Cell (English name: cell) does not have a unified definition. It is generally said that cell is the basic structural and functional unit of organism.Known divisionVirusesAll other organisms are composed of cells, but the life activities of viruses must also be reflected in cells.
Generally speaking, most microorganisms such as bacteria andprotozoanIt is composed of one cell, namelyUnicellular organism, higher plants andHigher animalIt is a multicellular organism.Cells can be divided intoProkaryotic cellAnd eukaryotic cells, but it was also proposed that they should be divided into three categories, namelyArchaeokaryocyteIndependent as a parallel category.The discipline of studying cells is calledCell Biology。
The cell body is extremely smallMicroscopeIt can be seen from below, with various shapes.It is mainly composed of nucleus and cytoplasm, with cell membrane on the surface.There arecell wall, often found in cytoplasmPlastid, there arechloroplastandvacuole, andmitochondrion。Animal cells have no cell wall and are often found in cytoplasmCentrosome, and higherplant cellNone in.Cells have the functions of movement, nutrition and reproduction.
Cells was founded by British scientistsRobert Hooke (Robert Hook, 1635-1703) was discovered in 1665.He used homemadeoptical microscopeObserve the thin slice of cork and find small spaces one by one after zooming in,[1]In EnglishcellThe meaning of this English word itself has the usage of a small room, so it is not a new vocabulary.The cells observed in this way have already died, and only the remaining plants can be seencell wall[2]Although he did not really see a unit of life (because there was no sign of life), later generations of scientists still believed that his contribution was indispensable and generally regarded him as the first person to discover cells.In fact, Dutch biologists were the first to discover living cellsLeeuwenhoek。
In 1674, Antony van Leeuwenhoek (1632-1723), with a lens made by himself that can magnify the image of an object by 200 times, observedred blood cell。In 1677, we saw it under the microscope for the first timespermThe existence of[3]。
French naturalist in 1809（Natural HistoryIt is the general name of biology, life science, etc. in the late 20th century)Lamarck(Jean Baptiste de Lamarck, 1744-1829) proposed that "all organisms are composed of cells, which contain some flowing 'liquid'." However, there is no specific observational evidence to support this statement.
In 1824, French botanist Henri Dutrochet (1776-1847) proposed in his paper that "cells are indeed the basic structure of organisms", and because plant cells have more cell walls than animal cells, it is easier to observe than animal cells when the observation technology is not yet mature, so this statement was first accepted by botanists[4]。
After 1830, with the development of industrial production, microscope production overcame the shortcomings of lens blurring and chromatic aberration. The resolution was increased to 1 μ m, and the microscope began to become popular.With the improved microscope, the cells and their contents were observed more clearly.In 1839, German botanist Matthias Schleiden (1804-1881) concluded from the observation of a large number of plants that all plants are composed of cells.At the same time, German zoologist Shiwang did a lot of research work on animal cells.At that time, due to the influence of Hooke, the observation of cells focused on cell walls rather than the contents of cells, so the understanding of animal cells without cell walls was much later than that of plant cells.Shi Wang has carried out a lot of research, the first to describe the similarity between animal cells and plant cells[5]。
In Schwann and GermanySchleidenIn the following ten years, scientists have successively discovered new evidence to prove that cells are all derived from the original cell division. The cell theory in the early 21st century can be roughly summarized as the following three points: cell is the structural unit of all living things, cell is the physiological unit of all living things, and cell is derived from the original cell division.
The word "cell" first appeared in the 1834 book "Phytological Enlightenment" by Japanese orchidologist Yutian Chuanrong Nunnery.
Chinese natural scientist Li Shanlan used "cell" as the Chinese translation of Cell in his book Botany in 1858[6]。Some scholars believe that Li Shanlan did not come into contact with "Botanical Enlightenment" at this time, so he invented it alone.
On the evening of May 10, 2023, the Tianzhou VI cargo spaceship launched 98 scientific experimental products, among which, in the field of space life science and biotechnology,Four scientific experiments will be carried out in the biotechnology laboratory cabinet of the Sky Quest Laboratory Module, including biomechanical research on the effects of microgravity environment on the interaction between cells and cell growth[18]。
Cellular structure
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cell wall
The cells of organisms classified as bacteria, fungi and plants all havecell wall(Cell Wall), andProtozoaSome organisms have this structure, but animals do not.
The main component of plant cell wall is cellulose, which forms a reticular outer wall through systematic weaving.Can be divided intoMesosol、Primary cell wall、Secondary cell wall。The mesothelium is the first space between the daughter cells of plant cells that have just completed division. Its main components arePectin(a kind of polysaccharide), and then the primary cell wall is formed on both sides of the mesothelium. The primary cell wall is mainly composed of pectin, lignin and a small amount of protein.The secondary cell wall is mainly composed of fibers composed of cellulose, which are arranged like lines at nearly right angles, and then bonded with polysaccharides such as lignin.
The fungal cell wall is composed ofchitin, cellulose and other polysaccharides, in which chitin is composed of carbohydrates andammonia, soft, elastic, andCalcium saltIf mixed, it will harden and formArthropodaOfExoskeleton。Chitin is insoluble inwater、alcohol、weak acidandweak alkaliAnd other liquids, with protection function.
The bacterial cell wall is composed ofPeptidoglycanMainly.
cell membrane
The inner side of the cell wall is closely attached to an extremely thin membrane, called cell membrane.This layer consists of protein molecules andPhospholipid bilayerThe film, water, oxygen and other small molecular substances can pass freely, while some ions and macromolecular substances cannot pass freely.Therefore, in addition to protecting the inside of cells, it also has the function of controlling the ingress and egress of substances: it neither allows useful substances to seep out of cells arbitrarily, nor allows harmful substances to enter cells easily.In addition, it can exchange information between cells.
The cell membrane is difficult to distinguish under the light microscope.useelectron microscopeObservation shows that cell membrane is mainly composed of protein molecules and lipid molecules.In the middle of the cell membrane is the phospholipid bilayer, which is the basic skeleton of the cell membrane.On the outside and inside of the phospholipid bilayer, there are manysphericalThey are embedded in the phospholipid molecular layer at different depths or covered on the surface of the phospholipid molecular layer.Most of these phospholipid molecules and protein molecules can flow, which can be said that the cell membrane has a certain fluidity.This structural feature of cell membrane is very important for it to complete various physiological functions.[7]
① Free diffusion: matter passes through simpleDiffusionEnter the cell.The concentration difference on both sides of the cell membrane and the nature of the diffused substance (e.gPrinciple of similarity compatibility，LiposolubilityThe substance is easier to enter and exit the cell) has an impact on the rate of free diffusion. Common substances that can conduct free diffusion areoxygen, carbon dioxideglycerol、ethanol、benzene、urea、cholesterol, water, ammonia, etc.
② Assisted diffusion: materials entering and leaving the cellCarrier proteinDiffusion.The concentration difference on both sides of the cell membrane and the type and number of carriers affect the rate of assisted diffusion.red blood cellThe absorption of glucose depends on the aid of diffusion.
（2）Active transportation: The substance is transported from the low concentration side to the high concentration side, which needs the assistance of carrier protein, and also needs to consume the cellschemical reactionEnergy released.Active transportation ensuresLiving cellAble to actively select and absorb what is needed according to the needs of life activitiesNutrientsDischarge metabolic wastes and substances harmful to cells.Various ions pass through the membrane from low concentration to high concentration by active transport.
Capable ofTransmembrane transportAll of them are ions and small molecules. When macromolecules enter and exit cells, they wrap macromolecular substancesVesicleSeparation from or fusion with cell membrane（EndocytosisandExocytosis）Macromolecules can enter and exit cells without transmembrane.[8]
cytoplasm
Protoplast
The thick and transparent substance wrapped in the cell membrane is called cytoplasm（Cytoplasm）。Some refractive particles can also be seen in the cytoplasm. Most of these particles have a certain structure and function, similar to various organs of organisms, so they are calledOrganelle。For example, in the green plantMesophyll cellMany green particles can be seen, which is a kind of organelle calledchloroplast。Photosynthesis of green plants is carried out in chloroplasts.In the cytoplasm, one or severalvacuole, which is filled with liquid, calledCytosol。stayMature plant cellsThe vacuoles are merged into oneCentral vacuoleIts volume takes up most of the whole cell.The cytoplasm is squeezed into a layer.Cell membrane andTonoplastAnd the cytoplasm between the two membranes is calledProtoplast。
The protoplasm layer of plant cells is equivalent to one layerSemipermeable membrane。When the concentration of the cell fluid is lower than the external concentration, the water in the cell fluid will enter the external solution through the protoplasm layer, causing the cell wall and protoplasm layer to shrink to a certain extent.Since the protoplasm layer has greater flexibility than the cell wall, when the cell continuously loses water, the protoplasm layer separates from the cell wall, that isPlasmolysis。When the concentration of the cell solution is greater than that of the external solution, the water in the external solution enters the cell solution through the protoplast layer to restore the protoplast layer, and the restoration of the plasmolysis gradually occurs.
The cytoplasm is not frozen and static, but moves slowly.In a cell with only one central vacuole, the cytoplasm often circulates around the vacuole, thus promoting the transport of substances in the cell and strengthening theOrganelleThe interrelationship between.Cytoplasmic movement is an energy consuming processLife phenomenon。The more vigorous the cell's life activity is, the faster the cytoplasmic flow is; on the contrary, the slower it is.cell deathAfter that, the flow of cytoplasm stops.
Cytoskeleton refers to the network structure of protein fibers in eukaryotic cellsMicrofilament、microtubuleandIntermediate fibreComposition.The microfilament determines the characteristics of the cell surface, enabling the cell to move and contract.Microtubules determine the location and function of membranous organellesvesicular transportTrack.Intermediate fibers make cells have tension and shear resistance.
CytoskeletonIt not only plays an important role in maintaining cell morphology, withstanding external forces, and maintaining the order of cell internal structure, but also participates in many important life activities, such as cytoskeleton traction during cell divisionChromosome segregation；In the transport of cellular material, various vesicles and organelles can be oriented along the cytoskeleton.
The cytoskeleton was discovered in the late 1960s.Mainly because of the earlyElectron microscope sample preparationIt is fixed at low temperature (0-4 ℃), and the cytoskeleton will be depolymerized at low temperature.Until glutaraldehyde fixed at room temperature, people gradually realized the objective existence of cytoskeleton.[9]
Organelle
Cells with plasmolysis
There are also some organelles in the cell, which have different structures and perform different functions to complete the life activities of the cell together.The structure of these organelles needs to be observed with an electron microscope.stayElectron microscopeThe cell structure observed under is calledSubmicrostructure。
①mitochondrion
Mitochondria (Mitochondria/Mitochonrion) Mitochondria are some linear, small rod or granular structures that can be used in living cellsJanus Green(Janus green) dyed in blue-green.stayelectron microscopeUnder observation, the surface of mitochondria is composed of double membranes.intimaIt forms some partitions inward, calledMitochondrial crista（Cristae）。There are abundantEnzyme system。Mitochondria areCellular respirationIt is the center of biological organismoxidationA major energy generating mechanism that can convert nutrients (such as glucose, fatty acidamino acidEnergy generated by oxidation, stored in ATP（Adenosine triphosphate）OfHigh-energy phosphate bondIt supplies the needs of other physiological activities of cells, so some people say that mitochondria are the "power plant" of cells.
Chloroplasts are important organelles in green plant cells, whose main function is photosynthesis.Chloroplasts consist of bilayer membranesGrana(Thylakoid) and matrix.thylakoidIt is a flat small capsule like structure. On the thylakoid membrane, there are pigments and enzymes necessary for photosynthesis.Many thylakoids overlap to form grana.The grana are filled with matrix, which contains enzymes related to photosynthesis.The matrix also contains DNA.[10]
Endoplasmic Reticulum is a kind of reticular pipeline system composed of membrane in cytoplasm, which is widely distributed inCytoplasmic matrixInside.It is related toCell membrane andnuclear membraneThe interconnection plays an important role in the synthesis and transportation of proteins, lipids and other substances in cells.Endoplasmic reticulum can be divided intoRough endoplasmic reticulumandSmooth endoplasmic reticulum。The rough endoplasmic reticulum has ribosomes attached to its surface, which can transport proteins. The smooth endoplasmic reticulum contains many enzymes, which are related to the synthesis and secretion of glycolipids and sterols.
Golgi Apparatus/Golgi Body is a reticular vesicle near the nucleus, which is a transportation and processing system within the cell.The protein transported by rough endoplasmic reticulum can be processed, concentrated and packaged into vesicles and lysosomes.
Ribosomes are ellipsoidal granular bodies, some attached to the outer surface of the endoplasmic reticulum membrane (supplying proteins on and outside the membrane), and some freeCytoplasmic matrixMedium (supplying protein in the membrane, directly forming spatial configuration under the action of enzymes in the cytoplasm matrix without Golgi apparatus), is an important base for protein synthesis.
Centrosome exists inanimalCells and someLower plantIn cells, because its position is close to the nucleus, it is called centrosome.Each centrosome is composed of twoCentrioleAnd its surrounding material composition, the centrosome of animal cells andmitosisThere is close relationship.Centriole is a fixed organelle with polar structure.In interphase cells, only one or two small granules can be seen after fixation and staining.Under the electron microscope, the centriole is a columnar body with a length of about 0.3 μ m~0.5 μ m and a diameter of about 0.15 μ m. It is composed of nine groups of small tubular subunits, and each subunit is generally composed of three microtubules.The arrangement direction of these tubes is parallel to the longitudinal axis of the column.
Vacuoles are vesicular structures in plant cells.The vacuoles in mature plant cells are very large, accounting for 90% of the total cell volume.The surface of the vacuole hasTonoplast。In vacuoleCytosolIt contains sugars, inorganic salts, pigments, proteins and other substances, which can reach very high concentrations.Therefore, it plays a role in regulating the internal environment of cells, and can make cells maintain certainosmotic pressure, keep the state of expansion.Animal cells also have small vacuoles.
⑧lysosome
Saccular body or vesicle, containing a variety of hydrolases, with autolysis and heterolysis.Autolysis refers to the process of lysosome digestion and decomposition of damaged and aging organelles in cells, while heterolysis refers to the process of digestion and decomposition of pathogenic microorganisms and their cell fragments engulfed by cells.Lysosomes are organelles with single-layer membrane and cystic structure in cells.It contains many kinds of hydrolases, which can decompose many substances.
In addition to the above structures, there are microfilaments, microtubules and other structures in the cytoplasm. Their main function is not just to support the cell skeleton, so as to maintain the cell shape, such as inRed blood cellMicrotubules arranged in bundles parallel to discoid cellsPeriphery, also like epithelial cellsmicrovilliMicrofilaments in;They also participate in cell movement, such as mitoticSpindle yarn, and ciliaFlagellumMicrotubule.In addition, there are variousInclusion, such asGlycogen, lipids, crystals, pigments, etc.[11]
nucleus
The cytoplasm contains a nearly spherical nucleus, which is composed of more viscous substances.The nucleus is usually located in the center of the cell. The nucleus of mature plant cells is often pushed to the edge of the cell by the central vacuole.There is a substance in the nucleus that is easy to beMagenta、Hematoxylin、Methyl green、Gentian violet solutionetc.alkaline dyeDye it dark, calledChromatin（Chromatin）。The substance used by organisms for breeding isgenetic materialIt's on the chromatin.When cellsmitosisThe chromatin spirals intochromosome。
Most cells have only one nucleus, and some cells have two or more nuclei, such asmuscle cells, hepatocytes, etc.The nucleus can be divided into nuclear membrane, chromatinkaryolymphandnucleolusFour parts.The nuclear membrane is connected to the endoplasmic reticulum, and chromatin is located atnuclear membraneAnd nucleolus.Chromatin is mainly composed of protein and DNA.DNA is an organic macromolecule, also calledDeoxyribonucleic acidIt is the genetic material of living things.During mitosis, the chromosome copies, and the DNA also copies into two copies, evenly distributed to twoDaughter cellIn this way, the number of chromosomes in offspring cells is constant, thus ensuring the stability of genetic characteristics of offspring.alsoRNARNA is a single strand of DNA formed during replication. It transmits information and controls the synthesis of proteins, including transfer ribonucleic acid（tRNA）, Messenger RNA（mRNA）And ribosomal RNA（rRNA）。The function of the nucleus is to preserve genetic material, control biochemical synthesis and cell metabolism, determine the performance of cells or body traits, and pass down genetic material from cell (or individual) generation to generation.However, the nucleus does not function in isolation, but interacts and depends on the cytoplasm to show a unified life process of cells.Nucleus controls cytoplasm;Cytoplasmic pairCell differentiation, development and heredity also play an important role[12]。
The size of cells from some different sources varies greatly:
Human oocyte: diameter 0.1mm;Ostrich egg cell: 5cm in diameter.
The size of cells of the same type is generally similar, and does not increase or decrease according to the size of individual organisms.
The size of an organ mainly depends on the number of cells, which is proportional to the number of cells, but independent of the size of cells. This phenomenon is called "the conservation law of cell volume".[13]
generality
Cellular structure
1. Allcell surfaceAll byPhospholipid bilayerAnd mosaic protein andGlycocalyx The biomembrane (note: cancer cells have no sugar coating and are easy to wander and spread), that is, cell membrane.
2. All cells contain two kinds of nucleic acids: DNA andRNA。
4. Asprotein synthesis's machine——ribosomeExists in all cells without exception.Ribosome is a necessary machine for protein synthesisCytogeneticsInformation flow plays an essential role in the transmission.
5. Basically, the proliferation of all cells divides in two.(A few are not, for example, some species of cyanobacteria generate new cells from old cells)
6. Some cells can self proliferate and inherit (highly differentiated cells cannot self proliferate)
7. Metabolism.
8. Cells are mobile, including the movement of cells themselves and the internalMaterial movement。
Note: The virus has no cellular structure.
type
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Eukaryotic cell
Eukaryotic cell(eucaryotic cell) refers to a cell containing a eukaryote (a nucleus surrounded by a nuclear membrane).hisChromosome numberMore than one can undergo mitosis.It can also carry out protoplasm flow andDeformation movement。While photosynthesis andOxidative phosphorylationThe action is carried out by chloroplasts and mitochondria respectively.Bactericidal andblue-green algaeExcept plant cells, all animal cells and plant cells belong to eukaryotic cells.Organisms composed of eukaryotic cells are called eukaryotes.In the nucleus of eukaryotic cells, DNA andHistoneThe nucleolus can be seen in the nucleus.In cytoplasmEndometrial systemIt is very developed, with endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes and other organelles, which perform specific functions respectively.
EukaryoteBiology includesAnimals and plants, tiny protozoa and single cellsseaweed, fungimoss and lichenEtc.Eukaryotic cells have one or more nuclei wrapped by double membranes,genetic materialIt is contained in the nucleus and exists in the form of chromosomes.Chromosomes are composed of a small amount of histones and some basic proteins rich in arginine and lysine.Eukaryotes reproduce sexually and undergo mitosis.
Prokaryotic cell
Prokaryotic cells have no nuclear membrane, and genetic material is concentrated at a low level without clear boundarieselectron densityZone, calledQuasinuclear（nucleoid）。DNA is a naked ring molecule, usually without binding protein. The diameter of the ring is about 2.5 nm, and the circumference is about tens of nanometers.majorityprokaryoteThere is no constant intima system, and the ribosome is 70S type. The organisms composed of prokaryotic cells are called prokaryotes, all of which are single celled organisms.
The cells that make up prokaryotes.The main feature of this kind of cells is that there is no obvious nucleus, no nuclear membrane and nucleolus, only nucleoid, and the evolutionary status is low.
Procaryotic/Prokaryotic cell refers to a cell without nuclear membrane and without mitosis, meiosis or amitosis.This kind of cell does not produce protoplasm flow and cannot be observedAmoebaLike motion.Flagellum has a single structure.Photosynthesis and oxidative phosphorylation are carried out in the cell membrane, without the differentiation of organelles such as chloroplast and mitochondrion, and only ribosomes.Organisms composed of such cells are called prokaryotes, which include all bacteria and cyanobacteria.That is, the cells that constitute lower organisms such as bacteria and cyanobacteria.It has no real nucleus, only prokaryotic or pseudonuclearGene band(or chromosome) is a circular double stranded single sequence deoxyribonucleic acid molecule (Circular DNA), with no histone binding to it, no nucleolus, and no nuclear envelope.The outer protoplasm contains 70S ribosomes and intermediates, and lacks Golgi, E.R., mitochondria and centrosomes.Transcription andTranslation(Transcription and translation) At the same time, the surrounding plasma membrane contains respiratory enzymes.Without mitosis and meiosis, after DNA replication, the cell divides into two.
Archaeokaryocyte
ArchaeokaryocyteArchaea(Archaebateria): It is a special kind of bacteria, which mostly live in extremeecological environmentMedium.It has some characteristics of prokaryotes, such as no nuclear membrane and inner membrane system;It also has the characteristics of eukaryotes, such as starting protein synthesis with methionine, ribosome pairingchloramphenicolInsensitiveRNAPolymerase is similar to eukaryotic cells, DNA has introns and binds histones;In addition, it has characteristics different from prokaryotic cells and eukaryotic cells, such as: lipids in cell membrane are unsaponifiable;Cell wall does not containPeptidoglycanSome are mainly protein, and some containHeteropolysaccharideSome are similar to peptidoglycan, but they do not contain cell wall acid, D-amino acid andDiaminoheptanedioic acid。
Thermophiles: those that can grow above 90 ℃High temperature environment。asStanford UniversityThe most suitable growth temperature of archaea discovered by scientists is 100 ℃, and it will be inactivated if it is below 80 ℃. A group of archaea discovered by the German research team of K. Stetter in the Italian seabed can live in a high temperature above 110 ℃, and the most suitable growth temperature is 98 ℃, and it will stop growing when it drops to 84 ℃;JA. Baross found that some bacteria isolated from the crater can live at 250 ℃.Thermophilic bacteria have a wide range of nutrients, most of which are heterotrophic bacteria, many of which can oxidize sulfur to obtain energy.
Extremely halophilic bacteria(Extremehalophiles): living in a high salinity environment, the salinity can reach 25%, such asdead seaAnd salt lakes.The cell wall of extreme halophilic bacteria is composed of glycoproteins rich in acid amino acids, and the integrity of this cell wall structure is maintained by ionic bonds. High Na+concentration is necessary for the combination between cell wall protein subunits and the maintenance of the integrity of cell structure.When transferred from high salt environment to low salt environment, on the one hand, cell wall proteins are depolymerized into protein monomers, making the cell wall lose integrity;On the other hand, the balance of ion concentration inside and outside the cell is broken, the cell absorbs water and expands, eventually causing cell wall rupture and complete autolysis of the cell.
Acidophiles: able to live in an environment with pH below 1, they are also thermophilic bacteria and live involcanoIn the acid hot water of the area, sulfur can be oxidized, and sulfuric acid can be discharged from the body as a metabolite.
Alkaliphiles: most of them live in saline lakes orAlkali lakeIn the alkali pool, the pH value of the living environment can reach above 11.5, and the optimal pH value is 8-10.[14]
Active process
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division
The process by which a cell divides into two cells.Cell scale before divisionmother cells, the new cell formed after division is calledDaughter cell。Cell division usually includesMitosisandCytokinesisTwo steps.In the process of nuclear division, the mother cell transmits genetic material to the daughter cell.In unicellular organisms, cell division is the reproduction of individuals. In multicellular organisms, cell division is the basis of individual growth, development and reproduction.[15]
differentiation
Cell differentiation refers to the process in which cells after division change in different directions in morphology, structure and function.Cell differentiation is the formation of different tissues. Cells before and after differentiation do not belong to the same type.Those cell groups with similar morphology, same structure and certain functions are called tissues.Different tissues form organs in a certain order.Various organs coordinate to form a system.Composition of various organs and systemsLife form。Canceration of cellsIt is an abnormal way of cell differentiation.Every normal cell hasProto oncogene。The cells with canceration were originally normal cells, because they were exposed to the outside worldCarcinogen(Carcinogenic factors includePhysical carcinogen, mainly refers to radiation, such as ultraviolet, X-ray, etc.), chemical carcinogens (such asAflatoxin、nitriteEtc.), biological carcinogen (RousSarcoma virus, hepatitis B virus, etc.), leading to the activation of proto oncogenes in cells, and the activated proto oncogenes control the canceration of cells.
Some changes have taken place in the morphology, results and functions of cancerous cells.
death
Skin cell
Cell death is the result of cell aging and the termination of cell life phenomenon.Including acute death (cell necrosis) and programmed death（Apoptosis）。The most obvious phenomenon of cell death is the coagulation of protoplasm.In fact, cell death is a gradual process, and it is difficult to decide when a cell has died.Unless it is killed instantly by using fixative and other factors.Identifying whether a cell is dead is usually done byLiving stainingTo identify.If neutral red staining is used, living cells onlyVacuole systemDye it red. If the dye diffuses and the cytoplasm and nucleus are dyed red, it indicates that the cell has died.The research of cell aging is only a part of the whole research of aging biology (gerontology, anthropology).The so-called Biology of senecence (or gerontology) is to study the phenomenon, process and law of biological aging.Its task is to reveal the characteristics of biological (human) aging, explore the causes and mechanisms of aging, and find ways to delay aging. The fundamental purpose is to extend the life span of biological (human).multicellular Organism cells can be divided into two categories according to the length of life, namelystem cellsAnd functional cells.Stem cells maintain the ability to divide throughout their life, and they will age and die when they reach the highest number of divisions.Such as epidermal germinal cells, blood generating stem cells, etc.
Apoptosis is an active process of automatically ending life determined by genes, which is also often called programmed cell death (PCD). Apoptosis cells will be engulfed by phagocytes.This hypothesis is based onHayflick limitation Proposed by: Hayflick in 1961Embryonic cellThe experiment of subculture was proposed.It refers to the normal appearance of cells at a certain stage of developmentNatural death, which is related to thePathologyDeath is fundamentally different.Apoptosis plays a key role in the normal development of multicellular organisms, the maintenance of homeostasis and the resistance to interference from various external factors.For example, the disappearance of tadpole tail, apoptosis of bone marrow and intestines, the development of vertebrate nervous system, and the formation of hands and feet during development.[16]
Reproductive mode
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Cell reproduction is achieved through cell division. Cells that divide continuously from the completion of one division to the completion of the next division are onecell cycle。There are four types of cell division, including three types of eukaryotic cells: mitosis, amitosis, and meiosis. The first two are somatic cell division;The division of prokaryotic cells is dichotomous.
mitosis
From the end of one division to the next divisionInterphaseMost of the cell cycle is in the interphase, accounting for 90%~95% of the cell. During the interphase, the cell completesDNA moleculeThe replication and synthesis of related proteins.After the end of the interdivision period, it enters the division period.The splitting period is a continuous process. For the convenience of research, people divide the splitting period into four periods: early stage, middle stage, late stage and late stage.
1. Early stage;It is the beginning of cell division.The cells are generally rounded,CentrosomeOfCentrioleSeparate and move towards the poles of the cell.Emitting filaments appear around.Nuclear enlargementDeoxyribonucleic acidIncrease, nuclear staining deepens, irregular chromatin forms filamentous chromosomes, and shortens and thickens.The nucleolus and nuclear membrane disappeared,NucleoplasmMixed with cytoplasm.
2. Medium term;The two centrosomes are close to the poles, and they are connected by wires in a spindle shape, calledSpindle。Chromosomes moved to the equator of the center of the cell and arranged in a star shaped manner;Later, the longitudinal chromosome split into two.
3. Later stage;The already split chromosomes are divided into two groups, moving from the equator to the centrosome of the poles, and the organelles are equally distributed.Towards the poles, the cell body begins to constrict transversely at the equator.
4. Late stage;Chromosomes move near the centrosomes of the poles, reunite, and becomeChromatin filamentThe nuclear envelope and nucleolus reappear.The cell body becomes narrower at the equator
The mitosis of plant cells is similar to that of animal cells.However, there is no centrosome in the cells of higher plants, and the spindle filaments are emitted from the cells at two levels.AnaphaseThe two cells are not separated by the inward depression of the cell membrane, but formed at the equator of the center of the cellCell plate。
Amitosis
Direct division is the earliest mode of cell division found in chicken embryos as early as 1841blood corpuscleSee in.Because this mode of division is the direct division of the nucleus and cytoplasm, it is called direct division.It is also called because there is no spindle during splittingAmitosis。Only some cells of some animals can undergo amitosis, such as red blood cells of frogs.
At the early stage of direct division, both spherical nuclei and nucleoli elongate.Then the nucleus further elongates into a dumbbell shape with a narrow central part.Finally, the nucleus divides, and then the cytoplasm also divides, and forms the cell membrane with the participation of the smooth endoplasmic reticulum.In direct division, the nuclear membrane and nucleolus do not disappear, and there is no chromosome, of course, there is no regular change in chromosome replication.However, this does not mean that chromatin has not undergone profound changes. In fact, chromatin also needs to be replicated and cells need to be enlarged.When the size of the nucleus doubles, the nucleus splits, and the genetic material in the nucleus is distributed to the daughter cells.As for how the genetic material DNA in the nucleus is distributed, further research is needed.
There have been different views on the issue of direct division for a long time.Some people think that direct division is not a normal way of cell proliferation, but an abnormal division phenomenon;Others maintain that direct division is one of the proliferation modes of normal cells, mainly seen in highly differentiated cells, such as hepatocytesRenal tubuleEpithelial cellsadrenal cortexCells, etc.
Meiosis
This form of cell division followsGametogenesisWhat appears, usually goes onSexual reproductionMost animals and plants have meiosis.The difference between meiosis and normal mitosis is that during meiosis, two consecutive mitoses are carried out, and the cell divides twice, of which the chromosome divides only once. As a result, the number of chromosomes is reduced by half.
The time of meiosis is fixed for each type of organism, but differentbiological groupIt can be different.It can be roughly divided into three types:Zygote meiosisOr initial meiosis, which occurs at the beginning of the fertilized eggcleavageThe result is an organism with half the number of chromosomes.This form of meiosis is only found in a fewLower organism。SecondSpore meiosisOr intermediate meiosis, which occurs inSporogenesisWhen the sporophyte andGametophyteBetween generations.This ishigher plantCharacteristics of.ThirdgameteMeiosisTerminal meiosis, is a feature of general animals, including allMetazoan, people and some protozoa.This meiosis occurs during the formation of gametes and the last two divisions at the mature stage during gamete formation, resulting inspermAnd eggs.
The specific process of meiosis is very complex, including two cell divisions.The prophase of the first division is relatively long, which is generally divided intoTenuous period, coupling periodPachytene stage, double track period, final change period, this earlier stage I (represents the first timeProphase）Then comes the middle stage I, late stage I and late stage I;After the meiotic interval (very short or invisible), enter the prophase II, metaphase II, anaphase II, and anaphase II, and some do not go through the interval.
In the process of meiosis, the cell divides twice, but the chromosome divides only once. As a result, the number of chromosomes is reduced by half.Generally speaking, the first split isHomologous chromosomeWhen separated, the number of chromosomes is reduced by half, which is called meiosis.The second split isSister chromatidWhen separated, the number of chromosomes does not decrease, which is equal division.But strictly speaking, this is general.It is not so simple to analyze from the perspective of heredity, because it involves the exchange and recombination of chromosomes.
Meiosis is of great significance to maintain the stability of the number of chromosomes in a species, and to the distribution and recombination of genetic material, which is extremely important to the evolution and development of organisms.[15]
Dichotomy
Of most microorganisms, such as bacteriaAsexual reproductionMode.In the process of division, the length of the cell first extends about twice, and the middle part is formed byCytoplasmIt forms a separation from the cell wall, divides chromosomes, distributes them to two cells, and finally forms two daughter cells with basically the same shape and size, and its proliferation speed is
The geometric progression of.After division, daughter cells may be separated from each other, or they may be connected together to form pairs, chains, clusters or folds.Yeast also propagates by dichotomous method[17]。
chemical composition
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The basic elements of cells are: OC、H、N、S、K、Ca、P、Mg，The four elements of O, C, H and N account for more than 90%.CytochemistrySubstance can be divided into two categories:InorganicAnd organic matter.In inorganic substances, water is the most important component, accounting for 75% - 80% of the total content of cellular substances.
1、 Water and inorganic salts
（1） Water is the most basic substance of protoplasm
Water not only has the largest content in cells, but also has some unique physical and chemical propertiesOrigin of lifeAnd forming cellsOrdered structureAspect plays a key role.It can be said that without water, there would be no life.Water exists in cells in two forms: one is free water, accounting for about 95%;The other isBound waterIt binds to protein through hydrogen bond or other bonds, accounting for about 4%~5%.With the growth and aging of cells, the water content of cells gradually decreases, but the water content of living cells will not be less than 75%.
The main role of water in cells is to dissolve inorganic substances, regulate temperature, participate in enzyme reaction, participate inMaterial metabolismAnd form an orderly structure of cells.
（2） Inorganic salt
The content of inorganic salts in cells is very low, accounting for about 1% of the total cell weight.Salt dissociates into ions in cells, and the concentration of ions can regulate osmotic pressure and maintainacid-base balanceBesides, there are many important functions.
The main anions are Cl -, PO4 - and HCO3 -, of whichPhosphate radicalIons are the most important in cell metabolism: ①energy metabolismPlays a key role in; ②yesnucleotide, phospholipid, phosphoprotein and phosphorylated sugar; ③Regulate acid-base balanceTissue fluidPH acts as a buffer.
There are thousands of organic substances in cells, accounting for more than 90% of the dry weight of cells. They are mainly composed of carbon, hydrogen, oxygen, nitrogen and other elements.Organic matter is mainly composed of four types of molecules, namely protein, nucleic acid, lipid and sugar, which account for more than 90% of the dry weight of cells.
（1） Protein
In life activities, protein is a kind of extremely important macromolecule, and almost all kinds of life activities are related to the existence of protein.Protein is not only the main structural component of cells, but also, more importantly, the proprietary biological catalyst enzyme is protein, so the metabolic activity of cells cannot be separated from protein.A cell contains about 104 proteins and 1011 molecules.
（2） Nucleic acid
Nucleic acid is the carrier molecule of biological genetic information, and all organisms contain nucleic acid.Nucleic acid is a macromolecule polymerized from nucleotide monomer.Nucleic acids can be divided into RNA and DNA.When the temperature rises to a certain height, the double strand of DNA will be dissociated into a single strand, which is called denaturation or melting. This temperature is called melting temperature (Tm). The melting temperature of DNA with different base composition is different, and the Tm of DNA with more G-C pairs (three hydrogen bonds) is higher;If there are more A-T pairs (two hydrogen bonds), Tm is lower.When the temperature drops below a certain temperature, the complementary single strand of denatured DNA can restore the double helix structure of DNA by forming hydrogen bonds between paired bases, a process called renaturation or annealing.
（3） Sugar
Existing sugars in cellsmonosaccharideThere are also polysaccharides.Monosaccharides in cells exist as raw materials for energy and sugar related compounds.The important monosaccharides are pentasaccharides (pentose) and hexasaccharides (hexose), and the most important pentasaccharides areRiboseThe most important hexose is glucose.Glucose is not only the key monosaccharide of energy metabolism, but also the main monomer of polysaccharide.
Polysaccharides play an important role in cell structure.Polysaccharides in cells can be basically divided into two categories: one is nutrition reserve polysaccharide;The other is structural polysaccharide.There are mainly two kinds of polysaccharides as food reserves, starch in plant cells and starch in animal cellsGlycogen（glycogen）。In eukaryotic cells, structural polysaccharides mainly include cellulose andchitin（chitin）。
（4） Lipids
Lipids include fatty acids, neutral fatssteroid, wax, glycerophosphate, sphingolipids, glycolipidsCarotenoidsEtc.Lipids are insoluble in water and soluble in non-polar organic solvents.
1. Neutral fat
① Glyceride: It is a triglyceride formed by combining the carboxyl group of fatty acid with the hydroxyl group of glycerol.Glycerides areAnimals and plantsThe main storage form of body fat.When carbohydrates, proteins or lipids are surplus in the body, they can be converted into glycerides for storage.Glyceride is an energy material, which can release twice as much energy as sugar or protein when oxidized.When nutrition is deficient, glyceride is used to provide energy.
② Wax: fatty acid with long chainAliphatic groupThe monohydric alcohol or sterol is esterified to form wax (such as beeswax).The hydrocarbon chain of wax is very long, and its melting point is higher than that of glycerol ester.There is no wax in cells, but some cells can secrete wax.For example:Plant epidermisWax membrane secreted by cells;HomopterousWax gland, such as cerumen glands in the external auditory canal of higher animals.
2. Phospholipid
Phospholipids are crucial to the structure and metabolism of cells. They are the basic components of biological membranes, and manyMetabolic pathwayParticipants of.Divided intoGlycerophospholipidAnd sphingomyelin.
3. Glycolipids
Glycolipids are also components of cell membranes, which are related to cell recognition and surface antigenicity.
4. Terpenoids and steroids
These two types of compounds are isoprenederivative, both contain no fatty acids.
The main terpenoids in biology include carotene andvitamin A, E, K, etc.There is also a polyterpene alcohol phosphate, which is the carrier of glycosyltransferase in the cytoplasm.
Steroids are also called steroids, in which cholesterol is the component of membrane.Other steroids are hormones, such asestrogen, androgenAdrenal hormoneEtc.
