23.2: Transport of water through xylem

Cohesion

Tendency for the same type of molecules to stick together due to intermolecular forces.

Hydrogen bonds in water

Oxygen atoms with partially negative charge in one water molecule can bind to a hydrogen atom with a partial positive charge in another water molecule. Each molecule can form up to 4 bonds

Surface Tension

Stronger attraction between water molecules due to hydrogen bonds creates a pull inwards generating surface tension = water surface can resist rupture

Apoplast

between the cell wall and plasma membrane - movement is rapid and unregulated as there is no selective barrier to cross

Symplast

Interconnected cytoplasm via plasmodesmata - movement is slow and regulated due to solutes having to cross cell membrane

Plasmodesmata

Membraned-lined tunnels that traverse the cell wall connecting cytoplasm of adjacent cells- equivalent to gap junctions in animal cells

Casparian Strip

layers of water impermeable suberin in cells of endodermis surrounding stele = prevents water from entering from apoplast = forces solutes to enter symplast before entering stele

Endodermis

Inner layer of root cortex where the Casparian strip is

Xylem

Composed of long tubular vessels of dead cells either vessel elements or tracheids

Tracheids

Spindle shaped with pits in secondary wall which water can move through with little resistance from one trachea to neighbours

Vessel Elements

Have larger diameter than tracheids and meet end to end and partially breakdown end walls to form an open pipeline for water conduction

why is transpiration a driving force for water movement in xylem

Water lost from leaves due to evaporation from leaf surface creates a negative pressure that pulls water upward through the xylem.

Transpiration-Cohesion Hypothesis

1. transpiration causes water loss

2. creates tension (negative pressure)

3. water pulled up through xylem with cohesion and adhesion allowing for a continuous water column from roots to leaves

Bulk Flow

Movement of solution from region of higher pressure potential to lower pressure potential

why are water molecules cohesive

ability to form hydrogen bonds = they stick together and create surface tension.

2 pathways of water and ions from soil to xylem

apoplast and symplast

why is xylem reinforced by lignin

to withstand the negative pressure during water transport.