Cell: definition, structure, classification
The structural unit of any organism iscell. The definition of this structure was first used by Robert Hooke when he studied the structure of tissues under a microscope. Now scientists have found a large number of different types of cells that are found in nature. The only organisms of the non-cellular structure are viruses.
Cell: definition, structure
A cell is a structural and morphofunctional unit of all living organisms. There are unicellular and multicellular organisms.
Most cells have the following structures: a cover device, a nucleus and a cytoplasm with organelles. The integuments can be represented by the cytoplasmic membrane and the cell wall. The nucleus and organelles have only a eukaryotic cell, the definition of which differs from the prokaryotic cell.
Cells of multicellular organisms form tissues,which, in turn, are part of the organs and systems of organs. They come in different sizes and can vary in form and function. It is possible to distinguish these small structures only with the help of a microscope.
What is a cell in biology. Prokaryotic cell definition
Such microorganisms as bacteria area bright example of prokaryotic organisms. This type of cells is simple in structure, since bacteria do not have a nucleus and other cytoplasmic organelles. Hereditary information of microorganisms is contained in a specialized structure - the nucleoid, and the functions of the organelles are performed by mesosomes, which are formed by invagination of the cytoplasmic membrane inside the cell.
What other features doesa prokaryotic cell? The definition says that the presence of cilia and flagella is also a characteristic feature of bacteria. This additional motor apparatus is different in different groups of microorganisms: someone has only one flagellum, someone has two or more. Infusorians have no flagella, but there are cilia along the entire periphery of the cell.
Inclusions play an important role in the life of bacteria, Because prokaryotic cells do not have organelles that can accumulate the necessary substances. Inclusions are in the cytoplasm and there they are compacted. If necessary, bacteria can use these accumulated substances for their own needs, in order to maintain normal life activity.
Eukaryotic cells are evolutionarily more developed than prokaryotic cells. They have all the typical organelles, as well as the core - the center of storage and transfer of genetic information.
The definition of the concept of "cell" accurately describesstructure of eukaryotes. Each cell is covered with a cytoplasmic membrane, which is represented by a bilipid layer and proteins. Above is glycocalyx, which is formed by glycoproteins and performs a receptor function. Plant cells also secrete a cell wall.
The cytoplasm of eukaryotes is colloidalsolution, which contains organelles, a cytoskeleton and various inclusions. Among the organelles, the endoplasmic reticulum (smooth and rough) is isolated, the Golgi apparatus, lysosomes, peroxisomes, mitochondria, and plant plastids. The cytoskeleton is represented by microtubules, microfilaments and intermediate microfilaments. These structures form a skeleton, and also participate in division. A direct role in this process is played by the center that any animal cell has. Definition, the presence of the cytoskeleton and the cell center in its thickness is possible only with the use of a powerful modern microscope.
The core is a two-membrane structure, the contentswhich is represented by karyolymph. It contains chromosomes that contain the DNA of the whole cell. The nucleus is responsible for the transcription of the organism's genes, and also controls the stages of division in mitosis, amitosis and meiosis.
Non-cellular forms of life
What is a cell in biology? The definition of this term can be used to describe the structure of almost any organism, but there are exceptions. So, viruses are the main representatives of the non-cellular form of life. Their organization is quite simple, because viruses are infectious agents that contain only two organic components: DNA or RNA, as well as a protein shell.
Viruses are peculiar parasites of cellsanimals and plants. After penetrating into the host cell, the viruses insert their nucleic acid into the nucleus DNA, after which the synthesis of the genes of the virus begins. As a result, the host cell becomes a kind of factory for the production of new virus particles, which in this way increase their number. After such manipulations, the eukaryotic cell most often dies.
Bacteria also suffer from virus attacks,which constitute a group of bacteriophages. Their body has the form of a dodecahedron, and the nucleic acid is "injected" into the bacterial cell by the tail process, represented by the contractile cover, inner core and basal plate.