The structural unit of any organism is a cell. 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. Viruses are the only non-cellular organisms.
Cell: definition, structure
A cell is a structural and morphofunctional unit of all living organisms. Distinguish between unicellular and multicellular organisms.
Most cells have the following structures: integumentary apparatus, nucleus and cytoplasm with organelles. Covers can be represented by a cytoplasmic membrane and a cell wall. Only the eukaryotic cell has a nucleus and organelles, the definition of which differs from the prokaryotic one.
Cells of multicellular organisms form tissues, which, in turn, are a component of organs and organ systems. They come in different sizes and may differ in form and function. These small structures can only be distinguished with a microscope.
What is a cell in biology. Prokaryotic cell definition
Microorganisms such as bacteria are a prime example of prokaryotic organisms. This type of cell is simple in structure, since bacteria lack a nucleus and other cytoplasmic organelles. The hereditary information of microorganisms is contained in a specialized structure - the nucleoid, and the functions of organelles are performed by mesosomes, which are formed by protrusion of the cytoplasmic membrane into the cell.
What other features does a prokaryotic cell have? The definition states that the presence of cilia and flagella is also a characteristic feature of bacteria. This additional motor apparatus differs in different groups of microorganisms: someone has only one flagellum, someone has two or more. Infusoria have no flagella, but there are cilia along the entire periphery of the cell.
Inclusions play a big role in the life of bacteria, because prokaryotic cells do not have organelles that are able to accumulate the necessary substances. Inclusions are located in the cytoplasm and are compacted there. If necessary, bacteria can use these accumulated substances for their needs in order to maintain normal life.
Eukaryotic cell
Eukaryotic cells are evolutionarily more advanced than prokaryotic cells. They have all the typical organelles, as well as the nucleus, the center for storing and transmitting genetic information.
Defining the term "cell" exactlydescribes the structure of eukaryotes. Each cell is covered with a cytoplasmic membrane, which is represented by a bilipid layer and proteins. Above is the glycocalyx, which is formed by glycoproteins and performs a receptor function. Plant cells also have a cell wall isolated.
The cytoplasm of eukaryotes is represented by a colloidal solution containing organelles, cytoskeleton and various inclusions. Among the organelles, the endoplasmic reticulum (smooth and rough), the Golgi apparatus, lysosomes, peroxisomes, mitochondria, and plant plastids are distinguished. The cytoskeleton is represented by microtubules, microfilaments and intermediate microfilaments. These structures form a scaffold and are also involved in division. The center, which any animal cell has, plays a direct role in this process. Determination, finding the cytoskeleton and the cell center in its thickness is possible only with the use of a powerful modern microscope.
The nucleus is a two-membrane structure, the contents of which are represented by karyolymph. It contains the chromosomes containing the DNA of the entire cell. The nucleus is responsible for the transcription of the body's genes, and also controls the stages of division during mitosis, amitosis and meiosis.
Non-cellular life forms
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. Thus, viruses are the main representatives of non-cellular forms of life. Their organization is quite simple, since viruses are infectious agents,which in their composition contain only two organic components: DNA or RNA, as well as a protein coat.
Viruses are peculiar parasites of animal and plant cells. After entering the host cell, viruses insert their nucleic acid into the DNA of the nucleus, after which the synthesis of the genes of the virus itself begins. As a result, the host cell becomes a kind of factory for the production of new viral particles, which thus increase their numbers. After such manipulations, the eukaryotic cell most often dies.
Bacteria are also attacked by viruses that make up the bacteriophage group. Their body has the shape of a dodecahedron, and the "injection" of the nucleic acid into the bacterial cell occurs with the help of the tail process, represented by the contractile sheath, internal rod and basal plate.
