Transport of water

Why do plants need water

• Photosynthesis

• Transport soluble mineral ions, sugars

• Cool plants

• Turgor pressure: support, cell expansion

How are root hair cells

• Microscopic can penetrate soil particles easily

• Large SA:V thousands growing on each root

• Thin surface layer

• Concentration of solutes in cytoplasm of root hair cells maintains water potential gradient

What is cohesion

Attraction between water molecule due to H-bonds

What is adhesion

Force of attraction between molecules of two different substances

What is the apoplast pathway

Movement of water through the gaps between cellulose fibres in cell wall due to cohesive properties of water

What is the symplast pathway

Water moves by osmosis through plasma membrane into cytoplasm and from one cell to the next via plasmodesma

How does water move from the root to the xylem

Cells of the Endodermis surrounding the Xylem contain a Casparian strip made of Suberin, which is hydrophobic. This blocks the apoplast pathway and forces water and dissolved ions to move through the symplast pathway into the cytoplasm. Cell membranes contain carrier proteins that selectively allow ions such as Nitrate ion to enter preventing any toxic substances from entering xylem. Mineral ions are then actively transported by endodermal cells from the cortex into the xylem, lowering the water potential in the xylem so water enters by osmosis.

What is root pressure

Active transport of mineral ions into the Xylem lowers the water potential in the xylem. Water then enters the xylem from surrounding cells by osmosis. This creates a pressure that pushes water up the stem for a short distance (a few metres)

What evidence is there for root pressure

• Root pressure disappears if cyanide (a metabolic poison which prevents ATP synthesis) is added to roots

• If O₂ levels/ glucose levels fall so does root pressure so must involve respiration

• Guttation, sap forced out of pores in leaves, even at low transpiration

How are the properties of water related to its transport in stem

Cohesive properties means water molecules attract each other, so chains of water molecules are pulled up. Adhesive properties allows water molecules to stick to walls of xylem by capillary action. Good solvent so can dissolve assimilates to be transported in phloem