CDS (II) 2023 General Knowledge · Question 46 of 120

Correct — B, Transpiration pull, cohesion and adhesion. Water reaches the top of a tall tree because it is pulled from above rather than pushed from below, and the pulling works only because the water column does not break. Transpiration supplies the pull: water evaporating from the leaf surfaces creates a suction in the xylem that draws the whole column upward. NCERT puts the division of labour plainly — the effect of root pressure in the transport of water is more important at night, while during the day, when the stomata are open, the transpiration pull becomes the major driving force in the movement of water in the xylem. Cohesion is what allows a pull applied at the top to be felt at the bottom: water molecules stick to one another through hydrogen bonding, so the column behaves like a thread that can be drawn up whole. Adhesion is the stickiness between water molecules and the hydrophilic walls of the xylem, which keeps the thread clinging to the narrow tubes and stops it slipping back. The three together are the cohesion-tension account of the ascent of sap, and the question names exactly those three.

• (a)Respiration and cohesion — Respiration is the release of energy from food and has no role in lifting water. The ascent of sap is a passive process driven by evaporation at the leaves, not by the plant spending energy to pump; cohesion is right, but the driving force named here is wrong.
• (c)Root pressure, cohesion and adhesion — The most tempting wrong option, because root pressure is real. It develops as ions are actively taken into the root and water follows by osmosis, and it matters most at night when transpiration is low. What it cannot do is account for the ascent in a tall tree during the day — the stem specifically says tall trees, and for those the transpiration pull is the major driving force.
• (d)Transpiration and adhesion — Names the right driving force and one of the two properties, but drops cohesion — and cohesion is the property that actually makes a pull from the top work. Without water molecules holding on to one another the column would snap and the suction would lift nothing.

Xylem is the tissue that carries water and dissolved minerals from the roots up to the stem and leaves, through tracheids and vessel elements joined end to end into continuous tubes. The flow is essentially one-way and upward. Two mechanisms are usually named. Root pressure is generated at the base when ions are actively absorbed into the root and water follows osmotically; it is a push, and it is modest. The cohesion-tension mechanism is a pull from the top: evaporation at the leaf pulls on the water column, and the column holds together because water molecules cohere to each other and adhere to the tube walls. Phloem, by contrast, carries the sugars made in the leaves and can move them in either direction.

Read the stem carefully — it says tall trees, and that word is doing the work. Any mechanism based on pushing from the roots runs into a height limit, whereas a pull applied at the leaves does not, provided the column stays intact. That is why the answer must contain transpiration pull, and why the properties named must include cohesion, the one that keeps the column from breaking under tension. A neat way to hold it is to picture drinking through a straw: the suction is transpiration, the unbroken liquid thread is cohesion, and the film clinging to the inside of the straw is adhesion. The honest qualification is that root pressure is not useless — it is genuinely important at night and in short plants, and it is what forces water out as droplets at leaf margins in the early morning. The claim in the correct option is only that the daytime ascent in a tall tree is carried by the transpiration pull with cohesion and adhesion, and that claim is well supported.

• NCERT: 'The effect of root pressure in transport of water is more important at night. During the day when the stomata are open, the transpiration pull becomes the major driving force in the movement of water in the xylem.'
• Cohesion is the attraction between water molecules themselves, produced by hydrogen bonding between the slightly negative oxygen of one molecule and the slightly positive hydrogen of another.
• Adhesion is the stickiness between water molecules and the hydrophilic cell walls of the xylem.
• Xylem is made of tracheids and vessel elements, with lignified walls, joined into continuous tubes; the flow is unidirectional and upward.
• Upward transport of water by xylem is considered to limit the maximum height that trees can reach.

• Choosing root pressure because it sounds like the obvious source of an upward push; the stem's word 'tall' is what rules it out as the main mechanism.
• Dropping cohesion from the answer. Adhesion alone cannot transmit a pull along a column — it is cohesion that holds the thread together.
• Bringing respiration into it. The ascent of sap is passive, powered by evaporation rather than by the plant's own energy.

As a name-the-mechanism item on the ascent of sap, or as a statements question contrasting root pressure with the transpiration pull.

• practice — not a real PYQ

  The upward movement of water in the xylem of a tall tree during the day is driven mainly by

  • (a)root pressure
  • (b)transpiration pull
  • (c)active pumping by xylem cells
  • (d)capillary rise alone

  Answer(b) transpiration pull — evaporation from the leaves creates the suction, and it is the major driving force by day when the stomata are open; root pressure matters more at night.

• practice — not a real PYQ

  The attraction between water molecules and the walls of the xylem vessels is called

  • (a)cohesion
  • (b)adhesion
  • (c)surface tension
  • (d)osmosis

  Answer(b) adhesion — adhesion is water sticking to the vessel walls, while cohesion is water molecules sticking to one another.