As a rule, it is proteins that are responsible for the functional activity of membranes.
Such proteins include a variety of enzymes, transport proteins, receptors, channels, proteins that form pores (aquaporins), that is, a variety of protein structures that provide the unique functions of each membrane.
Membrane proteins can be divided into three groups according to their biological role:
I - enzyme proteins with catalytic activity,
II - receptor proteins that specifically bind certain substances,
III - structural proteins.
Enzyme proteins
The most common among all membrane proteins. They include both integral (membrane ATPases) and peripheral (acetylcholinesterase, acid and alkaline phosphatases, RNase) proteins.
Enzymes are large molecules, while the sizes of molecules of substances (substrates) that enter into enzymatic reactions are usually thousands of times smaller. The enzyme interacts with the substrate with a small area of its surface - the active site. The specificity of an enzyme is always determined by the extent to which the surface of its active center corresponds to the surface of the substrate. This principle of structural correspondence is also widely used in the work of cell membrane proteins. In addition, it should be taken into account that the conformation of proteins penetrating the membrane depends on the membrane bilayer, so that their enzymatic activity is also controlled by membrane lipids. This control can be implemented due to both the effect on the affinity for substrates or their availability, and the effect on the life span (strength) of protein associates of membrane enzymes formed in the cell membrane.
Enzymes are part of both plasma and intracellular membranes. For example, on the outer membrane of the epithelial cells lining the digestive organs, there are enzymes that break down nutrients even before they get inside the cell (this process, discovered by the Russian physiologist A.M. Ugolev, is called “membrane digestion”). The outer membrane of liver cells contains more than 20 different enzymes.
Membrane enzymes need to be in contact with their surrounding lipids. When they are removed from the lipid environment (for example, when lipids are extracted from the membrane with non-polar solvents), the work of membrane enzymes is disrupted (the kinetics or the nature of the influence of foreign substances change, or it stops altogether). The activity of such membrane enzymes can be partially restored if lipid micelles are added to them.
An analysis of the nature of lipids that activate membrane enzymes demonstrates the absence of strict specificity - the determining factor is the hydrophilic-lipophilic coefficient of the lipid mixture. In some cases, it is possible to activate a delipidated enzyme even with a detergent. However, such a reactivated enzyme loses the ability to perceive regulatory signals from the outside, which controlled its work in the "living" membrane.
The activating effect of lipids on membrane enzymes can be at least twofold. First, in the presence of lipids, the shape of the membrane enzyme molecule can change, so that its active site becomes accessible to the substrate. Secondly, lipids can play the role of an organizer of an ensemble or conveyor consisting of many enzymes.
Membrane enzyme molecules contain large nonpolar hydrophobic regions. Therefore, in the aquatic environment, they aggregate, due to which most of the active centers are masked. In the presence of lipids, membrane enzymes are organized into ensembles surrounded by annular lipid molecules, and their enzymatic activity can be fully manifested. For the normal operation of membrane enzymes, it is essential that the lipids surrounding them be in a liquid state of aggregation.
Receptor proteins
Receptor proteins are proteins that specifically bind certain low molecular weight substances. When binding specific ligands, receptor proteins reversibly change their shape. These changes trigger responses inside the cell chemical reactions. In this way, the cell perceives various signals coming from the external environment and responds to them.
Receptor proteins and proteins that determine the cell's immune response, antigens, can also be both integral and peripheral components of the membrane.
Often, receptors are part of more complex membrane complexes containing executor proteins. For example, the cholinergic receptor receives a signal from a neurotransmitter and transmits it to a channel-forming protein. This reaction opens the membrane permeability to sodium and potassium ions and forms an excitatory potential.
TO membrane proteins include proteins that are embedded in or associated with the cell membrane or the membrane of a cell organelle. About 25% of all proteins are membrane proteins.
Biochemical classification
According to the biochemical classification, membrane proteins are divided into integral And peripheral.
- Integral membrane proteins are firmly embedded in the membrane and can only be removed from the lipid environment with the help of detergents or non-polar solvents. In relation to the lipid bilayer, integral proteins can be transmembrane polytopic or integral monotopic.
- Peripheral membrane proteins are monotopic proteins. They are either bound by weak bonds to the lipid membrane or are associated with integral proteins by hydrophobic, electrostatic, or other non-covalent forces. Thus, unlike integral proteins, they dissociate from the membrane when treated with an appropriate aqueous solution (eg, low or high pH, high salt concentration, or chaotropic agent). This dissociation does not require the destruction of the membrane.
Membrane proteins can be built into the membrane due to fatty acid or prenyl residues or glycosylphosphatidylinositol attached to the protein during their post-translational modification.
Another important point is the methods of attaching proteins to the membrane:
1. Binding to proteins immersed in the bilayer. Examples include the F1 part of H + -ATRase, which binds to the Fo part embedded in the membrane; some cytoskeletal proteins can also be mentioned.
2. Binding to the bilayer surface. This interaction is primarily electrostatic in nature (eg myelin basic protein) or hydrophobic (eg surfactant peptides and possibly phospholipases). On the surface of some membrane proteins there are hydrophobic domains that are formed due to the features of the secondary or tertiary structure. These surface interactions can be used in addition to other interactions such as transmembrane anchoring.
3. Binding with a hydrophobic "anchor"; this structure usually appears as a sequence of non-polar amino acid residues (for example, in cytochrome 65). Some membrane proteins use covalently linked fatty acids or phospholipids as anchors.
4. Transmembrane proteins. Some of them cross the membrane only once (for example, glycophorin), others - several times (for example, lactose permease; bacteriorhodopsin).
Membrane lipids
Membrane lipids are amphipathic molecules that spontaneously form bilayers. Lipids are insoluble in water, but readily soluble in organic solvents. In most animal cells, they make up about 50% of the mass of the plasma membrane. There are approximately 5 x 100 thousand lipid molecules in a 1 x 1 μm section of the lipid bilayer. Therefore, the plasma membrane of a small animal cell contains approximately 10 lipid molecules. There are three main types of lipids in the cell membrane:
1) phospholipids (the most common type); complex lipids containing glycerol, fatty acids, phosphoric acid and a nitrogenous compound.
A typical phospholipid molecule has a polar head and two hydrophobic hydrocarbon tails. The length of the tails varies from 14 to 24 carbon atoms in the chain. One of the tails usually contains one or more cis double bonds (unsaturated hydrocarbon), while the other (saturated hydrocarbon) has no double bonds. Each double bond causes a kink in the tail. These differences in tail length and saturation of hydrocarbon chains are important because they affect membrane fluidity.
Amphipathic molecules in an aqueous environment usually aggregate, with hydrophobic tails being hidden and hydrophilic heads remaining in contact with water molecules. Aggregation of this type is carried out in two ways: either by the formation of spherical micelles with tails turned inward, or by the formation of bimolecular films, or bilayers, in which hydrophobic tails are located between two layers of hydrophilic heads.
The two main phospholipids that are present in plasma are phosphatidylcholine (lecithin) and sphingomyelin. Synthesis of phospholipids occurs in almost all tissues, but the main source of plasma phospholipids is the liver. The small intestine also supplies plasma with phospholipids, namely lecithin, as part of the chylomicrons. Most of the phospholipids that enter the small intestine (including in the form of complexes with bile acids) are subjected to preliminary hydrolysis by pancreatic lipase. This explains why polyunsaturated lecithin added to food does not affect plasma phospholipid content of linoleate more than equivalent amounts of corn oil triglycerides.
Phospholipids are an integral component of all cell membranes. Phosphatidylcholine and sphingomyelin are constantly exchanged between plasma and erythrocytes. Both of these phospholipids are present in plasma as constituents of lipoproteins, where they play a key role in maintaining non-polar lipids such as triglycerides and cholesterol esters in a soluble state. This property reflects the amphipathic nature of phospholipid molecules - nonpolar fatty acid chains are able to interact with the lipid environment, and polar heads - with the aqueous environment (Jackson R.L. ea, 1974).
2) Cholesterol. Cholesterol is a sterol containing a four-ring steroid nucleus and a hydroxyl group.
This compound is found in the body both as a free sterol and as an ester with one of the long chain fatty acids. Free cholesterol is a component of all cell membranes and is the main form in which cholesterol is present in most tissues. The exceptions are the adrenal cortex, plasma and atheromatous plaques, where cholesterol esters predominate. In addition, a significant part of the cholesterol in the intestinal lymph and in the liver is also esterified.
Cholesterol is found in lipoproteins either in free form or as esters with long-chain fatty acids. It is synthesized in many tissues from acetyl-CoA and excreted in bile as free cholesterol or bile salts. Cholesterol is a precursor to other steroids, namely corticosteroids, sex hormones, bile acids, and vitamin D. It is a compound typical of animal metabolism and is found in significant amounts in animal products such as egg yolk, meat, liver and brain.
Eukaryotic plasma membranes contain a fairly large amount of cholesterol - approximately one molecule for each phospholipid molecule. In addition to regulating flow, cholesterol increases the mechanical strength of the bilayer. Cholesterol molecules are oriented in the bilayer in such a way that their hydroxyl groups are adjacent to the polar heads of phospholipid molecules.
3) glycolipids
Glycolipids are lipid molecules belonging to the class of oligosaccharide-containing lipids that are found only in the outer half of the bilayer, and their sugar groups are oriented towards the cell surface.
Glycolipids are sphingolipids in which a fatty acid residue is attached to the NH group of sphingazine, and the following groups are attached to the oxygen of sphingazine: oligosaccharide chains, Gal, Glc, GalNAc (neuraminic acid) - gangliosides. Gal or Glc are cerebrosides. sulfosaccharides Glc-SO3H, Gal-SO3H are sulfolipids.
Glycolipids are found on the surface of all plasma membranes, but their function is unknown. Glycolipids make up 5% of the lipid molecules of the outer monolayer and vary greatly between different types and even in different tissues of the same species. In animal cells, they are synthesized from sphingosine, a long amino alcohol, and are called glycosphingolipids.
Their structure is generally similar to the structure of phospholipids formed from glycerol. All glycolipid molecules differ in the number of sugar residues in their polar heads. One of the simplest glycolipids is galactocerebroside.
Classification
Membrane proteins can be classified according to topological or biochemical principles. The topological classification is based on the location of the protein in relation to the lipid bilayer. Biochemical classification is based on the strength of the interaction of the protein with the membrane.
Different categories of polytopic proteins. Membrane binding via (1) single transmembrane alpha helix, (2) multiple transmembrane alpha helices, (3) beta sheet structure.
Various categories of integral monotopic proteins. Membrane binding by (1) amphipathic alpha helix parallel to the plane of the membrane, (2) hydrophobic loop, (3) covalently linked fatty acid residue, (4) electrostatic interaction (direct or calcium mediated).
Topological classification
In relation to the membrane, membrane proteins are divided into poly- and monotopic.
- Polytopic or transmembrane proteins completely penetrate the membrane and thus interact with both sides of the lipid bilayer. As a rule, the transmembrane fragment of a protein is an alpha helix, consisting of hydrophobic amino acids (possibly from 1 to 20 such fragments). Only in bacteria, as well as in mitochondria and chloroplasts, transmembrane fragments can be organized as a beta-sheet structure (from 8 to 22 turns of the polypeptide chain).
- Integral monotopic proteins permanently embedded in the lipid bilayer, but connected to the membrane only on one side without penetrating to the opposite side.
Biochemical classification
According to the biochemical classification, membrane proteins are divided into integral And peripheral.
- Integral membrane proteins are firmly embedded in the membrane and can only be removed from the lipid environment with the help of detergents or non-polar solvents. In relation to the lipid bilayer, integral proteins can be transmembrane polytopic or integral monotopic.
- Peripheral membrane proteins are monotopic proteins. They are either bound by weak bonds to the lipid membrane or are associated with integral proteins by hydrophobic, electrostatic, or other non-covalent forces. Thus, unlike integral proteins, they dissociate from the membrane when treated with an appropriate aqueous solution (eg, low or high pH, high salt concentration, or chaotropic agent). This dissociation does not require the destruction of the membrane.
Membrane proteins can be built into the membrane due to fatty acid or prenyl residues or glycosylphosphatidylinositol attached to the protein during their post-translational modification.
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The proportion of protein in the total mass of the membrane can vary over a very wide range - from 18% in myelin to 75% in the mitochondrial membrane.
According to their location in the membrane, proteins can be divided into: integral And peripheral.
Integral proteins are generally hydrophobic and easily integrated into the lipid bilayer.
The interaction of such a protein with the membrane occurs in several stages. Protein first adsorbed on the surface of the bilayer changes its conformation by establishing hydrophobic contact with the membrane. Then comes insertion of the protein into the bilayer. The depth of penetration depends on the strength of the hydrophobic interaction and the ratio of hydrophobic and hydrophilic regions on the surface of the protein globule. The hydrophilic regions of the protein interact with the membrane layers on one or both sides of the membrane. The fixation of the protein globule in the membrane occurs due to electrostatic and hydrophobic interactions. The carbohydrate part of the protein molecules (if any) protrudes. Integral proteins, due to their close connection with the bilayer, have a significant effect on it: conformational rearrangements of the protein lead to a change in the state of lipids, the so-called deformation of the bilayer.
Peripheral proteins have a smaller depth of penetration into the lipid bilayer, and, accordingly, interact more weakly with membrane lipids, having a much lesser effect on them than integral ones.
According to the nature of interaction with the membrane, proteins are divided into monotopic, bitopic, polytopic :
monotopic proteins interact with the membrane surface (mono - one of the layers of lipids);
biotopic penetrate the membrane through (bi - two layers of lipids);
polytopic penetrate the membrane several times (multiple interaction with lipids).
It is clear that the former belong to peripheral proteins, and the latter and third to integral ones.
Membrane proteins can also be classified according to their function. In this regard, structural proteins are isolated:
proteins - enzymes;
proteins - receptors;
transport proteins.
Cell cytoskeletal proteins constitute a special group. Strictly speaking, these proteins are not components of the membrane, adjoining it from the cytoplasmic side. Cytoskeleton proteins are part of all its components: myofilaments contain actin protein molecules; microtubules contain tubulin protein, intermediate filaments also contain a more polymorphic protein complex. The cytoskeleton not only provides membrane elasticity and resists changes in cell volume, but, apparently, participates in various intra- and extracellular regulatory mechanisms.
As a rule, it is proteins that are responsible for the functional activity of membranes. These include a variety of enzymes, transport proteins, receptors, channels, and pores. Prior to this, it was believed that membrane proteins have exclusively β - folded structure of the secondary structure of the protein, but these works have shown that membranes contain a large number of α - helices. Further studies have shown that membrane proteins can penetrate deeply into the lipid bilayer or even penetrate it and their stabilization is carried out due to hydrophobic ...
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Lecture 5
The structure and functions of membrane proteins
Cell membranes contain protein from 20 to 80% (by weight). As a rule, it is proteins that are responsible for the functional activity of membranes. These include a variety of enzymes, transport proteins, receptors, channels, pores, etc. etc., which ensure the uniqueness of the functions of each membrane. The first advances in the study of membrane proteins were achieved when biochemists learned to use detergents to isolate membrane proteins in a functionally active form. These were works on the study of enzyme complexes of the inner membrane of mitochondria. Prior to this, it was believed that membrane proteins have exclusively β-folded structure (the secondary structure of the protein), but these works have shown that membranes contain a large number of α-helices. Much less common is the β-helix, which, however, is of great biological importance. The fact is that in the areas surrounded by lipids, the β-helix is a hollow cylinder, in the outer wall of which non-polar (hydrophobic) amino acid residues are concentrated, and in the inner wall - hydrophilic ones. Such a cylinder could form a channel in the membrane through which ions and water-soluble substances can freely pass. Further studies have shown that membrane proteins can penetrate deeply into the lipid bilayer or even permeate it and their stabilization is carried out due to hydrophobic interactions. There are at least four types of protein arrangement in membranes: The first type is transmembrane, when the protein permeates the entire membrane, and the hydrophobic region of the protein has an α-configuration. A similar arrangement in the membrane has a bacteriorhodopsin molecule from Halobacterium halobium its α-helices sequentially cross the bilayer; The second type is binding with the help of a hydrophobic anchor, when the protein has a short section consisting of hydrophobic amino acid residues near the carboxyl end. This is the so-called hydrophobic anchor, which can be removed by proteolysis, and the released protein becomes water-soluble. This arrangement in the membrane is inherent in many cytochromes. The third type is binding to the surface of the bilayer, when the interaction of proteins is primarily of an electrostatic or hydrophobic nature. This type of interaction can be used as an addition to other interactions, such as transmembrane anchoring. The fourth type is binding to proteins embedded in the bilayer, this is when some proteins bind to proteins that are located inside the lipid bilayer. For example, F 1 - part H + - ATPase, which binds to F0 - a part immersed in the membrane, as well as some proteins of the cytoskeleton.
At the core contemporary ideas The structure of membrane proteins is based on the idea that their polypeptide chain is folded so that a non-polar, hydrophobic surface is formed in contact with the non-polar region of the lipid bilayer. The polar domains of a protein molecule can interact with the polar heads of lipids on the surface of the bilayer. Many proteins are transmembrane and span the bilayer. Some proteins appear to be associated with the membrane only through their interaction with other proteins.
Many membrane proteins typically bind to the membrane through non-covalent interactions. However, there are proteins that are covalently linked to lipids. Many plasma membrane proteins belong to the class of glycoproteins. Carbohydrate residues of these proteins are always located on the outside of the plasma membrane.
Usually, membrane proteins are divided into external (peripheral) and internal (integral). In this case, the criterion is the degree of severity of processing required to extract these proteins from the membrane. Peripheral proteins are released when membranes are washed with buffer solutions of low ionic strength, low or, conversely, high pH, and in the presence of chelating agents (eg, EDTA) that bind divalent cations. It often happens that it is very difficult to distinguish peripheral membrane proteins from proteins bound to the membrane during isolation.
Detergents or even organic solvents must be used to release integral membrane proteins.
Many eukaryotic and prokaryotic membrane proteins are covalently linked to lipids that are attached to the polypeptide after translation.
Membrane proteins covalently linked to lipids
(BUT) Proteins to which myristic acid is attached
(B) Proteins to which palmitic acid is attached
(IN) Proteins with a glycosylphosphatidylinositol anchor
4. Adhesive molecule of nerve cells |
In some cases, these lipids act as a hydrophobic anchor by which the protein is attached to the membrane. In other cases, lipids are likely to function as an assistant in protein migration to the appropriate region of the cell or (as in the case of viral envelope proteins) in membrane fusion.
In prokaryotes, Brown's lipoprotein, the main lipoprotein of the outer membrane, is most fully characterized. E. coli . The mature form of this protein contains acylglycerol, which is linked by a thioether bond to N - terminal cysteine. Besides, N - the terminal amino acid is linked to the fatty acid by an amide bond. The membrane-bound form of penicillase attaches to the cytoplasmic membrane via N - terminal acylglycerol similar to membrane lipoproteins.
Eukaryotic membrane proteins are covalently linked to lipids, as shown in the table, and can be divided into three classes. Proteins of the first two classes, apparently, are localized mainly on the cytoplasmic surface of the plasma membrane, and proteins of the third class on the outer surface.
There are membrane proteins that are covalently linked to carbohydrates. These include surface proteins of cells mainly performing the functions of transport and reception. It is still unclear what is the matter here. Perhaps this is due to the fact that proteins need to be sorted when directed to the plasma membrane. Sugar residues can protect the protein from proteolysis or participate in recognition or adhesion. Therefore, sugar residues in membrane glycoproteins are localized exclusively on the outer side of the membrane.
Two main classes of oligosaccharide structures of membrane glycoproteins can be distinguished: 1) N - glycosidic oligosaccharides associated with proteins through the amide group of aspargine; 2) O-glycosidic oligosaccharides linked through the hydroxyl groups of serine and threonine. This class of oligosaccharides consists of three subclasses.
- A simple or mannose-enriched complex in which the oligosaccharide contains mannose and N - acetylglucosamine.
- A normal complex in which the mannose-enriched core has additional side branches containing other saccharide residues, such as sialic acid.
- Large complex that is associated with the anionic transporter of the erythrocyte membrane
Most membrane glycoprotein oligosaccharides belong to subclass 1 or 2.
Membrane proteins of bacteria
As noted above, proteins in the cytoplasmic membrane make up about 50% of its surface. Approximately 10% of the membrane is formed by tightly bound protein–lipid complexes. A molecule of any protein built into the membrane is surrounded by 45-130 or more lipid molecules. About half of the free lipids are associated with peripheral membrane proteins.
Protein composition cytoplasmic membrane bacteria is more diverse than lipid. So, in the cytoplasmic membrane E. coli K 12 about 120 different proteins have been found. Depending on the orientation in the membrane and the nature of the connection with the lipid bilayer, as noted above, proteins are divided into integral and peripheral. The peripheral proteins of bacteria include a number of enzymes such as NADH - dehydrogenase, malate dehydrogenase, etc., as well as some proteins that are part of the ATPase complex. This complex is a group of protein subunits located in a certain way, contacting the cytoplasm, periplasmic space and forming a channel in the membrane through which the proton passes. The section of the complex marked F1 , is immersed in the cytoplasm, a and c are components of the site F0 - the hydrophobic sides of the molecules are immersed in the membrane. Subunit b partially immersed in the membrane with its hydrophobic part and carries out the connection of the membrane and cytoplasmic parts of the enzyme complex, as well as the connection of ATP synthesis in the area F1 with the proton potential in the membrane. subunits a, b and c provide a proton channel. Other components of the complex ensure its structural and functional integrity.
to integral proteins E. coli, which lipids are necessary for the manifestation of enzymatic activity, can be attributed to succinate dehydrogenase, cytochrome b . The antibiotics gramicidin A, alamethicin, amphotericin and nystacin have very interesting properties. When interacting with the bacterial membrane, they become integral proteins (antibiotics are polypeptides and macrocycles).
Gramicidin A is a hydrophobic peptide composed of 15 L-D -amino acids. When embedded in the membrane, it forms channels that allow monovalent cations to pass through. This channel, which forms gramicidin A, is the most fully characterized. The channel is formed by two molecules of gramicidin A. As a result of the alternation L- and D - amino acids, a helix is formed in which the side chains are located outside, and carboxyl groups the core is inside the channel. This type of helix is not found in any other proteins and is formed due to the unusual alternation of stereoisomers of amino acids in gramicidin A. The gramicidin channel, as noted above, is cation-selective. Small inorganic and organic cations pass through it, while the permeability Cl is equal to zero.
Alameticin is a peptide antibiotic of 20 amino acid residues that can form electrically excitable channels in the membrane. The amino acid sequence of alamethicin includes the unusual residues -α-aminobutyric acid and L -phenylalanine. When bound to the membrane, unlike gramicidin A, it forms a pore. It is much smaller than the channel that gramicidin A forms. First of all, this is due to the fact that the space around the α-helix is too small for an ion to pass through it.
Marcolide antibiotics such as nystatin and amphotericin bind to cholesterol and form channels. The channels are formed by 8–10 molecules of these polyene antibiotics, through which, however, ions penetrate at low rates.
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