What is the primary difference between a bubble potometer and a mass potometer?

A bubble potometer measures the volume of water uptake by tracking the movement of an air bubble in a capillary tube, whereas a mass potometer measures the actual weight lost by the plant due to evaporation. The bubble potometer is generally faster and more sensitive for short-term experiments.

Why might the rate of water uptake measured by a potometer be higher than the actual rate of transpiration?

The potometer measures all water absorbed by the shoot, but not all of this water is transpired. Some water is used by the plant cells for photosynthesis and to maintain turgor pressure, leading to a slight overestimation of transpiration.

How does the effect of high humidity on a potometer reading compare to the effect of high wind speed?

High humidity decreases the transpiration rate by reducing the concentration gradient between the leaf and the air. Conversely, high wind speed increases the rate by moving saturated air away from the leaf surface, maintaining a steep concentration gradient.

What is the consequence of failing to dry the leaves of a plant shoot before starting a potometer experiment?

Water droplets on the leaf surface can block the stomatal pores and increase the local humidity. This artificially reduces the rate of transpiration, leading to inaccurate results that do not reflect the environmental conditions being tested.

Why is it a mistake to assemble the potometer apparatus in the air rather than underwater?

Assembling in air allows air bubbles to enter the xylem of the plant and the capillary tube of the device. These bubbles break the continuous column of water required for the cohesion-tension mechanism, stopping the movement of water and the bubble.

What happens to the results if the petroleum jelly seal around the plant stem is incomplete?

An incomplete seal allows air to be sucked into the apparatus instead of water being pulled from the capillary tube. This results in the air bubble moving slowly or not at all, regardless of the actual transpiration rate of the plant.

Define 'Transpiration' in the context of a potometer experiment.

Transpiration is the loss of water vapor from the aerial parts of a plant, primarily through the stomata in the leaves. In a potometer experiment, this process provides the 'pull' that moves the air bubble along the tube.

What is the function of the reservoir (or tap) in a bubble potometer?

The reservoir stores extra water and allows the user to reset the air bubble to the starting position on the scale. By opening the tap, water is released into the capillary tube, pushing the bubble back so that repeat measurements can be taken.

What does the 'rate of transpiration' represent mathematically in this practical?

It represents the volume or distance of water moved per unit of time, typically expressed in units such as $mm/min$ or $cm^3/min$. It is calculated by dividing the total distance the bubble moved by the duration of the observation.

Why does increasing the distance of a lamp from the plant decrease the rate of bubble movement?

Increasing the distance reduces light intensity, which causes stomata to close as the demand for photosynthesis drops. Fewer open stomata mean less water vapor can escape the leaf, reducing the tension that pulls water through the potometer.

Practical - Using a Potometer | OCR GCSE Science

Combined Science A Gateway / Biology

Practical Skills

Practical - Using a Potometer

Summary

A potometer is a specialized laboratory apparatus used to estimate the rate of transpiration by measuring the rate of water uptake in a leafy shoot. While it technically measures water absorption rather than direct evaporation, the two values are closely correlated, making it a standard tool for investigating how environmental factors like light, temperature, and humidity influence plant water loss.

1. Definition & Core Concepts

Potometer: A device designed to measure the rate of water uptake in a plant shoot, which serves as an indirect proxy for the rate of transpiration.
Transpiration-Uptake Relationship: In most conditions, over 95% of the water absorbed by a plant is lost through transpiration, meaning the speed of water movement in the potometer is a reliable indicator of evaporative loss.
Bubble Potometer: The most common variant, which utilizes the movement of a single air bubble along a graduated capillary tube to quantify the volume of water consumed over a specific time interval.

Diagram of a bubble potometer showing a plant shoot, capillary tube with an air bubble, and a water reservoir.

2. Underlying Principles

Cohesion-Tension Theory: As water evaporates from the mesophyll cells in the leaves, it creates a negative pressure (tension) that pulls the entire water column upward through the xylem due to the cohesive forces between water molecules.
Hydrostatic Continuity: Because the potometer system is completely filled with water and sealed airtight, any water lost at the leaf surface results in an immediate and equal displacement of the water column in the capillary tube.
Assumptions of Measurement: The method assumes that water uptake equals water loss; however, a small fraction of water is used for photosynthesis and maintaining cell turgidity, meaning the potometer slightly overestimates actual transpiration.

3. Methods & Techniques

Underwater Preparation: The plant shoot must be cut and the apparatus assembled underwater to prevent air from entering the xylem vessels, which would break the continuous water column and stop transpiration.
Airtight Sealing: All joints and bungs must be sealed with petroleum jelly (Vaseline) to ensure the system is vacuum-tight, as any air leak will prevent the bubble from moving accurately.
Leaf Preparation: The leaves must be gently dried before starting the experiment; surface water can block stomata and artificially lower the transpiration rate.
Data Collection: Introduce a single air bubble by briefly lifting the capillary tube out of the water, then measure the distance the bubble travels over a fixed time period (e.g., 10 minutes) using a ruler or integrated scale.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions