A cell is a level of organization of living matter, an independent biosystem that possesses the basic properties of all living things. So, it can develop, multiply, move, adapt and change. In addition, metabolism, a specific structure, and orderliness of structures and functions are inherent in any cells.
The science that studies cells is cytology. Its subject is the structural units of multicellular animals and plants, unicellular organisms - bacteria, protozoa and algae, consisting of only one cell.
If we talk about the general organization of the structural units of living organisms, then they consist of a shell and a nucleus with a nucleolus. Also in their composition are organelles of the cell, the cytoplasm. To date, a variety of research methods are highly developed, but the leading place is occupied by microscopy, which allows us to study the structure of cells and to study its basic structural elements.
What is an organoid?
Organoids (they are also called organelles) are the constant constituent elements of any cell, which make it integral and perform certain functions. These are the structures that are vital to maintaining its activities.
Organoids include the nucleus, lysosomes, the endoplasmic reticulum and the Golgi complex, vacuoles and vesicles, mitochondria, ribosomes, as well as the cell center (centrosome). This also includes the structures that form the cytoskeleton of the cell (microtubules and microfilaments), melanosomes. Separately, it is necessary to highlight the organelles of movement. These are cilia, flagella, myofibrils and pseudopods.
All these structures are interconnected and provide a coordinated activity of cells. That is why the question: "What is an organoid?" - you can answer that this is a component that can be equated to the organ of a multicellular organism.
Organoid classification
Cells differ in size and shape, as well as their functions, but at the same time they have a similar chemical structure and a single organization principle. At the same time, the question of what is an organoid and what kind of structure it is is quite debatable. So, for example, lysosomes or vacuoles are sometimes not classified as cellular organelles.
If we talk about the classification of these components of cells, then non-membrane and membrane organoids are distinguished. Non-membrane - this is the cell center and ribosomes. Motion organoids (microtubules and microfilaments) are also devoid of membranes.
The basis of the structure of membrane organelles is the presence of a biological membrane. Single and double membrane organoids have a shell with a single structure, which consists of a double layer of phospholipids and protein molecules. It separates the cytoplasm from the external environment, helps the cell maintain its shape. It is worth recalling that in
plant cells, in addition to the membrane, there is also an external cellulose membrane, which is called the cell wall. It performs a supporting function.
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 ability of organelles, then some of them are able to synthesize certain substances. So, important organoids of synthesis are mitochondria, in which ATP is formed. Ribosomes, plastids (chloroplasts) and rough endoplasmic reticulum are responsible for protein synthesis, smooth EPS - for the synthesis of lipids and carbohydrates.
Consider the structure and functions of organelles in more detail.
Nucleus
This organelle is extremely important, because when it is removed, the cells cease to function and die.
The core has a double membrane in which there are many pores. With the help of them, it is closely associated with the endoplasmic reticulum and cytoplasm. This organoid contains chromatin - chromosomes, which are a complex of proteins and DNA. Given this, we can say that the nucleus is the organelle that is responsible for maintaining the bulk of the genome.
The liquid part of the nucleus is called karyoplasm. It contains the vital products of the structures of the nucleus. The most dense zone is the nucleolus, which houses ribosomes, complex proteins and RNA, as well as phosphates of potassium, magnesium, zinc, iron and calcium. The nucleolus disappears before cell division and forms again in the last stages of this process.
Endoplasmic reticulum (reticulum)
EPS is a single-membrane organoid. It occupies half the cell volume and consists of tubules and cisterns, which are interconnected, 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 plasmalemma. This structure is integral and does not open into the cytoplasm.
The endoplasmic reticulum is smooth and granular (rough). On the inner shell of the granular EPS, ribosomes are located in which protein synthesis takes place. There are no ribosomes on the surface of the smooth endoplasmic reticulum, but here the synthesis of carbohydrates and fats takes place.
All substances that form in the endoplasmic reticulum are transferred through the system of tubules and tubules to their destinations, where they are accumulated and subsequently used in various biochemical processes.
Considering the synthesizing ability of EPS, the rough reticulum is located in cells whose main function is the formation of proteins, and smooth - in cells synthesizing carbohydrates and fats. In addition, calcium ions are accumulated in a smooth reticulum, which are necessary for the normal functioning of cells or the body as a whole.
It should also be noted that EPS is the site of formation of the Golgi apparatus.
Lysosomes, their functions
Lysosomes are cellular organelles that are represented by round-shaped single-membrane sacs with hydrolytic and digestive enzymes (proteases, lipases, and nucleases). The content of lysosomes is characterized by an acidic environment. Membranes of these formations isolate them from the cytoplasm, preventing the destruction of other structural components of cells. With the release of lysosome enzymes in the cytoplasm, cell self-destruction occurs - autolysis.
It should be noted that enzymes are primarily synthesized on a rough endoplasmic reticulum, after which they are transferred to the Golgi apparatus. Here they undergo modification, are packed into membrane vesicles and begin to separate, becoming independent components of the cell - lysosomes, which are primary and secondary.
Primary lysosomes are structures that separate from the Golgi apparatus, and secondary (digestive vacuoles) are those that are formed due to the fusion of primary lysosomes and endocytotic vacuoles.
Given this structure and organization, we can distinguish the main functions of lysosomes:
- digestion of various substances inside the cell;
- destruction of cell structures that are not needed;
- participation in cell reorganization processes.
Vacuoli
Vacuoles are spherical single-membrane organelles that are reservoirs of water and dissolved in it organic and inorganic compounds. The Golgi apparatus and EPS are involved in the formation of these structures.
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.
The vacuoles of the plant cell are large and occupy up to 90% of the volume. In a mature cell, there is only one vacuole, which occupies a central position. Its membrane is called tonoplast, and its contents are called cell 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 organoids supply the water needed for the photosynthesis process.
If we talk about the composition of cell juice, then it includes the following substances:
- reserve - organic acids, carbohydrates and proteins, individual amino acids;
- compounds that are formed during the life of the cells and accumulate in them (alkaloids, tannins and phenols);
- volatile and phytohormones;
- pigments, due to which the fruits, root crops and flower petals are painted in the corresponding color.
Golgi complex
The structure of the organelles called the “Golgi apparatus” is quite simple. In plant cells, they look like separate bodies with a membrane; in animal cells, they are represented by cisterns, tubules, and blisters. The structural unit of the Golgi complex is a dictiosome, which is represented by a stack of 4-6 "cisterns" and small vesicles that separate from them and are an intracellular transport system, and can also serve as a source of lysosomes. The number of dictiosomes can range from one to several hundred.
The Golgi complex is usually located near the core. In animal cells, near the cell center. The main functions of these organelles are as follows:
- 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, and also the Golgi apparatus together form a tubular-vacuolar system that divides the cell into separate sections with corresponding functions. In addition, this system provides continuous updating of membranes.
Mitochondria - cell energy stations
Mitochondria are double-membered organelles of rod-shaped, spherical or filiform, which synthesize ATP. They have an outer smooth surface and an inner membrane with numerous folds called cristae. It should be noted that the number of cristae in mitochondria can vary depending on the energy requirement of the cell. It is on the inner membrane that numerous enzyme complexes synthesizing adenosine triphosphate are concentrated. Here, the energy of chemical bonds is transformed into macroergic bonds of ATP. In addition, in the mitochondria, fatty acids and carbohydrates are broken down with the release of energy, which is accumulated and used on the processes of growth and synthesis.
The internal environment of these organelles is called a matrix. It contains circular DNA and RNA, small ribosomes. It is interesting that mitochondria are semi-autonomous organoids, since they depend on the functioning of the cell, but at the same time they can retain a certain independence. So, they are able to synthesize their own proteins and enzymes, as well as reproduce independently.
It is believed that mitochondria arose when aerobic prokaryotic organisms entered the host cell, which led to the formation of a specific symbiotic complex. So, mitochondrial DNA has the same structure as the DNA of modern bacteria, and protein synthesis in both mitochondria and bacteria is inhibited by the same antibiotics.
Plastids - plant cell organoids
Plastids are quite large organelles. They are present only in plant cells and are formed from progenitors - proplastids, contain DNA. These organelles play an important role in metabolism and are separated from the cytoplasm by a double membrane. In addition, an ordered system of internal membranes can form in them.
Plastids are of 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 able to move in the cytoplasm towards the light source. The main substance that is contained in chloroplasts is chlorophyll, with 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.
- Leukoplasts are colorless plastids, which under the influence of light turn into chloroplasts. These cellular components contain enzymes. Using 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 largest number of leukoplasts is concentrated in the cells of the underground organs of plants.
- Chromoplasts are derivatives of the other two types of plastids. They form carotenoids (during the destruction of chlorophyll), which are red, yellow or orange. Chromoplasts are the final stage in the transformation of plastids. Most of them are in fruits, petals and autumn leaves.
Ribosomes
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 cells of all types. These are organoids of animal and plant cells, bacteria. These structures form in the nucleus, after which they transfer to the cytoplasm, where they are freely placed or attached to the EPS. Depending on the synthesizing properties, ribosomes function alone or combine into complexes to form polyribosomes. In this case, these non-membrane organelles are bound by a molecule of messenger RNA.
The ribosome contains 4 r-RNA molecules that make up its framework, as well as various proteins. The main objective of this organoid is the collection of the polypeptide chain, which is the first stage of protein synthesis. 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.
Cell cytoskeleton
The cell cytoskeleton is formed by microtubules and microfilaments. 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 hyaloplasm and perform the following functions:
- create an elastic, but at the same time strong cell frame, which allows it to maintain its shape;
- take part in the distribution of cell chromosomes;
- provide movement of organelles;
- contained in the cell center, as well as in flagella and cilia.
Microfilaments are filaments that are located under the plasma membrane and are composed of actin protein or myosin. They can contract, resulting in a movement of the cytoplasm or protrusion of the cell membrane. In addition, these components are involved in the formation of constriction during cell division.
Cell Center (Centrosome)
This organelle consists of 2 centrioles and a centrosphere. Centriole is cylindrical in shape. Its walls are formed by three microtubules, which merge together through cross-linking. Centrioles are arranged in pairs at right angles to each other. It should be noted that the cells of higher plants lack these organoids.
The main role of the cell center is to ensure uniform distribution of chromosomes during cell division. It is also the center for organizing the cytoskeleton.
Organelles of movement
Organoids of movement include cilia, as well as flagella. These are miniature outgrowths in the form of hairs. Flagellum contains 20 microtubules. Its base is located in the cytoplasm and is called the basal body. The flagellum is 100 microns or more in length. Flagella, which have only 10-20 microns, are called cilia. When microtubules slip, the cilia and flagella are able to oscillate, causing the movement of the cell itself. The cytoplasm may contain contractile fibrils, which are called myofibrils - these are organoids of an animal cell. Myofibrils, as a rule, are located in myocytes - cells of muscle tissue, as well as in heart cells. They consist of smaller fibers (protofibrils).
It should be noted that the bundles of myofibrils consist of dark fibers - these are anisotropic disks, as well as light areas - these are isotropic disks. The structural unit of myofibrils is the sarcomere. This is the area between the anisotropic and isotropic disc, which has actin and myosin filaments. When they slip, the sarcomere contracts, which leads to the movement of the entire muscle fiber. It uses ATP energy and calcium ions.
With the help of flagella, protozoa and sperm of animals move. Cilia are the organ of movement of the ciliates.In animals and humans, they cover the airways and help get rid of small solid particles, such as dust. In addition, there are pseudopods that provide amoeboid movement and are elements of many unicellular and animal cells (for example, white blood cells).
Most plants cannot move in space. Their movements consist in growth, leaf movements and changes in the flow of cell cytoplasm.
Conclusion
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| there is | there is | allow the cell to maintain a certain shape, are an integral part of the cell center, cilia and flagella, provide movement of organelles |
lysosomes | there is | there is | digestion of substances inside the cell, destruction of its unnecessary structures, participation in the reorganization of cells, cause autolysis |
large central vacuole | there is | no | provides tension of the cell membrane, accumulates nutrients and waste products of the cell, volatile and phytohormones, as well as pigments, is a reservoir of water |
Golgi complex | there is | there is | secrets and accumulates proteins, lipids and carbohydrates, modifies the nutrients that enter the cell, is responsible for the formation of lysosomes |
cell center | there are, in addition to higher plants | there is | is the center of the organization of the cytoskeleton, provides uniform divergence of chromosomes during cell division |
myofibrils | no | there is | provide muscle contraction |
If we draw conclusions, then we can say that there are slight differences between the animal and plant cells. At the same time, the functional features and structure of the organoids (the table indicated above confirms this) has a general organization principle. The cell functions as a coherent and holistic system. At the same time, the functions of organelles are interconnected and aimed at optimal operation and maintenance of cell activity.