Cell: The Unit of Life | Hinglish Quality NEET Notes | Class 11 Biology
1. Introduction to the Cell
Jab hum aas-paas dekhte hain, toh hume living aur non-living cheezein dono nazar aati hain. Aapne socha hoga ki koi cheez living kyu hai aur koi non-living. Iska main difference ye hai ki living organisms me life ka basic unit, yaani cell, present hota hai.
Saare organisms cells se bane hote hain. Kuch organisms ek hi cell se bane hote hain aur unhe unicellular organisms kehte hain, jabki dusre, jaise humans, kai cells se bane hote hain aur unhe multicellular organisms kehte hain. Unicellular organisms independent existence ke capable hote hain aur saare essential life functions perform kar sakte hain. Koi bhi cheez agar complete cell structure ke bina ho, toh wo independently live nahi kar sakti. Isi liye cell ko saare living beings ka fundamental structural aur functional unit kaha jata hai.
Cell Ki discovery ki history Antonie Von Leeuwenhoek se shuru hua, jo pehle living cell ko observe aur describe karne wale scientist the. Baad me, Robert Brown ne nucleus discover kiya. Microscope ke invention aur electron microscope ke advancement ke baad, scientists cell ke saare structural details dekh paaye.
2. Cell Theory – Principles of Cell Structure and Function
1838 me, Matthias Schleiden, ek German botanist, ne kai plants study kiye aur paya ki saare plants alag-alag types ke cells se bane hote hain, jo milkar tissues banate hain. Lagbhag same time me, Theodore Schwann, ek German zoologist, ne animal cells study kiye aur observe kiya ki har cell ka ek thin outer layer hota hai, jise ab plasma membrane kaha jata hai. Schwann ne ye bhi note kiya ki cell wall ki presence plant cells ka unique feature hai. In observations ke basis par, Schwann propose kiya ki animals aur plants dono cells aur cell ke products se bane hote hain.
Milkar, Schleiden aur Schwann ne pehli cell theory formulate ki. Lekin, is theory me ye explain nahi kiya gaya tha ki new cells kaise bante hain. Baad me, 1855 me, Rudolf Virchow ne discover kiya ki cells divide karte hain aur new cells existing cells se hi arise hote hain (Omnis cellula-e cellula). Usne Schleiden aur Schwann ke ideas ko refine karke modern cell theory complete ki.
Aaj ke samajh ke hisaab se cell theory ke according:
- Saare living organisms cells aur cell ke products se bane hote hain.
- Saare cells pre-existing cells se bante hain.
3. Overview of Cell – Structure and Functions
Jab aap onion peel cells ya human cheek cells ko microscope ke under observe karte hain, toh aap cells ka basic structure notice karte hain. Onion cell, jo ek typical plant cell hai, ka distinct cell wall hota hai as outermost boundary, aur iske andar hota hai cell membrane. Human cheek cells, jaise ki zyadatar animal cells, me sirf cell membrane hi outer boundary ke roop me hoti hai. Dono plant aur animal cells ke andar ek dense, membrane-bound structure hota hai jise nucleus kehte hain, jo chromosomes contain karta hai, jisme genetic material DNA hota hai.
Jo cells membrane-bound nucleus rakhte hain, unhe eukaryotic cells kehte hain, aur jo membrane-bound nucleus nahi rakhte, unhe prokaryotic cells kehte hain. Dono types me, ek semi-fluid matrix hoti hai jise cytoplasm kehte hain, jo cell ke main activities ka site hota hai. Cytoplasm me kai chemical reactions hote hain jo cell ke living state ko maintain karte hain.
Nucleus ke alawa, eukaryotic cells me kai membrane-bound organelles hote hain jaise endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies, aur vacuoles. Prokaryotic cells me ye membrane-bound organelles nahi hote. Ribosomes, jo non-membrane-bound hote hain, saare cells me paye jate hain, cytoplasm me, rough ER par, aur organelles jaise chloroplasts (plants) aur mitochondria ke andar. Animal cells me ek aur non-membrane-bound organelle hota hai jise centrosome kehte hain, jo cell division me help karta hai.
Cells size, shape, aur function me bahut alag hote hain. Jaise, Mycoplasmas sabse chhote cells hain, approx 0.3 µm, jabki bacteria 3–5 µm ke range me hote hain. Sabse bada isolated single cell ostrich egg hai. Multicellular organisms me, human red blood cells approx 7 µm diameter ke hote hain, aur nerve cells bahut lambe ho sakte hain. Cells shape me bhi vary karte hain, jo disc-like, polygonal, columnar, cuboid, thread-like, ya irregular ho sakti hai, unke function ke hisaab se.
4. Prokaryotic Cells – Structure and Characteristics
Prokaryotic cells me aise organisms shamil hote hain jaise bacteria, blue-green algae, mycoplasma, aur PPLO (Pleuro Pneumonia Like Organisms). Ye cells generally eukaryotic cells se chhote hote hain aur tezi se multiply kar sakte hain. Ye shape aur size me bhi variation dikhate hain. Bacteria ke chaar main shapes hain: bacillus (rod-shaped), coccus (spherical), vibrio (comma-shaped), aur spirillum (spiral-shaped).
Jabki prokaryotes ke shapes aur functions kaafi diverse hote hain, unka basic organization similar hota hai. Zyadatar prokaryotic cells ke paas cell membrane ke bahar cell wall hoti hai, except mycoplasma me. Cytoplasm, ek semi-fluid matrix, cell ke andar fill hoti hai. Prokaryotic cells me well-defined nucleus nahi hota; unka genetic material naked DNA hota hai, jo nuclear membrane se enclosed nahi hota. Main genomic DNA usually single circular chromosome hota hai, lekin kai bacteria ke paas chhote circular DNA molecules bhi hote hain jise plasmids kehte hain. Plasmids bacteria ko special traits de sakte hain, jaise antibiotic resistance, aur biotechnology me foreign DNA ke saath bacterial transformation monitor karne ke liye use hote hain.
Eukaryotic cells ke opposite, prokaryotes me membrane-bound organelles nahi hote, except ribosomes. Unke paas unique inclusions aur ek special structure hota hai jise mesosome kehte hain, jo cell membrane ka infolding hota hai. Mesosome prokaryotic cells ka characteristic feature hai aur ye cellular processes jaise respiration aur cell division me help karta hai.
Cell Envelope – Structure and Modifications
Zyadatar prokaryotic cells, khaaskar bacteria, ke paas ek chemically complex cell envelope hota hai. Ye envelope teen tightly bound layers se bana hota hai: outermost glycocalyx, uske baad cell wall, aur phir plasma membrane. Jabki har layer ka apna specific function hota hai, ye saath milkar cell ke liye ek single protective unit ka kaam karte hain.
Bacteria ko Gram-positive aur Gram-negative me classify kiya ja sakta hai, based on unka cell envelope Gram staining technique ke response ke hisaab se. Gram-positive bacteria stain ko retain karte hain, jabki Gram-negative bacteria nahi karte.
Glycocalyx bacteria me composition aur thickness me vary karta hai. Kuch me ye loose sheath banata hai jise slime layer kehte hain, aur kuch me ye thick aur tough hota hai, jise capsule kehte hain. Cell wall bacterium ka shape determine karta hai aur structural support provide karta hai, jo bursting ya collapsing se bachata hai. Plasma membrane selectively permeable hota hai, external environment ke saath interact karta hai, aur structurally eukaryotic cells ke membrane jaisa hota hai.
Prokaryotes me ek unique structure mesosome hota hai, jo plasma membrane ke extensions se cytoplasm me banta hai. Ye infoldings vesicles, tubules, ya lamellae ke roop me appear hote hain aur cell wall formation, DNA replication, DNA ka daughter cells me distribution, respiration, secretion, aur surface area aur enzymatic activity ko increase karne me help karte hain. Kuch prokaryotes jaise cyanobacteria me aur bhi membranous extensions hote hain jise chromatophores kehte hain, jo pigments contain karte hain.
Bacteria motile ya non-motile ho sakte hain. Motile bacteria ke paas flagella hote hain, jo cell wall se thin filamentous extensions ke roop me nikalte hain. Flagellum ke teen parts hote hain: filament, hook, aur basal body. Filament sabse lamba part hota hai jo cell ke bahar extend karta hai. Dusre surface structures me pili aur fimbriae hote hain, jo movement ke liye use nahi hote. Pili long, tubular structures hote hain jo special proteins se bane hote hain, jabki fimbriae chhote bristle-like fibres hote hain. Ye bacteria ko surfaces se attach hone me help karte hain, jaise streams ke rocks ya host tissues.
Ribosomes and Inclusion Bodies – Structure and Functions
Prokaryotic cells me ribosomes plasma membrane ke saath associated hote hain. Ye chhote hote hain, lagbhag 15 nm by 20 nm, aur do subunits se bane hote hain: 50S aur 30S, jo saath milkar 70S prokaryotic ribosomes banate hain. Ribosomes protein synthesis ke sites hote hain. Kai baar, ek single mRNA ke saath kai ribosomes attach ho jate hain, jo ek chain banate hain jise polyribosome ya polysome kehte hain. Polysome me ribosomes milkar mRNA ko proteins me translate karte hain.
Inclusion bodies prokaryotic cells ke cytoplasm me reserve materials ka storage form hote hain. Ye membranes se surrounded nahi hote aur cytoplasm me freely lie karte hain. Examples me phosphate granules, cyanophycean granules, aur glycogen granules shamil hain. Kuch prokaryotes, jaise blue-green, purple, aur green photosynthetic bacteria, ke paas gas vacuoles hote hain jo buoyancy me help karte hain.
5. Eukaryotic Cells – Structure and Characteristics
Eukaryotic cells me sabhi protists, plants, animals, aur fungi shamil hote hain. Ye cells high level ki compartmentalisation dikhate hain kyunki cytoplasm me membrane-bound organelles present hote hain. Eukaryotic cells ka nucleus well-organised hota hai aur nuclear envelope se covered hota hai, aur unka genetic material chromosomes me arranged hota hai. Ye complex locomotory structures aur cytoskeleton bhi rakhte hain, jo cell ke shape, movement, aur intracellular transport me help karte hain.
Sabhi eukaryotic cells identical nahi hote. Plant aur animal cells me kuch important differences hote hain. Plant cells ke paas cell wall, plastids, aur large central vacuole hote hain, jo animal cells me nahi hote. Dusri taraf, animal cells me centrioles hote hain, jo lagbhag sabhi plant cells me absent hote hain. In differences ko samajhne se individual cell organelles ke structure aur function ko detail me study karna aasaan ho jata hai.
Cell Membrane – Structure and Functions
Cell membrane ka detailed structure 1950s me electron microscope ke invention ke baad zyada clearly study kiya gaya. Usse pehle, chemical studies, khaaskar human red blood cells (RBCs) par, scientists ko plasma membrane ka possible structure samajhne me help mili. In studies se pata chala ki cell membrane mainly lipids aur proteins se bana hota hai. Major lipids phospholipids hote hain, jo bilayer me arranged hote hain. Is bilayer me, polar heads bahar ki taraf hote hain, aur hydrophobic tails andar ki taraf point karte hain, jo nonpolar tails ko watery environment se protect karte hain. Phospholipids ke alawa, membrane me cholesterol, proteins, aur carbohydrates bhi hote hain. Protein aur lipid ka ratio alag cell types me vary karta hai; jaise, human RBCs me membrane me lagbhag 52% protein aur 40% lipids hote hain.
Membrane proteins ko unke position ke basis par classify kiya jata hai: peripheral proteins surface par hote hain, jabki integral proteins partially ya fully membrane me embedded hote hain. 1972 me Singer aur Nicolson ne fluid mosaic model propose kiya, jo cell membrane ka improved model hai. Ye model describe karta hai lipid bilayer ko quasi-fluid, jisse proteins lateral move kar sakte hain. Ye fluidity processes ke liye important hai jaise cell growth, intercellular junction formation, secretion, endocytosis, aur cell division.
Plasma membrane ka ek key function hai transport. Membrane selectively permeable hota hai, jo kuch molecules ko freely pass karne deta hai. Molecules ka energy ke bina move karna passive transport kehlata hai. Simple diffusion neutral solutes ko higher se lower concentration me move karne deta hai. Paani bhi is process se osmosis ke through move karta hai. Polar molecules, jo nonpolar lipid bilayer se nahi ja sakte, unhe carrier proteins ki help chahiye hoti hai membrane cross karne ke liye.
Kuch molecules apne concentration gradient ke against (lower se higher concentration) move karte hain using energy in the form of ATP. Is process ko active transport kehte hain, jaise Na⁺/K⁺ pump.
Cell Wall – Structure and Functions
A cell wall ek non-living, rigid structure hai jo plants aur fungi me plasma membrane ke outer covering ke roop me hoti hai. Cell wall cell ko uska shape deti hai, mechanical damage aur infections se protect karti hai, aur cell-to-cell interactions me bhi help karti hai. Ye ek barrier ke tarah kaam karti hai, jo unwanted macromolecules ko cell me enter karne se rokta hai.
Cell wall ka composition alag-alag organisms me vary karta hai. Algae me, cell wall cellulose, galactans, mannans, aur kabhi-kabhi minerals jaise calcium carbonate se bani hoti hai. Higher plants me, ye mainly cellulose, hemicellulose, pectins, aur proteins contain karti hai.
Young plant cell me, primary wall flexible hoti hai aur growth ke capable hoti hai. Jaise-jaise cell mature hota hai, secondary wall primary wall ke inner side me form hoti hai, jo stronger aur less flexible hoti hai. Adjacent cells ke beech me, ek layer hoti hai jise middle lamella kehte hain, jo mostly calcium pectate se bani hoti hai, aur glue ki tarah kaam karti hai, neighboring cells ko saath hold karti hai.
Cell wall aur middle lamella me aksar plasmodesmata hote hain, jo tiny channels hote hain jo neighboring cells ke cytoplasm ko connect karte hain, allowing communication aur materials ka transport.
Endomembrane System – Organelles and Their Functions
Ek eukaryotic cell me, bahut saare membrane-bound organelles apni structure aur function me distinct hote hain, lekin kuch organelles coordinated way me kaam karte hain aur inhe collectively endomembrane system kehte hain. Endomembrane system me endoplasmic reticulum (ER), Golgi complex, lysosomes, aur vacuoles shamil hote hain. Ye organelles protein aur lipid synthesis, modification, storage, aur transport jaise processes me saath kaam karte hain.
Dusre organelles jaise mitochondria, chloroplasts, aur peroxisomes ke specific functions hote hain jo endomembrane system se independent hote hain. Isliye, inhe endomembrane system ka part nahi mana jata.
Endoplasmic Reticulum (ER) – Structure and Functions
Electron microscopy se eukaryotic cells me ek network of tiny tubules dikhta hai jo cytoplasm me scattered hota hai, jise endoplasmic reticulum (ER) kehte hain. ER cell ke interior ko do compartments me divide karta hai: luminal compartment (ER ke andar) aur extra-luminal compartment (ER ke bahar ka cytoplasm).
ER ke surface par ribosomes attach ho sakte hain. Jab ribosomes present hote hain, to ise rough endoplasmic reticulum (RER) kehte hain, aur jab ribosomes absent hote hain, to ise smooth endoplasmic reticulum (SER) kehte hain.
RER un cells me especially prominent hota hai jo actively protein synthesis aur secretion me involved hote hain. Ye extensive hota hai aur nucleus ke outer membrane ke saath continuous hota hai, jo newly synthesized proteins ka smooth transport allow karta hai.
Dusri taraf, SER lipid synthesis ka main site hai. Animal cells me, steroid hormones bhi SER me produce hote hain.
Golgi Apparatus – Structure and Functions
Golgi apparatus ko sabse pehle Camillo Golgi ne 1898 me observe kiya tha as densely stained structures nucleus ke paas, aur ise kabhi-kabhi Golgi bodies bhi kehte hain. Ye flat, disc-shaped sacs se bana hota hai jise cisternae kehte hain, jo lagbhag 0.5 se 1.0 µm diameter ke hote hain aur ek dusre ke parallel stacked hote hain. Cisternae ki number alag-alag cells me vary kar sakti hai.
Golgi cisternae nucleus ke paas concentric manner me arrange hote hain aur do distinct faces hote hain: cis face (convex, forming face) aur trans face (concave, maturing face). Jabki ye faces alag structures aur functions rakhte hain, ye interconnected hote hain.
Golgi apparatus ka main function materials ko transport ke liye package aur modify karna hai, chahe cell ke andar ya bahar. Vesicles jo endoplasmic reticulum (ER) se materials leke aate hain, wo Golgi apparatus ke cis face se fuse karte hain aur trans face ki taraf move karte hain secretion ke liye. Ribosomes par synthesized proteins Golgi cisternae me modify hote hain release hone se pehle.
Golgi apparatus glycoproteins aur glycolipids ke formation ka bhi key site hai, jo various cellular functions ke liye essential hote hain.
Lysosomes – Structure and Functions
Lysosomes membrane-bound vesicles hote hain jo Golgi apparatus me packaging process ke through bante hain. Ye vesicles variety of hydrolytic enzymes se filled hote hain, jise hydrolases bhi kehte hain, jo include karte hain lipases (fats ko digest karte hain), proteases (proteins ko digest karte hain), aur carbohydrases (carbohydrates ko digest karte hain). Ye enzymes acidic environment me best kaam karte hain.
Lysosomes ka main function digestion hai. Ye carbohydrates, proteins, lipids, aur nucleic acids ko cell ke andar break down kar sakte hain. In enzymes ki wajah se, lysosomes ko aksar cell ke “suicide bags” kehte hain, kyunki ye worn-out organelles ya foreign material ko digest kar sakte hain jo cell me enter hota hai.
Vacuoles – Structure and Functions
Vacuole ek membrane-bound space hai jo cell ke cytoplasm me hota hai. Isme water, cell sap, waste products, aur dusre materials hote hain jo cell ke liye immediately useful nahi hote. Vacuole ek single membrane se surrounded hota hai jise tonoplast kehte hain. Plant cells me, vacuoles cell ke volume ka upto 90% occupy kar sakte hain, jo unhe bahut prominent banata hai.
Tonoplast ions aur dusre substances ko vacuole me transport karne me important role play karta hai, aksar concentration gradients ke against. Isse vacuole certain substances ka higher concentration cytoplasm ke comparison me maintain kar sakta hai.
Amoeba aur dusre protists me, specialized vacuoles specific functions perform karte hain. Contractile vacuole osmoregulation aur excretion me help karta hai, jabki food vacuoles food particles ko engulf karke digestion me help karte hain.
Mitochondria – Structure and Functions
Mitochondria (singular: mitochondrion) double membrane-bound organelles hote hain jo easily microscope me nahi dikhte, jab tak specially stain na kiya jaye. Mitochondria ki number, shape, aur size cell ke activity ke hisaab se vary karte hain. Typically, mitochondria sausage-shaped ya cylindrical hote hain, diameter 0.2–1.0 µm aur length 1.0–4.1 µm hoti hai.
Har mitochondrion ka ek outer membrane aur inner membrane hota hai, jo uske interior ko do compartments me divide karta hai: outer compartment aur inner compartment. Inner compartment dense substance se filled hota hai jise matrix kehte hain. Inner membrane infoldings banata hai jo cristae ke naam se jane jate hain, jo chemical reactions ke liye surface area increase karte hain. Dono membranes me specific enzymes hote hain jo mitochondrial functions ke liye essential hote hain.
Mitochondria aerobic respiration ka main site hain, jahan cellular energy (ATP) produce hoti hai, isliye inhe “powerhouses of the cell” kehte hain. Matrix me circular DNA, RNA, 70S ribosomes, aur protein synthesis ke liye saari machinery bhi hoti hai. Mitochondria independent replicate karte hain ek process ke through jise fission kehte hain.
Plastids – Structure and Functions in Plant Cells
Plastids wo organelles hain jo sabhi plant cells aur kuch protists jaise euglenoides me paye jate hain. Ye generally large hote hain aur microscope me visible hote hain. Plastids me specific pigments hote hain, jo plant ke alag-alag parts ko colour dete hain. Pigments ke basis par, plastids ko chloroplasts, chromoplasts, aur leucoplasts me classify kiya jata hai.
Chloroplasts me chlorophyll aur carotenoid pigments hote hain, jo photosynthesis ke liye light energy trap karne ke liye essential hain. Chromoplasts me fat-soluble carotenoids jaise carotene aur xanthophyll hote hain, jo flowers aur fruits ko yellow, orange, ya red colour dete hain. Leucoplasts colourless hote hain aur nutrients store karte hain: amyloplasts starch store karte hain, elaioplasts oils aur fats store karte hain, aur aleuroplasts proteins store karte hain.
Green plants me zyadatar chloroplasts leaves ke mesophyll cells me paye jate hain. Ye lens-shaped, oval, spherical, discoid, ya ribbon-like ho sakte hain, aur approx 5–10 µm length aur 2–4 µm width me measure karte hain. Inki number vary kar sakti hai, jaise Chlamydomonas me 1 per cell aur mesophyll cell me 20–40.
Jaise mitochondria, chloroplasts bhi double membrane-bound hote hain. Inner membrane ek space ko enclose karta hai jise stroma kehte hain, jo carbohydrate aur protein synthesis ke enzymes ke saath small circular DNA molecules aur 70S ribosomes bhi contain karta hai. Stroma ke andar flattened membranous sacs hote hain jise thylakoids kehte hain, jo grana (singular: granum) me stacked hote hain. Thylakoids stroma lamellae se connected hote hain, aur thylakoid ke andar ka space lumen kehlata hai. Chlorophyll pigments thylakoid membranes me embedded hote hain, jo photosynthesis ko enable karte hain.
Ribosomes – Structure and Functions
Ribosomes cell me small granular structures hote hain jo sabse pehle electron microscope ke under George Palade ne 1953 me dekhe. Ye ribonucleic acid (RNA) aur proteins se bane hote hain, aur inhe kisi membrane se surround nahi kiya jata.
Ribosomes dono prokaryotic aur eukaryotic cells me protein synthesis ka site hote hain. Eukaryotes me ribosomes 80S hote hain, jabki prokaryotes me 70S hote hain. Har ribosome do subunits se bana hota hai – ek larger subunit aur ek smaller subunit. 80S ribosomes ke liye, subunits 60S aur 40S hote hain, aur 70S ribosomes ke liye 50S aur 30S hote hain.
Yahan, ‘S’ (Svedberg unit) sedimentation rate ko measure karta hai, jo indirectly ribosome ke size aur density ke baare me batata hai. Size ke difference ke bawajood, dono 70S aur 80S ribosomes ka basic structure same hota hai, jisme do subunits saath kaam karte hain.
Cytoskeleton – Structure and Functions
Cytoskeleton ek complex network hai protein fibers ka jo cell ke cytoplasm me present hota hai. Ye mainly teen types ke filamentous structures se bana hota hai: microtubules, microfilaments, aur intermediate filaments.
Cytoskeleton ke kai important functions hain. Ye cell ko mechanical support deta hai, cell ke shape ko maintain karne me help karta hai, aur cell movement (motility) me bhi involved hota hai. Additionally, cytoskeleton intracellular transport me bhi role play karta hai, organelles aur vesicles ko cell ke andar move karne me help karta hai.
Short me, cytoskeleton cell ke andar scaffolding aur transport network ki tarah kaam karta hai, jo cell ko structure ke saath flexibility bhi provide karta hai.
Cilia and Flagella – Structure and Functions
Cilia aur flagella cell membrane se nikle hair-like projections hote hain jo movement me help karte hain. Cilia short aur numerous hote hain, aur oars ki tarah kaam karte hain, ya to cell ko ya phir uske aas-pass ke fluid ko move karne ke liye. Flagella longer hote hain aur primarily poore cell ke movement me help karte hain. Prokaryotic bacteria me bhi flagella hote hain, lekin unka structure eukaryotic flagella se different hota hai.
Electron microscopy dikhata hai ki dono cilia aur flagella plasma membrane se covered hote hain. Inka core, jise axoneme kehte hain, microtubules se bana hota hai jo length ke along arranged hote hain. Usually, periphery me nine doublets of microtubules hote hain aur center me do single microtubules hote hain. Is arrangement ko 9+2 pattern kehte hain.
Central microtubules ek central sheath se connected hote hain aur peripheral doublets se radial spokes (total nine spokes) ke through linked hote hain. Peripheral doublets bhi linkers se join hote hain. Dono cilia aur flagella basal bodies se arise hote hain, jo centrioles ke similar hote hain..
Centrosome and Centrioles – Structure and Functions
Centrosome ek cell organelle hai jo usually do cylindrical structures contain karta hai jise centrioles kehte hain. Ye centrioles ek gel-like material se surrounded hote hain jise pericentriolar material kehte hain. Centrosome me do centrioles ek dusre ke perpendicular arranged hote hain. Har centriole ka structure cartwheel-like hota hai, jo nine evenly spaced tubulin fibrils se bana hota hai. Har fibril ek triplet hota hai, aur ye triplets ek dusre se connected hote hain.
Centriole ke proximal end ka central part hub kehlata hai, jo protein se bana hota hai. Hub peripheral triplets se radial spokes ke through connected hota hai. Centrioles important hote hain kyunki ye cilia aur flagella ka basal body form karte hain, aur animal cells me cell division ke dauran spindle fibres banakar spindle apparatus create karte hain.
Nucleus – Structure and Functions
Nucleus ek important cell organelle hai jo sabse pehle Robert Brown ne 1831 me observe kiya tha. Baad me, nucleus ke andar ka material jo basic dyes se stain ho sakta tha, usse Flemming ne chromatin naam diya. Jab cell divide nahi kar raha hota, to uska nucleus interphase stage me hota hai aur isme long, thread-like nucleoprotein fibers ka network hota hai jise chromatin kehte hain, saath me nuclear matrix aur ek ya zyada round structures hote hain jise nucleoli kehte hain. Electron microscopy se scientists ne paaya ki nucleus ek nuclear envelope se surrounded hota hai, jo do parallel membranes se bana hota hai jo ek chhoti gap perinuclear space ke through separate hoti hain. Ye envelope nucleus aur cytoplasm ke beech barrier ka kaam karta hai. Envelope ka outer membrane usually endoplasmic reticulum se connected hota hai aur isme ribosomes attached hote hain. Envelope me chhote pores hote hain, jise nuclear pores kehte hain, jo RNA aur proteins ko nucleus aur cytoplasm ke beech move karne allow karte hain. Zyadatar cells me sirf ek nucleus hota hai, lekin kuch cells me zyada ho sakte hain, aur kuch mature cells jaise mammalian erythrocytes aur plant ke sieve tube cells me nucleus completely absent hota hai.
Nucleus ke andar, nuclear matrix ya nucleoplasm me chromatin aur nucleoli hote hain. Nucleolus ek spherical structure hai jahan ribosomal RNA (rRNA) actively produce hota hai. Jo cells actively proteins bana rahe hote hain, unme larger aur zyada nucleoli hote hain. Interphase me chromatin loose network ke roop me dikhta hai, lekin cell division ke dauran ye distinct chromosomes me organize hota hai. Chromatin DNA, histone proteins, non-histone proteins, aur RNA se bana hota hai. Ek single human cell me lagbhag 2 meters DNA hota hai, jo 46 chromosomes (23 pairs) me arranged hota hai.
Har chromosome ka ek primary constriction hota hai jise centromere kehte hain, jo disc-shaped kinetochores se flanked hota hai. Centromere chromosome ke do chromatids ko saath hold karta hai. Chromosomes ko centromere ke position ke basis par classify kiya jata hai. Metacentric chromosomes me centromere middle me hota hai, jisse do equal arms bante hain. Sub-metacentric chromosomes me centromere thoda off-center hota hai, jisse ek short arm aur ek long arm banta hai. Acrocentric chromosomes me centromere end ke paas hota hai, jisse ek bahut short arm aur ek long arm banta hai, aur telocentric chromosomes me centromere terminal hota hai. Kuch chromosomes me non-staining secondary constrictions bhi hote hain, jo small fragments ke roop me dikhti hain aur unhe satellites kehte hain.
Microbodies – Structure and Functions
Dono plant aur animal cells me kai chhote membrane-bound vesicles hote hain jise microbodies kehte hain. Ye microbodies cell ke andar small, round structures hote hain jo various enzymes ko store aur carry karte hain. In microbodies me present enzymes different chemical reactions me help karte hain, jo cell ke normal functioning ke liye essential hote hain. Halanki ye size me bahut chhote hote hain, microbodies cell metabolism aur dusre biochemical processes me important role play karte hain.
6. Chapter Overview – Key Concepts and Highlights
Sabhi living organisms cells ya cells ke groups se bane hote hain, aur ye cells shape, size, aur function me bahut different ho sakte hain. Membrane-bound nucleus aur dusre organelles ke presence ya absence ke basis par, cells ko eukaryotic ya prokaryotic classify kiya jata hai. Ek typical eukaryotic cell me cell membrane, nucleus, aur cytoplasm hota hai, jabki plant cells me membrane ke bahar cell wall bhi hoti hai. Plasma membrane selectively permeable hoti hai, jo kuch molecules ko cell me enter aur exit karne allow karti hai. Endomembrane system, jo endoplasmic reticulum (ER), Golgi complex, lysosomes, aur vacuoles include karta hai, substances ke transport, storage, aur processing me help karta hai. Har organelle ka specific function hota hai. For example, centrosome aur centriole cilia aur flagella ka basal body banate hain, jo cell movement me help karte hain, aur animal cells me centrioles cell division ke dauran spindle apparatus bhi form karte hain.
Nucleus me nucleoli aur chromatin network hota hai. Ye sirf organelles ke activities control nahi karta, balki heredity me bhi major role play karta hai. Endoplasmic reticulum tubules ya flattened sacs jise cisternae kehte hain, se bana hota hai, aur ye do forms me exist karta hai: rough ER (ribosomes ke saath) aur smooth ER (ribosomes ke bina). ER substances ke transport, protein synthesis, aur lipoproteins aur glycogen ke production me help karta hai. Golgi body flattened membranous sacs se bana hota hai jo cell ke secretions ko package aur transport karta hai. Lysosomes single-membrane vesicles hote hain jisme digestive enzymes hote hain jo saare types ke macromolecules ko break down karte hain. Ribosomes protein synthesis ke liye responsible hote hain aur cytoplasm me free ya ER se attached ho sakte hain.
Mitochondria ko cell ka “powerhouse” kehte hain kyunki ye oxidative phosphorylation ke through ATP produce karte hain. Inka double membrane hota hai, jisme outer membrane smooth hota hai aur inner membrane folds banata hai jise cristae kehte hain. Plastids pigment-containing organelles hote hain jo sirf plant cells me paye jate hain. Chloroplasts green plastids hain jo photosynthesis ke liye responsible hote hain, jisme grana light reactions ka site aur stroma dark reactions ka site hota hai. Dusre plastids, jise chromoplasts kehte hain, me carotene aur xanthophyll jaise pigments hote hain. Nucleus khud nuclear envelope se surrounded hota hai, jo double membrane aur nuclear pores ke saath nucleoplasm aur chromatin ko enclose karta hai.
Conclusion me, cell life ka basic structural aur functional unit hai, jisme har organelle apna specific role perform karta hai jo cell ke survival aur function ko maintain karta hai.
