At first glance, it seems that modern taxonomy has already identified all the main taxa and has no contentious issues. But this is not at all true. Have you heard of such a systematic unit as protists? If not, then our article is for you.
Discovery story
Protists are a concept that was first introduced into science by the German naturalist Ernst Haeckel. It happened in 1886. At that time, two kingdoms of wildlife were already known : Plants and Animals. All other living organisms scientist and attributed to the protists. However, science did not stand still. The taxonomists identified new leading characters and created taxa. And in 1969, the American ecologist Roberg Whittaker described protists in a modern sense. By the way, this scientist is called the author of the "five kingdoms" system. Such a classification of all living things is relevant today.
Characteristic Protists
Protists include all organisms whose body does not form real tissues. Moreover, it does not matter how many cells they are formed. The structure of protists is characterized by the presence of a nucleus. Among plants, algae belong to this group. Heterotrophic protists are represented by the simplest animals and mushroom-like organisms.
Algae structure
We begin the description of protists with the very first plants that appeared on the planet - algae. Among them there are unicellular representatives. These are chlamydomonas and chlorella. Despite the fact that their entire body is represented by a single cell, they carry out all the processes of life. This is breathing through the membrane, movement using a flagellum, autotrophic nutrition, reproduction by halving or spore formation. Multicellular algae are more diverse. In their body, the cells are connected anatomically, but do not form tissues. Such structures are called thallus, or thallus.
Heterotrophic protists
This group includes species that can only eat ready-made organic substances. Heterotrophic protists are unicellular, or protozoa. Despite the name, their structure is also quite complicated. One of the most common representatives of protozoa is the ciliates shoe. Like all animals, their surface apparatus is represented by a plasma membrane and a pellicle representing a compacted layer of the cytoplasm. The permanent organelles of these protists are the digestive and contractile vacuoles. The former carry out the enzymatic breakdown of organic substances, and the latter regulate the osmotic pressure and water-salt metabolism.
In ciliates, there is even a sexual process that takes place in the form of conjugation. In this case, the two animals come closer, a cytoplasmic bridge is formed between them, along which the nuclei exchange genetic information. Organelles of heterotrophic protist movement are very diverse. In ciliates, these are numerous cilia, in euglena it is the only flagellum. But the amoeba protea forms an unstable protrusion of the cytoplasm, called pseudopods, or pseudopodia.
Mushroom-like protists
This group of protists reminds real mushrooms remotely. For example, the body of the labyrinthula is represented by mesh wandering plasmodia. And the cell wall of oomycetes consists of hard cellulose. In addition, in the process of asexual reproduction, they form motile zoospores. These signs are also characteristic of the hyphochitridia half-row, most of which are intracellular parasites of algae and invertebrate animals.
Unique properties
Protists are organisms that have very unusual symptoms. These include the pseudopodia of mushroom-like labyrinths. They merge with similar structures of neighboring cells, forming a whole network. Representatives of the chrysophyte order are equipped with a special outgrowth called the haptoneme. It consists of microtubules surrounded by a channel of the endoplasmic reticulum. In dinophytic algae, a nuclear structure unique to them is observed, the chromosomes of which are always in a spiral state.
What is polyphilia
Very often, protists are called a polyphyletic group, or taxon. This means that its composition includes organisms that have proven kinship with representatives of other systematic units that are not included in this. Thus, protozoa belong to the animal kingdom, and algae to the plant kingdom. Polyphyletic taxa are not part of modern taxonomy, since their representatives do not have a common ancestor. Examples of such groups are cold-blooded animals or autotrophic bacteria.
So, protists are eukaryotic organisms that do not form real tissues. Among them there are single and multicellular species, auto and heterotrophs. Algae, protozoa, and fungus-like organisms are considered modern representatives of protists.