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

How do tall trees get enough water? Redwoods are the tallest species of tree in the world. How does a plant this size get water from its roots to the branches at the top? What experiment could show that water travels up stems? The tallest redwood ever measured was 120 metres tall. That is six times the height of the Angel of the North! The water will travel up the stem, or trunk, of the tree.

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

Water movement out of stomata

Opening and closing of stoma

Transport of water up the xylem There are 3 pathways: Root pressure (-minor) Capillarity (-minor) Cohesion tension (major)

Root pressure is caused by the mineral ions which are actively transported into xylem vessels in the root by endodermal cells. This makes the water potential of the xylem more negative and causes water to enter the xylem by osmosis. Capillarity is responsible for some water creeping up the xylem vessels simply because they are very narrow.

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 When water evaporates from a leaf mesophyll cell, this cell’s water potential (WP) will become more negative. Water from an adjacent cell will then move into this cell by osmosis as a result of the WP difference between them. This ‘chain’ of WP differences continues back to the xylem sap. Water moves smoothly and continuously along this WP gradient. The continual movement of water from the roots to the leaves is often called the Transpiration Stream. This can be thought of as the movement of water from a less negative to more negative water potential. Water moves from less negative in soil to the more negative air surrounding leaves. This provides a very steep gradient from soil to air and is one of the driving forces of the transpiration stream.

Cohesion-tension theory

Putting it all together Transport in xylem transpiration – pathways transpiration – pathways Private life of plants - xylem, phloem.wmv

Extension and Homework 1.AQA AS Biology textbook pg 195 Summary question 1 Application questions 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; ORSap 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]

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