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, multiply, move, adapt and change. In addition, any cells are characterized by metabolism, specific structure, orderliness of structures and functions.
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, they consist of a shell and a nucleus with a nucleolus. They also include cell organelles, cytoplasm. To date, a variety of research methods are highly developed, but microscopy occupies a leading position, which allows you to study the structure of cells and explore its main structural elements.
What is an organoid?
Organoids (they are also called organelles) are permanent constituent elements of any cell thatmake it complete and perform certain functions. These are the structures that are vital to keeping it going.
Organoids include the nucleus, lysosomes, the endoplasmic reticulum and the Golgi complex, vacuoles and vesicles, mitochondria, ribosomes, and the cell center (centrosome). This also includes structures that form the cytoskeleton of the cell (microtubules and microfilaments), melanosomes. Separately, it is necessary to single out the organelles of movement. These are cilia, flagella, myofibrils and pseudopods.
All these structures are interconnected and ensure the 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 an organ of a multicellular organism.
Classification of organelles
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 principle of organization. At the same time, the question of what an organoid is and what structures it is is quite debatable. For example, lysosomes or vacuoles are sometimes not classified as cell organelles.
If we talk about the classification of these cell components, then non-membrane and membrane organelles are distinguished. Non-membrane - this is the cell center and ribosomes. The organelles of movement (microtubules and microfilaments) also lack membranes.
The structure of membrane organelles is based on the presence of a biological membrane. Single-membrane and double-membrane organelles have a shell with a single structure, which consists ofdouble layer of phospholipids and protein molecules. It separates the cytoplasm from the external environment, helps the cell to maintain its shape. It is worth remembering that in addition to the membrane, in plant cells there is also an outer 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 organelles of synthesis are mitochondria, in which ATP is formed. Ribosomes, plastids (chloroplasts) and the rough endoplasmic reticulum are responsible for the synthesis of proteins, the smooth ER is responsible for the synthesis of lipids and carbohydrates.
Let's consider the structure and functions of organelles in more detail.
Core
This organelle is extremely important because when it is removed, cells cease to function and die.
The nucleus 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 it is the nucleus that 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 products of vital activity of the structures of the nucleus. The densest zone is the nucleolus, which houses ribosomes, complex proteins andRNA, as well as potassium, magnesium, zinc, iron and calcium phosphates. The nucleolus disappears before cell division and is formed again in the last stages of this process.
Endoplasmic reticulum (reticulum)
EPS is a single-membrane organelle. It occupies half the volume of the cell and consists of tubules and cisterns that are connected to each other, as well as to 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). Ribosomes are located on the inner shell of the granular ER, in which protein synthesis takes place. There are no ribosomes on the surface of the smooth endoplasmic reticulum, but carbohydrate and fat synthesis takes place here.
All substances that are formed in the endoplasmic reticulum are transported through the system of tubules and tubules to their destinations, where they are accumulated and subsequently used in various biochemical processes.
Given the synthesizing ability of EPS, the rough reticulum is located in cells whose main function is the formation of proteins, and the smooth reticulum is located in cells that synthesize carbohydrates and fats. In addition, calcium ions accumulate in the smooth reticulum, which are necessary for the normal functioning of cells or the body as a whole.
It should also be noted that the ER is the site of the formation of the Golgi apparatus.
Lysosomes, their functions
Lysosomes are cellular organelles,which are represented by single-membrane round-shaped sacs with hydrolytic and digestive enzymes (proteases, lipases and nucleases). The content of lysosomes is characterized by an acidic environment. The membranes of these formations isolate them from the cytoplasm, preventing the destruction of other structural components of cells. When the enzymes of the lysosome are released into the cytoplasm, the cell self-destructs - autolysis.
It should be noted that enzymes are primarily synthesized on a rough endoplasmic reticulum, after which they move 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, while secondary (digestive vacuoles) are those that form as a result of the fusion of primary lysosomes and endocytic vacuoles.
Given this structure and organization, we can distinguish the main functions of lysosomes:
- digestion of various substances inside the cell;
- destruction of cellular structures that are not needed;
- participation in cell reorganization processes.
Vacuoles
Vacuoles are single-membrane organelles of a spherical shape, which are reservoirs of water and organic and inorganic compounds dissolved in it. The Golgi apparatus and EPS are involved in the formation of these structures.
In an animal cell vacuolesLittle. 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 a 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 the tonoplast, and its contents are called cell sap. The main functions of plant vacuoles are to ensure the tension of the cell membrane, the accumulation of various compounds and waste products of the cell. In addition, these plant cell organelles supply the water required for the process of photosynthesis.
If we talk about the composition of cell sap, then 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 colored in the corresponding color.
Golgi complex
The structure of organoids 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 bladders. The structural unit of the Golgi complex is the dictyosome, which is represented by a stack of 4-6 "tanks" 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 dictyosomes can vary from one to severalhundreds.
The Golgi complex is usually located near the nucleus. 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 the formation of lysosomes.
It should be noted that ER, lysosomes, vacuoles, and the Golgi apparatus together form a tubular-vacuolar system that divides the cell into separate sections with corresponding functions. In addition, this system ensures constant renewal of the membranes.
Mitochondria are the energy stations of the cell
Mitochondria are two-membrane organelles of rod-shaped, spherical or filamentous shape that synthesize ATP. They have a smooth outer surface and an inner membrane with numerous folds called cristae. It should be noted that the number of cristae in mitochondria may vary depending on the energy requirement of the cell. It is on the inner membrane that numerous enzyme complexes that synthesize adenosine triphosphate are concentrated. Here, the energy of chemical bonds is converted into macroergic bonds of ATP. In addition, mitochondria break down fatty acids and carbohydrates with the release of energy, which is accumulated and used for growth and synthesis.
The internal environment of these organelles is called the matrix. She iscontains circular DNA and RNA, small ribosomes. Interestingly, mitochondria are semi-autonomous organelles, since they depend on the functioning of the cell, but at the same time they can maintain a certain independence. So, they are able to synthesize their own proteins and enzymes, as well as reproduce on their own.
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 organelles
Plastids are fairly 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 are of three types:
- Chloroplasts are the most numerous plastids responsible for photosynthesis, which produces organic compounds and free oxygen. These structures have a complex structure and are able to move in the cytoplasm towards the light source. The main substance 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, as they are capable ofindependent division and synthesis of their own proteins.
- Leucoplasts are colorless plastids that turn into chloroplasts when exposed to light. 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 capable of accumulating 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 of plastid transformation. Most of them are in fruits, petals and autumn leaves.
Ribosome
What is an organelle 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 organelles of animal and plant cells, bacteria. These structures are formed in the nucleus, after which they pass into the cytoplasm, where they are placed freely or attached to the EPS. Depending on the synthesizing properties, ribosomes function alone or combine into complexes, forming polyribosomes. In this case, these non-membrane organelles are bound by a messenger RNA molecule.
The ribosome contains 4 rRNA molecules that make up its framework, as well as various proteins. The main task of this organoid is to assemble the polypeptide chain, which is the first step in 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 an individual cell are synthesized by ribosomes, which are located in the cytoplasm. It should be noted that ribosomes are also found in mitochondria and plastids.
Cytoskeleton of a cell
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 frame of the cage, which allows it to keep its shape;
- take part in the process of 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 consist of the protein actin or myosin. They can contract, resulting in 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. Cylindrical centriole. Its walls are formed by three microtubules, which merge with each other through cross-links. Centrioles are arranged in pairs at right angles to each other. It should be noted that the cells of higher plants lack these organelles.
The main role of the cell center is to ensure an even distribution of chromosomes during cell division. It is also the center of organization of the cytoskeleton.
Organelles of movement
The organelles of movement include cilia, as well as flagella. These are tiny growths in the form of hairs. The flagellum 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. Flagella that are only 10-20 microns in size are called cilia. When microtubules slide, cilia and flagella are able to oscillate, causing movement of the cell itself. The cytoplasm may contain contractile fibrils called myofibrils - these are organelles of an animal cell. Myofibrils, as a rule, are located in myocytes - muscle tissue cells, as well as in heart cells. They are made up of smaller fibers (protofibrils).
It should be noted that myofibril bundles consist of dark fibers - these are anisotropic disks, as well as light areas - these are isotropic disks. The structural unit of the myofibril is the sarcomere. This is the area between the anisotropic and isotropic disk, which has actin and myosin filaments. When they slide, the sarcomere contracts, which leads to the movement of the entire muscle fiber. Atthis uses the energy of ATP and calcium ions.
Protozoa and spermatozoa of animals move with the help of flagella. Cilia are the organ of movement of the ciliates-shoes. In animals and humans, they cover the airways and help to get rid of small solid particles, such as dust. In addition, there are also pseudopods that provide amoeboid movement and are elements of many unicellular and animal cells (for example, leukocytes).
Most plants cannot move in space. Their movements are growth, leaf movements and changes in the flow of the cytoplasm of cells.
Conclusion
Despite all the variety of cells, they all have a similar structure and organization. The structure and functions of organelles are characterized by identical properties, ensuring the normal functioning of both a single cell and the whole organism.
This pattern can be expressed as follows.
Table "Organoids of eukaryotic cells"
Organoid |
Plant cell |
Animal cage |
Main Functions |
| core | is | is | DNA storage, RNA transcription and protein synthesis |
| endoplasmic reticulum | is | is | synthesis of proteins, lipids and carbohydrates, accumulation of calcium ions, formation of the Golgi complex |
| mitochondria | is | is | synthesis of ATP, own enzymes and proteins |
| plastids | is | no | participation in photosynthesis, accumulation of starch, lipids, proteins, carotenoids |
| ribosomes | is | is | gathering the polypeptide chain (protein synthesis) |
| microtubules and microfilaments | is | 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 | is | is | digestion of substances inside the cell, destruction of its unnecessary structures, participation in cell reorganization, cause autolysis |
| large central vacuole | is | no | provides tension in the cell membrane, accumulates nutrients and waste products of the cell, phytoncides and phytohormones, as well as pigments, is a reservoir of water |
| Golgi complex | is | is | secretes and accumulates proteins, lipids and carbohydrates, modifies the nutrients that enter the cell,responsible for the formation of lysosomes |
| cell center | there is, except for higher plants | is | is the center of the organization of the cytoskeleton, ensures uniform divergence of chromosomes during cell division |
| myofibrils | no | is | ensure muscle contraction |
If we draw conclusions, we can say that there are minor differences between an animal and a plant cell. At the same time, the functional features and structure of organelles (the table above confirms this) has a general principle of organization. The cell functions as a harmonious and integral system. At the same time, the functions of organelles are interconnected and aimed at optimal operation and maintenance of the cell's vital activity.
