There are about 70 thousand species of unicellular animals (Protozoa). Their body consists of one cell, which performs all the vital functions inherent in the whole organism: movement, nutrition, respiration, excretion, etc.
The shape and size of the cells of protozoa are diverse. They can be viewed only with the help of magnifying devices. The average size of protozoa is 0.1–0.5 mm (there are individuals of about 0.01 mm, but there are also larger organisms several millimeters in size). There are also colonial forms of protozoa. Some protozoa do not have a constant body shape, while in others it is relatively constant.
Single-celled animals live mainly in a liquid environment – in sea and fresh water, moist soil, in other organisms. Under unfavorable conditions (drying up of water bodies, rising and falling temperatures), protozoa can form a dense protective shell, turning into a cyst.
Cyst is a temporary form of existence adapted for experiencing adverse conditions.
With the onset of favorable conditions, the animal leaves the cyst membrane and continues its vital activity.
The unicellular organism is delimited from the external environment by the cell membrane; inside the cell there is a cytoplasm with one or more nuclei and organelles.
Organelles are permanent cell structures that perform specific functions.
Organelles of movement of protozoa are represented by pseudopods, flagella, cilia.
Pseudopods (pseudopodia) are inconsistent outgrowths of the cytoplasm that do not have a dense cell membrane.
Pseudopodia are formed due to the overflow of cytoplasm inside the cell and provide not only the movement of the unicellular, but also the capture of food.
Cilia and flagella provide faster movement than pseudopodia.
Digestive and contractile vacuoles are characteristic of protozoan cells.
Vacuoles are cavities in cells, delimited by a membrane and filled with a liquid with substances dissolved in it.
Contractile vacuoles are found mainly in freshwater protozoa. They remove excess water from the organism of the simplest. The concentration of salts in fresh water is much lower than in the cytoplasm of the protozoan cell. Therefore, according to the laws of physics, the water surrounding the protozoan organism enters the cell. To avoid rupture of the cell, excess water is removed from the cell through the contraction of the vacuole. And together with water, metabolic products that are harmful to the body are also excreted.
Digestive vacuoles provide the digestion of food that enters the cell. Undigested food debris in some organisms can be excreted anywhere on the cell surface, while in others they are excreted through special organelles.
Single-celled animals feed on ready-made organic substances (bacteria, unicellular algae and other unicellular organisms). This type of diet is called heterotrophic.
For unicellular organisms, in addition to the heterotrophic type of nutrition, the mixotrophic type is also inherent. Forms with this type of nutrition in the light are capable of photosynthesis – the creation of organic substances from inorganic ones (autotrophic type of nutrition), and in the dark they switch to a heterotrophic type of nutrition.
Mixotrophic type of nutrition is a mixed type of nutrition, in which the body is simultaneously able to eat both auto- and heterotrophic.
The simplest are characterized by irritability. The reactions of protozoa are manifested in the form of movement towards the source of irritation or in the opposite direction. Such reactions are called taxis.
Irritability is the ability to react in a certain way to changes in the environment.
Taxis are motor reactions in response to a stimulus (stimulus) towards it or away from it.
Taxis in the opposite direction from the stimulus (for example, from a crystal of table salt) – negative (left); towards the stimulus (for example, to a group of bacteria that feed on unicellular organisms) – positive (right).
Reproduction of protozoa is predominantly asexual. First, the nucleus divides in two, then the cytoplasm. Asexual type of reproduction refers to cell division into two or more cells (multiple division, or schizogony).
Some protozoa have sexual reproduction.
Division is transverse (for example, in ciliates-shoes) and longitudinal (for example, in green euglena).