Chapter 2: Cell-The Building Block of Life Quick Revision notes | Class 9th Science (Exploration) notes

In-text Questions

Activities · "What if" · Pause & Ponder

Activity 2.1 · numerical

Estimate the size of a cell. Field-of-view diameter = 5 mm aur 25 onion-peel cells line mein hain. Ek cell ka size nikalo, aur microscope ka magnification (eyepiece 10X, objective 10X).

Step-by-step

Field diameter convert karo: 5 mm × 1000 = 5000 µm (kyunki 1 mm = 1000 µm)
Ek cell ka size = field diameter ÷ cells ki sankhya: 5000 µm ÷ 25 = 200 µm
Total magnification = eyepiece × objective: 10X × 10X = 100X

→ Ek onion cell ≈ 200 µm, 100× bada dikh raha hai

In-text · §2.2.1

Alveoli ki cell membrane O₂ aur CO₂ ke movement ko kaise control karti hai?

Answer

Alveolar cell membrane selectively permeable hoti hai. Gases diffusion se cross karti hain — har gas zyada concentration se kam concentration ki taraf jaati hai. O₂ alveolar air mein zyada aur blood mein kam hai, isliye O₂ blood mein diffuse hoti hai; CO₂ blood mein zyada aur air mein kam hai, isliye bahar nikalti hai. Thin lipid–protein membrane choti gas molecules ko pass karne deti hai par dusri cheezein rokti hai.

Activity 2.2 · experiment

Aloo plain water mein (Beaker A) vs 20% salt/sugar solution mein (Beaker B). Weight change ke baare mein kya observe aur infer karte ho?

Observation & Inference

Beaker A — plain water

Aloo phool jaata hai
Weight badhta hai
Paani cells andar jaata hai

Beaker B — salt/sugar solution

Aloo sikar jaata hai
Weight kam hota hai
Paani cells se bahar jaata hai

Yeh osmosis se hota hai — selectively permeable membrane se paani ka movement zyada paani (dilute) se kam paani (concentrated) ki taraf. Membrane paani ko pass karne deti hai par salt ya sugar molecules ko nahi.

Fig. Osmosis: paani andar (A) → aloo phooleta hai; paani bahar (B) → sikarता है।

What if…

Mung bean seeds 12 ghante paani mein bhigo ke phir concentrated solution mein rakh dein to kya hoga?

Answer

12 ghante paani mein bhigone ke baad seeds phool jaate hain aur turgid ho jaate hain. Concentrated (hypertonic) solution mein rakhne par surrounding solution mein seed cells se kam paani hota hai. Osmosis se paani ab cells se bahar jaata hai. Seeds sikar jaate hain, jhurri padh jaati hai aur firmness kho dete hain, aur sprouting slow ho jaati hai kyunki cells ko chahiye paani kho jaata hai.

What if…

Agar cell ko alag concentration ke solutions mein rakha jaaye — isotonic, hypotonic aur hypertonic — to kya hoga?

Teen cases

Solution	Bahar vs andar	Paani ka direction	Cell
Isotonic	barabar	koi net movement nahi	koi change nahi
Hypotonic	bahar zyada dilute	cell mein jaata hai	phool jaati hai
Hypertonic	bahar zyada concentrated	cell se bahar jaata hai	sikar jaati hai

Fig. Solution concentration ka cell par asar.

In-text · §2.2.2

Plant, fungal aur bacterial cells mein cell wall ki zaroorat kya hai?

Answer

Rigid shape aur mechanical strength deta hai, taki cell wind aur rain ka dab sah sake.
Nazuk cell membrane aur contents ki raksha karta hai.
Cell ko burst hone se bachata hai jab zyada paani andar aaye (hypotonic surroundings mein over-swelling rokta hai).
Poore plant/organism ko seedha aur firm rakhne mein madad karta hai kyunki yeh organisms harsh conditions se hil nahi sakte.

Activity 2.3 · investigate

Onion/Rhoeo peel cells vs cheek cells. Plant cells box-shaped kyun hain aur cheek cells irregular? 20% sugar solution mein plant cells boundary kyun hold karti hain jabki cheek cells poori tarah sikar jaati hain?

Answer

Shape difference: Plant (onion/Rhoeo) cells mein rigid cell wall hoti hai jo fixed, box-like shape deti hai. Cheek (animal) cells mein cell wall nahi hoti; sirf flexible membrane se bounced, yeh irregular, rounded shapes lete hain.

Sugar solution mein: Dono osmosis se paani kho deti hain. Plant cell mein rigid wall outer boundary hold karti hai jabki inner contents (membrane + cytoplasm) pull away hote hain — ek gap banta hai. Cheek cell, wall na hone se, as a whole sikar jaati hai.

In-text · Fig 2.10

Fig. 2.10 mein kaun se cells prokaryotic hain aur kaun se eukaryotic?

Answer

Prokaryotic: bacterial cell (a) — koi well-defined nucleus nahi (sirf nucleoid) aur koi membrane-bound organelles nahi.

Eukaryotic: plant cell (b) aur animal cell (c) — dono mein true membrane-bound nucleus aur membrane-bound organelles hote hain.

Activity 2.4 · Table 2.1

Bacterial, plant aur animal cells compare karo — Table 2.1 fill karo (presence ✓ / absence ✕).

Cell structure	Bacterial	Plant	Animal
Cell membrane	✓	✓	✓
Cell wall	✓	✓	✕
Cytoplasm	✓	✓	✓
Well-defined nucleus	✕	✓	✓
Nucleoid (primitive nucleus)	✓	✕	✕
Membrane-bound organelles	✕	✓	✓

Threads of Curiosity

Mature RBCs mein nucleus nahi hota. Kya aur bhi koi cells hain bina nucleus ke?

Answer

Mature sieve-tube cells plants ke phloem mein — nucleus kho deti hain par food transport ke liye alive rehti hain.
Mature mammalian red blood cells — haemoglobin ke liye extra space.
Platelets — blood mein cell fragments, nucleus nahi.
Eye lens fibre cells — transparent rehne ke liye nucleus kho deti hain.

Activity 2.5 · skills

Onion root-tip squash. Kya cells similar hain? Alag kyun dikhti hain? Pehla stage kaun sa aata hai?

Answer

Root-tip cells sab same nahi hain. Growing tip continuously cell division se divide karti hai, isliye alag cells alag stages mein pakdi jaati hain — varied internal appearances dikhti hain. Pehla stage woh hai jab cell divide nahi ho rahi aur ek intact, well-defined nucleus dikhti hai loose chromatin ke saath — chromosomes condense hone aur cell split hone se pehle.

Pause & Ponder · 1

Plants (fixed) ko cell wall ki zaroorat kyun hai jabki animals (moving) ko nahi?

Answer

Plants ek jagah jamirikd hain, isliye wind, rain ya dusri stresses se bhaag nahi sakte. Support, seedha rehne aur shape hold karne ke liye cell wall chahiye.

Animals ghoomte hain, isliye unhe flexibility chahiye — flexible, wall-less cells tissues ko bend, stretch aur shape change karne deti hain. Rigid wall movement rokegi, isliye animal cells mein sirf membrane hoti hai.

Pause & Ponder · 2

Agar plant cell ki cell wall cell membrane jaisi flexible ho jaaye to kya hoga?

Cell fixed shape aur rigidity kho degi.
Zyada paani mein bahut zyada phool sakti hai aur burst bhi ho sakti hai, kyunki inflow resist karne wala kuch nahi.
Plant seedha nahi reh sakegi — patte, tana aur phool jhuk jaayenge.
Rigid wall ki dii protection aur firm support kho jaayegi.

Pause & Ponder · 3

Aloo ke dono tukdon ko roughly equal size ka kyun kaatna chahiye aur initial weight record kyun karna chahiye?

Answer

Yeh ek fair test banata hai. Equal-sized pieces mein similar surface area aur cells hote hain, isliye sirf liquid alag hoti hai. Initial weight record karna baseline deta hai, taaki experiment ke baad change measure ho sake aur confirm ho ki woh osmosis se hua, pieces ke unequal hone se nahi.

Pause & Ponder · 4

Kya white flowers mein koi pigment hota hai? Karan do.

Answer

White petals mein aam taur par leucoplasts hote hain — plastids jinmein koi coloured pigment nahi hota. Yeh white isliye dikhte hain kyunki sabhi wavelengths of light ko reflect/scatter karte hain. Rang dene wale pigment ke terms mein, white flowers mein essentially koi nahi hota; colourless plastids aur reflected light unhe white banate hain.

Pause & Ponder · 5

In clues se well-labelled cell diagram banao: nucleus = dark round body; ER = network from nuclear envelope; mitochondria & chloroplasts = rod-shaped.

Schematic plant cell

Fig. Schematic plant cell — diye gaye clues se banaya gaya.

Pause & Ponder · 6

Cell mein ek bade mitochondrion ki jagah bahut saare chhote kyun hote hain? Surface area se kya relation hai?

Answer

Energy reactions mitochondrial membranes (cristae) par hoti hain. Bahut saare chhote mitochondria milke bahut bada total membrane surface area dete hain ek bade sphere se zyada — zyada surface matlab respiration reactions ke liye zyada jagah, isliye zyada energy (ATP).

Chhote units ko cell mein exactly wahan bheja ja sakta hai jahan energy chahiye, aur agar ek damage ho toh dusre kaam karte rehte hain. (Geometrically, bada hone par volume surface area se tez badhta hai — ek giant mitochondrion ka surface-to-volume ratio bahut kam hoga.)

Pause & Ponder · 7

Agar skin cells mitosis ki jagah meiosis se divide hoon to cut par kya hoga?

Answer

Cut heal karne ke liye original jaisi nayi skin cells chahiye — yahi mitosis deta hai (do identical daughter cells, full chromosome set ke saath).

Meiosis se char cells banti hain, har ek mein aadhe chromosomes aur genetic variation hota hai. Aise cells normal skin cells ki tarah kaam nahi kar sakte, isliye wound theek nahi hoga — nayi cells defective hongi aur healthy skin tissue rebuild nahi kar paayengi.

Revise, Reflect, Refine

End-of-chapter exercise — poore 16 questions

Exercise · 1

Differentiate karo: (i) Cell membrane & cell wall (permeability) (ii) RER & SER (structure) (iii) Chloroplasts & chromoplasts (pigments).

Pair	Pehla	Doosra
Membrane vs Wall (permeability)	Cell membrane: selectively permeable (sirf kuch substances pass)	Cell wall: fully permeable (paani aur dissolved minerals freely pass)
RER vs SER (structure)	RER: surface par ribosomes → rough dikhta hai	SER: koi ribosome nahi → smooth dikhta hai
Chloroplast vs Chromoplast (pigments)	Chloroplast: green pigment chlorophyll hota hai	Chromoplast: yellow/orange/red pigments hote hain (chlorophyll nahi)

Exercise · 2 · MCQ

Cell X (pure water) phool gaya; Cell Y (concentrated salt) sikar gaya. Kaunsa statement sahi hai?

(i) Salt molecules Cell Y mein chale gaye, isliye woh sikar gaya.
(ii) Paani X mein gaya, aur Y se salt andar aane se zyada paani bahar gaya.
(iii) Paani Cell X mein aur Cell Y se bahar gaya cell membrane ke through.
(iv) Solute movement ne dono cells mein osmosis kiya.

✓ Sahi: option (iii)

Osmosis sirf paani ke selectively permeable membrane cross karne ke baare mein hai. Pure water (hypotonic) mein paani X mein jaata hai → phoolti hai; salt solution (hypertonic) mein paani Y se bahar jaata hai → sikarti hai. Salt khud membrane cross nahi karta — (i) aur (ii) galat hain; (iv) solute movement ko galat blame karta hai.

Exercise · 3

Fig. 2.20 mein parts (a)–(g) identify karo aur functions (i)–(vii) se match karo.

Organelle → function match

Fig. 2.20 (redrawn). Apni textbook figure ke saath labels match karo.

Part	Organelle	Function
(b)	Nucleus	(i) Cell ki sabhi activities control karta hai
(a)	Mitochondrion	(ii) Cellular respiration ki jagah
(g)	Vacuole	(iii) Storage organelle jo rigidity bhi deta hai
—	Cell membrane	(iv) Cell contents ko surroundings se alag karta hai
(e)	Cell wall	(v) Structural rigidity deta hai
(d)	Golgi apparatus	(vi) ER se materials pack & store karta hai
(f)	Chloroplast	(vii) Khana banane mein madad karta hai

Exact letter har part ka printed figure mein arrow position par depend karta hai — organelle→function pairs use karo.

Exercise · 4 · MCQ

Kaun sa option sahi tarah "plant cells mein present" aur "animal cells mein absent" pair karta hai?

(i) Leucoplast | Cell wall
(ii) Mitochondria | Ribosome
(iii) Cell wall | Golgi apparatus
(iv) Lysosome | Endoplasmic reticulum

✓ Sahi: option (i)

Leucoplasts plant cells mein hote hain, aur cell wall animal cells mein absent hoti hai — dono halves sahi hain. Baaki fail hote hain: ribosomes, Golgi aur ER sab animal cells mein present hain.

Exercise · 5

Renu kehti hai sab plant parts (roots bhi) mein plastids hain; Rohit kehta hai roots mein nahi kyunki underground hain. Kaun sahi hai? Justify karo.

✓ Renu sahi hai

Roots mein plastids hote hain — sirf chloroplasts nahi hote. Roots underground aur non-photosynthetic hone ki wajah se colourless plastids yaani leucoplasts (e.g. amyloplasts) hote hain jo starch store karte hain. Rohit ne "plastids" ko "chloroplasts" samjha: photosynthesis roots mein absent hai, par dusre types ke plastids still present hain.

Exercise · 6

Mitochondria aur chloroplasts structurally aur functionally kaise similar aur alag hain?

Similarities

Dono double-membrane-bound organelles hain.
Dono mein apna DNA aur ribosomes hain — kuch apne proteins bana sakte hain.
Dono energy se linked hain aur endosymbiotic origin share karte hain.

Differences

	Mitochondrion	Chloroplast
Found in	sab eukaryotic cells	sirf green plant cells
Function	energy release (cellular respiration → ATP)	light capture se food banata hai (photosynthesis)
Pigment	koi nahi	chlorophyll (green)
Inner structure	folded cristae	stroma + disc-shaped membranes

Exercise · 7 · MCQ

Kaun sa organelles ka pair DNA contain karta hai?

(i) Chloroplasts, Ribosomes
(ii) Mitochondria, Nucleus
(iii) Golgi bodies, Ribosomes
(iv) Nucleus, Lysosomes

✓ Sahi: option (ii)

DNA nucleus, mitochondria aur chloroplasts mein milta hai. Ribosomes mein RNA hota hai (DNA nahi), aur Golgi aur lysosomes mein koi DNA nahi — isliye (ii) Mitochondria + Nucleus woh pair hai jahan dono mein DNA hai.

Exercise · 8

Gajar plain water vs concentrated salt mein (Fig 2.21). (i) Hypothesis? (ii) Improvements? (iii) Water wali gajar crunchy kyun rehti hai aur salt wali limp kyun ho jaati hai?

(i) Hypothesis: Plant tissue surrounding solution ki concentration ke hisaab se osmosis se paani gain ya lose karta hai (paani zyada concentrated side ki taraf jaata hai).

(ii) Improvements: Equal size aur mass ki gajar use karo; pehle aur baad mein weigh karo; liquid ki same volume, same temperature aur same time rakho; several replicate karo reliable results ke liye.

(iii) Plain water (hypotonic) mein paani gajar cells mein jaata hai → turgid → firm aur crunchy. Salt solution (hypertonic) mein paani cells se nikalta hai → turgor kho deti hai (plasmolysis) → gajar limp aur rubbery ho jaati hai.

Exercise · 9

Bacterial vs animal cells mein in structures ki presence/absence batao.

Structure	Bacterial cell	Animal cell
Chromosome	Present (single, circular)	Present
Nucleus (well-defined)	Absent	Present
Mitochondria	Absent	Present
Golgi complex	Absent	Present
Chromoplasts	Absent	Absent

Chromoplasts sirf plant-only plastids hain, isliye dono bacterial aur animal cells mein absent hain.

Exercise · 10

Potato-cup experiment: A empty, B sugar, C salt, D sugar in a boiled potato. (i) B aur C mein paani kyun aata hai? (ii) A kyun chahiye? (iii) A aur D mein paani kyun nahi aata?

(i) Sugar/salt andar hypertonic banata hai. Beaker ka paani living potato cells ke through osmosis se zyada concentration ki taraf jaata hai, isliye B aur C ke hollow mein collect hota hai.

(ii) Cup A control hai. Andar koi solute nahi, isliye koi paani nahi aana chahiye. Yeh prove karta hai ki B aur C mein paani added solute (osmosis) ki wajah se aaya — kisi aur wajah se nahi.

(iii) A mein koi concentration gradient nahi, isliye net water movement ka koi reason nahi. D mein aloo boil hua tha — cells dead hain aur membranes destroy ho gayi hain, isliye woh selectively permeable nahi rahi aur osmosis ho nahi sakti, chahe sugar ho.

Exercise · 11 · MCQ

Woh pair identify karo jo galat tarah organelle ko function se match karta hai.

(i) Ribosome — Protein synthesis
(ii) SER — Lipid and cellulose synthesis
(iii) Lysosome — Digestion of foreign agents

✗ Galat pair: option (ii)

SER lipids synthesise karta hai, par cellulose nahi banata — cellulose cell wall ke liye banta hai, SER se nahi. Isliye (ii) galat matched pair hai. Options (i) aur (iii) sahi hain.

Exercise · 12

Agar eukaryotic cell ke sabhi mitochondria remove kar diye jaayein to kya hoga?

Answer

Mitochondria "powerhouses" hain jo cellular respiration se ATP (cell ki energy currency) banate hain. Unke bina cell apna main energy supply kho deti hai, isliye transport, molecules build karna ya divide karna jaise kaam nahi ho sakte. Cell weak ho jaayegi aur eventually mar jaayegi. (Cytoplasm mein anaerobic glycolysis se thoda energy aata — cell sustain karne ke liye bahut kam.)

Exercise · 13

Humans mein tumour formation kaunsa phenomenon rokta hai? Kya plants bhi tumours develop kar sakte hain? Explain karo.

Answer

Contact inhibition normally tumour formation rokti hai animals mein: cells divide karna band kar deti hain jab neighbouring cells ko touch karti hain. Cancer tab hota hai jab cells yeh control kho deti hain.

Plants: Plant cells mein rigid cell walls hain aur contact inhibition nahi hoti, isliye abnormal growths (galls/crown gall) ban sakti hain. Par rigid walls aur koi circulatory system na hone ki wajah se, abnormal cells spread (metastasise) nahi ho sakti poore plant mein jaise animal cancer cells karte hain — plant "tumours" localised rehte hain.

Exercise · 14

Cell membrane proteins aur lipids se bani hai. Kaun se organelles ise synthesise karte hain? Synthesis se membrane tak ka path labelled diagram mein dikhao.

Answer

Proteins RER ke ribosomes banate hain; lipids SER banata hai. Yeh vesicles mein pack hoke Golgi apparatus mein modify aur sort hote hain, phir vesicles mein plasma membrane se fuse ho jaate hain.

Fig. Membrane components ka plasma membrane tak ka path.

Exercise · 15

Agar gametes mitotic divisions se bane to kya hoga?

Answer

Mitosis full chromosome number rakhta hai. Isliye mitosis se bane gametes mein complete set hoga aadhe ki jagah. Fertilisation mein do aisi gametes milne par chromosome number har generation mein double ho jaata — kabhi constant nahi rehta.

Isse serious genetic disorders aur likely non-viable offspring hoge. Meiosis ki genetic variation bhi nahi hogi. Yahi wajah hai ki gametes meiosis se banne chahiye (chromosomes half karke aur genes shuffle karke).

Exercise · 16

Kisan Deepa amla aur lemons ko salt/sugar/jaggery se pickles, murabbas aur sharbat mein preserve karti hai. (i) Scientific concept? (ii) High salt/sugar spoilage microbes kaise rokta hai? (iii) Healthy preservation recipe suggest karo. (iv) Scientific values?

(i) Concept: Osmosis — khane ko high-solute (hypertonic) environment mein rakh ke preserve karna jo microbes se paani kheenchta hai (plasmolysis).

(ii) High salt ya sugar surroundings ko bacteria aur fungi ke liye hypertonic banata hai. Paani osmosis se microbial cells se bahar khicha jaata hai, isliye woh paani kho dete hain, sikar jaate hain (plasmolyse) aur grow ya multiply nahi kar sakte. Itna kam free water hone par spoilage organisms survive nahi kar sakte — khana zyada der tak tika rehta hai.

(iii) Healthy recipe: e.g. amla murabba refined sugar ki jagah thodi jaggery ke saath, ya lemon-ginger pickle rock salt aur thodi mustard oil se preserve kiya gaya; dono achhe store hote hain aur nutrients dete hain.

(iv) Scientific values: Khane ki barbaadi kam karna, sustainability aur food security, scientific knowledge ko daily life mein apply karna, traditional knowledge aur science ka combination, local economy ko support karna.