Explain transpiration and the cohesion-tension theory

4.9.4 · Biology › Plant Biology

Transpiration KYA hai?

KYA move ho raha hai? Liquid water roots se upar jaata hai → stem → leaves; leaf par pahunchkar vapour ban jaata hai aur escape kar jaata hai.

YE HOTA HI KYUN HAI? Bahar ki hawa (usually) leaf ke andar ki air spaces se zyada dry hoti hai. Paani hamesha high water potential → low water potential ki taraf diffuse karta hai. Leaf ki air spaces ke andar water potential high hai (humid); bahar low hai (dry). Isliye paani bahar jaata hai.

Plant paani khone ko kyun "sehti hai"

Paani khona wasteful lagta hai — neeche steel-man se clarity milegi. Faayde ye hain:

Ye paani + dissolved minerals ko upar khichta hai (transpiration stream).
Evaporation leaf ko thanda karta hai (sweat ki tarah).
Ye cells ke support ke liye turgor maintain karta hai aur photosynthesis machinery ko hydrated rakhta hai.

Photosynthesis ke liye $C O_{2}$ ko andar aane dene ke liye stomata kholne hi padte hain — paani ka loss iska anaawashyak "ticket ka daam" hai.

Paani asal mein upar KAISE jaata hai: Cohesion-Tension Theory

Hum mechanism ko step by step paani ke physical properties se DERIVE karte hain.

Step 1 — Evaporation tension create karta hai (KYUN?) Mesophyll cells ki moist cell walls se paani evaporate hota hai. Molecules ke jaane ke baad, bacha hua paani wall ke tiny pores mein curved menisci banata hai. Curved meniscus mein surface tension ek pulling force (negative pressure = tension) generate karta hai jo uske peeche ke paani par act karta hai.

Ye step kyun? Kyunki narrow pore se paani hatane par surface curved reh jaata hai, aur curvature = tension. Ye poore system ki engine hai.

Step 2 — Cohesion pull ko transmit karta hai (KYUN?) Water molecules polar hote hain → ek doosre ke saath hydrogen bonds banate hain. Isliye ye ek chain ke links ki tarah behave karte hain. Top link ko khincho aur puri chain neeche se khich jaati hai.

Ye step kyun? Cohesion ke bina column toot jaata aur sirf top wala hissa move karta.

Step 3 — Adhesion column ko gravity ke against thaami rehti hai (KYUN?) Paani narrow xylem vessels ki hydrophilic walls ke saath bhi H-bonds banata hai (adhesion). Ye column ko support karne mein madad karta hai aur, chote diameter ke saath milkar, capillary action provide karta hai.

Step 4 — Root uptake jo kho gaya use replace karta hai Tension roots tak transmit hoti hai, wahan $Ψ$ kam kar deti hai, isliye paani root hairs mein osmosis se enter karta hai.

$Net direction: Ψ_{so i l} > Ψ_{r oo t} > Ψ_{s t e m} > Ψ_{l e a f} > Ψ_{ai r}$

Paani is water-potential gradient ke neeche flow karta hai, ek arrow at a time.

Figure — Explain transpiration and the cohesion-tension theory

Physics: kitni pull chahiye?

Worked examples

Transpiration rate ko affect karne wale factors

Factor ↑	Rate par effect	KYUN
Light	↑	Stomata photosynthesis ke liye kholte hain
Temperature	↑	Zyada KE → tez evaporation, steeper $Ψ$ gradient
Wind	↑	Humid boundary layer hatata hai, gradient steep rehta hai
Humidity	↓	Bahar ki hawa zyada wet → chota $Ψ$ gradient

Common mistakes (Steel-man + fix)

Recall Feynman: 12-saal ke bachche ko samjhao

Socho ek bahut lamba straw ek juice box mein ghusa hai, jo puri tree ke upar tak jaata hai. Jab sun juice ke top ko sukha deta hai, tiny bits invisible steam ki tarah ud jaate hain. Paani apne aap se bahut sticky hota hai — jaise magnets ki chain. Toh jab top wala bit jaata hai, wo agle bit ko kheenchta hai, jo agle ko kheenchta hai, seedha roots tak. Wo khinchaav soil se fresh paani upar le aata hai. Koi push nahi kar raha — sukhta leaf hi sucking kar raha hai, aur paani ki stickiness chain ko tootne nahi deti.

Flashcards

Transpiration kya hai?

Aerial plant parts se water vapour ka loss (mainly stomata ke through) evaporation aur diffusion dwara.

Leaf mein evaporation kahan hoti hai?

Mesophyll cells ki moist walls se air spaces mein, phir stomata ke through bahar.

Cohesion-tension theory mein tension kya generate karta hai?

Leaf cell walls se paani ka evaporation (curved menisci negative pressure create karte hain).

Kaunsi property paani ko ek continuous column banane deti hai jo toot nahi ta?

Cohesion — water molecules ke beech hydrogen bonding.

Yahaan cohesion aur adhesion mein kya fark hai?

Cohesion = water–water H-bonds (pull transmit karta hai); adhesion = water–xylem-wall H-bonds (column support karta hai, capillarity mein madad karta hai).

Water potential ke terms mein paani flow kis direction mein karta hai?

High Ψ → low Ψ: soil > root > stem > leaf > air.

Height h ke water column ko hold karne ke liye pressure kitna chahiye?

P = ρgh (column ke weight/area se derive hota hai).

Transpire karte waqt xylem water positive ya negative pressure mein hota hai?

Negative pressure (tension) mein — column stretch ho raha hota hai.

Kyun narrow xylem capillarity se paani zyada upar uthata hai?

h = 2γcosθ/(ρgr), isliye h ∝ 1/r; chota radius → zyada rise.

Transpiration ke do fayde batao.

Paani+minerals upar kheenchta hai (transpiration stream); leaf ko thanda karta hai; turgor maintain karta hai.

Wind transpiration kyun badhata hai?

Humid boundary layer hatata hai, water-potential gradient steep rakhta hai.

Steel-man fix: kya roots paani ko treetop tak pump karti hain?

Nahi — root pressure bahut weak hota hai; top se tension ka pull (transpiration) dominate karta hai.

Connections

Water potential and osmosis
Xylem and phloem structure
Stomata and guard cell mechanism
Photosynthesis and gas exchange
Hydrogen bonding in water
Capillary action and surface tension
Root pressure and guttation