the pores in the nuclear envelope, the endoplasmic reticulum is responsible for the in/out transport of compounds.
Animal-like protists or protozoan are primarily identified by method of movement such as:
Ciliates Paramecium from BiologyCorner.com
In addition, parasitic protists such as sporozoans also fall under animal-like, but are categorized by the way they survive and diseases brought about in hosts.
Animal-like protozoan are also heterotrophic and contain organelles such as a cell membrane and food vacuole.
Considered the foundation of many aquatic food chains, responsible for over forty percent of photosynthesis that occurs in salt and fresh bodies of water, as well as essential to the production of oxygen in the atmosphere, plant-like protista are classified into three phylum:
Although each contains chlorophyll, organisms such as Dinoflagelles have properties that are both animal and plant like.
This overlap is exclusive to Eukaryotes in the Protista Kingdom.
Fungus-like protists have cells walls similar to plants, which contain chitin, but possess the animal-like function of heterotrophy. They release spores into the air to reproduce and have the ability to move, although this might only happen once during a lifespan. Requiring a moist environment to survive, the three types of fungus-like protists are:
Fungus-like organisms also have instances of overlapping. For example, certain slime molds are the result of stressed amoebas merging into a pseudoplasomodium (slug); this fungal “body” is able to relocate and reproduce by releasing spores.
All types of protista organisms can be studied under a simple light microscope and some, like fungus, can be seen with the naked eye.
Microscopy studies can be as easy as using a pipette to drop pond water onto a slide and viewing live paramecium as they move in their natural environment.
Advanced techniques such as dark field illumination or phase contrast are used to view structures in greater detail. Comparing images from each technique also yields valuable information.
Unicellular Eukaryotes, protists provide a foundation for viewing multi-cellular plants and animals. Often used in introductory microscope experiments, the first image many students see through a lens is an amoeba or paramecium.
Although members of the Kingdom Protista do not contain
all the organelles found in plants and animals, they do contain a
well-defined nucleus, providing a foundation for more advanced
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