With the appearance of compartments, the eukaryotic cell receives not only obvious advantages, but also a number of problems. One of them is sorting and delivering the right compounds to the right organelles. First of all, it concerns proteins. The fate of the synthesized proteins is different and depends on the places of their subsequent functioning. There are two main routes of protein transport, which begin in different places in the cytoplasm. Rice. 1.2.

The first transport branch works with proteins that are intended for plastids, mitochondria, the nucleus and peroxisomes - that is, for all cell compartments, except for the organelles of the endomembrane system. The synthesis of these proteins occurs on the free ribosomes of the cytosol. Proteins destined for transport contain sorting signals directing them to the appropriate organelles. Such signals are usually one or more regions of the protein, which are called signal or leader peptides. The organelle membrane contains a special translocator protein that “recognizes” the signal peptide. The molecule of the transported protein must unfold in order to “pass” through the “eye of the needle” of the translocator protein like a thread of an unfolded ball. Table 1.1. some characteristics of various sorting signals are presented. This protein transport pathway is sometimes called cytosolic. It should be noted that most proteins synthesized on free cytosol ribosomes do not have sorting signals and remain in the cytosol as permanent components.

The other transport branch is used for secreted proteins, as well as for proteins destined for the organelles of the endomembrane system and the plasma membrane. The synthesis of these proteins also begins on the ribosomes of the cytosol, but after the initiation of translation, the ribosomes attach to the ER membrane, and a rough ER is formed. The resulting proteins are co-translated into the ER. This means that immediately after the synthesis of the next section of the polypeptide chain, it crosses the ER membrane. After synthesis, some of the proteins enter the ER lumen, while others remain fixed in the membrane and become ER transmembrane proteins. This transport branch is often referred to as secretory way cells.

Table 1.1. Signal sequences for protein transport in plant cells.

The two branches of transport differ in the scheme of transportation. The paths of cytosolic transport of proteins are parallel, that is, proteins from the cytosol are immediately sent to the desired organelle. Usually, no more than one to two minutes pass from the moment the protein is released into the cytosol until it enters the organelle.

The transport of proteins along the secretory pathway occurs sequentially - from organelle to organelle. Before reaching the final destination, the protein can visit several organelles (ER, different parts of the AG). The journey from the ER membrane to the destination can take tens, if not hundreds of minutes. In the process of transfer, proteins can undergo significant modifications (primarily in AG). On final stages transport can occur in parallel - into the vacuole, periplasmic space or into the plasmalemma.

Finally, the two pathways of protein transport differ in the mechanism by which molecules are transported. For the cytosolic pathway, only a monomolecular mechanism of protein transport is possible, in which each protein molecule individually crosses the membrane through the corresponding translocator. The secretory pathway is characterized by a vesicular mechanism of transport of protein molecules, which is mediated by transport vesicles (vesicles). Vesicles are laced from one compartment, and certain molecules are captured from its cavity. The vesicles then fuse with another compartment, delivering their contents into it. During vesicular transport, proteins do not cross any membranes; transport can only occur between topologically equivalent compartments. The vesicular transport mechanism is selectively controlled by special proteins that act as sorting signals. A protein enters the transport vesicle if its sorting signal binds to a receptor on the vesicle membrane. At present, some sorting signals in proteins are known, while most of their complementary membrane receptors are not.

1.2. The plant cell is the result of a double symbiosis.

The strategy for the existence of higher plants is primarily due to their two main properties - the phototrophic type of nutrition and the lack of active mobility. These two properties left their imprint on all levels of organization of the plant organism, up to the cellular level.

In addition to the features common to all eukaryotic cells, plant cells have a number of features. The main ones are:

the presence of plastids; the presence of vacuoles; having a rigid cell wall.

The structure diagram of a typical plant cell is shown in fig. 1.3.

The presence of plastids is associated primarily with the phototrophic type of plant nutrition. Plastids, like mitochondria, have their own genome. Thus, another feature of a plant cell is that it combines three relatively autonomous genetic systems: nuclear (chromosomal), mitochondrial, and plastid. The presence of three genomes is a consequence of the symbiotic origin of plant cells. At the same time, the plant cell, unlike other eukaryotic cells, was formed from at least three initially independent forms:

1) "host" organism, genetic apparatus which has moved to the core;

2) a heterotrophic bacterium (similar to rhodospirilla), which served as the precursor of mitochondria;

3) an ancient bacterium with oxygenic photosynthesis (similar to the cyanobacterium synechocystis), which became the ancestor of plastids.

Long-term coevolution of symbionts has led to a redistribution of functions between them and their genetic systems, with many mitochondrial and plastid DNA genes being transferred to the nucleus.

The ribosome is a mini-factory for the production of proteins.

One of the most complex processes carried out by living beings is, perhaps, the synthesis of proteins - the most important structural and functional "building blocks" of any organism. A true understanding of the molecular processes that underlie it could shed light on incredibly long-standing events related to the mystery of the origin of Life itself ...

In all living organisms, from the simplest bacteria to humans, proteins are synthesized by special cellular devices called ribosomes. In these unique factories, a protein chain is formed from individual amino acids.

There are a lot of ribosomes in cells that conduct intensive protein synthesis: for example, one bacterial cell contains about 10 thousand of these minifactories, which make up to 30% of the total dry mass of the cell! The cells of higher organisms contain fewer ribosomes - their number depends on the type of tissue and the level of cell metabolism.

The ribosome synthesizes protein average speed 10-20 amino acids per second. The accuracy of translation is extremely high - the erroneous inclusion of the "wrong" amino acid residue in the protein chain averages one amino acid per 3 thousand links (with an average human protein chain length of 500 amino acid residues), that is, only one error per six proteins.

About the genetic code

The program that specifies the sequence of amino acid residues in a protein is recorded in the cell genome: about half a century ago it was found that the amino acid sequences of all proteins are directly encoded in DNA using the so-called genetic code. According to this code, universal for all living organisms, each of the twenty existing amino acids has its own codon- a trio of nucleotides, which are the elementary units of the DNA chain. Any protein is encoded in DNA by a certain sequence of codons. This sequence is called genome.

How does this genetic information get to the ribosome? On a separate gene, as on a matrix, a chain of another information molecule is synthesized - ribonucleic acids (RNA). This gene copying process, called transcription, is carried out by special enzymes - RNA polymerases.

But the RNA obtained in this way is not yet a template for protein synthesis: certain “non-coding” pieces of the nucleotide sequence are cut out from it (the process splicing).

The result is messenger RNA (mRNA), which is used by ribosomes as a program for protein synthesis. The synthesis itself, i.e. the translation of genetic information from the language of the nucleotide sequence of mRNA into the language of the amino acid sequence of a protein is called translation.

Decoding and synthesis

In eukaryotic cells, one mRNA is usually translated by many ribosomes at once, forming the so-called polysomes, which can be clearly seen using electron microscopy, which makes it possible to obtain an increase of tens of thousands of times.

How do amino acids, which are the building blocks for protein synthesis, enter the ribosome? Back in the 50s of the last century, special “carriers” were discovered that deliver amino acids to the ribosome - short (less than 80 nucleotides long) transport RNA (tRNA). A special enzyme attaches an amino acid to one of the ends of the tRNA, and each amino acid corresponds to a strictly defined tRNA. Protein synthesis on the ribosome includes three main stages: the beginning, the elongation of the polypeptide chain and the end.

The ribosome itself - one of the most complexly organized molecular machines of the cell - consists of two unequal parts, the so-called subparticles (small and large). It can be easily divided into parts by centrifugation at ultra-high speeds in special test tubes with a solution of sucrose, the concentration of which increases from top to bottom. Since the small subparticle is twice as light as the large one, they move from the top of the tube to the bottom at different speeds.

The small subparticle is responsible for decoding genetic information. It is made up of high molecular weight ribosomal RNA (rRNA) and several dozen proteins (about 20 in prokaryotes and more than 30 in eukaryotes).

A large subunit responsible for the formation of a peptide bond between amino acid residues consists of several rRNAs: one high molecular weight and one (or two in the case of eukaryotes) low molecular weight, as well as several dozen proteins (more than 30 in prokaryotes and up to 50 in eukaryotes). The scale of ribosome activity can be judged at least by the fact that ribosomal RNA makes up about 80% of all cell RNA, tRNA transporting amino acids - about 15%, while messenger RNA carrying information about the protein sequence - only 5%!

It should be noted that ribosomal proteins are endowed with many other, additional functions that can manifest themselves on different stages cell viability. For example, the human ribosomal protein S3, one of the key proteins of the mRNA binding site on the ribosome, is also involved in the “repair” of DNA damage (Kim et al., 1995), participates in apoptosis(programmed cell death) (Jung et al., 2004) and also protects against degradation of heat shock protein (Kim et al., 2006).

In addition, too intense synthesis of some ribosomal proteins may indicate the development of malignant transformation of the cell. For example, a significant increase in the level of five ribosomal proteins was found in colon tumor cells (Zhang et al., 1999). Recently, the staff of the Laboratory of the Structure and Function of Ribosomes of the ICBFM SB RAS discovered a new mechanism for autoregulation of the biosynthesis of ribosomal proteins in humans, based on the principle feedback. The uncontrolled synthesis of ribosomal proteins, which is characteristic of tumor cells, is probably caused by failures in this mechanism. Further research in this area is of particular interest not only for scientists, but also for physicians.

Works as a "ribozyme"

Surprisingly, despite the billions of years of evolution that separate bacteria and humans, the secondary structure of ribosomal RNAs differs little between them.

Until recently, little was known about how rRNA is folded in subparticles and how it interacts with ribosomal proteins. A revolutionary shift in understanding the structure of the ribosome on molecular level occurred at the turn of the new millennium, when, using X-ray diffraction analysis, it was possible to decipher the structure of the ribosomes of the simplest organisms and their model complexes with mRNA and tRNA at the level of individual atoms. This made it possible to understand the molecular mechanisms of decoding genetic information and the formation of bonds in a protein molecule.

It turned out that both the most important functional centers of the ribosome - both decoding on the small subparticle and responsible for the synthesis of the protein chain on the large subparticle - are formed not by proteins, but by ribosomal RNA. That is, the ribosome works like ribozymes - unusual enzymes that do not consist of proteins, but of RNA.

Ribosomal proteins, however, also play an important role in the functioning of the ribosome. In the absence of these proteins, ribosomal RNAs are completely unable to either decode genetic information or catalyze the formation of peptide bonds. Proteins provide the complex "folding" of rRNA in functional centers necessary for the work of the ribosome, serve as "transmitters" of changes in the spatial structure of the ribosome necessary in the course of work, and also bind various molecules that affect the speed and accuracy of the process of protein synthesis.

The very working scheme of the protein cycle is, in principle, the same for the ribosomes of all living beings. However, it is still unknown to what extent the molecular mechanisms of ribosome functioning are similar in different organisms. Particularly lacking information about the device functional centers ribosomes of higher organisms, which are much less studied than the ribosomes of protozoa.

This is due to the fact that many of the methods successfully used to study prokaryotic ribosomes turned out to be inapplicable to eukaryotes. Thus, crystals suitable for X-ray diffraction analysis cannot be obtained from ribosomes of higher organisms, and their subparticles cannot be “assembled” in a test tube from a mixture of ribosomal proteins and rRNA, as is done in protozoa.

From lowest to highest

And yet, there are ways to obtain information about the structure of the functional centers of ribosomes in higher organisms. One of these methods is the method chemical affinity crosslinking, developed 35 years ago in the Department of Biochemistry of the Research Institute of Chemistry of the Siberian Branch of the USSR Academy of Sciences (now the ICBFM of the Siberian Branch of the Russian Academy of Sciences) under the guidance of Academician D. G. Knorre.

The method is based on the use of short synthetic mRNAs carrying chemically active ("cross-linking") groups in the chosen position, which can be activated at the right time (for example, by irradiating with soft ultraviolet light).

The advantage of this method is that the linking group can be attached to almost any mRNA nucleotide residue and, as a result, detailed information about its environment on the ribosome can be obtained. Using a set of short mRNAs with different positions of the linking group, we were able to identify ribosomal proteins and rRNA nucleotides of the human ribosome that form a channel for reading genetic information during translation.

For the first time, it has been experimentally shown that all rRNA nucleotides of a small human ribosomal particle adjacent to mRNA codons are located in conservative regions of the secondary structure of the rRNA molecule. Moreover, their location coincides with the position of the corresponding nucleotides in the secondary structure of ribosome rRNA in lower organisms. This led to the conclusion that this part of the ribosomal RNA of the small subunit constitutes an evolutionarily conservative "core" (core) of the ribosome, structurally identical in all organisms.

On the other hand, a number of fundamental differences have been found in the structure of the mRNA-binding ribosome channel in humans and lower organisms. It turned out that in higher organisms, ribosomal proteins play a much greater role in the formation of this channel than in prokaryotes, in addition, proteins that do not have “twins” (homologues) in lower organisms also participate in this.

Why, despite the fact that the function of the ribosome remained practically unchanged in the process of evolution, did specific features appear in the organization of the decoding center of ribosomes in higher organisms? This is probably due to the more complex and multistage regulation of protein synthesis in eukaryotes compared to prokaryotes, during which the ribosomal proteins of the mRNA-binding channel can interact not only with mRNA, but also with various factors that affect the efficiency and accuracy of translation. Whether this is the case will be shown by further research.

1. The main stages of protein biosynthesis. Genetic code

2. Regulation of gene expression

1. The main stages of protein biosynthesis. Genetic code

Protein biosynthesis in cells is a sequence of matrix-type reactions, during which the successive transfer of hereditary information from one type of molecule to another leads to the formation of polypeptides with a genetically determined structure.

Protein biosynthesis is First stage implementation, or expression of genetic information. The main matrix processes that ensure the biosynthesis of proteins include DNA transcription And mRNA translation. DNA transcription consists in rewriting information from DNA to mRNA (messenger or messenger RNA). Translation of mRNA is the transfer of information from mRNA to a polypeptide. general characteristics The matrix synthesis reactions are given in Chapter 3. The sequence of matrix reactions in protein biosynthesis can be represented as Scheme 1.

The diagram shows that the genetic information about the structure of a protein is stored as a sequence of DNA triplets. In this case, only one of the DNA chains serves as a template for transcription (such a chain is called transcribed). The second strand is complementary to the transcribed strand and is not involved in mRNA synthesis.

The mRNA molecule serves as a template for polypeptide synthesis on ribosomes. mRNA triplets that code for a particular amino acid are called codons. Translation is carried out by tRNA molecules. Each tRNA molecule contains anticodon- a recognition triplet in which the nucleotide sequence is complementary to a specific mRNA codon. Each tRNA molecule is capable of carrying a strictly defined amino acid. The combination of tRNA with an amino acid is called aminoacyl-tRNA.

The tRNA molecule in general conformation resembles a clover leaf on a petiole. "Top of the sheet" carries anticodon. There are 61 types of tRNA with different anticodons. An amino acid is attached to the "leaf petiole" (there are 20 amino acids involved in the synthesis of the polypeptide on ribosomes). Each tRNA molecule with a certain anticodon corresponds to a strictly defined amino acid. At the same time, a certain amino acid usually corresponds to several types of tRNA with different anticodons. The amino acid covalently attaches to tRNA with the help of enzymes - aminoacyl-tRNA synthetases. This reaction is called tRNA aminoacylation.

On ribosomes, the anticodon of the corresponding aminoacyl-tRNA molecule is attached to a specific mRNA codon with the help of a specific protein. This binding of mRNA and aminoacyl-tRNA is called codon dependent. Amino acids are linked together on ribosomes by peptide bonds, and the released tRNA molecules go in search of free amino acids.

Let us consider in more detail the main stages of protein biosynthesis.

Stage 1. DNA transcription . On the transcribed DNA strand, a complementary mRNA strand is completed using DNA-dependent RNA polymerase. The mRNA molecule is an exact copy of the non-transcribed DNA chain, with the difference that instead of deoxyribonucleotides it contains ribonucleotides, which include uracil instead of thymine.

Stage 2. Processing (maturation )mRNA . The synthesized mRNA molecule (primary transcript) undergoes additional transformations. In most cases, the original mRNA molecule is cut into separate fragments. Some fragments - introns- are cleaved to nucleotides, and others - exons are fused into mature mRNA. The process of connecting exons "without knots" is called splicing.

Splicing is characteristic of eukaryotes and archaebacteria, but sometimes also occurs in prokaryotes. There are several types of splicing. Essence a alternative splicing is that the same regions of the original mRNA can be both introns and exons. Then one and the same DNA region corresponds to several types of mature mRNA and, accordingly, several different forms of the same protein. Essence trans splicing consists in joining exons encoded by different genes (sometimes even from different chromosomes) into one mature mRNA molecule.

Stage 3. mRNA translation . Translation (like all matrix processes) includes three stages: initiation(Start), elongation(continued) and termination(ending).

Initiation. The essence of initiation is the formation of a peptide bond between the first two amino acids of the polypeptide.

Initially formed initiating complex, which includes: a small subunit of the ribosome, specific proteins (initiation factors) and a special initiator methionine tRNA with the amino acid methionine - Met-tRNA Met. The initiation complex recognizes the beginning of the mRNA, attaches to it, and slides to the point of initiation (beginning) of protein biosynthesis: in most cases, this start codon AUG. Between the start codon of mRNA and the anticodon of methionine tRNA, codon-dependent binding occurs with the formation of hydrogen bonds. Then the large subunit of the ribosome is attached.

When combining subunits, a complete ribosome is formed, which carries two active centers (sites): A – site (aminoacyl, which serves to attach aminoacyl-tRNA) and R -section (peptidyltransferase, which serves to form a peptide bond between amino acids).

Initially, Met-tRNA Met is located on A –section, but then moves to R -plot. On the freed A The –site receives aminoacyl-tRNA with an anticodon that is complementary to the mRNA codon following the AUG codon. In our example, this is Gly-tRNA Gly with the anticodon CCG, which is complementary to the GHC codon. As a result of codon-dependent binding, hydrogen bonds are formed between the mRNA codon and the aminoacyl-tRNA anticodon. Thus, two amino acids are adjacent to the ribosome, between which a peptide bond is formed. covalent bond between the first amino acid (methionine) and its tRNA breaks.

After the formation of a peptide bond between the first two amino acids, the ribosome shifts by one triplet. As a result, translocation (movement) of the initiating methionine tRNA Met occurs outside the ribosome. The hydrogen bond between the start codon and the anticodon of the initiator tRNA is broken. As a result, free Met tRNA is cleaved off and goes in search of its amino acid.

The second tRNA, together with the amino acid (in our example, Gly-tRNA Gly), as a result of translocation, is on R -section, and A - the site is freed.

Elongation. The essence of elongation is the addition of subsequent amino acids, that is, the extension of the polypeptide chain. The work cycle of the ribosome during elongation consists of three steps: codon-dependent binding of mRNA and aminoacyl-tRNA to A – site, formation of a peptide bond between the amino acid and the growing polypeptide chain and translocation with release A -plot.

On the freed A – the site enters aminoacyl-tRNA with an anticodon corresponding to the next mRNA codon (in our example, it is Tyr-tRNA Tyr with the AUA anticodon, which is complementary to the UAU codon).

On the ribosome, two amino acids are next to each other, between which a peptide bond is formed. The link between the previous amino acid and its tRNA (in our example, between glycine and Gly tRNA) is broken.

Then the ribosome moves one more triplet, and as a result of the translocation of the tRNA that was on R -site (in our example, Gly tRNA), is outside the ribosome and is cleaved off from mRNA. A - the site is released, and the working cycle of the ribosome begins anew.

Termination. The essence of termination is the completion of the synthesis of the polypeptide chain.

Eventually, the ribosome reaches an mRNA codon that no tRNA (and no amino acid) matches. There are three such nonsense codons: UAA (“ocher”), UAG (“amber”), UGA (“opal”). At these mRNA codons, the working cycle of the ribosome is interrupted, and the growth of the polypeptide stops. The ribosome, under the influence of certain proteins, is again divided into subunits.

Protein modification.

As a rule, the synthesized polypeptide undergoes further chemical transformations. The original molecule can be cut into separate fragments; then some fragments are crosslinked, others are hydrolyzed to amino acids. Simple proteins can combine with a wide variety of substances to form glycoproteins, lipoproteins, metalloproteins, chromoproteins, and other complex proteins. In addition, amino acids already in the composition of the polypeptide can undergo chemical transformations. For example, an amino acid proline, which is part of the protein procollagen, oxidized to hydroxyproline. As a result of procollagen formed collagen- the main protein component of connective tissue.

Protein modification reactions are not matrix-type reactions. These biochemical reactions are called stepped.

Energy of protein biosynthesis. Protein biosynthesis is a very energy intensive process. During aminoacylation of tRNA, the energy of one bond of the ATP molecule is expended, with codon-dependent binding of aminoacyl-tRNA, the energy of one bond of the GTP molecule is consumed, and when the ribosome moves one triplet, the energy of one bond of another GTP molecule is consumed. As a result, about 90 kJ / mol is spent on attaching an amino acid to a polypeptide chain. Hydrolysis of the peptide bond releases only 2 kJ/mol. Thus, during biosynthesis, most of the energy is irretrievably lost (dissipated in the form of heat).

1. 1. PURPOSE OF THE LESSON: to study the structure of the interphase nucleus in fixed preparations. Consider the features of the structure of cell nuclei with different functional activity. The main components of the nucleus are: the nuclear membrane (karyolemma), chromatin, nucleolus, nuclear juice. Under light microscopy, the nuclear envelope presents a clear line outlined from the side of the nucleus and cytoplasm. When considering the scheme of the ultramicroscopic structure of the nucleus, one should pay attention to the structural features of the karyolemma, to the connection of its membranes with the endoplasmic reticulum of the cytoplasm. Understand the morphological characteristics of chromatin and its chemical composition. Chromatin in the nucleus can be in the form of clumps - condensed chromatin or dispersed - dispersed chromatin. The different state of chromatin is an indicator of the biosynthetic activity of the cell. Cells that actively synthesize protein have a nucleus with dispersed chromatin and a well-developed nucleolus. In the nuclei of cells that do not synthesize protein, the chromatin is condensed, the nucleoli are poorly visible.
2. 2. Control questions: 1. Core. The concept of the interphase nucleus. Structural Components nuclei according to light and electron microscopy: nuclear membrane, chromatin, nucleolus, nuclear juice. The value and functions of the nucleus in the life of the cell. 2. Nuclear-cytoplasmic ratios in cells with different levels of metabolism. 3. Structure of the nuclear envelope in SM and EM. Molecular organization and functional value nuclear lamina. 4. Nuclear pore and nuclear pore complex. Participation in nuclear import and export of substances. 5. Chromatin of the interphase nucleus. Euchromatin and heterochromatin. Chromatin as an indicator of the biosynthetic activity of a cell. 6. Molecular organization DNA in chromosomes. Levels of chromatin packing. The role of histone proteins in providing the structure of chromatin and the implementation of genetic information. 7. Nucleolus. The structure of the nucleolus in SM and EM. The main components of the nucleolus. The role of the nucleolus in the synthesis of rRNA and the formation of ribosomes. 8. Synthesis and transport of biopolymers in the cell. Cellular conveyor in protein synthesis. Morphological characteristic cells that synthesize proteins. 9. Cell conveyor in the synthesis of carbohydrates and lipids. Morphological characteristics of a cell synthesizing carbohydrates and lipids.
3. 3. Preparation 1. Core structures. Ovary. Stained with hematoxylin-eosin. Under low magnification, make a general overview of the micropreparation, find a growing follicle with an egg. Under high magnification, find a large rounded cell - an egg and examine the structure of the nucleus. Pay attention to the nuclear membrane, the nucleolus, the state of chromatin. Draw an egg and label the structures of the interphase nucleus. Study the electron diffraction pattern of the nucleus. Sketch the structure of the karyolemma and the nuclear pore complex.
4. 4. Preparation 1. Core structures. Ovary. Egg. Hematoxylin-eosin staining
5. 5. Specimen 2. Pancreas. Stained with hematoxylin-eosin. A cell that synthesizes protein. Under low magnification, make a general overview of the micropreparation and find the exocrine part of the pancreas. Under high magnification, consider one cell, paying attention to the presence of the nucleolus and euchromatin in the nucleus, note the basophilia of the cytoplasm in the basal part of the cell and oxyphilia in the apical part.
6. 6. Preparation 2. Pancreas. Hematoxylin-eosin staining. Cells that synthesize proteins
7. 7. Preparation 3. Liver. Glycogen in liver cells. CHIC reaction. A cell that synthesizes carbohydrates. Under low magnification, make a general overview of the micropreparation and find a group of hepatocytes. Under high magnification, consider clumps of red-violet glycogen in the cytoplasm of the hepatocyte.
8. 8. Preparation 3. Liver. Glycogen in liver cells. CHIC reaction. A cell that synthesizes carbohydrates.
9. 9. Preparation 4. Lipid inclusions in liver cells. Stained with osmic acid. A cell that synthesizes lipids. Under low magnification, make a general overview of the micropreparation and find a group of hepatocytes. Under high magnification, consider the cytoplasm of the hepatocyte, paying attention to the black-colored lipid droplets.
10. 10. Preparation 4. Lipid inclusions in liver cells. Stained with osmic acid. Cells that synthesize lipids.

Eukaryotic cells have a developed system internal structures surrounded by membranes called organelles

Each organelle has a unique composition of (glyco)proteins and (glyco)lipids and performs a specific set of functions.

Each organelle contains one or more membrane-bound compartments.

Organelles perform their functions autonomously or in groups

During endocytosis and exocytosis, transported proteins (cargo proteins) are transported between compartments through transport vesicles, which are formed by budding from the surface of the organelle and then fuse with the target membrane of the acceptor compartment.

Transport vesicles can selectively include transported material and exclude those components that should remain in the organelle from which the vesicles are formed.

Selective incorporation into vesicles is provided by signals present in the primary structure of the protein or in the carbohydrate structure

Transport vesicles contain proteins that guide them to their destinations and binding sites. Subsequently, the vesicles fuse with the acceptor site of the membrane

One of characteristic features eukaryotic cell is the presence in it of a developed system of internal structures surrounded by membranes called organelles. Eukaryotic cells are characterized by the presence of membranes that divide their internal contents into functionally different compartments, while all cells of living organisms have an outer two-layer membrane.

One of the advantages compartmentalization is that the cell has the ability to create the necessary environment to perform functions that require a certain chemical composition environment.

Illustrated structure and diversity organelle, having a membrane, which are usually present in a eukaryotic cell (in this case, in a typical animal cell). Each organelle contains one or more compartments. For example, the endoplasmic reticulum (ER) is one compartment; on the contrary, the Golgi apparatus consists of several compartments surrounded by membranes that have certain biochemical functions.

Mitochondria are characterized by two compartment, matrix and intermembrane space containing a set of specific macromolecules.

The cytosol can be considered one compartment, limited by the plasma membrane and in contact with the outer part of the membrane of all intracellular organelles. The cytoplasm consists of the cytosol and organelles. Similarly, the nucleoplasm is bounded by the inner nuclear membrane.

Each organelle contains unique set of proteins(both membrane and soluble), lipids and other molecules necessary to perform its functions. Some lipids and proteins are covalently linked to oligosaccharides. As cells grow and divide, their new components must be synthesized, which are necessary for growth, division, and the final distribution of intracellular material between two daughter cells. During cell differentiation and development, as well as in response to external factors such as stress, organelle components are synthesized.

However Components are not always formed in the organelle where they function. Typically, various macromolecules are formed at sites specially designed for their synthesis. For example, most proteins are formed on the ribosomes of the cytosol, which is the optimal environment for ribosome function and protein synthesis.

The following question arises: how the components organelle get into their places of operation? Since the early 1970s this question was central in cell biology. As shown in the figure below, there are at least eight major types of organelles, each made up of hundreds or thousands of different proteins and lipids.

All these molecules must be transported into organelles in which they perform their functions. Most are formed in the cytosol, and therefore the question arises: how are they delivered to the corresponding organelles or exit the cell if they belong to secreted proteins? In many cases, the answer to this question is the presence of special signals in the protein molecule, usually called sorting signals or addressing signals. They are short sequences of amino acids present in the primary structure of those proteins that should not be localized in the cytosol. Each destination address of a protein molecule is associated with one or more various types signals.

Sort signals are recognized special cell systems as the protein moves towards its destination. As shown in the figure below, there are two main transport mechanisms: exocytosis (or secretory pathway) and endocytosis, in which material (cargo) is transported from the cell and into the cell, respectively.

For all newly synthesized proteins, intended for secretion from the cell, or for entry into organelles by exo- or endocytosis, there is a common entry point on the ER membrane. Signal sequences serve as signals for protein translocation across the ER membrane. In this chapter, we will look at the sorting signals that direct proteins to their destinations.

Being in EPR, the protein cannot be transported through the cytoplasm, and the only way for it to reach other organelles surrounded by membranes is vesicular transport. Transport vesicles are primarily composed of proteins and lipids and are said to "bud" from the membrane. After a vesicle has budded, it fuses with the next compartment in its path. The compartment from which the vesicle originated is usually called the donor compartment (or source compartment) and the destination (or target) compartment is usually called the acceptor compartment.

Transport vesicles directly or indirectly transfer proteins from the ER to all other compartments on the way of exo- or endocytosis. During endocytosis, vesicles form on the plasma membrane. These vesicles transport the material contained in them to endosomes, from which other vesicles are formed, carrying the material to other compartments. Thus, the composition of transport vesicles differs depending on their origin and destination compartment.

Vesicular transport creates a problem for the organelles with which the vesicles exchange. For normal functioning, a certain internal composition of organelles must be maintained. However, how can this be achieved if the vesicles change this composition all the time? The scale of the problem becomes apparent when calculating transport efficiency. Through the endocytosis pathway, the amount of membrane proteins and lipids equivalent to their total content in the plasma membrane can be transported through the organelles in less than an hour. When compared to the time it takes to synthesize a new organelle (usually one day), this speed is impressive.

Solution to this Problems related to the selectivity of the transport process. When budding, only those proteins that need to be transported pass into the vesicle. Resident proteins of the organelle do not enter the vesicle. The vesicle holds these proteins and passes them on to the next vesicle in its path. To maintain homeostasis between organelles, by its nature, vesicular transport must always be bidirectional, i.e. components of the donor compartment must not be continuously transferred to the acceptor compartment.