4.9.5
Intuition Ek-line pictureSugar ko phloem mein source (jaise ek leaf) par pump kiya jaata hai. Jahan sugar jaati hai, water osmosis se saath follow karta hai . Isse source end par pressure build up hota hai. Sink par (jaise root ya fruit), sugar bahar nikalti hai, water bahar follow karta hai, aur pressure drop ho jaata hai. High pressure, low pressure ki taraf push karta hai — toh sugary sap bulk mein flow karta hai source se sink tak, jaise hose mein paani behta hai jab aap ek end ko dabate ho.
Translocation organic solutes ki movement hai (mainly sucrose , amino acids bhi) phloem ke through source se sink tak.
Source = jahan sugar banti ya release hoti hai (photosynthesising leaf, storage organ jo starch mobilise kar raha ho).Sink = jahan sugar use ya store hoti hai (roots, fruits, growing shoots, tubers).
Intuition Source vs sink ekrole hai, fixed identity nahi
Ek potato tuber summer mein sink hoti hai (starch store karta hai) lekin spring mein source ban jaati hai (naye shoot ko feed karta hai). Phloem flow ki direction depend karti hai ki pressure kahan sabse zyada hai — isliye phloem transport poore plant mein bidirectional hota hai (alag tubes, alag times), unlike xylem jo hamesha upar ki taraf jaata hai.
Xylem transpiration pull (negative pressure) use karta hai. Phloem alag hai: yeh ek living, positive-pressure system hai. Is engine ke do coupled tricks hain:
Source par sugar ki active loading (ATP chahiye).
Osmosis baaki kaam free mein karti hai.
Intuition Sugar loading se water andar kyun aa jaata haiOsmosis water ko high water potential → low water potential ki taraf move karti hai. Solute (sucrose) add karna water potential girata hai. Toh jab sucrose ko phloem sieve tube mein pump kiya jaata hai, uska water potential neighbouring xylem se neeche chala jaata hai, aur water osmosis se rush karke andar aata hai . Wahi water hai jo pressure create karta hai.
Maano Ψ Ψ s Ψ p
SOURCE par:
Sucrose ko companion cells / sieve tubes mein active transport se load kiya jaata hai (often ek proton–sucrose cotransport jo ATP se bane H + Active kyun? Sugar ko ek pehle se sugary tube mein apne concentration gradient ke against jaana hota hai.
Loading se Ψ s Ψ
Paas ke xylem se osmosis ke zariye water andar aata hai → volume badhta hai → turgor pressure Ψ p . Kyun? Sieve tube ek band-ish pipe hai; paani andar aane par kahi jaane ki jagah na ho toh pressure build hota hai.
SINK par:
4. Sucrose unload hoti hai (respiration mein use hoti hai, ya starch mein convert hoti hai jo osmotically inactive hoti hai).
5. Ψ s turgor pressure Ψ p .
Beech mein:
6. Ab Ψ p ( source ) > Ψ p ( sink ) pressure gradient poore sap solution ka bulk (mass) flow source se sink tak drive karta hai.
Worked example Worked scenario — ek leaf root ko feed kar rahi hai
Leaf sucrose load karti hai → leaf sieve tube Ψ s = − 1.5 Ψ p = + 0.8
Root sucrose unload karti hai → root sieve tube Ψ p = + 0.2
Step: driver calculate karo. Kyun? Direction higher Ψ p Δ Ψ p = 0.8 − 0.2 = + 0.6 leaf → root flow karta hai. ✔
Worked example Worked scenario — spring mein potato
Tuber starch mobilise karta hai → sucrose release hoti hai → tuber source ban jaata hai, Ψ p
Naya shoot sugar fast respire karta hai → sink , Ψ p
Step: pressure kahan sabse zyada hai? Tuber mein. Yeh matter kyun karta hai? Flow ab tuber → shoot chalti hai, summer ki reverse direction. Same tubes, opposite direction — prove karta hai ki phloem flow pressure follow karta hai, gravity nahi.
Intuition Hum jaante kaise hain?
Aphid stylet experiment : ek aphid ek sieve tube ko pierce karta hai; sap apne aap pressure ke under ooze karta hai — prove karta hai ki phloem contents positive pressure mein hain. Source ke paas tubes se sap zyada tezi se ooze karta hai (higher pressure). Yeh convincing kyun hai? Predicted pressure gradient ka direct measurement.Radioactive 14 C O 2 ko leaf ko dene par labelled sugar roots tak move karti dikhti hai — source→sink direction confirm karta hai.
Common mistake "Sugar apne concentration gradient ke neeche diffusion se move karta hai."Yeh sahi kyun lagta hai: solutes usually high→low diffuse karte hain, aur phloem mein source par actually high sugar hoti hai. Fix: sugar poore solution ke bulk flow se move karti hai, ek pressure gradient se drag ho ke — water , sugar nahi, apna potential follow kar raha hai. Loading par sugar ek pehle se high-sugar tube mein against gradient bhi jaati hai (ATP use karke).
Common mistake "Phloem bhi xylem vessels ki tarah dead tubes hain."Yeh sahi kyun lagta hai: dono lambi transport tubes hain. Fix: phloem sieve tube elements living hote hain (halanki woh nucleus kho dete hain) aur ATP aur loading ke liye living companion cells par depend karte hain. Phloem ko maar do aur translocation ruk jaati hai — prove karta hai yeh ek active, living process hai. Xylem transport (transpiration) dead cells mein kaam karta hai.
Common mistake "Water potential aur pressure ek hi cheez hain."Yeh sahi kyun lagta hai: dono paani ko push karte hain. Fix: Ψ = Ψ s + Ψ p Ψ p ek component hai. Source par, Ψ negative hota hai (low, water attract karta hai) jabki Ψ p positive (high, flow push karta hai) hota hai. Yeh dono opposite direction mein move kar sakte hain.
Common mistake "Phloem hamesha neeche ki taraf flow karta hai."Yeh sahi kyun lagta hai: hum leaves ko roots feed karte imagine karte hain. Fix: flow kisi bhi sink ki taraf jaata hai — growing shoot tips aur fruits tak upar bhi. Direction = pressure gradient, gravity nahi.
Recall Feynman: 12-saal ke bachche ko samjhaoImagine karo do water balloons ek straw se jude hue hain. Tum pehle balloon (leaf) mein sugar-water inject karte rehte ho. Sugar thirsty hoti hai, isliye woh water suck karti hai — balloon phool jaata hai aur tight aur high-pressure ho jaata hai. Doosra balloon (root) mein se sugar nikaali ja rahi hai, isliye woh water bahar jaane deta hai aur floppy aur low-pressure ho jaata hai. Ek tight balloon hamesha floppy waale mein push karta hai, isliye sugary water straw mein se leaf se root tak whoosh ho jaata hai. Plant bas ek end par sugar load karta rehta hai aur doosre end par nikalta rehta hai, aur flow kabhi nahi rukta. Isko ulta karo (root spring mein shoot ko feed kare) aur water doosri taraf whoosh karta hai!
Mnemonic Poori chain yaad karo"Load → Lower → Lure → Lift → Let-go"
Load sugar (active) → water potential Lower ho jaata hai → osmosis se water ko Lure karta hai andar → source par pressure Lift hota hai → sink Let ta hai sugar & water ko jaane → pressure girta hai → flow source→sink.
Translocation kya hai? Organic solutes (mainly sucrose) ki phloem ke through source se sink tak movement.
Translocation mein source ko define karo. Woh region jahan sugar banti ya release hoti hai (e.g. photosynthesising leaf, mobilising storage organ).
Sink ko define karo. Woh region jahan sugar use ya store hoti hai (roots, fruits, growing tissues, tubers).
Source par sugar loading ko active transport kyun kaha jaata hai? Sucrose apne concentration gradient ke against phloem mein jaati hai, ATP chahiye (often H⁺–sucrose cotransport ke zariye).
Sucrose load hone par sieve tube mein water potential ka kya hota hai? Yeh girta hai (zyada negative ho jaata hai) kyunki solute water potential girata hai.
Phloem mein source par water kyun andar aata hai? Osmosis: lowered water potential neighbouring xylem se water ko andar khiinchti hai.
Phloem flow ko directly kya drive karta hai? Ek hydrostatic (turgor) pressure gradient, source par high aur sink par low → bulk/mass flow.
Water potential equation likhon. Ψ = Ψs + Ψp (solute potential + pressure potential).
Sugar source se sink tak sirf diffuse kyun nahi ho sakti? Metres tak diffusion bahut slow hoti hai; bulk flow poore solution ko saath mein, quickly move karta hai.
Aphid stylet experiment pressure-flow ko kaise support karta hai? Cut sieve tube se sap apne aap pressure mein ooze karta hai, prove karta hai ki phloem mein positive hydrostatic pressure exist karta hai.
Phloem sieve tube elements living hain ya dead? Living (lekin enucleate), living companion cells se support hote hain jo ATP supply karte hain.
Xylem ke one-way hone ke jabki phloem transport poore plant mein bidirectional kyun hai? Flow kisi bhi sink ki taraf pressure gradient follow karta hai, isliye direction source/sink location par depend karti hai, gravity par nahi.
Ek aisi tissue ka example do jo sink se source switch karti hai. Ek potato tuber: summer mein sink (starch store karta hai), spring mein source (naye shoot ko feed karta hai).
Water potential (Ψ = Ψs + Ψp) Osmosis and turgor pressure Xylem and transpiration — contrast: dead cells, negative pressure, one-wayActive transport and proton pumps — sucrose loading mechanismPhotosynthesis — source sugar banata haiCompanion cells and sieve tube structure Sources and sinks in plants
Active loading of sucrose
High turgor pressure at source
Bulk pressure-flow of sap
Sink - sugar used or stored
