Question 1

What is transpiration pull?

Accepted Answer

Transpiration pull is the tension created in the xylem when water evaporates from mesophyll cell walls into the air spaces and out through the stomata. Because water molecules stick together (cohesion) and to the xylem walls (adhesion), this tension pulls the entire continuous water column upward from the roots. It is the main mechanism of ascent of sap in tall plants.

Question 2

Why can root pressure not explain ascent of sap in tall trees?

Accepted Answer

Root pressure is a positive hydrostatic pressure of at most about 1–2 atmospheres, enough to raise water roughly 10–20 metres. Tall trees exceed 50–100 metres, far beyond what root pressure can push. Root pressure is also absent during rapid daytime transpiration and is mainly observed at night, where it explains gutation rather than bulk ascent.

Question 3

How do guard cells open a stoma?

Accepted Answer

Potassium ions (K⁺) are actively transported into the guard cells, lowering their water potential (Ψ). Water therefore enters osmotically, the guard cells become turgid, and because their inner walls are thicker than the outer walls, they bow outward, widening the pore between them. Closing reverses the sequence: K⁺ exits, Ψ rises, water leaves, and the cells become flaccid.

Question 4

What is water potential?

Accepted Answer

Water potential (Ψ) measures the free energy of water and is expressed in megapascals (MPa). Pure water at standard conditions has Ψ = 0, the reference value. It has two components, Ψ = Ψs + Ψp, where Ψs (solute potential) is always negative and Ψp (pressure potential) may be positive in turgid cells. Water always moves from higher Ψ to lower Ψ.

Question 5

Why is the inner wall of a guard cell thicker than the outer wall?

Accepted Answer

The unevenly thickened walls make guard cells function like hinged beams. When the turgid cell expands, the rigid thick inner wall resists stretching while the thinner elastic outer wall stretches more, forcing the cell to bow outward. This bows the two guard cells apart and opens the pore — simple inflation alone would not create an opening.

Transpiration Pull: Water Is Pulled, Not Pushed

Practice

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