Living organisms that have a single cell eukaryote are rated as protists. The protist kingdomTherefore, it is the one formed by this class of living beings.
Despite this definition provided by the dictionary of the Royal Spanish Academy (RAE), there are experts who also include multicellular organisms in the protist kingdom. In this sense, all eukaryotic-type organisms that, due to their characteristics, are not part of the kingdoms of fungi, plants and animals, would make up the protist kingdom.
These differences in criteria mean that there is no single definition of the protist kingdom. Biologists cannot agree on which organisms are protists and which should not be included in this list. classification.
There are those who divide the Protist kingdom into three sub-kingdoms: archaezoa, euglenozoa Y protozoa. The archaezoa sub-kingdom includes those protists that do not have mitochondria. In the euglenozoa subkingdom, also called chromist, protists appear that have mitochondria and that, in addition, perform the photosynthesis. In the subkingdom protozoa (or rotozoan), finally, there are protists with mitochondria and without photosynthetic capacity.
Difficulties in classification are due to the fact that protists sometimes resemble plants, while others resemble animals. However, the members of the protist kingdom are not plants or animals.
It can be said, in the broadest sense of the concept, that protists are unicellular or multicellular microorganisms that, for the most part, feed in a autotrophic, and that, on the other hand, cannot be included in the three remaining kingdoms of eukaryotes: that of plants (Plantae), that of animals (Animalia) and that of fungi (Fungi). It is important to note that there are no fully air-adapted protists, so even those that do not inhabit the air Water they depend on humidity to survive.
The beings whose nutrition It is considered autotrophic are those that can carry out the synthesis of the fundamental substances for their metabolism starting from inorganic substances, such that they do not need the presence of other organisms to feed themselves. In other words, the individuals of the protist kingdom produce their own food based on inorganic substances.
In addition, an autotrophic organism is capable of generating its organic matter and cell mass exclusively starting from carbon dioxide (inorganic substance) to obtain carbon, and using light or certain chemicals to get energy. Protists, on the other hand, can be both heterotrophic and photosynthetic autotrophs, although many of them combine both forms of nutrition.
In the first group, the heterotrophs, we also enter the animals, since they are those individuals who must take advantage of the matter third parties for the manufacture of complex organic molecules. To obtain energy, they break the molecules of their prey.
Regarding your structure, protists do not have any type of tissue, although brown and red algae exhibit a degree of complexity that approaches tissue, which includes the presence of plasmodesmata (the name given to the continuous units of cytoplasm that maintain cells of the multicellular organisms connected to each other).
There are many multicellular individuals of the protist kingdom that present cell walls of various types of composition, while some unicellular, such as the diatoms, are usually covered with a teak (a resistant structure that can be compared to a shell, since it is found on the outside of the cells of certain protists), or have reinforcements or scales.
The reproduction of the protists can be sexual, with gametes, or asexual (in this case we speak of clonal reproduction), although the most frequent is that they alternate both types of reproduction. It should be clarified that in neither of them there is an embryo.