Artificial Transpiration Lab (Potometer)
Measure the rate of water loss in a simulated plant cutting using a virtual potometer. Adjust environmental physics variables including Temperature, Wind Speed, and Humidity to determine their exact multiplicative effects on evaporation.
PLANT TRANSPORT: THE COHESION-TENSION THEORY
Plants move water from their roots to their leaves—sometimes hundreds of feet high—without any mechanical pumps. This is driven by **transpiration**, the evaporation of water through the **stomata** in leaves. As water molecules evaporate, they pull on the chain of water molecules below them, all the way down to the roots.
COHESION, ADHESION, AND TENSION
1. **Cohesion**: Water molecules stick to each other via hydrogen bonding, creating an unbroken column. 2. **Adhesion**: Water molecules stick to the cellulose walls of the **xylem**, preventing the column from slipping back down. 3. **Tension**: The 'negative pressure' created by evaporation at the leaf surface pulls the column upward. This process is passive and requires no energy (ATP) from the plant.
HOW TO USE THIS VISUALIZATION
1. **Adjust Humidity**: Increase the humidity and watch the transpiration rate slow down. 2. **Open Stomata**: Watch the guard cells swell to open the pores. Note the trade-off: enters for photosynthesis, but water escapes. 3. **Trace the Molecules**: Follow the hydrogen-bonded water chain as it moves through the xylem fibers. **Try This**: Turn on the 'Wind' setting. Observe how the rate of transpiration increases. Why does moving air speed up evaporation at the leaf surface?
AP EXAM CONNECTION
Unit: Unit 1: Chemistry of Life (Topic 1.1)
Learning Objective: SYI-1.A
COMMON MISCONCEPTIONS
- Thinking plants 'pump' water (it is a passive pull).
- Believing water moves through the phloem (phloem moves sugars).
- Confusing transpiration with respiration.
KEY TAKEAWAYS
- Transpiration is driven by solar energy (evaporation).
- Hydrogen bonding (cohesion) is essential for water transport.
- Stomata regulate the water-gas exchange trade-off.
- Xylem transport is passive.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): A scientist coats the bottom of a leaf with petroleum jelly, blocking the stomata. What happens to the rate of water transport?
Show Answer & Explanation
Answer: The rate will stop or decrease significantly.
Explanation: Transpiration is the 'engine' that pulls water up. If evaporation cannot occur at the stomata, the tension required to move the water column is lost.
Q2 (CONCEPTUAL): Which property of water allows it to move upward against gravity in a narrow tube?
Show Answer & Explanation
Answer: Adhesion and Cohesion (Capillary Action).
Explanation: Adhesion to the xylem walls and cohesion between water molecules create the pull needed to overcome gravity.
