A cell is a level of organization of living matter,an independent biosystem that has the basic properties of all living things. So, it can develop, reproduce, move, adapt and change. In addition, any cells are characterized by a metabolism, a specific structure, the orderliness of structures and functions.
The science that deals with the study of cells iscytology. Its subject are the structural units of multicellular animals and plants, unicellular organisms - bacteria, protozoa and algae, consisting of only one cell.
If we talk about the overall organization of structuralunits of living organisms, they consist of a shell and a nucleus with a nucleolus. Also in their composition are cell organelles, cytoplasm. To date, a variety of research methods are highly developed, but the leading place is occupied by microscopy, which allows studying the structure of cells and investigating its basic structural elements.
What is an organelle?
Organoids (they are also called organelles) -Constant constituent elements of any cell that make it holistic and perform certain functions. These are the structures that are vital for the maintenance of its activities.
Organoids include the nucleus, lysosomes,endoplasmic reticulum and Golgi complex, vacuoles and vesicles, mitochondria, ribosomes, as well as the cell center (centrosome). This also includes the structures that form the cytoskeleton cells (microtubules and microfilaments), melanosomes. Separately it is necessary to allocate organoids of movement. These are cilia, flagella, myofibrils and pseudopods.
All these structures are interrelated and providecoordinated activity of cells. That is why the question: "What is an organoid?" - You can answer that it is a component that can be equated to the organ of a multicellular organism.
Classification of organelles
Cells differ in size and shape, and alsotheir functions, but they have a similar chemical structure and a single principle of organization. At the same time, the question of what an organoid is and what structures it is, is quite controversial. For example, lysosomes or vacuoles sometimes do not belong to cellular organelles.
If we talk about the classification of these componentscells, then non-membrane and membrane organelles are isolated. Nemembrannye - a cell center and ribosomes. Organoids of movement (microtubules and microfilaments) are also devoid of membranes.
Membrane organelles include EPS, lysosomes and mitochondria, as well as lysosomes and plastids. Their membranes can differ only in the set of proteins.
If we talk about the functional abilityorganelles, some of them are capable of synthesizing certain substances. So, the important organoids of synthesis are mitochondria, in which ATP is formed. Ribosomes, plastids (chloroplasts) and a rough endoplasmic reticulum are responsible for the synthesis of proteins, smooth EPS - for the synthesis of lipids and carbohydrates.
Consider the structure and functions of organoids in more detail.
This organelle is extremely important, because when it is removed, cells cease to function and die.
The liquid part of the nucleus is called karyoplasm. It contains the products of vital activity of the core structures. The most dense zone is the nucleolus, which contains ribosomes, complex proteins and RNA, as well as phosphates of potassium, magnesium, zinc, iron and calcium. The nucleolus disappears before cell division and is formed again at the last stages of this process.
Endoplasmic reticulum (reticulum)
EPS is a single-membrane organoid. It occupies half the volume of the cell and consists of tubules and cisterns that are connected together, as well as with the cytoplasmic membrane and the outer shell of the nucleus. The membrane of this organoid has the same structure as the plasmalome. This structure is holistic and does not open into the cytoplasm.
The endoplasmic reticulum is smooth andgranular (rough). On the inner shell of granular EPS are placed ribosomes, in which the synthesis of proteins. On the surface of the smooth endoplasmic reticulum there are no ribosomes, but here the synthesis of carbohydrates and fats is carried out.
Given the synthesizing ability of EPS,the rough reticulum is placed in cells whose main function is the formation of proteins, and the smooth function in cells that synthesize carbohydrates and fats. In addition, calcium ions are accumulated in the smooth reticulum, which are necessary for the normal functioning of cells or the organism as a whole.
It should also be noted that EPS is the location of the Golgi apparatus.
Lysosomes, their functions
Lysosomes are cellular organoids thatare represented by single-membrane pouches of round shape with hydrolytic and digestive enzymes (proteases, lipases and nuclease). The content of lysosomes is characterized by an acidic medium. Membranes of these formations isolate them from the cytoplasm, preventing the destruction of other structural components of the cells. When the enzymes of the lysosome are released into the cytoplasm, self-destruction of the cell occurs - autolysis.
It should be noted that the enzymes are primaryare synthesized on a rough endoplasmic reticulum, after which they are transferred to the Golgi apparatus. Here they undergo a modification, they are packed into membrane vesicles and begin to separate, becoming independent components of the cell - lysosomes, which are primary and secondary.
Given such a structure and organization, it is possible to single out the main functions of lysosomes:
- digestion of various substances inside the cell;
- destruction of cellular structures that are not needed;
- participation in the processes of cell reorganization.
Vacuoles are single-membrane organellesspherical forms, which are reservoirs of water and organic and inorganic compounds dissolved in it. The formation of these structures involved the Golgi apparatus and EPS.
There are few vacuoles in the animal cell. They are small and occupy no more than 5% of the volume. Their main role is to ensure the transport of substances throughout the cell.
Vacuoles of plant cells are large and occupy up to90% of the volume. In a mature cell, there is only one vacuole, which occupies a central position. Its membrane is called tonoplast, and the contents are called cellular juice. The main functions of plant vacuoles are to ensure the tension of the cell membrane, the accumulation of various compounds and waste cells. In addition, these plant cell organelles supply the water necessary for the process of photosynthesis.
If we talk about the composition of the cell sap, it includes the following substances:
- reserve - organic acids, carbohydrates and proteins, individual amino acids;
- compounds that are formed during the life of cells and accumulate in them (alkaloids, tannins and phenols);
- phytoncides and phytohormones;
- Pigments, due to which fruits, roots and flower petals are painted in the appropriate color.
The structure of organoids called "apparatusGolgi "is quite simple. In plant cells they look like separate bodies with a membrane, in the cells of animals they are represented by cisterns, tubules and blisters. The structural unit of the Golgi complex is a diktiosome, which is represented by a stack of 4-6 "cisterns" and small vesicles, which are separated from them and are an intracellular transport system, and can also serve as a source of lysosomes. The number of diktiosomes can range from one to several hundred.
- secretion and accumulation of proteins, lipids and saccharides;
- modification of organic compounds entering the Golgi complex;
- This organoid is the site of lysosome formation.
It should be noted that EPS, lysosomes, vacuoles, andAlso, the Golgi apparatus together form a tubular-vacuolar system, which divides the cell into separate parts with corresponding functions. In addition, this system provides a permanent update of the membranes.
Mitochondria - energy cell stations
Mitochondria are biomembrane organoidsrodlike, globular or filiform, which synthesize ATP. They have an outer smooth surface and an internal membrane with numerous folds, which are called cristae. It should be noted that the number of cristae in the mitochondria can vary depending on the cell's need for energy. It is on the inner membrane that numerous enzyme complexes synthesizing adenosine triphosphate are concentrated. Here the energy of the chemical bonds is converted into the macroergic bonds of ATP. In addition, in the mitochondria splits fatty acids and carbohydrates with the release of energy, which accumulates and is used for growth and synthesis processes.
It is believed that the mitochondria originated in the case ofin the host cell of aerobic prokaryotic organisms, which led to the formation of a specific symbiotic complex. Thus, mitochondrial DNA has the same structure as the DNA of modern bacteria, and the synthesis of proteins in both mitochondria and bacteria is inhibited by identical antibiotics.
Plastids are plant cell organoids
Plastids are quite large organelles. They are present only in plant cells and are formed from precursors - proplastids, contain DNA. These organelles play an important role in metabolism and are separated from the cytoplasm by a double membrane. In addition, they can form an ordered system of internal membranes.
Plastids come in three types:
- Chloroplasts are the most numerous plastids,responsible for photosynthesis, in which organic compounds and free oxygen are formed. These structures have a complex structure and are capable of moving in the cytoplasm towards the light source. The main substance that is contained in chloroplasts is chlorophyll, through which plants can use the energy of the sun. It should be noted that chloroplasts like mitochondria are semi-autonomous structures, since they are capable of independent division and synthesis of their own proteins.
- Leukoplast - colorless plastids, which underlight turns into chloroplasts. These cellular components contain enzymes. With the help of them, glucose is converted and accumulated in the form of starch grains. In some plants, these plastids are able to accumulate lipids or proteins in the form of crystals and amorphous bodies. The greatest number of leukoplasts is concentrated in the cells of the underground organs of plants.
- Chromoplasts are derivatives of the other two typesplastids. They form carotenoids (with the destruction of chlorophyll), which have a red, yellow or orange color. Chromoplasts are the final stage of plastid transformation. Most of them in fruits, petals and autumn leaves.
What is an organoid called a ribosome? Ribosomes are called non-membrane organelles, consisting of two fragments (small and large subunits). Their diameter is about 20 nm. They are found in all types of cells. They are organelles of animals and plant cells, bacteria. These structures form in the nucleus, after which they pass into the cytoplasm, where they are placed freely or attached to EPS. Depending on the synthesizing properties, the ribosomes function alone or combine into complexes, forming polyribosomes. In this case, these non-membrane organelles are bound by an information RNA molecule.
The ribosome contains 4 molecules of r-RNA thatmake up its frame, as well as various proteins. The main task of this organoid is the collection of a polypeptide chain, which is the first stage in the synthesis of proteins. Those proteins that are formed by the ribosomes of the endoplasmic reticulum can be used by the whole organism. Proteins for the needs of a single cell are synthesized by ribosomes, which are located in the cytoplasm. It should be noted that ribosomes are also found in mitochondria and plastids.
The cellular cytoskeleton is formed by microtubules andmicrofilaments. Microtubules are cylindrical formations with a diameter of 24 nm. Their length is 100 μm-1 mm. The main component is a protein called tubulin. It is incapable of contraction and can be destroyed by colchicine. Microtubules are located in the hyaloplasma and perform the following functions:
- create an elastic, but at the same time, a strong cell frame that allows it to retain its shape;
- participate in the process of chromosome distribution of the cell;
- provide the movement of organelles;
- are contained in the cellular center, as well as in flagella and cilia.
Microfilaments - threads that are placed underplasma membrane and consist of an actin or myosin protein. They can contract, resulting in the movement of the cytoplasm or protrusion of the cell membrane. In addition, these components are involved in the formation of a constriction in cell division.
The cell center (centrosome)
This organelle consists of 2 centrioles andcentrospheres. The centriole is cylindrical. Its walls are formed by three microtubules, which merge with each other through cross-linking. Centrioles are arranged in pairs at right angles to each other. It should be noted that cells of higher plants are deprived of these organoids.
The main role of the cell center is to ensure an even distribution of chromosomes during cell division. It is also the center of the organization of the cytoskeleton.
Organelles of movement
To organoids of movement carry cilia, and alsoflagella. These are miniature outgrowths in the form of hairs. The flagella contains 20 microtubules. Its base is located in the cytoplasm and is called the basal body. The length of the flagellum is 100 μm or more. Flagellites, which have only 10-20 microns, are called cilia. When slipping microtubules, cilia and flagella are able to oscillate, causing the movement of the cell itself. In the cytoplasm, contractile fibrils, called myofibrils, can be contained - they are organelles of the animal cell. Myofibrils, as a rule, are located in myocytes - cells of the muscle tissue, as well as in the heart cells. They consist of smaller fibers (protofibrils).
With the help of flagella, the simplest andspermatozoa of animals. Cilia are the organ of movement of the infusoria-shoe. In animals and humans, they cover the airway respiratory tract and help to get rid of small solid particles, for example, from dust. In addition, there are still pseudopods that provide amoeboid movement and are the elements of many unicellular and animal cells (eg, leukocytes).
Most plants can not move in space. Their movements consist in growth, movement of leaves and changes in the cytoplasm of cells.
Despite the diversity of cells, all of themhave a similar structure and organization. The structure and functions of organoids are characterized by identical properties, ensuring the normal functioning of both the individual cell and the whole organism.
This regularity can be expressed as follows.
Table "Organoids of the eukaryote cell"
DNA storage, RNA transcription and protein synthesis
synthesis of proteins, lipids and carbohydrates, the accumulation of calcium ions, the formation of the Golgi complex
synthesis of ATP, native enzymes and proteins
participation in photosynthesis, accumulation of starch, lipids, proteins, carotenoids
collection of the polypeptide chain (protein synthesis)
microtubules and microfilaments
allow the cell to retain a certain shape, are an integral part of the cell center, cilia and flagella, provide movement of organelles
the digestion of substances within the cell, the destruction of its unnecessary structures, participation in the reorganization of cells, cause autolysis
large central vacuole
provides the tension of the cell membrane, accumulates nutrients and cell life products, phytoncides and phytohormones, as well as pigments, is a reservoir of water
secrete and accumulate proteins, lipids and carbohydrates, modify the nutrients that enter the cell, is responsible for the formation of lysosomes
there are, except for higher plants
is the center of the organization of the cytoskeleton, ensures a uniform divergence of chromosomes in cell division
provide a reduction in muscle tissue
If we draw conclusions, then we can say thatthere are insignificant differences between the animal and plant cells. In this case, the functional features and structure of organoids (the table above, confirms this) has a general principle of organization. The cell functions as a coherent and integrated system. In this case, the functions of organoids are interrelated and are aimed at optimal work and maintenance of cell vital activity.