How to Use a Potometer (Transpiration)

Q: Why MUST the stem of the leafy shoot be cut underwater before being attached to a potometer?

Correct Answer: A) To prevent air bubbles entering xylem vessels and disrupting the transpiration pull. Explanation: Cutting underwater ensures water enters xylem rather than air, preserving cohesion-tension continuity. Mark Scheme: 1 mark for preventing air locks in xylem / preserving continuous water column.

Q: What is the purpose of the water reservoir tap located above the potometer capillary tube?

Correct Answer: A) To push the air bubble back to the start of the scale for repeated measurements. Explanation: Opening the reservoir tap releases water that resets the air bubble back to zero for repeat trials. Mark Scheme: 1 mark for resetting the air bubble to allow repeat measurements.

Q: Which environmental condition DECREASES the rate of transpiration measured by a potometer?

Correct Answer: A) High atmospheric humidity. Explanation: High humidity lowers the water vapour concentration gradient between leaf air spaces and outside air, slowing diffusion. Mark Scheme: 1 mark for High humidity.

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Key Learning Objectives

Explain why leafy shoots must be cut underwater and attached to a potometer underwater to prevent air locks in the xylem.
Calculate water uptake rates using capillary tube cross-sectional area and bubble distance over time (Volume = πr²d).
Analyse the effects of wind speed, temperature, humidity, and light intensity on transpiration rates.

Topic Summary & Core Notes

Master the required practical investigation of transpiration rates using a bubble potometer. Cover airtight sealing, capillary measurements, reservoir resetting, and controlling environmental variables.

Key Rules, Formulas & Definitions

Potometer Volume Rate

Volume (mm³) = π × r² × d; Rate = Volume ÷ Time (mm³/min)

Assumed Equivalence

Rate of Water Uptake ≈ Rate of Transpiration (assuming minimal water retained in cells/photosynthesis)

2 Step-by-Step Worked Examples

Worked Example 1

Worked Example 1: Calculating Transpiration Rate

Question: In a potometer practical, an air bubble moves 45 mm in 15 minutes through a capillary tube of internal radius 0.5 mm. Calculate the rate of water uptake in mm³/minute.

Step-by-Step Solution:

Step 1: Calculate volume displaced: V = π × r² × d = π × (0.5 mm)² × 45 mm = 3.14159 × 0.25 × 45 = 35.34 mm³.
Step 2: Calculate rate per minute: Rate = 35.34 mm³ ÷ 15 min = 2.36 mm³/min.

Final Answer: 2.36 mm³/min

Senior Examiner Insight: Show full formula substitution and round to 3 significant figures.

Worked Example 2

Worked Example 2: Experimental Precautions

Question: Give two critical precautions required when setting up a bubble potometer to guarantee valid results.

Step-by-Step Solution:

Step 1: Preventing air entry: Cut shoot and assemble potometer completely underwater to prevent air entering xylem vessels and breaking cohesive water columns.
Step 2: Airtight seal: Apply waterproof petroleum jelly (Vaseline) around all rubber bungs and joints to ensure an airtight seal.

Final Answer: 1. Cut and assemble the leafy shoot underwater to prevent air locks in xylem. 2. Seal all joints with petroleum jelly to maintain an airtight system.

Senior Examiner Insight: Use the term 'air lock in xylem vessels' rather than just 'bubbles get in'.

Common Pitfalls & Examiner Warnings

Common Mistake: Assuming water uptake strictly equals transpiration

A small fraction (<2%) of absorbed water is retained for cell turgidity and split during photosynthesis.

Common Mistake: Cutting the shoot in air

Tension in xylem pulls air into vessels, permanently breaking the continuous water column.

Interactive Self-Check Quiz (3 Questions)

Question 1 of 3

Why MUST the stem of the leafy shoot be cut underwater before being attached to a potometer?

A) To prevent air bubbles entering xylem vessels and disrupting the transpiration pull

B) To wash away surface bacteria on the stem

C) To cool down the leaf tissues

D) To activate stomatal guard cells

Click to Reveal Answer & Mark Scheme

Correct Answer: A) To prevent air bubbles entering xylem vessels and disrupting the transpiration pull

Detailed Explanation: Cutting underwater ensures water enters xylem rather than air, preserving cohesion-tension continuity.

Mark Scheme & Scoring Points: 1 mark for preventing air locks in xylem / preserving continuous water column.

Question 2 of 3

What is the purpose of the water reservoir tap located above the potometer capillary tube?

A) To push the air bubble back to the start of the scale for repeated measurements

B) To measure root pressure directly

C) To add liquid fertiliser to the shoot

D) To adjust water pH

Click to Reveal Answer & Mark Scheme

Correct Answer: A) To push the air bubble back to the start of the scale for repeated measurements

Detailed Explanation: Opening the reservoir tap releases water that resets the air bubble back to zero for repeat trials.

Mark Scheme & Scoring Points: 1 mark for resetting the air bubble to allow repeat measurements.

Question 3 of 3

Which environmental condition DECREASES the rate of transpiration measured by a potometer?

A) High atmospheric humidity

B) Increased wind velocity

C) Elevated temperature

D) Bright illumination

Click to Reveal Answer & Mark Scheme

Correct Answer: A) High atmospheric humidity

Detailed Explanation: High humidity lowers the water vapour concentration gradient between leaf air spaces and outside air, slowing diffusion.

Mark Scheme & Scoring Points: 1 mark for High humidity.

Created by Fiaraz Iqbal

Former Headteacher & Senior Science / Maths Examiner

Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.

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