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Difficulty: MediumAnalyzing Experimental Procedures and Apparatus

Experiment 1

Students set up a capillary potometer to measure the rate of water uptake (transpiration) by a leafy plant shoot under different light intensities. The apparatus consists of a plant stem fitted tightly into a water-filled glass tube connected horizontally to a calibrated capillary tube containing a single air bubble. Attached above the capillary tube is a water reservoir controlled by a stopcock valve.

Based on the described experimental setup, which of the following best explains the purpose of including the water reservoir and stopcock valve in this apparatus?

  1. To reset the position of the air bubble back to the zero mark on the capillary scale between measurement trialsAnswer
  2. B
    To serve as the primary source of oxygen gas required to drive transpiration in the leaves
  3. C
    To alter the water pressure in the tubing as the main independent variable of the experiment
  4. D
    To collect and directly measure the volume of water vapor evaporating from the leaf stomata

Answer

The water reservoir and stopcock valve allow liquid water to enter the capillary tube, resetting the air bubble to the zero position for repeated trials.
In a potometer experiment, as the plant shoot transpires, it draws water from the capillary tube, pulling the air bubble toward the plant. To conduct multiple trials or test different conditions, the air bubble must be returned to its baseline position. Opening the stopcock allows water from the reservoir to flow into the tube, pushing the bubble back to the starting point.

Step-by-Step Solution

1
Analyze how water uptake is measured by the potometer
As the plant shoot transpires water, it pulls water from the horizontal capillary tube, causing the air bubble to move along the calibrated scale toward the plant stem.
Understanding the movement of the indicator bubble shows what happens during a single test run.
2
Determine why the bubble needs to be repositioned
After a measurement trial, the bubble has moved far down the tube and must be returned to the start mark before changing light conditions for the next trial.
Multiple trials require standardized starting conditions.
3
Identify the structural function of the reservoir and stopcock
Opening the stopcock releases water from the reservoir into the main tube, pushing the bubble back to the start position without introducing additional air or disassembling the apparatus.
This confirms that the reservoir serves as a resetting mechanism.

Key Concept

Function of components in a capillary potometer apparatus
Estimated Time:1m 0s
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Topics

Analyzing Experimental Procedures and Apparatus

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