2.7.3
Ek leaf green dikhti hai kyunki woh green light ko use karne mein buri hai. Photosynthesis ko drive karne wale pigments coloured molecules hote hain jo kuch wavelengths ko absorb karte hain aur kuch ko reflect/transmit karte hain — aur jo colour hum dekhte hain woh bacha hua light hai. Photosynthesis samajhne ke liye tumhe yeh samajhna hoga ki har pigment kaun se colours pakadta hai, kyun pakadta hai, aur kaise hum ise measure karte hain (absorption vs action spectrum).
Ek pigment ek aisa molecule hai jo visible light ko specific wavelengths par absorb karta hai. Absorbed light energy ek electron ko excite karti hai, aur yahi energy ultimately photosynthesis ko power karti hai.
Plants mein main pigment classes:
Pigment
Colour
Strongly absorb karta hai
Role
Chlorophyll a blue-green
~430 & ~662 nm (red + blue)
Primary pigment; reaction centre
Chlorophyll b
yellow-green
~453 & ~642 nm
Accessory; capture ko wider banata hai
Carotenoids (carotenes, xanthophylls)
orange/yellow
~450–500 nm (blue)
Accessory + photoprotection
Itne saare pigments kyun? Sunlight mein saare colours hote hain. Akela ek pigment (chlorophyll a ) bahut saari green-blue light miss kar deta hai. Accessory pigments un wavelengths ko absorb karte hain jo chlorophyll a miss kar deta hai aur energy usse de dete hain — bilkul ek bucket brigade ki tarah jo ek naale ko paani supply karti hai.
Ek pigment ek photon tab hi absorb karta hai jab photon ki energy molecule ke ek electronic energy gap se exactly match kare.
Key consequence: E ∝ 1/ λ high energy. Long wavelength (red, ~660 nm) = low energy.
Worked example Ek red photon (662 nm) ki energy calculate karoλ = 662 nm = 662 × 1 0 − 9 m E = 662 × 1 0 − 9 ( 6.626 × 1 0 − 34 ) ( 3 × 1 0 8 ) ≈ 3.0 × 1 0 − 19 J Yeh step kyun? nm → m convert karne se SI units consistent rehti hain isliye answer joules mein aata hai. Ek blue 430 nm photon ≈ 4.6 × 1 0 − 19
Definition Absorption spectrumEk graph jo dikhata hai ki ek pigment har wavelength par kitna light absorb karta hai . Peaks wahan hote hain jahan pigment light pakadta hai, valleys (jaise green ~500–600 nm) wahan hoti hain jahan nahi pakadta.
Inhe compare kyun karein? Agar ek pigment sach mein photosynthesis drive karta hai, toh jo light woh absorb karta hai wahi light photosynthesis rate ko boost karni chahiye. Isliye action spectrum saare pigments ke combined absorption spectrum se closely match karta hai — yeh historic proof hai (Engelmann's experiment) ki chlorophyll a main driver hai, red aur blue peaks ke saath aur ek green dip ke saath.
Worked example Engelmann's experiment (Yeh clever kyun hai)Ek filamentous alga ko spectrum se lit kiya gaya (prism-split light). Aerobic bacteria add kiye gaye.
Observation: bacteria red aur blue regions mein cluster ho gaye.Yeh step kyun? Bacteria O₂ ki taraf swim karte hain. O₂ sabse tezi se wahan banta hai jahan photosynthesis sabse tezi hoti hai → isliye action spectrum red aur blue par peak karta hai, exactly wahan jahan chlorophyll absorb karta hai. Brilliant isliye kyunki bacteria khud hi meter hain.
Accessory pigments khud photochemistry nahi karte. Woh ek photon absorb karte hain aur excitation energy ko resonance transfer ke zariye reaction centre chlorophyll a tak pass karte hain, jo akele ek excited electron ko electron transport chain mein bhejta hai.
"Cars Brake In Yellow Caution" → C hlorophyll a = primary; B , I n... accessory: Chl-b , Carotenoids (yellow) energy feed karte hain. Yeh bhi yaad rakho: "Blue & Red, Chlorophyll's well-fed; Green is shed."
Common mistake "Leaves green hoti hain kyunki chlorophyll green light absorb karta hai."Yeh sahi kyun lagta hai: colour = jo wahan hai, intuitively. Fix: jo colour hum dekhte hain = wavelengths jo reflected/transmitted hoti hain, yaani absorb NAHI hoti. Green woh colour hai jo chlorophyll reject karta hai.
Common mistake "Absorption spectrum aur action spectrum ek hi cheez hain."Yeh sahi kyun lagta hai: dono almost identical dikhte hain. Fix: absorption = ek pigment dwara light li gayi ; action = rate of photosynthesis . Woh sirf correlate karte hain; action spectrum thoda broader hota hai kyunki accessory pigments bhi contribute karte hain.
Common mistake "Blue aur red light mein same energy hoti hai kyunki dono kaam karte hain."Yeh sahi kyun lagta hai: dono photosynthesis peaks dete hain. Fix: E = h c / λ kaam karte hain, lekin per-photon energy alag hoti hai.
Common mistake "Carotenoids bekar extras hain."Yeh sahi kyun lagta hai: chlorophyll sab kuch karta hua lagta hai. Fix: carotenoids blue-green light capture karte hain AUR chlorophyll ko oxidative damage se protect karte hain (photoprotection ) excess energy quench karke.
Recall Self-test (answers chhupao)
Leaf green kyun hoti hai? → Woh green reflect/transmit karti hai; chlorophyll wahan poorly absorb karta hai.
Chlorophyll a ke do wavelength peaks? → ~430 nm (blue), ~662 nm (red).
Wavelength aur energy ko link karne wala formula? → E = h c / λ
Action vs absorption spectrum? → rate of photosynthesis vs amount of light absorbed.
Carotenoids ka role? → accessory capture + photoprotection.
Recall Feynman: ek 12-saal ke bacche ko explain karoSocho sunlight har colour ke crayons ka ek dabba hai. Ek leaf mein tiny "catchers" hote hain jinhein pigments kehte hain. Main catcher, chlorophyll, sirf red aur blue crayons ko pasand karta hai — woh unhe pakadta hai aur unki energy se khana banata hai. Ushe green pasand nahi, isliye green tumhari aankhon mein bounce karta hai — isliye leaves green dikhti hain! Doosre catchers (yellow wale) un crayons ko pakadke chlorophyll ko de dete hain jo chlorophyll miss kar gaya, taaki kuch waste na ho. Hum prove kar sakte hain ki red/blue crayons sabse zyada khana banate hain kyunki tiny oxygen-loving bacteria wahan swim karte hain jahan leaf sabse zyada oxygen bana rahi hoti hai — aur woh red aur blue light par bheed lagate hain.
Ek leaf green kyun dikhti hai? Chlorophyll red aur blue light absorb karta hai lekin green ko reflect/transmit karta hai; reflected green hamaari aankhon tak pahunchti hai.
Primary photosynthetic pigment kya hai? Chlorophyll a (reaction-centre pigment).
Chlorophyll a maximally kaun si wavelengths absorb karta hai? ~430 nm (blue) aur ~662 nm (red).
Absorption spectrum define karo. Ek graph jo har wavelength par ek pigment dwara absorb ki gayi light ki matra dikhata hai.
Action spectrum define karo. Ek graph jo har wavelength par rate of photosynthesis dikhata hai.
Absorption aur action spectra closely match kyun karte hain? Kyunki pigments dwara absorb ki gayi light hi photosynthesis drive karti hai; match prove karta hai ki chlorophyll main driver hai.
Photon energy ko wavelength se relate karne wala formula? E = hc/λ.
Blue ya red light mein per photon zyada energy kaun si hoti hai? Blue (shorter wavelength → higher energy).
Carotenoids ke do roles? Blue-green light absorb karna (accessory capture) aur photoprotection provide karna.
Engelmann's experiment ne kya demonstrate kiya? Ki photosynthesis (O₂ release) red aur blue light mein peak karta hai, chlorophyll absorption se match karta hai.
Accessory pigments energy kaise contribute karte hain? Woh extra wavelengths absorb karte hain aur excitation energy reaction-centre chlorophyll a ko transfer karte hain.
Light Reactions of Photosynthesis Photosystems I and II Electromagnetic Spectrum and Photon Energy Chloroplast Structure Engelmann's Experiment Carotenoids and Photoprotection
E inversely prop to lambda
widen capture, pass energy to
Photon energy E=hc/lambda
