AQA Biology - Topic 13.8 - Movement of water up stems

Firstly - apologies that these blogs will not be in order, I realise that this may be confusing if you are looking for a particular topic, I am just posting in the order that I am using for myself.

MOVEMENT OF WATER UP STEMS

The main force that pulls water up the stem of a plant is evaporation of water from leaves - TRANSPIRATION.

Movement Of Water Out Through Stomata

• Humidity of atmosphere is usually less than that of air spaces next to stomata.
• When stomata are open, water vapour molecules diffuse out of air spaces into surrounding air.
• Changing the size of stomatal pores means plants can control their rate of transpiration.

 

Movement Of Water Across The Cells Of A Leaf

• Water is lost from mesophyll cells by evaporation from their surfaces to the air spaces.
• This is replaced by water from xylem by apoplastic or symplastic pathways.
• In symplastic pathway, water movement occurs because:

         -Mesophyll cells lose water to air spaces.

         -These cells now have lower water potential so water enters by osmosis from neighbouring cells

         -This loss of water lowers those cells water potential.

         - In turn, they take water from neighbours by osmosis.

A water potential gradient is established that pulls water from the xylem, across the mesophyll and into the atmosphere.

Movement of water up the stem in the xylem

The two main factors responsible for the movement of water up xylem, from roots to leaves, are cohesion-tension and root pressure.

Root pressure is explained in topic 13.7.

The cohesion-tension throry:

-Water evaporates from leaves as a result of transpiration.

-Water molecules form hydrogen bonds between one another (cohesion)

-Water forms a continuous, unbroken pathway across mesophyll cells and down xylem.

-As water evaporates from mesophyll cells to air spaces beneath stomata, more molecules of water are drawn up behind it as a result of this cohesion. (Imagine sucking water up a straw!!)

-Water pulled up xylem as a result of transpiration pull.

-Transpiration pull puts xylem under tension - there is negative pressure within the xylem.

Transpiration pull can raise water 100m or more up trees.

Evidence for cohesion-tension theory:

• Diameter of tree trunks change according to the rate of transpiration. During the day (transpiration greatest), there is more tension (more negative pressure) in the xylem - this causes the trunk to shrink in diameter.
• At night (transpiration lowest), less tension in xylem - trunk diameter increases.
• If a xylem vessel breaks and air enters, the tree can't draw up water as the continuous column of water is broken so molecules no lobger stick together.
• When a xylem vessel is broken, water doesn't leak out as it would if it were under pressure, instead, air is drawn in - consistent with it being under tension.

-Transpiration pull is passive so does not require energy for it to take place.

-The xylem vessels through which water passes are dead so cannot actively move water.

-As they are dead, their end walls can break down, meaning xylem forms a series of continuos, unbroken tubes from roots to leaves, which is essental to the cohesion-tension theory.

-Energy is nevertheless needed to drive transpiration - this energy comes from the sun.

Any questions - just ask! :)