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Protists are abundant and diverse in nearly all habitats. They contribute 4 gigatons (Gt) to Earth's biomass—double that of animals (2 Gt), but less than 1% of the total. Combined, protists, animals, archaea (7 Gt), and fungi (12 Gt) make up less than 10% of global biomass, with plants (450 Gt) and bacteria (70 Gt) dominating. [166]
A protist (/ ˈ p r oʊ t ɪ s t /) is any eukaryotic organism (one with cells containing a nucleus) that is not an animal, plant, or fungus.The protists do not form a natural group, or clade, since they exclude certain eukaryotes with whom they share a common ancestor; [a] but, like algae or invertebrates, the grouping is used for convenience.
All eukaryotes apart from animals, plants and fungi are considered protists. [1] Its field of study therefore overlaps with the more traditional disciplines of phycology , mycology , and protozoology , just as protists embrace mostly unicellular organisms described as algae , some organisms regarded previously as primitive fungi , and protozoa ...
Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms. [ 9 ] [ 10 ] This more constrained definition excludes all brown , the multicellular red and green algae , and, sometimes, slime molds (slime molds excluded when multicellularity is defined ...
In protists, all-or-none action potentials occur almost exclusively in association with ciliary membranes, [100] [101] [102] with the exception of some non-ciliated diatoms. [ 103 ] [ 104 ] Graded potentials occur in amoebae , also for movement control.
The third domain, Eukaryota, includes all multicellular organisms as well as many unicellular protists and protozoans that are microbes. Some protists are related to animals and some to green plants. Many multicellular organisms are also microscopic, namely micro-animals, some fungi, and some algae, but these are generally not considered ...
They are similar to Radiolaria, but they are distinguished from them by lacking central capsules and other complex skeletal elements, although some produce simple scales and spines. [5] They may be found in both freshwater and marine environments.
By 1981, Cavalier-Smith had divided all the eukaryotes into nine kingdoms. [20] In it, he created Chromista for a separate kingdom of some protists. [21] Most chromists are photosynthetic. This distinguishes them from most other protists which lack photosynthesis. In both plants and chromists photosynthesis takes place in chloroplasts.