The structural unit of any organism is a cell. The definition of this structure was first used by Robert Hook 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 non-cellular organisms are viruses.
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. The integument can be represented by a cytoplasmic membrane and cell wall. Only a eukaryotic cell has a nucleus and organelles, the definition of which is different from prokaryotic.
Cells of multicellular organisms form tissues, which, in turn, are a component of organs and organ systems. They come in different sizes and may vary in shape and function. You can distinguish these small structures only with a microscope.
What is a cell in biology. Determination of prokaryotic cell
Microorganisms such as bacteria are a prime example of prokaryotic organisms. This type of cell is simple in structure, because bacteria lack a nucleus and other cytoplasmic organelles. The hereditary information of microorganisms is enclosed in a specialized structure - the nucleoid, and the functions of the organelles are performed by the mesosomes, which are formed by invading 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 is different in different groups of microorganisms: someone has only one flagellum, someone has two or more. Ciliates do not have flagella, but cilia are present along the entire periphery of the cell.
Inclusions play a large role in the life of bacteria, because prokaryotic cells do not have organelles that can 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 core - the center for storing and transmitting genetic information.
The definition of "cell" accurately describes the structure of eukaryotes. Each cell is covered with a cytoplasmic membrane, which is represented by a bilipid layer and proteins. On top is the glycocalyx, which is formed by glycoproteins and performs a receptor function. In plant cells, the cell wall is also secreted.
The eukaryotic cytoplasm is represented by a colloidal solution in which there are organelles, the cytoskeleton and various inclusions. Among the organoids, the endoplasmic reticulum (smooth and rough), the Golgi apparatus, lysosomes, peroxisomes, mitochondria, as well as plant plastids are distinguished. The cytoskeleton is represented by microtubules, microfilaments and intermediate microfilaments. These structures form the framework, and also participate in the division. A direct role in this process is played by the center that any animal cell has. Determination, finding the cytoskeleton and the cell center in its thickness is possible only using a powerful modern microscope.
The nucleus is a two-membrane structure, the contents of which are represented by karyolymph. It contains 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. 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 the protein coat.
Viruses are a kind of parasite of animal and plant cells. After penetration into the host cell, viruses embed their nucleic acid in the DNA of the nucleus, after which the synthesis of 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 also suffer from the attack of viruses that make up the group of bacteriophages. Their body has the form of a dodecahedron, and the “injection” of nucleic acid into the bacterial cell occurs with the help of the tail process, represented by the contractile sheath, inner shaft and basal lamina.