Photosynthesis in Higher Plants: Class 11 Biology Chapter 11 Hinglish mein

Punjab mein kisi wheat field ke kinaare khade ho jao, to tum use do tarah se study kar sakte ho. Ek ecologist fasal, weeds, aphids aur unke beech daana chugti mynas ko ginta hai. Ek biochemist kuch aur dekhta hai: anginat chloroplasts jo sunlight, hawa aur paani ko sugar mein badal rahe hain. Dono hi biology hain. Yeh unit doosra nazariya leti hai. Yeh samjhati hai ki ek flowering plant apna khaana kaise banata hai (yeh chapter), usse energy kaise release karta hai (Chapter 12) aur us energy ko grow aur develop hone mein kaise use karta hai (Chapter 13). Har baar hum poore plant ki kisi ghatna ko cells aur molecules tak trace karte hain, aur dekhte hain ki aas-paas ka mahaul use kahan tez karta hai aur kahan rokta hai.

Lunch plate ke khaane ko peeche ki taraf trace karo, to tum hamesha ek green plant tak pahunchoge: chawal aur dal seedhe, aur dahi ek gaay ke zariye jisne chaara khaya tha. Green plants autotrophs hain: woh simple inorganic raw materials se apna organic khaana khud banate hain. Animals, fungi aur zyadatar bacteria heterotrophs hain, jinhe woh khaana khaana padta hai jo pehle kisi autotroph ne banaya ho.

Photosynthesis do kaaranon se zaroori hai. Yeh woh darwaaza hai jisse Sun ki energy Earth ki lagbhag har food chain mein aati hai, aur yeh hawa mein oxygen ko lagaataar bharta rehta hai jo humein aur zyadatar organisms ko saans lene ke liye chahiye. Is chapter mein hum is machinery ko kholte hain: light pakadne wale pigments, light ko chemical energy mein badalne wali membranes, aur woh enzymes jo us energy ko carbon fix karne mein kharch karte hain.

11.1 Hum Kya Jaante Hain?

Pichhli classes se tum teen requirements jaante ho: chlorophyll, light aur carbon dioxide. Har ek ko ek hi trick se dikhaya gaya tha. Leaf ke ek hisse ko sirf ek requirement se vanchit karo, phir leaf se poochho ki usne starch banaya ya nahi. Iodine jawab deta hai, kyunki woh starch ko blue-black rang deta hai aur decolourised leaf ke baaki hisse ko halka brown-yellow chhod deta hai.

Fig. 11.1 mein ek variegated leaf (safed patches wali green leaf) par black paper ki ek patti clip ki gayi hai. Kuch ghante sunlight mein rehne ke baad sirf woh hisse starch ke liye stain hote hain jo green bhi the aur dhake bhi nahi the. Ek doosri leaf ko aadha ek sealed bottle mein ghusaya jaata hai jismein potassium hydroxide ($\mathrm{K}\mathrm{O}\mathrm{H}$) mein bhigi cotton hai, jo andar ki hawa se $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ absorb kar leti hai. Sirf bahar wala aadha hissa starch banata hai. Dono halves ko same light, chlorophyll aur paani mila, isliye $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ki kami hi woh akela fark hai jo result ko samjha sakta hai.

11.2 Shuruaati Experiments

Photosynthesis ki hamari samajh simple experiments ki ek chain se judkar bani. Har experiment ne ek sawaal ka jawab diya aur agla sawaal khada kar diya.

Joseph Priestley (1733–1804), ek English clergyman jinhe chemistry ka bahut shauk tha, ne 1770s ki shuruaat mein bell-jar tests ki ek mashhoor series ki. Hawa se bhara ek sealed jar ek candle ko jalaye rakh sakta tha, ya ek mouse ko zinda rakh sakta tha, lekin sirf thodi der; uske baad woh dono mein se kisi ko support nahi karta tha. Lekin agar mint ki ek tehni kuch din jar mein saath rehti, to wahi hawa phir se dono ke liye theek ho jaati. Priestley ka explanation: flames aur saans lene wale animals hawa se jo bhi nikaalte hain, green plants use waapas daal dete hain. 1774 mein unhone us gas ko isolate kiya jise aaj hum oxygen kehte hain.

Jan Ingenhousz (1730–1799), England mein kaam karne wale ek Dutch mool ke physician, ne woh sawaal poochha jo Priestley ne nahi poochha tha: kya isse fark padta hai ki plant yeh kaam kab karta hai? Jar test ko sunlight aur andhere mein dohrakar unhone paaya ki plant hawa ko sirf light mein theek karta hai (Fig. 11.2). Water plants ne yeh baat saaf dikha di. Tez dhoop mein unki green leaves aur stems se chhote-chhote bubbles nikalte the, andhere mein bilkul nahi, aur non-green hisson se kabhi nahi; yeh gas oxygen nikli. Matlab hawa ko theek karne wale process ko sunlight chahiye aur yeh sirf green tissue mein hota hai.

Unnisvi sadi ke beech ke aas-paas German botanist Julius von Sachs ne is gas exchange ko khaane se joda. Leaves ko iodine se test karke unhe starch sirf us green tissue mein mila jo light mein raha tha, aur unhone chlorophyll ko cell ke andar alag-alag bodies, yaani chloroplasts, tak trace kiya. Plant ke badhte samay green tissue jo glucose banata hai, uska zyadatar hissa isi starch ke roop mein store hota hai.

Light ke kaun se rang photosynthesis chalate hain? 1882 mein German physiologist T. W. Engelmann (1843–1909) ne ek khoobsurat trick se iska jawab diya (Fig. 11.3). Ek prism se unhone green alga Cladophora ke ek filament par ek chhota spectrum daala, aise paani mein jismein aerobic bacteria the. Yeh bacteria oxygen ki taraf tairte hain, isliye yeh zinda oxygen detectors ka kaam karte the. Woh spectrum ke blue aur red hisson ke neeche ikattha ho gaye aur green ke neeche kam rahe. Wavelength ke against photosynthetic output ke plot ko action spectrum kehte hain. Engelmann ka spectrum sabse pehla measure kiya gaya tha, aur Section 11.4 mein tum dekhoge ki iske do peaks un rangon se match karte hain jinhe chlorophylls a aur b sabse achhe se absorb karte hain.

Unnisvi sadi ke doosre half tak basic baatein maan li gayi thin: light mein plants $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ aur paani se carbohydrate banate hain aur oxygen chhodte hain. Overall badlaav ko aise likha gaya

Oxygen kahan se aati hai: $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ se ya paani se? Microbiologist Cornelius van Niel (1897–1985) ne purple aur green sulphur bacteria ko study karke yeh guththi suljhayi. Yeh oxygen release kiye bina photosynthesis karte hain, paani ki jagah hydrogen sulphide ($\mathrm{H}{\phantom{X}}_2\mathrm{S}$) use karte hain aur peeche sulphur ya sulphate chhodte hain. Inhe plants se compare karke unhe ek general pattern dikha: har case mein light ek hydrogen donor, $\mathrm{H}{\phantom{X}}_2\mathrm{A}$, ko energy deti hai taaki woh $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ko carbohydrate mein reduce kare, aur donor ka baaki hissa, A, free ho jaata hai.

Sulphur bacterium ke liye donor $\mathrm{H}{\phantom{X}}_2\mathrm{S}$ hai aur product A sulphur hai. Green plant ke liye donor paani hai, isliye A oxygen hi hona chahiye: plant jo $\mathrm{O}{\phantom{X}}_2$ release karta hai woh paani se aati hai, $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ se nahi.11Iski pushti 1941 mein algae ko ${}^{18}\mathrm{O}$ se labelled paani dekar hui. NCERT ise radioisotope technique kehta hai; asal mein ${}^{18}\mathrm{O}$ ek stable heavy isotope hai, jise uske mass se detect kiya jaata hai. Ek glucose ke liye equation balance karne par, jismein har oxygen molecule paani se aata hai, humein milta hai

Equation (11.3) darjanon alag steps ke inputs aur outputs ka jod hai; yeh ek akela reaction nahi hai.

11.3 Photosynthesis Kahan Hota Hai?

Zyadatar kaam leaves karti hain, lekin sirf unka hi haq nahi hai: koi bhi green tissue photosynthesis kar sakta hai. Young green stems, flower bud ke green sepals, kachche fruits jaise hari mirch aur kachche aam, aur Opuntia jaise cacti ke chapte green stems, sab mein chloroplasts hote hain aur yeh carbon fix karte hain.

Leaf ke andar zyadatar chloroplasts mesophyll mein hote hain (inki structure tum Chapter 8 mein padh chuke ho), jo cell ke beech mein tairne ki jagah cell walls ke saath line mein lage hote hain. Light badalne par woh walls ke saath apni position badal sakte hain (Fig. 11.4).

Har chloroplast do membranes mein lipta hota hai, ek outer aur ek inner. Andar ek teesra membrane system hota hai: thylakoids naam ki chapti thailiyan, jo sikkon ki tarah grana mein stacked hoti hain aur bina stack wali stroma lamellae se judi hoti hain. Inke chaaron taraf stroma hota hai, protein se bhara ek fluid jismein ribosomes, starch grains aur lipid droplets bhi hote hain.

Chloroplast do kamron wali factory ki tarah kaam karta hai. Thylakoid membranes (grana aur stroma lamellae) power room hain: yeh light pakadti hain aur uski energy ko do portable forms, ATP aur NADPH, mein badalti hain. Stroma assembly room hai: iske enzymes ATP aur NADPH ko $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ se sugar banane mein kharch karte hain, aur kuch sugar ko starch ke roop mein store karte hain. Power room sirf tab chalta hai jab us par light padti hai, isliye iske steps ko light reactions, ya photochemical reactions, kehte hain.

Assembly room ka apna koi light switch nahi hai; jab power room ATP aur NADPH bhejna band karta hai to yeh bas ruk jaata hai. Aadat se iske steps ko ab bhi dark reactions kaha jaata hai, halaanki carbon reactions behtar naam hai: zinda leaf mein yeh din mein, light reactions ke saath-saath chalte hain, aur light jaane ke thodi der baad ruk jaate hain.

11.4 Photosynthesis Mein Kitne Tarah Ke Pigments Shaamil Hote Hain?

Ek spinach leaf ko thode acetone mein kuchlo to tumhe ek green liquid milta hai. Kya yeh green ek hi substance hai ya kai? Paper chromatography is sawaal ka jawab deti hai (Fig. 11.5): paper ki patti par dheere-dheere chadhta solvent extract ke har pigment ko alag doori tak le jaata hai, isliye mixture alag-alag bands mein phail jaata hai.

Leaf extract chaar bands mein alag hota hai. Chlorophyll a bright ya blue-green hota hai aur chlorophyll b yellow-green; xanthophylls yellow hote hain, aur carotenes yellow se orange tak hote hain. NCERT is aakhri band ko carotenoids kehta hai; sahi mein xanthophylls aur carotenes dono carotenoid ke hi types hain.

Pigments rangeen hote hain kyunki woh light ki kuch wavelengths absorb karte hain aur kuch nahi. Chlorophylls blue aur red ko zyada absorb karte hain aur green ka bada hissa waapas laut jaane dete hain, isiliye leaves humein green dikhti hain. Plant world mein chlorophyll a se zyada maatra mein koi pigment nahi hai.

Koi pigment har wavelength ko kitna absorb karta hai, iske graph ko uska absorption spectrum kehte hain. Har wavelength ke neeche photosynthesis kitni tez chalta hai, iske graph ko action spectrum kehte hain. Fig. 11.6 inhe saath mein dikhata hai. Match kamaal ka hai: action spectrum ke peaks blue aur red mein hain, theek wahin jahan chlorophyll a sabse zyada absorb karta hai (lagbhag 430 aur 660 nm). Yahi saboot hai ki chlorophyll a ko photosynthesis ka chief pigment kaha jaaye.

Lekin match perfect nahi hai. Lagbhag 450 aur 500 nm ke beech, aur green ke kuch hisse mein, photosynthesis us se tez chalta hai jitna akele chlorophyll a ka absorption chalne deta. Yeh extra accessory pigments se aata hai: chlorophyll b, xanthophylls aur carotenes, jo thylakoid membranes mein hi hote hain. Yeh un wavelengths ko absorb karte hain jinhe chlorophyll a theek se use nahi kar paata aur energy chlorophyll a ko de dete hain, jisse leaf jitni light use kar sakti hai uski range chaudi ho jaati hai. Yeh chlorophyll a ko photo-oxidation se bhi bachaate hain, yaani absorbed light energy se hone wala woh damage jab us energy ko safely use nahi kiya ja sakta.

11.5 Light Reaction Kya Hai?

Light reactions, jinhe photochemical phase bhi kehte hain, mein inputs light aur paani hain aur teen outputs hain: oxygen aur do energy se bhare carriers, ATP aur NADPH. Beech mein absorbed light ki energy paani ko split karne aur uske electrons ko $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$ tak dhakelne mein use hoti hai.

Yeh kaam thylakoid membrane ke bade protein complexes karte hain. Pigments do photosystems mein organised hote hain, jinhe PS I aur PS II kehte hain, aur har ek apne light-harvesting complex (LHC) ke aas-paas bana hota hai. Numbers ko kaam ka order mat samjho: PS I ko yeh naam isliye mila kyunki woh pehle discover hua tha, lekin light reactions mein PS II usse pehle kaam karta hai.

Akela ek chlorophyll molecule poori sunlight mein bhi har second sirf gine-chune photons pakadta hai, jo chemistry ko busy rakhne ke liye bahut kam hain. Isliye ek photosystem dish aerial ki tarah kaam karta hai (Fig. 11.7). Kai tarah ke saikdon pigment molecules, jinhe proteins apni jagah pakde rehte hain, antenna banate hain. Inmein se jo bhi photon absorb karta hai, woh energy ek molecule se doosre molecule tak dish ke centre ki taraf aage badhata hai. Wahan ek khaas chlorophyll a molecule hota hai, reaction centre, jo photosystem ka akela molecule hai jo sach mein ek electron deta hai, jise ek primary acceptor pakad leta hai. Kyunki antenna pigments alag-alag wavelengths absorb karte hain, isliye dish light ka ek chauda band bhi ikattha karti hai, jitna akela chlorophyll a nahi kar paata.

Har reaction centre thode alag red ke liye tuned hota hai. PS I ka reaction-centre chlorophyll a 700 nm par sabse achha absorb karta hai, isliye use P700 kehte hain; PS II wala 680 nm par sabse achha absorb karta hai aur use P680 kehte hain.

11.6 Electron Transport

Fig. 11.8 mein ek electron ke peechhe chalo. Uske safar mein do chadhaai hain, dono light se chalti hain, aur do utaar. Chadhaai 1: 680 nm ki red light PS II mein P680 ko excite karti hai, aur energised electron ko ek primary acceptor pakad leta hai. Utaar 1: acceptor use ek electron transport chain ko deta hai jismein cytochromes hote hain. Har carrier electron ko pichhle wale se thoda zyada kaske pakadta hai (woh redox-potential scale par neeche hota hai), isliye har hand-over par energy release hoti hai, aur uska kuch hissa ATP banane mein use hota hai. Electron bina kuch khoye PS I tak pahunchta hai.

Chadhaai 2: PS I par 700 nm ki light P700 ko excite karti hai aur ek electron ko doosre acceptor tak uthaati hai, jo aur bhi ooncha hai. Utaar 2: wahan se electron dhalaan par $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$ tak jaata hai, jo $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}\mathrm{H}+\mathrm{H}{\phantom{X}}^{+}$ mein reduce hota hai. Har carrier ko redox-potential scale par plot karo to upar–neeche–upar–neeche wala raasta karvat lete Z jaisa dikhta hai, isiliye iska naam Z scheme hai.

11.6.1 Paani Ka Splitting

Ek electron dene ke baad P680 electron ka bhookha reh jaata hai, aur yeh itna strong oxidising agent hai ki paani se electrons le sake, jo cell mein lagbhag aur koi nahi kar sakta. PS II par ek water-splitting complex (jismein manganese hota hai) do water molecules se ek baar mein chaar electrons ikattha karta hai:

Matlab leaf jo bhi $\mathrm{O}{\phantom{X}}_2$ molecule chhodti hai, woh PS II par do water molecules ke roop mein shuru hua tha. Bharpaai ki chain scheme mein ulti chalti hai: paani PS II ko bharta hai, aur PS II se aaye electrons, transport chain ke baad, PS I ko bharte hain.

Protons aur oxygen kahan dikhte hain? Water-splitting complex thylakoid membrane ke andar wale face par hota hai, lumen (thylakoid ke andar ki jagah) ki taraf. Isliye protons aur oxygen lumen mein release hote hain. Oxygen diffuse hokar chloroplast se bahar nikal jaati hai; protons wahin rehte hain aur, jaise Section 11.6.3 dikhata hai, kaam mein lagaye jaate hain.

11.6.2 Cyclic aur Non-cyclic Photo-phosphorylation

Cells oxidisable substances se release hui energy ko ATP jaise molecules ke chemical bonds mein store karte hain. ADP aur inorganic phosphate (${\mathrm{P}}_{\mathrm{i}}$) se ATP banane ko phosphorylation kehte hain, aur mitochondria aur chloroplasts dono yeh karte hain. Jab energy light se aati hai, to is process ko photophosphorylation kehte hain.

Z scheme mein trace kiya gaya flow ek-tarfa hai. Electrons paani se nikalte hain, PS II, chain aur PS I se guzarte hain, aur NADPH mein pahunchte hain; woh kabhi waapas nahi aate. Aise ek-tarfa flow mein bana ATP non-cyclic photophosphorylation ka product hai, aur iske saath hamesha NADPH aur release hui oxygen bhi aati hai.

PS I ke paas ek doosra option bhi hai. Uska excited electron $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$ tak jaane ki jagah transport chain mein waapas bheja ja sakta hai, aur is tarah P700 par lautkar phir se excite ho sakta hai (Fig. 11.8 ka step 6). Chain ka har chakkar ab bhi ATP ke liye energy release karta hai, lekin kuch reduce nahi hota aur paani split nahi hota, isliye yeh cyclic photophosphorylation sirf ATP deta hai. Do situations ise badhawa deti hain. Stroma lamellae mein PS I hota hai lekin na PS II aur na NADP reductase enzyme, isliye wahan sirf loop ka hi raasta khula hai; aur 680 nm se lambi light P700 ko excite kar sakti hai lekin P680 ko nahi, isliye aisi light mein PS I akela kaam karta hai.

11.6.3 Chemiosmotic Hypothesis

Electrons ka flow asal mein ATP kaise banata hai? Iska jawab, jo Peter Mitchell ne 1961 mein diya, chemiosmosis hai. Electron transport ki energy pehle membrane ke aar-paar proton concentration ke fark ke roop mein store hoti hai, aur phir us gradient ke saath waapas behte protons ATP synthesis chalate hain. Respiration bhi yahi idea use karta hai (Chapter 12), lekin protons alag jagah ikattha hote hain: mitochondria mein woh do membranes ke beech ki jagah mein jama hote hain, jabki chloroplasts mein thylakoids ke andar, lumen mein.

Light reactions ke teen processes gradient banate hain (Fig. 11.9):

1. Water splitting. Yeh membrane ki lumen wali side par hota hai, isliye iske protons seedhe lumen mein release hote hain.
2. Ek hydrogen carrier. PS II ka primary acceptor stroma side ke paas hota hai. Yeh har electron ko ek simple electron carrier ko nahi, balki poore hydrogen atoms ke carrier (plastoquinone) ko deta hai. Electron lete waqt carrier stroma se ek proton bhi leta hai; jab woh lumen side ke paas electron aage deta hai, to woh proton lumen mein chhod deta hai.
3. NADP reductase. Yeh enzyme stroma side par hota hai. $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}{\phantom{X}}^{+}$ ko $\mathrm{N}\mathrm{A}\mathrm{D}\mathrm{P}\mathrm{H}+\mathrm{H}{\phantom{X}}^{+}$ mein reduce karne ke liye yeh PS I ke acceptor ke electrons aur stroma se liye gaye protons use karta hai.

Teeno ko jodo to lumen mein protons badhte hain jabki stroma mein ghatte hain. Lumen stroma se kaafi zyada acidic ho jaata hai, aur unke beech ki membrane mein ab energy ka store hota hai, jaise dam ke peechhe ruka paani.

Dam mein ek hi spillway hai. Protons sirf ATP synthase enzyme se hi waapas ja sakte hain, aur raaste mein woh jo energy chhodte hain usse ATP banta hai. Enzyme ke do parts hain aur do kaam. Channel, $\mathrm{C}{\mathrm{F}}_0$, membrane ke aar-paar hota hai, aur protons isse lumen se stroma tak diffuse hote hain (facilitated diffusion). Catalytic head, $\mathrm{C}{\mathrm{F}}_1$, stroma wale face par hota hai, isiliye ATP stroma mein banta hai, theek wahin jahan Calvin cycle ko iski zaroorat hai. $\mathrm{C}{\mathrm{F}}_0$ se behte protons $\mathrm{C}{\mathrm{F}}_1$ ke shape mein baar-baar badlaav laate hain, aur har badlaav ADP aur ${\mathrm{P}}_{\mathrm{i}}$ ko jodkar ATP banane mein madad karta hai.

Short mein, dam ko ek deewar (aisi membrane jise protons aasani se paar na kar sakein), ek pump (electron transport jo protons ko lumen mein le jaata hai), paani ka dabaav (gradient) aur ek turbine (ATP synthase) chahiye. Isse bana ATP, electron flow ke NADPH ke saath, turant stroma mein $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ fix karne aur sugars banane mein kharch ho jaata hai.

11.7 ATP aur NADPH Kahan Use Hote Hain?

Photosynthesis kar rahi ek leaf ko andhere mein rakho aur uska $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ uptake dekho. Yeh turant nahi rukta: thodi der chalta rehta hai aur phir khatam ho jaata hai, aur light waapas aane par phir shuru ho jaata hai. $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ fix karne wale reactions, yaani biosynthetic phase, ko khud light nahi chahiye, yeh saaf hai. Inhe chahiye woh ATP aur NADPH jo light reactions lagaataar dete rehte hain, aur yeh tab tak chalte hain jab tak woh stock khatam na ho jaaye. Oxygen, light reactions ka teesra product, yahan koi role nahi nibhaati: woh bas diffuse hokar chloroplast se bahar chali jaati hai.

$\mathrm{C}\mathrm{O}{\phantom{X}}_2$ sabse pehle kis compound mein jaati hai? Calvin ki team ne radioactive ${}^{14}\mathrm{C}$ se iska jawab diya (chapter ki shuruaat ka box dekho). Algal cells ko labelled $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ke saath sabse kam exposure ke baad lagbhag saari radioactivity ek hi compound mein thi, 3-phosphoglyceric acid (PGA), teen carbon atoms wala ek organic acid. Unke group ne aage chalkar raaste ka har step map kiya, jo ab unke naam se jaana jaata hai.

Kya PGA hamesha pehla hota hai? Kai species ke surveys ne kaha nahi. Maize, sugarcane aur unke rishtedaar plants mein label sabse pehle ek four-carbon organic acid, oxaloacetic acid (OAA), mein mila. Isliye plants ko $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ fixation ke pehle stable product se group kiya jaata hai: ${\mathrm{C}}_3$ plants pehle three-carbon PGA banate hain (${\mathrm{C}}_3$ pathway), aur ${\mathrm{C}}_4$ plants pehle four-carbon OAA banate hain (${\mathrm{C}}_4$ pathway). Dono groups aur tarah se bhi alag hain (Section 11.8).

11.7.1 $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ka Primary Acceptor

Thoda hisaab puzzle khada karta hai. Agar ek $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ (ek carbon) ek acceptor se judta hai aur pehle product, PGA, mein teen carbons hain, to seedha guess hai ki acceptor mein do carbons honge. Researchers ne saalon tak aisa acceptor dhoondha. Asli acceptor hairaan karne wala nikla: ek five-carbon ketose sugar, ribulose-1,5-bisphosphate (RuBP). Ek $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ plus ek RuBP se chhah carbons bante hain, jo turant PGA ke do molecules mein toot jaate hain.

11.7.2 Calvin Cycle

RuBP pehle step mein kharch ho jaata hai, isliye pathway tab tak aage nahi chal sakta jab tak koi use dobara na banaye, aur Calvin ke group ne paaya ki baad ke steps theek yahi karte hain. Isliye pathway ek loop hai, Calvin cycle. Har photosynthetic plant ise chalata hai, chahe ${\mathrm{C}}_3$ ho, ${\mathrm{C}}_4$ ho ya koi aur; ${\mathrm{C}}_4$ plants bas aage ek extra hissa jod dete hain (Section 11.8). Hum loop ko teen stages mein baantte hain (Fig. 11.10).

1. Carboxylation, sabse zaroori step. RuBisCO $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ko RuBP se jodta hai, aur unstable six-carbon product turant do PGA mein toot jaata hai (equation (11.5)), isliye $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ab ek stable organic compound ka hissa hai. Enzyme ka poora naam, RuBP carboxylase-oxygenase, batata hai ki yeh $\mathrm{O}{\phantom{X}}_2$ ko bhi jod sakta hai (Section 11.9).
2. Reduction. PGA ko ATP se phosphorylate aur NADPH se reduce karke ek three-carbon sugar phosphate, triose phosphate, banaya jaata hai. Har fix hue $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ke liye yeh stage 2 ATP aur 2 NADPH use karta hai. Kuch triose phosphate glucose, sucrose aur starch banane ke liye nikaal liya jaata hai.
3. Regeneration. Baaki triose phosphate ko rearrange karke RuBP dobara banaya jaata hai, taaki cycle $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ lena jaari rakh sake. Iske liye har $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ par 1 aur ATP chahiye.

Ab poora bill jodo. Har chakkar ek carbon fix karta hai, isliye six-carbon glucose ke liye chhah chakkar lagte hain. Har chakkar mein reduction mein 2 ATP aur regeneration mein 1 ATP lagta hai, kul 3 ATP, plus 2 NADPH; equation (11.6) poore glucose ka bill hai. 3 : 2 ka ratio dekho. Sirf ek-tarfa electron flow ATP aur NADPH is ratio mein nahi deta, aur extra ATP shayad cyclic photophosphorylation se aata hai, jo NADPH ke bina ATP banata hai.

11.8 ${\mathrm{C}}_4$ Pathway

Maize, sugarcane, sorghum aur garam, sookhe tropical regions ki kai grasses ${\mathrm{C}}_4$ plants hain. Yeh apne pehle product ke roop mein OAA banate hain, phir bhi sugar tak inka main raasta Calvin cycle hi hai. To inhe alag kya banata hai? Yeh zyada biomass banate hain, garmi jhel lete hain, bahut tez light se bhi faayda uthaate rehte hain, inmein lagbhag koi photorespiration nahi hota, aur inki leaf anatomy anokhi hoti hai. Hum anatomy se shuru karte hain.

Maize leaf ko aar-paar kaato aur ek vein ko dekho (Fig. 11.11). Har vascular bundle ke chaaron taraf asaadharan roop se bade cells ka ek ring hota hai, bundle sheath, jo kabhi-kabhi kai layers mota hota hai. Dhyaan se dekho to teen baatein saaf dikhti hain: yeh cells chloroplasts se bhare hote hain, inki walls itni moti hoti hain ki gases paar na kar sakein, aur yeh bina air spaces ke ek doosre se sate hote hain. Mesophyll cells ring se bahar ki taraf phaile hote hain, aur poora pattern ek wreath (haar) jaisa dikhta hai, jo German shabd Kranz ka matlab hai; isiliye Kranz anatomy.

Ise ek ${\mathrm{C}}_3$ dicot leaf, jaise bean leaf, se compare karo. Iska mesophyll air spaces wali palisade aur spongy layers banata hai, aur har vein ke aas-paas ke cells chhote aur pheeke hote hain. Wheat jaisi ${\mathrm{C}}_3$ grass mein bhi, jiska mesophyll layered nahi hota, yeh wreath nahi hota.

Iski biochemistry M. D. Hatch aur C. R. Slack ne Australia mein 1960s ke beech mein samjhi, isliye ${\mathrm{C}}_4$ raaste ko Hatch and Slack pathway kehte hain. Yeh bhi ek cycle hai. Ise Fig. 11.12 mein follow karo:

1. Pakadna. Mesophyll cell mein enzyme PEP carboxylase (PEPcase) $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ko ek three-carbon acceptor, phosphoenolpyruvate (PEP), se jodta hai, jisse four-carbon acid OAA banta hai. In cells mein RuBisCO nahi hota, isliye saari $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ PEPcase ko hi milti hai.
2. Bhejna. OAA jaldi se ek zyada stable four-carbon acid (maize mein malic acid, kuch doosri species mein aspartic acid) mein badal jaata hai, jo plasmodesmata se hokar bundle sheath mein jaata hai.
3. Chhodna. Sheath mein ek enzyme four-carbon acid se $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ko alag karta hai (decarboxylation), aur peeche ek three-carbon hissa bachta hai, maize mein pyruvate.
4. Waapsi. Pyruvate mesophyll mein waapas jaata hai, jahan woh dobara PEP banta hai, aur loop poora ho jaata hai.

Ab kaam ka batwaara saaf hai. Mesophyll, jiske paas PEPcase hai lekin RuBisCO nahi, sirf $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ pakadta hai aur andar bhejta hai. Bundle sheath, jismein bahut RuBisCO hai lekin PEPcase nahi, woh $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ leta hai aur us par Calvin cycle chalata hai. Isliye dono tarah ke plants apni sugar same Calvin cycle se banate hain: ${\mathrm{C}}_3$ leaf ise apne poore mesophyll mein chalati hai, ${\mathrm{C}}_4$ leaf sirf apne bundle sheath mein.

11.9 Photorespiration

${\mathrm{C}}_4$ plants $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ pump karne ki itni mehnat kyun karte hain? Iska jawab us enzyme ki ek kamzori mein hai jo Calvin cycle ka pehla step chalata hai (equation (11.5)). Earth par RuBisCO se zyada maatra mein koi enzyme nahi hai, kuch had tak isliye ki yeh dheere kaam karta hai, isliye leaf ko iski bahut zyada maatra chahiye.

Iski active site dono mein se koi bhi gas, $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ya $\mathrm{O}{\phantom{X}}_2$, le leti hai (Fig. 11.13); isiliye carboxylase-oxygenase. Barabar maatra di jaaye to yeh $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ko kahin zyada baar chunta hai, lekin yeh chunaav ek competition hai jo is baat se tay hota hai ki aas-paas har gas ke kitne molecules hain. Mixture badlo to result badal jaata hai.

Hawa mein $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ se kahin zyada $\mathrm{O}{\phantom{X}}_2$ hoti hai, isliye ${\mathrm{C}}_3$ leaf mein RuBisCO ki kuch mulaqaatein $\mathrm{O}{\phantom{X}}_2$ se hoti hain, aur har ek ka matlab hai ek $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ jo fix nahi hui. Har baar jab $\mathrm{O}{\phantom{X}}_2$ jeetti hai, RuBP do PGA ki jagah ek PGA aur ek two-carbon phosphoglycolate mein toot jaata hai. Oxygen kharch karne wala yeh raasta, jo sirf light mein chalta hai, photorespiration hai.

Plant ke liye yeh sirf nuksaan hai. Jo carbon pehle hi fix ho chuka tha, woh phir se $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ke roop mein nikal jaata hai, phosphoglycolate ko bachaane mein ATP kharch hota hai, aur isse na sugar, na ATP, na NADPH milta hai. Photorespiration ka kya kaam hai, agar koi hai bhi, yeh abhi tak pata nahi.

Yahin par ${\mathrm{C}}_4$ pump ka faayda milta hai. Mesophyll lagaataar bundle sheath ko four-carbon acids pahunchata rehta hai, aur unka decarboxylation $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ko us se tez release karta hai jitni tezi se woh moti walls se leak ho sakti hai. RuBisCO ke aas-paas $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ hawa ke level se kahin upar chali jaati hai, competition $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ke favour mein jhuk jaata hai, aur oxygenation lagbhag gaayab ho jaata hai. Photorespiration na ke barabar hone se ${\mathrm{C}}_4$ plants apna fix kiya carbon kam khote hain, isiliye woh itne productive hote hain, aur garmi mein bhi yeh faayda bana rehta hai, jab ${\mathrm{C}}_3$ plants sabse zyada photorespire karte hain.

11.10 Photosynthesis Ko Prabhavit Karne Wale Factors

Wheat ka ek khet doosre se zyada paidawaar kyun deta hai? Iska bada jawab photosynthesis ka rate hai, aur yeh rate factors ke do sets par nirbhar karta hai. External factors aas-paas ka mahaul hain: sunlight, temperature, $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ concentration aur paani. Internal factors plant ke apne hain: uske paas kitni leaves hain aur woh kitni badi, purani aur sahi angle par hain; unmein kitne mesophyll cells aur chloroplasts hain; unmein kitna chlorophyll hai; aur unke andar kitni $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ hai. Yeh plant ke genes aur uski growth par nirbhar karte hain.

Yeh sab ek saath kaam karte hain, phir bhi kisi bhi pal rate ko aam taur par inmein se ek hi rokta hai, limiting factor: woh jo leaf ki zaroorat ke mukable sabse kam maatra mein hai. Railway ticket ki line ek achhi misaal hai. Agar sirf ek counter khula hai, to line us clerk ki raftaar se chalti hai, aur zyada tickets print karne se woh tez nahi hogi. F. F. Blackman ne 1905 mein is idea ko ek law bana diya.

11.10.1 Light

Fig. 11.14 ke curve ko left se right padho. Shuru mein (W ke paas) $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ fixation ka rate light intensity ke seedhe anupaat mein chadhta hai: light double, rate double, isliye light limiting hai. Phir chadhaai chapti ho jaati hai (X, Y), isliye nahi ki light ka matlab khatam ho gaya, balki isliye ki ab kuch aur, aam taur par $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, limit kar raha hai. Leaves is plateau tak hairaan karne wali jaldi pahunch jaati hain (NCERT light saturation ko poori sunlight ka lagbhag 10 per cent batata hai), isliye nature mein light mainly chhaanv mein ya ghane forest canopy ke neeche wale plants ko limit karti hai. Bilkul right mein (Z), itni tez light jo chlorophyll ko tod de, rate ko neeche le aati hai.

Sirf intensity hi sab kuch nahi hai. Light ki quality (uski wavelengths, Section 11.4) aur roz ke exposure ki duration bhi maayne rakhti hai.

11.10.2 Carbon Dioxide Ka Concentration

Tomatoes aur capsicum (shimla mirch) ugaane wale greenhouse growers apni fasal ke aas-paas ki hawa mein extra $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ pump karte hain aur zyada fruit paate hain. Yeh kyun kaam karta hai? Kyunki $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ woh factor hai jo photosynthesis ko sabse zyada baar limit karta hai: hawa mein yeh sirf lagbhag 0.04 per cent hoti hai.22Global average $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ 2024 mein 420 µL per litre (0.042 per cent) ko paar kar gayi aur abhi bhi badh rahi hai. NCERT purani range 0.03–0.04 per cent batata hai. Lagbhag 0.05 per cent tak, zyada $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ ka matlab hai tez fixation, lekin isse bahut upar ke levels lambe samay tak rahein to plants ko nuksaan pahunchate hain.

${\mathrm{C}}_3$ aur ${\mathrm{C}}_4$ plants is baat mein alag hain ki woh kitni extra $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ use kar sakte hain. Dhimi light mein dono respond nahi karte, kyunki light limiting hai. Tez light mein dono karte hain, ek had tak: ${\mathrm{C}}_4$ plant lagbhag 360 $\mu \mathrm{L}{\mathrm{L}}^{-1}$ par saturate ho jaata hai, jabki ${\mathrm{C}}_3$ plant 450 $\mu \mathrm{L}{\mathrm{L}}^{-1}$ ke baad tak respond karta rehta hai. Isliye aaj ki hawa ${\mathrm{C}}_4$ fasal ko uske maximum ke paas rakhti hai lekin ${\mathrm{C}}_3$ fasal ko ab bhi peeche rokti hai, aur tomatoes aur capsicum ${\mathrm{C}}_3$ plants hain.

11.10.3 Temperature

Carbon reactions enzymes chalate hain, aur enzyme activity garmi ke saath ek optimum tak badhti hai aur uske baad girti hai, isliye temperature in reactions ko bahut control karta hai; light se chalne wale steps kahin kam sensitive hain. Optima plants mein alag-alag hote hain. ${\mathrm{C}}_4$ plants, apne $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ pump ke saath, thirties (°C) tak tez hote rehte hain, jabki ${\mathrm{C}}_3$ plants kaafi thande temperatures par peak karte hain. Habitat bhi maayne rakhta hai: tropics ke liye adapted plant ka optimum temperate climate ke liye adapted plant se zyada hota hai.

11.10.4 Paani

Garam dopahar mein murjhaata plant dheere photosynthesis karta hai, lekin isliye nahi ki uske paas split karne ke liye paani nahi hai: light reactions plant dwara liye gaye paani ka bahut chhota hissa hi use karte hain. Nuksaan indirect hai. Paani bachaane ke liye plant apne stomata band kar leta hai, jisse $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ bhi andar nahi aa paati, aur uski jhuki leaves light ko kam surface dikhati hain aur unka metabolism dheema ho jaata hai.