Understanding Water Movement in Plants: Transpiration and Xylem, Slides of Water Resources Planning and Management

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13.8 Movement of water

up stems

Learning outcomes

Students should be able to understand the following:

 What transpiration is

 The process by which water moves through a leaf

 How water moves up the xylem

Candidates should be able to:

 Describe the roles of root pressure and cohesion

tension in moving water through the xylem

Movement of water up a stem

Transpiration

 The loss of water from the leaves of a plant is called transpiration, and the resulting flow of water through the plant is called the transpiration stream. The transpiration stream is important because:

 it carries water for photosynthesis to the palisade cells in the leaves

 the water carries essential mineral salts in solution

 evaporation from the leaves has a cooling effect

Opening and closing of stoma

Transport of water up the xylem

There are 3 pathways:

 Root pressure ( -minor )

 Capillarity ( -minor )

 Cohesion tension ( major )

Cohesion-tension theory

 The mass flow of water through the xylem relies on 2 important properties of water:

 Cohesion – the water molecules tend to stick together

 Adhesion – the water molecules also tend to stick to the inside of the xylem vessels.

Cohesion-tension theory

Extension and Homework

1. AQA AS Biology textbook pg 195

Summary question 1

Application questions 1-

2. Complete the exam style question about

transpiration and cohesion-tension

Mark scheme - exam style question

(a) Suitable accepted evidence, 1 mark for evidence and 1 mark for explanation EITHER (only) upward pressure could force liquid water out of leaves; OR Sap exuding from a cut, rooted stem; (only) upward force could make this happen; 2 (b) (Note: max. two for any component) (i) Evaporation from leaves during daytime only/mainly; tension/negative pressure (on water) in xylem creates inward pull (on walls of xylem vessel); xylem vessels become narrower; due to adhesion of water molecules (to walls of xylem vessels); (ii) root pressure gives outward force/push on walls of xylem vessels; tree would become wider/stay same diameter; xylem vessels become wider/stay same diameter; max 3 [5]