NCERT Solutions Class 11 Biology Chapter 8 – Cell: The Unit of Life

Har living organism — chahe unicellular ho ya multicellular — ek ya zyada cells se bana hota hai, aur cell hi wo sabse chhoti unit hai jo independently saare essential life-processes (nutrition, respiration, excretion, growth, reproduction) perform kar sakti hai. Isliye cell ko life ki structural (banawat ki) aur functional (kaam karne ki) — dono tarah se basic unit maana jaata hai.

Sabse fundamental difference nucleus ka hai — prokaryotic cell me true, membrane-bound nucleus nahi hota (genetic material nucleoid region me nanga/naked hota hai), jabki eukaryotic cell me DNA ek double-membrane wale nuclear envelope se bound, well-organised nucleus ke andar hota hai. Isi ek difference se dono cell types ki baaki saari properties (organelle presence, size, ribosome type) branch out hoti hain.

Plasma membrane har substance ko equally cross nahi hone deti — ye kuch molecules (jaise small non-polar molecules, gases) ko freely pass hone deti hai, jabki bade ya polar/charged molecules ke liye specific transport proteins/channels ki zaroorat hoti hai. Isi selective control ki wajah se cell apna internal environment (ions, nutrients) stable rakh paata hai, isliye ise "selectively permeable" kaha jaata hai.

Cell wall plant cell ko mechanical strength, rigidity aur definite shape deta hai, aur cell ko osmotic lysis (bahut zyada paani lene se phatna) se protect karta hai. Ye plasma membrane ke bahar ek non-living, rigid layer hoti hai, mainly cellulose se bani, jo neighbouring cells ke saath support aur protection dono provide karti hai.

Ribosomes vo cell organelle hain jahan protein synthesis (translation) hoti hai — mRNA ki genetic information ko read karke amino acids ko sahi sequence me joda jaata hai, jisse polypeptide chain (protein) banti hai. Ye RNA aur protein se milkar bane hote hain, kisi membrane se bound nahi hote, aur ER (rough ER) ya nuclear envelope se attached ya cytoplasm me free floating mil sakte hain — isi wajah se inhe "protein factory" bola jaata hai.

Centrosome ek organelle hai jisme do cylindrical structures — centrioles — hoti hain, jo ek dusre se right angle par arranged hoti hain. Har centriole 9 peripheral microtubule triplets (9+0 arrangement) se bani hoti hai, aur cartwheel jaisi spokes structure se connected hoti hai.

Centrosome cell division ke time spindle fibre formation me role play karta hai, aur cilia/flagella ke base pe basal body banane me bhi centriole involved hota hai.

Statement (a) galat hai.

Correction: Cell ki discovery Robert Hooke ne 1665 me ki thi — usne cork ki thin slice ko compound microscope se dekha aur dead cell walls ke honeycomb-jaisi structure ko "cell" naam diya. Robert Brown ne 1831 me cell nucleus discover kiya tha, cell nahi.

Baaki teeno statements sahi hain — (b) Schleiden (plants) aur Schwann (animals) ne cell theory propose ki, (c) Virchow ne "Omnis cellula-e cellula" (naye cells pre-existing cells se hi bante hain) explain kiya, aur (d) unicellular organisms me ek hi cell saari life-processes (nutrition, respiration, excretion, reproduction) perform karti hai.

Sahi option: (b) pre-existing cells se.

Cell theory ke third postulate (Virchow, 1855) ke according, sabhi naye cells sirf ek pre-existing cell ke division (mitosis/binary fission) se hi banti hain — koi cell spontaneously non-living matter se generate nahi hoti. Isse "spontaneous generation" theory reject ho gayi thi.

Correct matching:

• (a) → (iii) Lysosome — hydrolytic enzymes (lipase, protease, carbohydrase) se fat aur dusre macromolecules ko hydrolyse karta hai.
• (b) → (i) Mitochondria — oxidative phosphorylation se ATP generate karta hai, isliye "powerhouse of the cell" kehlata hai.
• (c) → (ii) Ribosome — mRNA ka translation karke protein synthesis karta hai.
• (d) → (iii) Lysosome — damaged cell me apne hi enzymes release karke cell ko digest kar deta hai, isliye "suicidal bags" kehlata hai.
• (e) → (iv) Glycocalyx — prokaryotic cell wall ke bahar capsule (thick, organised) ya slime layer (loose, unorganised) ke roop me present hota hai.

• (a) False — ribosomes RNA aur protein ke bane hote hain; ye membrane-bound nahi hote.
• (b) True — plasmodesmata plant cells ke beech cell wall se guzarne wale cytoplasmic connections hain, jo material transport allow karte hain.
• (c) False — cell Robert Hooke ne discover ki thi, 1665 me.
• (d) False — cell theory Schleiden, Schwann aur Virchow ne di thi, Watson aur Crick ne DNA structure diya tha.
• (e) False — Mycoplasma jaise prokaryotes me cell wall absent hota hai.
• (f) True — plant cell vacuoles ek single membrane, tonoplast, se bound hote hain.

Cellulose plant cell wall ka main structural polysaccharide hai (β-D-glucose units ki long chains se bana). Ye primary cell wall aur secondary cell wall (jahan present ho) dono me microfibrils ke form me hota hai, jo cell ko mechanical strength, rigidity aur definite shape deta hai.

Golgi apparatus, endoplasmic reticulum se aane wale materials (proteins, lipids) ko cis face pe receive karta hai, cisternae ke andar unhe modify karta hai (glycosylation, sulphation jaisi post-translational modifications), aur phir trans face se membrane-bound vesicles me package karke cell ke different destinations tak bhejta hai — jaise plasma membrane, lysosome, ya secretion ke liye cell ke bahar. Is receive → modify → sort → dispatch pathway ki wajah se ise distribution aur packaging centre kehte hain.

Mesosome, prokaryotic cell (bacteria) ki plasma membrane ka andar ki taraf fold hokar bana hua extension hai, jo vesicles, tubules ya lamellae ke form me hota hai.

Functions:

• Cell wall formation aur DNA replication me help karta hai.
• Daughter cells ke beech DNA distribution ko coordinate karta hai cell division ke time.
• Respiratory enzymes yahan concentrate hone se cell ki respiration efficiency badhata hai.

• Membrane-bound nucleus absent hota hai — genetic material nucleoid region me nangi (naked, histone-free) circular DNA ke form me hota hai; extra chromosomal DNA plasmid ke roop me bhi ho sakta hai.
• Membrane-bound organelles (mitochondria, ER, Golgi, plastid) absent hote hain.
• Size generally 1–10 μm range me hota hai (eukaryotic cells se chhota).
• Cell wall present hota hai (mostly), jo peptidoglycan (murein) ka bana hota hai — exception: Mycoplasma.
• Ribosomes 70S type ke hote hain.
• Flagella (agar present) simple structure ke hote hain, jisme microtubules absent hote hain.

Multicellular organisms me alag-alag cells specific functions ke liye specialised ho jaate hain — jaise muscle cells contraction ke liye, nerve cells signal-transmission ke liye, RBCs oxygen transport ke liye. Similar function wale cells milkar tissue banate hain, tissues milkar organ, aur organs milkar organ-system banate hain.

Is specialisation (division of labour) se har cell-type apna specific kaam zyada efficiently perform karta hai, jabki unicellular organism (jaise Amoeba) ko saari life-activities (nutrition, respiration, excretion, reproduction) akele ek hi cell se karni padti hain.

Cell theory ke according cell hi life ki smallest structural aur functional unit hai, kyunki:

• Sabhi living organisms — unicellular ho ya multicellular — ek ya zyada cells se bane hote hain.
• Cell independently sabhi essential life-processes (metabolism, growth, response to stimuli, reproduction) perform kar sakti hai — unicellular organisms (bacteria, Amoeba) me ye saari activities ek hi cell me hoti hain.
• Naye cells sirf pre-existing cells ke division se hi bante hain — koi cell spontaneously non-living material se nahi banti.

Isliye cell ko life ki basic structural aur functional unit mana jata hai.

Nuclear pores, nuclear envelope (double membrane structure) me present chhidra hote hain jo outer aur inner nuclear membrane ke local fusion se bante hain — inke chaaron taraf proteins ka octagonal arrangement nuclear pore complex (NPC) banata hai.

Function: Ye selective aur regulated transport channel ka kaam karte hain — RNA molecules aur proteins ka nucleoplasm se cytoplasm me, aur cytoplasm se nucleoplasm me movement isi ke through hota hai.

Lysosome: single membrane-bound vesicle hai jisme hydrolytic enzymes (lipase, protease, carbohydrase) hote hain jo acidic pH pe active hote hain. Iska main kaam intracellular aur extracellular material ka digestion hai, aur worn-out organelles ka autophagy/autolysis — damage hone par ye cell ko hi digest kar deta hai, isliye "suicide bags" kehlata hai.

Vacuole: bhi membrane (tonoplast) se bound hota hai, lekin iska kaam mainly storage hai — plant cell me large central vacuole cell sap (water, salts, sugars, organic acids, pigments) store karke turgor pressure maintain karta hai; Amoeba me contractile vacuole osmoregulation aur food vacuole digestion ke liye kaam karta hai.

Toh structurally dono membrane-bound sacs hain, lekin lysosome ka role "digestive/degradative" hai jabki vacuole ka role "storage/regulatory" hai.

• Nuclear envelope: double membrane (outer + inner) jisme nuclear pores hote hain; outer membrane ER se continuous hoti hai aur usme ribosomes attached ho sakte hain.
• Nucleoplasm: matrix jisme chromatin material aur nucleolus suspended hote hain.
• Nucleolus: membrane-less spherical structure, rRNA synthesis aur ribosome subunit assembly ki site hai.
• Chromatin: DNA aur histone proteins ka network hota hai, jo cell division ke time condense hokar chromosomes banata hai.

Nucleus cell ki genetic information (DNA) ko store karta hai aur cell ki saari activities control karta hai.

• Cell sap store karta hai — water, amino acids, sugars, organic acids aur waste products isme hote hain.
• Turgor pressure maintain karke cell ko rigid banata hai (turgidity), jo plant support ke liye zaroori hai.
• Osmoregulation me help karta hai.
• Kuch pigments (jaise anthocyanins) store karke petals/fruits ko colour deta hai.
• Waste aur toxic substances ko cytoplasm se isolate karta hai.

(Diagram notebook me draw karna hai — neeche description ke saath label points diye gaye hain.)

• Mitochondria ek double membrane-bound organelle hai.
• Outer membrane smooth hoti hai.
• Inner membrane andar ki taraf fold hokar cristae banati hai, jo surface area badhati hai (isi surface pe electron transport chain hoti hai).
• Matrix me mitochondrial DNA (circular), 70S ribosomes, aur respiration ke enzymes hote hain.
• Inner membrane pe F1 particles (oxysomes/ATP synthase) present hote hain jo oxidative phosphorylation se ATP synthesize karte hain.

Apna DNA aur 70S ribosomes hone ki wajah se mitochondria ko "semi-autonomous organelle" bhi kaha jata hai.

• Chromosome, DNA aur histone proteins (chromatin) ka condensed form hai, jo cell division ke time light microscope me visible hota hai.
• Har chromosome me ek primary constriction — centromere — hota hai, jispe disc-shaped structure kinetochore attach hota hai.
• Centromere ki position ke basis pe chromosomes metacentric, sub-metacentric, acrocentric ya telocentric type ke hote hain.
• Kuch chromosomes me ek secondary constriction bhi hota hai jise satellite region kehte hain.
• Chromosome number species-specific aur constant hota hai — somatic cells me diploid (2n), gametes me haploid (n).