How It Works: The Transpiration-Cohesion-Tension Theory
Part of Plant Transport Systems — GCSE Biology
This how it works covers How It Works: The Transpiration-Cohesion-Tension Theory within Plant Transport Systems for GCSE Biology. Xylem and phloem structure, water and sugar transport, root hair adaptations, translocation, and practical investigations It is section 9 of 17 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 9 of 17
Practice
19 questions
Recall
24 flashcards
How It Works: The Transpiration-Cohesion-Tension Theory
Water travels from roots to leaves without any pump — the plant uses physics alone. Here is the chain of events:
- Transpiration pull: Water evaporates from the surfaces of mesophyll cells inside the leaf. It then diffuses through air spaces and exits via open stomata. This continual loss creates a region of lower water potential at the top of the xylem.
- Tension: As water leaves the xylem at the top, it creates a negative pressure (tension) that acts like suction, pulling the water column upward.
- Cohesion: Water molecules are attracted to each other by hydrogen bonds — they "stick together." Because of this cohesion, the entire column of water moves as a continuous thread rather than breaking apart under tension.
- Adhesion: Water also sticks to the inner walls of xylem vessels. This adhesion prevents the water column from pulling away from the walls and helps support the column against gravity.
- Root uptake: As water is pulled out of root xylem, the lowered water potential draws more water in from soil by osmosis through root hair cells — maintaining a continuous stream.
Crucially, this entire process requires no metabolic energy from the plant. It is powered entirely by solar energy driving evaporation at the leaf surface.