The endoplasmic reticulum is a set of membrane channels and cavities that penetrate the entire cell. On the granular ER, protein synthesis occurs (granules are ribosomes), on the smooth ER, the synthesis of lipids and carbohydrates occurs. Inside the ER channels, synthesized substances accumulate and are transported throughout the cell.
The Golgi apparatus is a stack of flat membrane cavities surrounded by vesicles. Through the EPS channels, substances enter the AG, where they accumulate and are chemically modified. The finished substances are then placed in vials and sent to their destination.
Lysosomes are vesicles filled with digestive enzymes. Formed in the Golgi apparatus. After the fusion of the lysosome with the phagocytic vesicle, a digestive vacuole is formed. In addition to food, lysosomes can digest unnecessary parts of the cell or entire cells.
Ribosomes are the smallest organelles of the cell; they consist of two subunits; their chemical composition is rRNA and proteins; they are formed in the nucleolus. Function - protein synthesis.
The cell center consists of two centrioles, which form the spindle during cell division. During interphase, centrioles are the center of organization of microtubules that form the cytoskeleton.
Tests
1. Unified protein biosynthesis apparatus
A) endoplasmic reticulum and ribosomes
B) mitochondria and cell center
B) chloroplasts and Golgi complex
D) lysosomes and plasma membrane
2. In ribosomes located on the granular membranes of the endoplasmic reticulum,
A) photosynthesis
B) chemosynthesis
B) ATP synthesis
D) protein biosynthesis
3. During the process of mitosis, the cell center is responsible for
A) formation of a fission spindle
B) chromosome spiralization
B) protein biosynthesis
D) movement of cytoplasm
4. A process takes place in the ribosomes of an animal cell
A) protein biosynthesis
B) synthesis of carbohydrates
B) photosynthesis
D) ATP synthesis
5. What function does the cell center perform in a cell?
A) takes part in cell division
B) is the keeper of hereditary information
B) is responsible for protein biosynthesis
D) is the center of template synthesis of ribosomal RNA
6. In ribosomes, unlike the Golgi complex, there is
A) oxidation of carbohydrates
B) synthesis of protein molecules
B) synthesis of lipids and carbohydrates
D) oxidation of nucleic acids
7. What function does the cell center perform in a cell?
A) forms the large and small subunits of ribosomes
B) forms spindle filaments
C) synthesizes hydrolytic enzymes
D) accumulates ATP in interphase
8. The centriole, as a cell organelle, is
A) primary chromosome constriction
B) structural unit of the Golgi apparatus
B) structural unit of the cell center
D) small ribosomal subunit
9. A system of flat tanks with tubes extending from them ending in bubbles is
A) core
B) mitochondria
B) cell center
D) Golgi complex
10. In the Golgi complex, unlike chloroplasts, there is
A) transport of substances
B) oxidation of organic substances to inorganic ones
B) accumulation of substances synthesized in the cell
D) synthesis of protein molecules
11. The similarity between the functions of lysosomes and mitochondria lies in what happens in them
A) synthesis of enzymes
B) synthesis of organic substances
B) reduction of carbon dioxide to carbohydrates
D) breakdown of organic substances
12. Organic substances in the cell move to organelles via
A) vacuole system
B) lysosomes
B) endoplasmic reticulum
D) mitochondria
13. The similarity between the endoplasmic reticulum and the Golgi complex is that in their cavities and tubules
A) protein molecules are synthesized
B) substances synthesized by the cell accumulate
C) substances synthesized by the cell are oxidized
D) the preparatory stage of energy metabolism is carried out
14. Lysosomes in the cell are formed in
A) endoplasmic reticulum
B) mitochondria
B) cell center
D) Golgi complex
15. The Golgi complex is NOT involved in
A) formation of lysosomes
B) ATP formation
B) accumulation of secrets
D) transport of substances
16. Lysosome enzymes are formed in
A) Golgi complex
B) endoplasmic reticulum
B) plastids
D) mitochondria
17. In animal cells, polysaccharides are synthesized in
A) ribosomes
B) lysosomes
B) endoplasmic reticulum
D) core
18. Macromolecules of organic substances in the cell are broken down into monomers in
A) endoplasmic reticulum
B) lysosomes
B) chloroplasts
D) mitochondria
19. Membrane system of tubules that permeates the entire cell
A) chloroplasts
B) lysosomes
B) mitochondria
D) endoplasmic reticulum
20. The Golgi complex in a cell can be recognized by the presence of
A) cavities and tanks with bubbles at the ends
B) branched system of tubules
B) cristae on the inner membrane
D) two membranes surrounding many grains
21. What function do lysosomes perform in a cell?
A) break down biopolymers into monomers
B) oxidize glucose to carbon dioxide and water
C) carry out the synthesis of organic substances
D) carry out the synthesis of polysaccharides from glucose
22. The endoplasmic reticulum can be recognized in a cell by
A) a system of interconnected cavities with bubbles at the ends
B) the set of grains located in it
B) a system of interconnected branched tubules
D) numerous cristae on the inner membrane
23. The movement of substances in the cell is carried out with the participation
A) endoplasmic reticulum
B) lysosomes
B) mitochondria
D) chloroplasts
24. Substances synthesized in the cell accumulate and then are excreted
A) core
B) mitochondria
B) ribosomes
D) Golgi complex
25. What organelles are involved in the packaging and removal of substances synthesized in the cell?
A) vacuoles
B) Golgi apparatus
B) lysosomes
D) endoplasmic reticulum
26) In which organelle does the accumulation, packaging and removal of digestive enzymes from the cell take place?
A) in the cell center
B) in the ribosome
B) in the endoplasmic reticulum
D) in the Golgi complex
27. In which organelle do proteins, fats and carbohydrates synthesized in the cell accumulate?
A) lysosome
B) Golgi complex
B) ribosome
D) mitochondria
28. Lysosome is
A) a system of interconnected tubules and cavities
B) an organelle delimited from the cytoplasm by one membrane
B) two centrioles located in the dense cytoplasm
D) two interconnected subunits
Ribosome (from “RNA” and soma - body) is a cellular non-membrane organelle that carries out translation (reading the mRNA code and synthesizing polypeptides).
Eukaryotic ribosomes are located on the membranes of the endoplasmic reticulum (granular ER) and in the cytoplasm. Ribosomes attached to membranes synthesize protein “for export,” and free ribosomes synthesize protein for the needs of the cell itself. There are 2 main types of ribosomes - prokaryotic and eukaryotic. Mitochondria and chloroplasts also contain ribosomes, which are similar to the ribosomes of prokaryotes.
The ribosome consists of two subunits - large and small. In prokaryotic cells they are designated 50S and 30S subunits, in eukaryotic cells - 60S and 40S. (S is a coefficient that characterizes the sedimentation rate of the subunit during ultracentrifugation). Subunits of eukaryotic ribosomes are formed by self-assembly in the nucleolus and enter the cytoplasm through the pores of the nucleus.
Ribosomes in eukaryotic cells consist of four strands of RNA (three rRNA molecules in the large subunit and one rRNA molecule in the small one) and approximately 80 different proteins, i.e. they represent a complex complex of molecules held together by weak, non-covalent bonds. (Ribosomes in prokaryotic cells consist of three strands of RNA; two strands of rRNA are in the large subunit and one rRNA is in the small subunit). The process of translation (protein biosynthesis) begins with the assembly of an active ribosome. This process is called translation initiation. Assembly occurs in a strictly ordered manner, which is ensured by the functional centers of ribosomes. All centers are located on the contacting surfaces of both ribosomal subunits. Each ribosome works like a large biochemical machine, or more precisely, like a superenzyme, which, firstly, correctly orients the participants (mRNA and tRNA) of the process relative to each other, and secondly, catalyzes reactions between amino acids.
Active sites of ribosomes:
1) mRNA binding center (M-center);
2) peptidyl center (P-center). The initiating tRNA binds to this center at the beginning of the translation process; at subsequent stages of translation, tRNA moves from the A-center to the P-center, holding the synthesized part of the peptide chain;
3) amino acid center (A-center) – the site of binding of the mRNA codon with the anticodon of the tRNA carrying the next amino acid.
4) peptidyl transferase center (PTP center): it catalyzes the binding reaction of amino acids. In this case, another peptide bond is formed, and the growing peptide is lengthened by one amino acid.
Scheme of protein synthesis on ribosomes of the granular endoplasmic reticulum.
(Figure from the book Cell Biology, Vol.II)
Schematic representation of a polyribosome. Protein synthesis begins with the binding of a small subunit at the location AUG-codon in a messenger RNA molecule (figure from the book Cell Biology, Vol.II).
Endoplasmic reticulum
Endoplasmic reticulum (syn. endoplasmic reticulum) – organelle of a eukaryotic cell. In cells of different types and under different functional states, this component of the cell may look different, but in all cases it is a labyrinthine extended closed membrane structure, built from communicating tube-like cavities and sacs called cisterns. Outside the membranes of the endoplasmic reticulum there is cytosol (hyaloplasm, the main substance of the cytoplasm), and the lumen of the endoplasmic reticulum is a closed space (compartment) communicating through vesicles (transport vesicles) with the Golgi complex and the environment external to the cell. The endoplasmic reticulum is divided into two functionally distinct structures: the granular (rough) endoplasmic reticulum and the smooth (agranular) endoplasmic reticulum.
The granular endoplasmic reticulum, in protein-secreting cells, is represented by a system of numerous flat membrane cisterns with ribosomes on the outer surface. The membrane complex of the granular endoplasmic reticulum is associated with the outer membrane of the nuclear envelope and the perinuclear (perinuclear) cistern.
In the granular endoplasmic reticulum, the synthesis of proteins and lipids for all cell membranes occurs, lysosome enzymes are synthesized, and the synthesis of secreted proteins is also carried out, i.e. intended for exocytosis. (The remaining proteins are synthesized in the cytoplasm on ribosomes not associated with ES membranes.) In the lumen of the granular ES, the protein is surrounded by a membrane, and the resulting vesicles are separated (budding off) from the ribosome-free regions of the ES, which deliver the contents to another organelle - the Golgi complex - by fusion with its membrane.
That part of the ES, on the membranes of which there are no ribosomes, is called the smooth endoplasmic reticulum. The smooth endoplasmic reticulum does not contain flattened cisterns, but is a system of anastomosing membrane channels
ovs, bubbles and tubes. The smooth network is a continuation of the granular network, but does not contain ribophorins - glycoprotein receptors with which the large subunit of ribosomes connects and is therefore not associated with ribosomes.
The functions of the smooth endoplasmic reticulum are diverse and depend on the cell type. The smooth endoplasmic reticulum is involved in the metabolism of steroids, such as sex hormones. Controlled calcium channels and energy-dependent calcium pumps are localized in its membranes. The cisterns of the smooth endoplasmic reticulum are specialized for the accumulation of Ca 2+ in them by constant pumping of Ca 2+ from the cytosol. Similar Ca 2+ depots exist in skeletal and cardiac muscles, neurons, eggs, endocrine cells, etc. Various signals (for example, hormones, neurotransmitters, growth factors) influence cell activity by changing the concentration of the intracellular messenger - Ca 2+. In the smooth endoplasmic reticulum of liver cells, the neutralization of harmful substances (for example, acetaldehyde formed from alcohol), the metabolic transformation of drugs, the formation of most of the cell's lipids and their accumulation occur, for example, in fatty degeneration. The ES cavity contains many different component molecules. Among them, chaperone proteins are of great importance.
Chaperones(English letters - an elderly lady accompanying a young girl at balls) - a family of specialized intracellular proteins that ensure rapid and correct folding of newly synthesized protein molecules. Binding with chaperones prevents aggregation with other proteins and thereby creates conditions for the formation of the secondary and tertiary structure of the growing peptide. Chaperones belong to three protein families, the so-called heat shock proteins ( hsp 60, hsp 70, hsp90). The synthesis of these proteins is activated under many stresses, in particular during heat shock (hence the nameh eart shocked protein is a heat shock protein, and the number indicates its molecular weight in kilodaltons). These chaperones prevent denaturation of proteins at high temperatures and other extreme factors. By binding to abnormal proteins, they restore their normal conformation and thereby increase the survival of the organism during a sharp deterioration in the physicochemical parameters of the environment.
Card No. 1
Task 1. Choose the correct answer.
- 1) microtubules
2) many chloroplasts
3) many mitochondria
4) systems of branched tubules - The similarity between the functions of chloroplasts and mitochondria lies in what happens in them
1) synthesis of ATP molecules
2) synthesis of carbohydrates
3) oxidation of organic substances
4) lipid synthesis - In ribosomes located on the granular membranes of the endoplasmic reticulum,is happening
1) photosynthesis
2) chemosynthesis
3) ATP synthesis
4) protein biosynthesis - All organelles and the cell nucleus are interconnected by
1) shells
2) plasma membrane
3) cytoplasm
4) vacuoles - The cytoplasm in the cell does NOT take part in
1) transport of substances
2) placement of organoids
3) DNA biosynthesis
4) communication between organelles - The Golgi complex is NOT involved in
1) formation of lysosomes
2) ATP formation
3) accumulation of secrets
4) transport of substances - In animal cells, polysaccharides are synthesized in
1) ribosomes
2) lysosomes
3) endoplasmic reticulum
4) core - What function does the cell center perform in a cell?
1) takes part in cell division
2) is the custodian of hereditary information
3) responsible for protein biosynthesis
4) is the center of template synthesis of ribosomal RNA - Specify the organelle in which selective transport of substances occurs
1) chloroplast
2) mitochondria
3) Golgi complex
4) plasma membrane - The term cell was introduced
1) M. Schleiden 2) R. Hooke 3) T. Schwann 4) R. Virchow
Task 2.
Look at the cells shown in the picture. Determine which letters represent prokaryotic and eukaryotic cells. Provide evidence for your point of view.
Card No. 2
Task 1. Choose the correct answer.
- What is the role of cytoplasm in a plant cell
1) protects the contents of the cell from adverse conditions
2) provides selective permeability of substances
3) communicates between the nucleus and organelles
4) ensures the entry of substances into the cell from the environment - In the Golgi complex, unlike chloroplasts, there is
1) transport of substances
2) oxidation of organic substances to inorganic ones
3) accumulation of substances synthesized in the cell
4) synthesis of protein molecules - The similarity between the functions of lysosomes and mitochondria lies in what happens in them
1) synthesis of enzymes
2) synthesis of organic substances
3) reduction of carbon dioxide to carbohydrates
4) breakdown of organic substances - The mobility of plasma membrane protein molecules ensures
1) transport of substances into the cell
2) its stability
3) its complete permeability
4) cell interconnection
- Main function of mitochondria
1) DNA reduplication
2) protein biosynthesis
3) ATP synthesis
4) synthesis of carbohydrates - Protein synthesis occurs in
1) Golgi apparatus
2) ribosomes
3) smooth endoplasmic reticulum
4) lysosomes
7.
The formation of lysosomes and the growth of membranes of the endoplasmic reticulum occurs due to the activity
1) vacuoles
2) cell center
3) Golgi complex
4) plastid
8. The main properties of the plasma membrane include
1) impenetrability
2) contractility
3) selective permeability
4) excitability and conductivity
9. What function does the cell center perform in a cell?
1) forms the large and small subunits of ribosomes
2) forms spindle filaments
3) synthesizes hydrolytic enzymes
4) accumulates ATP in interphase
10
. The nucleus in a plant cell was discovered
1) A. Levenguk
2) R. Hooke
3) R. Brown
4) I. Mechnikov
Task 2. Find errors in the given text, correct them, indicate the numbers of the sentences in which they are made, write down these sentences without errors.
1. All living organisms - animals, plants, fungi, bacteria, viruses - consist of cells.
2. All cells have a plasma membrane.
3. Outside the membrane, the cells of living organisms have a rigid cell wall.
4. All cells have a nucleus.
5. The cell nucleus contains the genetic material of the cell - DNA molecules.
Structure of the endoplasmic reticulum
Definition 1
Endoplasmic reticulum(ER, endoplasmic reticulum) is a complex ultramicroscopic, highly branched, interconnected system of membranes that more or less evenly penetrates the mass of the cytoplasm of all eukaryotic cells.
EPS is a membrane organelle consisting of flat membrane sacs - cisterns, channels and tubes. Thanks to this structure, the endoplasmic reticulum significantly increases the area of the internal surface of the cell and divides the cell into sections. It's filled inside matrix(moderately dense loose material (synthesis product)). The content of various chemical substances in the sections is not the same, therefore, various chemical reactions can occur in a cell, both simultaneously and in a certain sequence, in a small volume of the cell. The endoplasmic reticulum opens in perinuclear space(the cavity between two caryolem membranes).
The membrane of the endoplasmic reticulum consists of proteins and lipids (mainly phospholipids), as well as enzymes: adenosine triphosphatase and enzymes for the synthesis of membrane lipids.
There are two types of endoplasmic reticulum:
- Smooth (agranular, aES), represented by tubes that anastomose with each other and do not have ribosomes on the surface;
- Rough (granular, grES), also consisting of interconnected cisterns, but they are covered with ribosomes.
Note 1
Sometimes they also allocate passing or transient(tES) endoplasmic reticulum, which is located in the area of transition of one type of ES to another.
Granular ES is characteristic of all cells (except sperm), but the degree of its development varies and depends on the specialization of the cell.
GRES of epithelial glandular cells (pancreas, producing digestive enzymes, liver, synthesizing serum albumin), fibroblasts (connective tissue cells producing collagen protein), plasma cells (producing immunoglobulins) is highly developed.
Agranular ES predominates in adrenal cells (synthesis of steroid hormones), in muscle cells (calcium metabolism), in the cells of the fundic glands of the stomach (release of chlorine ions).
Another type of EPS membranes are branched membrane tubes containing a large number of specific enzymes inside, and vesicles - small vesicles surrounded by a membrane, mainly located next to the tubes and cisterns. They ensure the transfer of those substances that are synthesized.
EPS functions
The endoplasmic reticulum is an apparatus for the synthesis and, partly, transport of cytoplasmic substances, thanks to which the cell performs complex functions.
Note 2
The functions of both types of EPS are associated with the synthesis and transport of substances. The endoplasmic reticulum is a universal transport system.
The smooth and rough endoplasmic reticulum with its membranes and contents (matrix) perform common functions:
- separation (structuring), due to which the cytoplasm is distributed in an orderly manner and does not mix, and also prevents random substances from entering the organelle;
- transmembrane transport, due to which necessary substances are transferred through the membrane wall;
- synthesis of membrane lipids with the participation of enzymes contained in the membrane itself and ensuring the reproduction of the endoplasmic reticulum;
- Due to the potential difference that arises between the two surfaces of the ES membranes, it is possible to ensure the conduction of excitation impulses.
In addition, each type of network has its own specific functions.
Functions of smooth (agranular) endoplasmic reticulum
The agranular endoplasmic reticulum, in addition to the named functions common to both types of ES, also performs functions unique to it:
- calcium depot. In many cells (in skeletal muscles, in the heart, eggs, neurons) there are mechanisms that can change the concentration of calcium ions. Striated muscle tissue contains a specialized endoplasmic reticulum called the sarcoplasmic reticulum. This is a reservoir of calcium ions, and the membranes of this network contain powerful calcium pumps that can release large amounts of calcium into the cytoplasm or transport it into the cavities of the network channels in hundredths of a second;
- lipid synthesis, substances such as cholesterol and steroid hormones. Steroid hormones are synthesized mainly in the endocrine cells of the gonads and adrenal glands, in the cells of the kidneys and liver. Intestinal cells synthesize lipids, which are excreted into the lymph and then into the blood;
detoxification function– neutralization of exogenous and endogenous toxins;
Example 1
Kidney cells (hepatocytes) contain oxidase enzymes that can destroy phenobarbital.
organelle enzymes take part in glycogen synthesis(in liver cells).
Functions of the rough (granular) endoplasmic reticulum
In addition to the listed general functions, the granular endoplasmic reticulum is also characterized by special ones:
- protein synthesis at the State Power Plant has some peculiarities. It begins on free polysomes, which subsequently bind to ES membranes.
- The granular endoplasmic reticulum synthesizes: all proteins of the cell membrane (except for some hydrophobic proteins, proteins of the internal membranes of mitochondria and chloroplasts), specific proteins of the internal phase of membrane organelles, as well as secretory proteins that are transported throughout the cell and enter the extracellular space.
- post-translational modification of proteins: hydroxylation, sulfation, phosphorylation. An important process is glycosylation, which occurs under the action of the membrane-bound enzyme glycosyltransferase. Glycosylation occurs before the secretion or transport of substances to certain parts of the cell (Golgi complex, lysosomes or plasmalemma).
- transport of substances along the intramembrane part of the network. Synthesized proteins move through the gaps of the ES to the Golgi complex, which removes substances from the cell.
- due to the participation of the granular endoplasmic reticulum The Golgi complex is formed.
The functions of the granular endoplasmic reticulum are associated with the transport of proteins that are synthesized in ribosomes and located on its surface. Synthesized proteins enter the EPS, fold and acquire a tertiary structure.
The protein that is transported to the cisterns changes significantly along the way. It can, for example, be phosphorylated or converted into a glycoprotein. The usual route for a protein is through the granular ER into the Golgi apparatus, from where it either exits the cell, goes to other organelles of the same cell, such as lysosomes), or is deposited as storage granules.
In liver cells, both granular and non-granular endoplasmic reticulum take part in the processes of detoxification of toxic substances, which are then removed from the cell.
Like the outer plasma membrane, the endoplasmic reticulum has selective permeability, as a result of which the concentration of substances inside and outside the reticulum channels is not the same. This has implications for cell function.
Example 2
There are more calcium ions in the endoplasmic reticulum of muscle cells than in its cytoplasm. Leaving the channels of the endoplasmic reticulum, calcium ions trigger the process of contraction of muscle fibers.
Formation of the endoplasmic reticulum
The lipid components of the endoplasmic reticulum membranes are synthesized by enzymes of the reticulum itself, while the protein components come from ribosomes located on its membranes. The smooth (agranular) endoplasmic reticulum does not have its own protein synthesis factors, therefore it is believed that this organelle is formed as a result of the loss of ribosomes by the granular endoplasmic reticulum.
Endoplasmic reticulum (ER), or endoplasmic reticulum (ER), was discovered only with the advent of the electron microscope. EPS is found only in eukaryotic cells and is a complex system of membranes that form flattened cavities and tubes. All together it looks like a network. EPS refers to single-membrane cell organelles.
The membranes of the ER extend from the outer membrane of the nucleus and are similar in structure to it.
The endoplasmic reticulum is divided into smooth (agranular) and rough (granular). The latter is dotted with ribosomes attached to it (this is why the “roughness” arises). The main function of both types is related to the synthesis and transport of substances. Only the rough one is responsible for protein, and the smooth one is responsible for carbohydrates and fats.
In terms of its structure, the ER is a set of paired parallel membranes that penetrate almost the entire cytoplasm. A pair of membranes forms a plate (the cavity inside has different widths and heights), but the smooth endoplasmic reticulum has a more tubular structure. Such flattened membrane sacs are called EPS tanks.
Ribosomes located on the rough ER synthesize proteins that enter the ER channels, ripen (acquire a tertiary structure) there and are transported. In such proteins, a signal sequence (consisting mainly of non-polar amino acids) is first synthesized, the configuration of which corresponds to the specific EPS receptor. As a result, the ribosome and the endoplasmic reticulum communicate. In this case, the receptor forms a channel for the passage of the synthesized protein into the EPS tanks.
Once the protein enters the endoplasmic reticulum channel, the signal sequence is separated from it. After this, it collapses into its tertiary structure. When transported along the EPS, the protein acquires a number of other changes (phosphorylation, formation of a bond with carbohydrate, i.e., conversion into a glycoprotein).
Most of the proteins found in the rough ER then enter the Golgi apparatus (complex). From there, proteins are either secreted from the cell, or enter other organelles (usually lysosomes), or are deposited as storage granules.
It should be borne in mind that not all cell proteins are synthesized on the rough ER. A part (usually a smaller one) is synthesized by free ribosomes in the hyaloplasm; such proteins are used by the cell itself. In them, the signal sequence is not synthesized because it is unnecessary.
The main function of the smooth endoplasmic reticulum is lipid synthesis(fat). For example, the EPS of the intestinal epithelium synthesizes them from fatty acids and glycerol absorbed from the intestine. The lipids then enter the Golgi complex. In addition to intestinal cells, smooth ER is well developed in cells that secrete steroid hormones (steroids are classified as lipids). For example, in adrenal cells, interstitial cells of the testes.
The synthesis and transport of proteins, fats and carbohydrates are not the only functions of the EPS. In baking, the endoplasmic reticulum is involved in detoxification processes. A special form of smooth ER - sarcoplasmic reticulum - is present in muscle cells and ensures contraction by pumping calcium ions.
The structure, volume and functionality of the cell's endoplasmic reticulum is not constant throughout the cell cycle, but is subject to certain changes.
Endoplasmic reticulum
Endoplasmic reticulum (ER) is a system of interconnected or separate tubular channels and flattened cisterns located throughout the cytoplasm of the cell. They are delimited by membranes (membrane organelles). Sometimes tanks have expansions in the form of bubbles. ER channels can connect to the surface or nuclear membranes and contact the Golgi complex.
In this system, smooth and rough (granular) EPS can be distinguished.
Rough XPS
Ribosomes are located on the channels of the rough ER in the form of polysomes. Here, the synthesis of proteins occurs, mainly produced by the cell for export (removal from the cell), for example, the secretions of glandular cells. The formation of lipids and proteins of the cytoplasmic membrane and their assembly also occur here. Densely packed cisterns and channels of granular EPS form a layered structure, where protein synthesis occurs most actively. This place is called ergastoplasma.
Smooth XPS
There are no ribosomes on the smooth ER membranes. The synthesis of fats and similar substances (for example, steroid hormones), as well as carbohydrates, takes place here. The channels of the smooth ER also transport the finished material to the place of its packaging into granules (to the area of the Golgi complex). In liver cells, smooth ER takes part in the destruction and neutralization of a number of toxic and medicinal substances (for example, barbiturates).
In the striated muscle, the tubules and cisterns of the smooth ER deposit calcium ions.
Golgi complex
The lamellar Golgi complex is the packaging center of the cell. It is a collection of dictyosomes (from several tens to hundreds and thousands per cell). Dictyosome- a stack of 3-12 flattened oval-shaped cisterns, along the edges of which there are small bubbles (vesicles). Larger expansions of the tanks give rise to vacuoles, which contain a reserve of water in the cell and are responsible for maintaining turgor. The lamellar complex gives rise to secretory vacuoles, which contain substances intended for removal from the cell. In this case, the secretion entering the vacuole from the synthesis zone (ER, mitochondria, ribosomes) undergoes some chemical transformations here.
The Golgi complex gives rise to primary lysosomes. Dictyosomes also synthesize polysaccharides, glycoproteins and glycolipids, which are then used to build cytoplasmic membranes.
N. S. Kurbatova, E. A. Kozlova "Lecture notes on general biology"
Endoplasmic reticulum(endoplasmic reticulum) was discovered by C. R. Porter in 1945.
This structure is a system of interconnected vacuoles, flat membrane sacs or tubular structures that create a three-dimensional membranous network within the cytoplasm. The endoplasmic reticulum (ER) is found in almost all eukaryotes. It connects organelles with each other and transports nutrients. There are two independent organelles: granular (granular) and smooth non-granular (agranular) endoplasmic reticulum.
Granular (rough or granular) endoplasmic reticulum. It is a system of flat, sometimes dilated cisterns, tubules, and transport vesicles. The size of the cisterns depends on the functional activity of the cells, and the width of the lumen can range from 20 nm to several microns. If the cistern expands sharply, it becomes visible under light microscopy and is identified as a vacuole.
The cisterns are formed by a two-layer membrane, on the surface of which there are specific receptor complexes that ensure attachment of ribosomes to the membrane, translating polypeptide chains of secretory and lysosomal proteins, cytolemma proteins, etc., that is, proteins that do not merge with the contents of the karyoplasm and hyaloplasm.
The space between the membranes is filled with a homogeneous matrix of low electron density. The outside of the membrane is covered with ribosomes. Ribosomes are visible under electron microscopy as small (about 20 nm in diameter), dark, almost round particles. If there are many of them, this gives a granular appearance to the outer surface of the membrane, which served as the basis for the name of the organelle.
On membranes, ribosomes are located in the form of clusters - polysomes, which form rosettes, clusters or spirals of various shapes. This feature of the distribution of ribosomes is explained by the fact that they are associated with one of the mRNAs, from which they read information and synthesize polypeptide chains. Such ribosomes are attached to the ER membrane using one of the sections of the large subunit.
In some cells, the granular endoplasmic reticulum (ER) consists of rare scattered cisterns, but can form large local (focal) accumulations. Poorly developed gr. EPS in poorly differentiated cells or in cells with low protein secretion. Clusters gr. EPS are found in cells that actively synthesize secretory proteins. With an increase in the functional activity of the cisternae, the organelles become multiple and often expand.
Gr. EPS is well developed in the secretory cells of the pancreas, the main cells of the stomach, in neurons, etc. Depending on the type of cells, the group. EPS can be distributed diffusely or localized in one of the poles of the cell, while numerous ribosomes stain this zone basophilically. For example, in plasma cells (plasmocytes) there is a well-developed group. EPS causes the bright basophilic color of the cytoplasm and corresponds to areas of concentration of ribonucleic acids. In neurons, the organelle is located in the form of compactly lying parallel cisterns, which under light microscopy is manifested as basophilic granulation in the cytoplasm (chromatophilic substance of the cytoplasm, or tigroid).
In most cases on gr. EPS synthesizes proteins that are not used by the cell itself, but are released into the external environment: proteins of the exocrine glands of the body, hormones, mediators (protein substances of endocrine glands and neurons), proteins of the intercellular substance (proteins of collagen and elastic fibers, the main component of the intercellular substance). Proteins formed by gr. EPS are also part of lysosomal hydrolytic enzyme complexes located on the outer surface of the cell membrane. The synthesized polypeptide not only accumulates in the ER cavity, but also moves and is transported through channels and vacuoles from the site of synthesis to other parts of the cell. First of all, such transport occurs in the direction of the Golgi complex. With electron microscopy, good development of the ER is accompanied by a parallel increase (hypertrophy) of the Golgi complex. In parallel with it, the development of nucleoli increases, and the number of nuclear pores increases. Often in such cells there are numerous secretory inclusions (granules) containing secretory proteins, and the number of mitochondria increases.
Proteins that accumulate in the cavities of the ER, bypassing the hyaloplasm, are most often transported to the Golgi complex, where they are modified and become part of either lysosomes or secretory granules, the contents of which remain isolated from the hyaloplasm by the membrane. Inside the tubules or vacuoles gr. EPS modification of proteins occurs, binding them to sugars (primary glycosylation); condensation of synthesized proteins with the formation of large aggregates - secretory granules.
On ribosomes gr. EPS synthesize membrane integral proteins that are embedded in the thickness of the membrane. Here, from the side of the hyaloplasm, lipids are synthesized and incorporated into the membrane. As a result of these two processes, the ER membranes themselves and other components of the vacuolar system grow.
The main function of gr. EPS is the synthesis of exported proteins on ribosomes, isolation from the contents of the hyaloplasm inside membrane cavities and transport of these proteins to other parts of the cell, chemical modification or local condensation, as well as the synthesis of structural components of cell membranes.
During translation, ribosomes are attached to the membrane c. EPS in the form of a chain (polysome). The ability to contact the membrane is provided by signaling sites that attach to special EPS receptors - mooring protein. After this, the ribosome binds to a protein that fixes it to the membrane, and the resulting polypeptide chain is transported through membrane pores, which open with the help of receptors. As a result, protein subunits find themselves in the intermembrane space c. EPS. An oligosaccharide (glycosylation) can join the resulting polypeptides, which is cleaved from dolichol phosphate attached to the inner surface of the membrane. Subsequently, the contents of the lumen of the tubules and cisterns gr. EPS, with the help of transport vesicles, is transferred to the cis-compartment of the Golgi complex, where it undergoes further transformation.
Smooth (agranular) EPS. It may be related to gr. The ER is a transition zone, but, nevertheless, it is an independent organelle with its own system of receptor and enzymatic complexes. It consists of a complex network of tubules, flat and dilated cisterns and transport vesicles, but if in gr. The EPS is dominated by cisterns, but in the smooth endoplasmic reticulum (smooth EPS) there are more tubules with a diameter of about 50...100 nm.
Smooth to membranes. Ribosomes are not attached to the EPS, which is due to the lack of receptors for these organelles. Thus smooth. Although the ER is a morphological continuation of the granular reticulum, it is not just the endoplasmic reticulum, on which there are currently no ribosomes, but is an independent organelle to which ribosomes cannot attach.
Glad. EPS is involved in the synthesis of fats, the metabolism of glycogen, polysaccharides, steroid hormones and some drugs (in particular, barbiturates). During the famine. EPS undergo the final stages of the synthesis of all cell membrane lipids. The membranes are smooth. EPS contains lipid-transforming enzymes - flippases, which move fat molecules and maintain the asymmetry of lipid layers.
Glad. EPS is well developed in muscle tissues, especially striated ones. In skeletal and cardiac muscles, it forms a large specialized structure - the sarcoplasmic reticulum, or L-system.
The sarcoplasmic reticulum consists of mutually connecting networks of L-tubules and marginal cisterns. They entwine special contractile muscle organelles - myofibrils. In striated muscle tissue, the organelle contains a protein, calsequestrin, which binds up to 50 Ca2+ ions. In smooth muscle cells and non-muscle cells in the intermembrane space there is a protein called calreticulin, which also binds Ca2+.
Thus smooth. EPS is a reservoir of Ca2+ ions. At the moment of cell excitation during depolarization of its membrane, calcium ions are removed from the ER into the hyaloplasm, the leading mechanism that triggers muscle contraction.
This is accompanied by contraction of cells and muscle fibers due to the interaction of actomyosin or actomyosin complexes of myofibrils. At rest, Ca2+ is reabsorbed into the lumen of the tubules. EPS, which leads to a decrease in calcium content in the cytoplasmic matrix and is accompanied by relaxation of myofibrils. Calcium pump proteins regulate transmembrane ion transport.
An increase in the concentration of Ca2+ ions in the cytoplasmic matrix also accelerates the secretory activity of non-muscle cells and stimulates the movement of cilia and flagella.
Glad. EPS deactivates various substances harmful to the body due to their oxidation with the help of a number of special enzymes, especially in liver cells. Thus, with some poisonings, acidophilic zones (not containing RNA) appear in the liver cells, completely filled with smooth endoplasmic reticulum.
In the adrenal cortex, in the endocrine cells of the gonads. EPS is involved in the synthesis of steroid hormones, and key enzymes of steroidogenesis are located on its membranes. Such endocrinocytes are smooth. EPS has the appearance of abundant tubules, which in cross section are visible as numerous vesicles.
Glad. EPS is formed from gr. EPS. There is smoothness in some areas. EPS forms new lipoprotein membrane areas devoid of ribosomes. These areas can grow, break off from granular membranes and function as an independent vacuolar system.
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ENDOPLASMIC RETICULUM
ENDOPLASMIC RETICULUM, endoplasmic reticulum (from endo...
Granular endoplasmic reticulum
and plasma), organelle of a eukaryotic cell. Discovered by K. Porter in 1945 in the endoplasm of fibroblasts. It is a system of small vacuoles and tubules connected to each other and bounded by a single membrane. The membranes of the endoplasmic reticulum, 5-7 nm thick, in some cases directly pass into the outer nuclear membrane. The derivatives of endoplasmic reticulum are microbodies, and in plant cells - vacuoles. There are smooth (agranular) and granular endoplasmic reticulum. The smooth endoplasmic reticulum is devoid of ribosomes. Consists of highly branched tubules and small vacuoles with a diameter of 50–100 nm. Apparently, it is a derivative of the granular endoplasmic reticulum; in some cases, their membranes directly pass into each other. Functions: synthesis of triglycerides and the formation of most of the cell's lipids, accumulation of lipid droplets (for example, in fatty degeneration), exchange of some polysaccharides (glycogen), accumulation and removal of toxic substances from the cell, synthesis of steroid hormones. A sarcoplasmic reticulum is formed in muscle fibers, which, by releasing and accumulating calcium ions, causes contraction and relaxation of the fiber. It is most developed in cells that secrete non-protein products (adrenal cortex, gonads, parietal cells of the glands of the fundus of the stomach, etc.). The granular endoplasmic reticulum has ribosomes on its membranes. It consists of tubules and flattened cisterns; in many cells it forms a branched network that penetrates most of the cytoplasm. The main function is the synthesis of proteins on ribosome complexes attached to the outside of the membrane - polyribosomes. Proteins are mainly synthesized and either excreted from the cell or transformed in the Golgi complex. Synthesized proteins enter the cavities of the granular endoplasmic reticulum, where ATP-dependent transport of proteins occurs and their modification and concentration can occur. It is most developed in cells with protein secretion (pancreas, salivary glands, plasma cells, etc.) and is practically absent in embryonic undifferentiated cells.
ENDOPLASMIC RETICULUM (ER)
Endoplasmic reticulum (ER)- a system of flattened, tubular, vesicular structures bounded by a membrane. The name is due to the fact that its numerous elements (cisterns, tubes, bubbles) form a single, continuous three-dimensional network.
The degree of development of the EPS varies in different cells, and even in different parts of the same cell, and depends on the functional activity of the cells.
There are two types of EPS (Fig. 4):
granular EPS (grEPS) And
smooth, or agranular EPS (aEPS), which are interconnected in the transition region.
Fig.4.
Granular EPS formed by membrane tubes and flattened cisterns, on the outer (facing the hyaloplasm) surface of which ribosomes are located. Attachment of ribosomes occurs due to the integral receptor proteins of the grEPS membranes - ribophorins. These same proteins form hydrophobic channels in the grEPS membrane for the penetration of the synthesized protein chain into the lumen of the cisterns.
Main function of GREPS: segregation(department) of newly synthesized protein molecules from hyaloplasm.
Thus, GREPS provides:
protein biosynthesis, intended for export from the cell;
biosynthesis of lysosome enzymes
biosynthesis of membrane proteins.
Protein molecules accumulate inside the lumen of the cisterns, acquire a secondary and tertiary structure, and also undergo initial post-translational changes– hydroxylation, sulfation, phosphorylation and glycosylation (addition of oligosaccharides to proteins to form glycoproteins).
GREPS is present in all cells, but this network is most developed in cells specialized in protein synthesis, such as pancreatic cells that produce digestive enzymes; connective tissue fibroblasts that synthesize collagen; plasma cells producing immunoglobulins. In these cells, GREP elements form parallel clusters of cisterns; at the same time, the lumen of the tanks is often expanded. All these cells are characterized by pronounced basophilia of the cytoplasm in the area where the elements of the grEPS are located.
Agranular EPS is a three-dimensional network of membrane tubes, tubules, vesicles, on the surface of which no ribosomes.
Functions of agrEPS
Participation in the synthesis of lipids, including membrane lipids, cholesterol and steroids;
Glycogen metabolism;
Neutralization and detoxification of endogenous and exogenous toxic substances;
Accumulation of Ca ions (especially in the specialized form of aER - the sarcoplasmic reticulum of muscle cells).
Agreps is well developed:
In cells that actively produce steroid hormones - cells of the adrenal cortex, interstitial glandulocytes of the testicle, cells of the corpus luteum of the ovary.
In liver cells, where its enzymes participate in the metabolism of glycogen, as well as in processes that ensure the neutralization and detoxification of endogenous biologically active substances (hormones) and exogenous harmful substances (alcohol, drugs, etc.).
GOLGI COMPLEX – a membrane organelle formed by three main elements (Fig. 5): clusters of flattened cisterns, small (transport) vesicles and condensing vacuoles.
The complex of these elements is called dictyosome.
Fig.5.
Tanks have the appearance of curved discs with slightly expanded peripheral sections. Tanks form a group in the form of a stack of 3-30 elements. Vesicles and vacuoles break off from the peripheral expansions of the cisterns.
Bubbles– small (diameter 40-80 nm), membrane-surrounded spherical elements containing moderate electron density.
Endoplasmic reticulum
Vacuoles– large (diameter 0.1-1.0 µm), spherical formations, separated from the mature surface of the Golgi complex in some glandular cells. The vacuoles contain a secretory product that is in the process of condensation.
The Golgi complex has polarity: each dictyosome has two surfaces:
emerging (immature, or cis surface) And
mature (trans surface).
The convex cis surface faces the EPS and is connected to it by a system of small transport bubbles that detach from the EPS. Thus, proteins in transport vesicles penetrate the cis surface.
Each group of medial cisterns within the stack has a different enzyme composition, and each group has its own protein processing reactions. The processed substances are released in vacuoles from the concave trans surface.