9.1.2 The Transpiration Stream

• The transport of water occurs in xylem vessels, one of the vascular tissues found within plants
• The cohesive property of water, together with the structure of the xylem vessels, allows water to be transported under tension from the soil to the leaves

Cohesion between water molecules

• Within water molecules the oxygen atom has a slight negative charge while the hydrogen atoms have a slight positive charge; this difference in charge across the molecule means that water is a polar molecule
• As a result of the polarity of water, hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules
  • This force of attraction between water molecules is known as cohesion
• Water molecules are also attracted to the hydrophilic surface of the cell walls on the interior of xylem vessels
  • This attraction between molecules of a different type is known as adhesion

Xylem vessels

• Xylem vessels are formed from long lines of cells that are connected at each end
• As the xylem vessels develop, the cell walls between the connected cells degrade and the cell contents are broken down
  • This forms mature xylem vessels that are long, continuous, hollow tubes
  • Mature xylem vessels are non-living cells
• The walls of xylem vessels are thickened with cellulose and strengthened with a polymer called lignin
  • This means xylem vessels are extremely tough and can withstand very low internal pressures, i.e. negative pressure caused by suction, without collapsing in on themselves

Cohesion between water molecules and adhesion of water molecules to xylem cell walls allow water to be pulled up xylem vessels in one continuous column

• When water evaporates from the surfaces of cells inside a leaf during transpiration, more water is drawn from the nearest xylem vessels to replace the water lost by evaporation
  • Water molecules adhere to the cell walls of plant cells in the leaf, enabling water to move through the cell walls
    • e.g. water moves through the cell walls of the xylem into other cells of the leaf
  • The loss of water from the xylem vessels generates low pressure within the xylem 
• The low pressure generated in the xylem when water moves into the cells in the leaves creates a pulling force throughout the xylem vessels that is transmitted, via cohesion between water molecules, all the way down the stem of the plant and to the ends of the xylem in the roots
  • This is known as transpiration-pull and it allows water to be moved upwards through the plant, against the force of gravity
  • This is sometimes known as the cohesion-tension theory of transpiration
• At the same time, forces of adhesion between the water molecules and the walls of the xylem vessels ensure that there are no air bubbles or spaces inside the xylem vessels 
• This continuous upwards flow of water in the xylem vessels of plants is known as the transpiration stream

The movement of water through xylem vessels is due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules

NOS: Use models as representations of the real world; mechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues

• Models are often used to study complex living systems which may be too complicated to observe in reality, or to demonstrate the mechanisms behind the processes that take place inside living systems
• It is possible to model the mechanisms involved in water transport in the xylem using simple apparatus including
  • Blotting paper or filter paper
  • Porous pots
  • Capillary tubing

Blotting or filter paper

• Blotting paper or filter paper has the ability to absorb water
• When a piece of blotting paper or filter paper is placed with the end touching the water, the water will rise up along the length of the paper 
  • Adhesive and cohesive forces cause water to be drawn into and through the paper
• This can be compared to the movement of water up a xylem vessel
• A strength of this model is that, like xylem vessels, the paper is made of cellulose
  • Adhesion occurs between water molecules and the interior cellulose surface of xylem vessels

Water transport in the xylem can be modelled using blotting paper or filter paper

Porous pots

• A porous pot is a partially permeable container made from a material full of microscopic pores through which water can pass
  • An example of a material containing these pores is terracotta clay that has been fired
• Porous pots can be used to model the evaporation of water that occurs from the leaves of a plant
  • If the porous pot is filled with water, adhesion between water molecules and the material of the pot occurs, causing water to fill the pores within the material, from where it can evaporate
    • This is similar to the way that water molecules adhere to cell walls inside a leaf
  • If the porous pot is connected to a water filled glass tube that is also dipped in a container of water, the negative pressure created by water evaporating from the surface of the pot, together with cohesion, causes water to be drawn up the glass tube, just as it would to form the transpiration stream within a xylem vessel

Water transport in the xylem can be modelled using porous pots

Capillary tubing

• When a very narrow glass tube known as a capillary tube is dipped into water, water can flow up the tube despite the opposing force of gravity purely due to the adhesion of water molecules with the inner surface of the tube and cohesion between water molecules
  • This phenomenon is known as capillary action
  • The thinner the tube, the higher the water will rise
• Although the movement of water up xylem vessels is also driven by evaporation of water from the cells of the leaf, which is not reflected by this model, and xylem vessels are much thinner than a capillary tube, this model still shows how important adhesion and cohesion are in the transport of water up xylem vessels
• The fact that water has these properties can be further demonstrated by
  • Dipping one capillary tube in dyed water and another capillary tube into liquid mercury
    • As adhesion does not occur between mercury and the glass of the capillary tube, and there is no cohesion between mercury atoms, capillary action in this tube cannot occur and mercury does not rise up the tube

Modelling the mechanisms involved in water transport in the xylem using capillary tubing