cell wall

Used to protect and support cells

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Cell wall is located at cell membrane The outer layer is thick, tough and slightly elastic, and its composition is clay complex. Some species also have polysaccharide Capsule , playing a protective role. Capsule It can also be used as a cell Nutrients , on Nutritional deficiency It can be used by cells.^[1]

Chinese name: cell wall
Foreign name: cell wall

Location: Outside the cell membrane, next to the cell membrane
Limits: Prokaryotes and Eukaryotes
Time of occurrence: About 3.5 billion years ago

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plant cell

Fossil research shows that the earth appeared about 3.5 billion years ago Prokaryotic cell About 1.2 to 1.4 billion years ago Eukaryotic cell 。 There are two main hypotheses about the origin of eukaryotic cells: one is“ Endosymbiosis hypothesis ”It is believed that various parts of eukaryotic cells originate from several symbiotic prokaryotic cells, and aerobic bacteria penetrate Heterotrophic Anaerobic prokaryote Becomes mitochondrion ， blue-green algae Penetrate into chloroplast ， Spirochete Penetrate into flagella and cilia; First“ plasma membrane Inner fold Hypothesis ", it is believed that with the increase of the volume of the ancient prokaryotes, the plasma membrane will be folded to form endoplasmic reticulum, etc Organelle , and nuclear membrane cause nucleus Etc; At the same time, DNA It replicates into many copies, and the plasma membrane folds around the DNA, finally forming organelles with double membranes, such as mitochondria, chloroplasts, etc. The shape and size of cells vary with the species of organisms, even though the shape of cells in different parts of the same organism is different. Isolated cells are usually spherical or nearly spherical, but because cell surface Tension or Protoplasm Viscosity and Nonuniformity The shape of cells sometimes changes. The cells constituting the tissue are affected by the mechanical force between adjacent cells and directional The restriction of often takes on different forms. Generally speaking, cell morphology is related to physiological function Closely related. The size of different kinds of cells varies greatly. Bacterial cells are generally 0.5~1.5 microns in diameter and 1~5 microns in length, seed plant The diameter of the young cells of, Mature cell The diameter is 15-65 μ m. Higher animal Of Histiocyte Usually plant cell Small, but Egg cell They are generally large. The largest known cell is the ostrich egg cell, with a diameter of about 5cm. yes , we have nerve cell The protuberance of can be more than 1 meter long. The smallest cell is mycoplasma Cells, only 0.1 microns in diameter. Unicellular organism It is composed of one cell. multicellular organisms The number of cells varies greatly. About

That is, one million billion cells; About adults

× 16 means 1600 trillion cells, nearly 100 types. Although biological cell types are diverse, they have roughly the same Basic structure ， Eukaryotic cell Of Submicrostructure Can be divided into surface structure And internal structure. Surface structure includes cell membrane And the outer material layer, such as the cell wall of plant cells and some Animal cell Of Cell envelope ； The internal structure includes the nucleus and cytoplasm 。 The nucleus consists of nuclear membrane Nucleoplasm , nucleolus and Chromatin form. In cytoplasm Undifferentiated Translucency of Colloidal state The solution is called matrix; Various fine structures with special functions are called organelles, such as mitochondria Plastid 、 Endoplasmic reticulum 、 ribosome 、 Golgi apparatus 、 lysosome 、 Centrosome 、 microtubule 、 Microfilament , flagella and cilia, etc; The cytoplasm often contains many Metabolites Formed particles, such as Starch granule 、 Aleurone granule , fat drops, glycogen granules, etc Metaplasm 。 Cells are Life activities The basic process of some life activities, such as Material metabolism 、 energy conversion , movement, development, reproduction and heredity are all based on cells.

Cell wall of Gram positive bacteria

Peptidoglycan

structure

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The cell wall is divided into three layers, namely Intercellular layer (middle floor) Primary wall and Secondary wall 。 The intercellular layer binds adjacent cells together to form tissue. The primary wall is on both sides of the intercellular layer plant cell Both. The secondary wall is inside the primary wall, which is divided into three layers: outer (S1), middle (S2) and inner (S3). Some plants have Tumor layer , also differentiated with special structures, such as Pit and Tumor Etc. The pit is the area of intercellular material flow, while the tumor is the protrusion on the inner layer of the secondary wall.

The structure of cell wall is generally divided into the following three layers

1. The intercellular layer: cell division When new cells are produced, they form a common membrane between two adjacent cells. Its main component is soft colloidal particles Pectin 。 Intercellular layer It can not only stick adjacent cells together, but also buffer the extrusion between cells without blocking Cell growth 。

Plant cell wall under electron microscope

2. Primary wall: after the formation of intercellular layer at the end of cell division, Protoplast It secretes cellulose hemicellulose And a small amount of pectin added to the intercellular layer to form the primary wall of cells. The primary wall is elastic and can increase its area with the growth of cells. The cell wall formed during cell growth is called primary wall, and all plant cells have Primary wall 。

3. Secondary wall: After the cell stops growing, the protoplast continues to secrete cellulose and other substances, which are added in the primary wall to thicken the cell wall. This thickened cell wall is called the secondary wall. The secondary wall is added to the primary wall. The thicker the secondary wall, the smaller the cell cavity in the wall. Secondary wall It is only found in some cells of plants. Thick wall Of Fibroblast 、 Stone cell 、 Tracheid And vessels, etc.

The main component of cell wall is cellulose, which forms the framework of cell wall and contains other substances. stay electron microscope As you can see below, this frame consists of layers of cellulose Microfilament , short for Microfibril Each layer of microfibrils is basically arranged in parallel. Each layer is added, the orientation of microfibrils is different, so the arrangement of microfibrils between layers is staggered Networking 。 The space between microfilaments is usually filled with other materials.

In addition, in some Plant epidermis In the cell wall, there are wax, cutin Corky 。 It is often deposited in some mature and thickened cell walls Lignin 。 stay Gramineae 、 Equisetaceae Vegetal Epidermal cell The wall contains silicon. In the cell wall of fungi chitin 。

Secondary wall

There are pits on the cell wall because during the cell growth, the secondary wall extends continuously with the cell growth, but the wall thickening is uneven, forming many thin wall areas, called Nascent pit field ； When cells produce secondary walls, the thickening is also uneven. Generally, the primary pit field is no longer thickened, and the cell wall forms a pit structure. Adjacent cell wall Pit It often corresponds to topographic texture Hole pair 。 There are single holes and Marginal pit Two. There are usually many Plasmodesmata Through the pit, plasmodesmata and cytoplasm In Endoplasmic reticulum Connection, so as to communicate the material exchange between cells, which is conducive to water transport. Therefore, the pit on the cell wall is a channel for cell to cell communication, which makes the whole plant Life activities China can become an organic unity.

The formation of new cell walls is at the end of cell division Equatorial surface Up, split mother cells Preformation Film-forming body 。 When the chromosome divides into two poles, Golgi apparatus Detached Vesicle And microtubule Gather on the equatorial plane to become Cell plate 。 New polysaccharide substances are gradually formed when they are deposited on the cell plate Intercellular layer 。 Then some cellulose components are synthesized in the cell Microfibril The primary wall occurs when it is deposited on both sides of the intercellular layer. When cells mature and stop growing, new cellulose and hemicellulose The secondary wall is formed when lignin is added to the primary wall in succession. Each layer of primary wall is added, the direction of micro fiber arrangement can be different (longitudinal or transverse), forming an irregular interlaced network, which is called multi network growth. As a result of this thickening, the mechanical support of the whole plant body has a foundation.

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Cell wall Intercellular layer It is basically composed of pectin.

If plant tissue For pectin in pectinase When decomposed, the cells will disperse, because the primary wall is composed of water hemicellulose , pectin, cellulose, protein and lipid form. Coleoptile , stems, leaves, hairs and other primary walls average value See table. About 90% of the components constituting the cell wall are polysaccharides, and about 10% are proteins, enzymes and fatty acids. Polysaccharides in cell wall are mainly cellulose, hemicellulose and pectin Classes, which are defined by glucose 、 Arabinose 、 Galacturonic acid And so on. Secondary cell wall There are a lot of Lignin 。

And animal Extracellular matrix In a sense, the chemical composition of cell wall is collagen protein 、 Adhesin 、 amino Polysaccharide and Proteoglycan 。^[2]

Comparison Diagram of Plant and Animal Cells

Bacterial cell wall The main components are Peptidoglycan 。

Fungal cell wall The main components in chitin , cellulose glucan 、 Mannan These polysaccharides are polymers of monosaccharides.

formation

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The formation of cell wall is a variety of organelles Coordinative action Results. The formation of new cell walls begins cell division The late or early stage of. During cell division, between two sets of chromosomes, that is mother cells Of Equatorial plate There are many secretions of different sizes on the surface Vesicle (secret vesicles) are gathered together irregularly Golgi apparatus And endoplasmic reticulum, which is rich in various sugars that make up the cell wall. With the movement of microtubules in the direction perpendicular to the equatorial plate of the cell, they gradually orderly arrange into sheets to form a membrane last. Film-forming body The vesicles in the cell fuse and connect with each other to form the plasma membrane of the cell Inclusion Connected Cell plate This is the embryonic middle structure. After cell plate composition, Golgi apparatus Vesicle transport Wall building substances are released outside the plasma membrane to enrich the newly formed wall. When there is Pectin And a small amount of cellulose molecules are continuously filled and mixed to form the middle layer, and cellulose and hemicellulose And other substances form soft Primary wall At this time, two daughter cells are formed. Since then, cellulose molecules have been layered and directionally deposited on the inner side of the primary wall of most cells, and they are crisscrossed and reinforced clearly, which is an important basis for enhancing the support ability of plants. The directional layered deposition of cellulose molecules is related to the activity of microtubules. Colchicine can prevent the formation of microtubules and inhibit the directional arrangement of cellulose molecules. Another important role of microtubules is to keep some channels on the newly formed cell plate, that is, participate in Plasmodesmata The formation of protoplasm can maintain contact between two daughter cells.

It can be seen that the formation of cell wall is gradually completed in the process of division, growth and differentiation of living cells. During cell division and new cell wall formation Golgi apparatus , endoplasmic reticulum and microtubule Besides participation auxin And a variety of enzymes, and all activities rely on mitochondria to provide energy, which reflects the characteristics of various parts of the cell to cooperate with each other to complete life activities.

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Bacterial cell wall

Bacterial cell wall The main components are Peptidoglycan (peptidoglycan), also known as Mucopeptide (mucopetide)。 Cell wall mechanical strength It depends on the existence of peptidoglycan. The ability to synthesize peptidoglycan is unique to prokaryotes. Peptidoglycan is composed of two aminosaccharides, n-acetylglucosamine and n-acetylcytosolic acid, through β - 1.4 Glycosidic bond Connect the polysaccharide scaffold formed by interval arrangement. Connect the tetrapeptide side chain on the n-acetyl muramic acid molecule, Peptide chain Between Peptide bridge Or peptide chain to form a highly mechanical Reticular structure 。 The peptidoglycan scaffolds of various bacterial cell walls are the same Side chain Its composition and connection mode vary with the strain.

Usually Gram staining technology Bacterial cell walls can be divided into two types, Gram G+and G -. Gram staining (Gram stay) Yes Denmark Doctor Gram（ Hans Christian Gram) used the procedures listed in Table 2-3 to stain bacteria in 1884, and the results showed that bacteria could be divided into two types due to different color development, namely gram-positive And negative.

Bacterial cell wall

Gram positive bacteria Tetrapeptide side chain of amino acid It is composed of d-Bing-d-gu-r-l-Lai-d-c. One more d-alanine at the end of the initially synthesized peptide chain Residue 。 Peptide bridge is one of five glycine Peptide chain of, one end is linked with Side chain In the third place Lysine Connect with the other end at Transpeptidase Under the action of, the d-alanine at the end of the side chain of another pentapeptide is removed and connected with the d-alanine at the fourth position of the side chain. The X-ray examination shows that the polysaccharide chain of peptidoglycan is a hard and spirally coiled long rod. Because it is spirally shaped, the peptide chain connected to it extends to the four directions, which restricts the cross-linking Peptide chain Can be crosslinked. but staphylococcus The peptide bridge of Plasticity So that the distant peptide chains can also be cross-linked, with the cross-linking rate of 90%, forming a solid and dense three-dimensional network structure. gram-negative Escherichia coli The third amino acid in the tetrapeptide side chain of Diaminoheptanedioic acid (dap), which is directly linked with d-alanine in the adjacent tetrapeptide side chain by peptide chain, and has low cross-linking rate. There is no pentapeptide cross bridge, forming two-dimensional Plane structure Therefore, its structure is better than that of Gram positive grapevine birch.

The cell wall of bacteria is tough and elastic, which protects bacteria against low permeability environment and bears 5~25 atmospheric osmotic pressure And make the cells of bacteria not easy to break under the low permeability environment; Cell wall plays an important role in maintaining the inherent morphology of bacteria; Allowable moisture and solubility with diameter less than 1nm Small molecule Free passage, and Substance exchange of There are many kinds of cells on the cell wall Antigenic determinant , which determines the antigenicity 。

Bacterial cell wall structure

Gram positive bacteria cell wall

Any substance that can destroy the structure of peptidoglycan or inhibit its synthesis can damage the cell wall to deform or kill bacteria, for example, lysozyme can cut off the linkage between the β - 1.4 glycosidic bonds between n-acetylglucosamine and n-acetyl muriatic acid in peptidoglycan, destroy the peptidoglycan scaffold, and cause bacterial lysis. penicillin and cephalosporin It can compete with bacteria to synthesize the transpeptidase required for cell wall process, inhibit the connection between d-alanine and pentapeptide bridge on the tetrapeptide side chain, so that bacteria can not synthesize complete cell wall, which can lead to bacterial death. Human and animal cells have neither cell wall structure nor peptidoglycan, so lysozyme And penicillin Human cells None Toxic effect 。 In addition to the basic component of peptidoglycan, gram-positive bacteria and Gram negative bacteria They also have their own special structural components.

The cell wall of Gram positive bacteria is relatively thick, and its chemical composition is simple, generally 90% peptidoglycan and 10% Phosphoteichoic acid 。 The cell wall is basically composed of a thick layer of peptidoglycan, in which phosphoteichoic acid is interspersed. Phosphotrichoic acid can be divided into two types, one is covalently bound with peptidoglycan molecules, called metolithic acid, and the other is linked with Cell plasma membrane Connected, called Membrane teichoic acid or Lipoteichoic acid 。 The space between cell wall and plasma membrane is called Periplasmic gap 。

Gram negative bacteria cell wall

The cell wall of gram-negative bacteria is thinner than that of gram-positive bacteria, and its chemical composition is more complex. It can be divided into two layers: adventitia And a thin peptidoglycan layer. There is an obvious space between the outer membrane and the cytoplasmic membrane, which is called periplasmic space. The main components of the outer membrane are lipopolysaccharide （lipopolysaccharide, LPS ）LPS by Core polysaccharide , O-specific Branched chain and lipid A composition. Adventitia removal phospholipid It also contains polysaccharide and protein 。 Lipids and polysaccharides form the outer layer of the outer membrane, called LPS layer, which is closely connected with lipids.

Cell wall of plant cell

The main components of plant cell wall are cellulose and pectin 。 Plant cell wall It is one of the main characteristics that distinguish plant cells from animal cells. It consists of three parts:

1、 Intercellular layer

Intercellular layer also called Mesosol 。 Located between two adjacent cells, it is a membrane shared by two adjacent cells, and its main component is pectin. It helps to stick adjacent cells together, and can buffer the squeeze between cells.

2. Primary wall

After primary parietal cell division Protoplast Cell wall formed by secretion. It exists in all living plant cells. be located Intercellular layer Inside. It is usually thin, about 1-3 microns thick. With great plasticity, it can not only keep cells in a certain shape, but also Cell growth And extension. The main components are cellulose, hemicellulose, and structural protein Exists. After the cell forms the primary wall, if there is no new Parietal layer Accumulation, the primary wall is their permanent cell wall. as Parenchyma Cells.

3、 Secondary wall

The cell wall layer that continues to accumulate inside the primary wall of some plant cells after they stop growing. It is located between the plasma membrane and the primary wall. The main component is cellulose, and there is often wood. It is usually thick, about 5-10 microns, and hard, so that the cell wall has great mechanical strength. When most cells with secondary walls are mature, Protoplast Death. Fiber and Stone cell It is a typical cell with secondary wall. In plant protoplast culture pectinase and cellulase Enzyme of Mixed liquid handle plant tissue To destroy Intercellular layer And removing the cellulose outer wall of cells to obtain free naked protoplasts.

Cell wall of fungi

Fungal cell wall The main components in chitin 。

The thickness of fungal cell wall is about 100-250nm, and it occupies cells Dry matter 30%. The main component of cell wall is polysaccharide, followed by protein lipid 。 In different groups of fungi, the types of cell wall polysaccharides are different. The polysaccharide of fungal cell wall mainly contains chitin（ chitin ）, cellulose glucan 、 Mannan These polysaccharides are polymers of monosaccharides, such as chitin, which is composed of N-acetylglucosamine molecules Glucoside Key connection Resulting from Polysaccharide 。 Lower fungi The cell wall component of, yeast The bacteria are mainly dextran, while the higher fungi are mainly chitin Mainly. The components of the cell wall of a fungus are not fixed, and the components of the cell wall are obviously different at different growth stages.

Yeast cell wall The thickness of yeast cell wall is 0.1~0.3 μ m, accounting for Cell dry weight 18%~30%, mainly composed of D-glucan and D-mannan, containing a small amount of protein, fat and minerals. Approximately equal amounts of dextran and mannan occupy the cell wall dry weight 85%. When Cell senescence After that, the cell wall weight will be doubled. Although it has a certain tenacity, its tenacity keeps the yeast in a special shape. his chemical composition It is special, mainly composed of yeast cellulose, and its structure is similar Sandwich 。 The outer layer is mannan, accounting for 40%~45% of the dry weight of cell wall. The middle layer is a layer of protein molecules, accounting for about 10% of the dry weight of the cell wall. Some of them are With Cell wall binding enzymes exist in the form of. The inner layer is glucan. Yeast glucan It is an insoluble branched polymer, the main chain is bound by β - 1,6 glycoside bond, and the branch chain is bound by β - 1,3 glycoside bond. As the inner layer of the cell wall, it maintains the strength of the cell wall when the cell is in the Hypertonic It can maintain the elasticity of cells when they contract under the environment of.

function

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1. Maintain cell shape, control cell growth, increase cell wall mechanical strength And bear the internal protoplast due to vacuole Produced by absorbing water turgor So that the cell has a certain shape, which not only protects Protoplast And maintain the relationship between organs and plant The inherent form of. In addition, the wall controls the growth of cells, because the premise of cell expansion and elongation is to make the cell wall relax and stretch irreversibly.^[2]

2. Material transportation and information transfer

Cell wall allows ions, polysaccharides, etc Small molecule And low molecular weight protein By, and will macromolecule Or microorganisms. Therefore, the cell wall is involved in material transport and reduction Transpiration Prevent moisture loss（ Secondary wall , surface wax, etc.), plant water potential regulation and a series of physiological activities. The size of pits or plasmodesmata on the cell wall Physiological age And metabolic activity, so cell wall has a regulatory effect on the transport of intercellular substances. In addition, cell walls are also chemical signals (hormones growth regulator Etc.) Physical signal (electric wave, pressure, etc.) the medium and path of transmission.^[2]

3. Defense and Resistance

Some in cell wall oligosaccharide Fragment can induce phytoalexins The formation of phytoalexin can also regulate other physiological processes. This oligosaccharide fragment with regulatory activity is called oligosaccharin (oligosaccharin)。 Put a kind of Geng Glucoside When oligosaccharides are applied to soybean cells, they will inhibit synthesis mould Growing antibiotics Antibiotics are produced by activation of. The functions of various oligosaccharides are complex and diverse, such as Protease inhibitor Inducing factors play a role in plant resistance to diseases and pests; Some oligosaccharides can make plants produce Anaphylaxis Death makes Pathogen No further diffusion; Other oligosaccharides are involved in regulating plant Morphogenesis 。 In cell wall Extensin In addition to being a structural component, it also has the function of disease prevention and stress resistance. For example, after a resistant cucumber variety is infected with a mold hydroxyproline Of Content ratio Sensitive varieties increased rapidly.^[2]

4. Other functions

Enzymes in the cell wall are widely involved in the synthesis, transfer, hydrolysis, delivery of extracellular substances into cells and defense of cell wall polymers.

5. Participate in the mutual adhesion between cells, namely“ Plasmodesmata ”。^[2]

It is found that cell wall is also involved in plant and Rhizobium Mutual symbiotic nitrogen fixation Identification function In addition Polygalacturonase and lectin May also be involved stock And scion grafting Identify reactions 。 It should be noted that not all cell walls have the above functions, and the cell wall functions of each type of cell are determined by its specific composition and structure.

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