High School Life Science/Protists

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 invertebraes. In some systems of biological classifications, 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.

Although Ernst Haeckel set up the Kingdom Protista in 1866, this kingdom was not accepted by the scientific world until the 1960s. These unique organisms can be so different from each other that sometimes Protista is called the “junk drawer" kingdom. Just like a junk drawer, which contains items that don't fit into any other category, this kingdom contains the eukaryotes that cannot be put into any other kingdom. Therefore, protists can seem very different from one another.

Most protists are so small that they can be seen only with a microscope. Protists are mostly unicellular (one-celled) eukaryotes. A few protists are multicellular (many-celled) and surprisingly large. For example, kelp is a multicellular protist that can grow to be over 100-meters long.

Multicellular protists, however, do not show cellular specialization or differentiation into tissues. That means their cells all look the same and, for the most part, function the same. On the other hand, your cells often are much different from each other and have special jobs. General Characteristics of Protista
 * 1) They are mostly unicellular but some are multicellular and colonial organisms.
 * 2) They are either free-living or parasitic.
 * 3) They have aerobic mode of respiration and have mitochondria for cellular respiration.
 * 4) They are true eukaryotes and are nucleated.
 * 5) They have (9+2) arrangement of flagella and have membranous organelles.
 * 6) They reproduce both sexually (syngamy) and asexually.
 * 7) They are grouped into 3 categories: animal like (protozoa), fungus like, and plant-like.
 * 8) According to the categories, they have different modes of nutrition, like heterotrophy or autotropohy.
 * 9) Plant-like protists (algae) have chlorophyll and accessory pigments, named xanthophylls, phycobilins, and carotene.