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Biology Question 118 – NEET-UG 2023

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Given below are two statements : Statement I : The forces generated by transpiration can lift a xylem-sized column of water over 130 meters height. Statement II : Transpiration cools leaf surfaces sometimes 10 to 15 degrees, by evaporative cooling. In the light of the above statements, choose the most appropriate answer from the options given below :

Transpiration is a vital physiological process in plants involving the loss of water vapor from aerial parts, primarily leaves.

Step 1: Analyze Statement I: Transpiration Pull and Water Ascent✦ Active

Statement I claims that forces generated by transpiration can lift a xylem-sized column of water over 130 meters height. The cohesion-tension-transpiration pull model is the most accepted theory for water transport in plants. Transpiration creates a strong negative pressure (tension) in the xylem, which, coupled with the cohesive and adhesive properties of water, can pull water up to the top of the tallest trees. Giant redwood trees, for instance, can exceed 100 meters in height, and the transpiration pull is sufficient to lift water to these heights. Therefore, a height of over 130 meters is biologically plausible under optimal conditions.

💡 Teacher's Secret Hint

Recall the mechanism of water transport in tall plants, specifically the role of transpiration pull and the properties of water.

Step 2: Analyze Statement II: Transpirational Cooling○ Expand

Statement II states that transpiration cools leaf surfaces sometimes 10 to 15 degrees by evaporative cooling. Transpiration involves the evaporation of water from the leaf surface. Evaporation is an endothermic process, meaning it absorbs heat from its surroundings (the leaf). This absorption of latent heat of vaporization leads to a significant cooling effect on the leaf surface. This cooling mechanism is crucial for preventing leaves from overheating, especially under intense sunlight. A temperature reduction of 10 to 15 degrees Celsius is a well-documented and commonly cited range for the cooling effect of transpiration.

💡 Teacher's Secret Hint

Consider the physical principle of evaporative cooling and its biological significance for plants.

Step 3: Conclusion○ Expand

Based on the analysis of both statements, Statement I is correct as transpiration pull can indeed lift water to very significant heights, and Statement II is also correct as evaporative cooling due to transpiration effectively cools leaf surfaces by 10 to 15 degrees Celsius. Therefore, both statements are correct.

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