Transport mechanisms

The apoplast pathway involves water moving through the non-living spaces in the cell walls (cellulose) and intercellular spaces. Lignin in xylem walls also contributes to this pathway, providing structural support.

The Casparian strip, made of suberin, is an impermeable layer in the endodermis cell walls. It forces water and ions to enter the symplast pathway, allowing the plant to control the uptake of minerals.

Transpiration is the evaporation of water from leaves followed by diffusion of water vapor into the atmosphere. This creates a tension, pulling water up the xylem in a continuous stream from the roots.

Hydrogen bonding between water molecules causes cohesion (water molecules sticking together) and adhesion (water molecules sticking to xylem walls). Cohesion-tension from transpiration pulls water upwards, while adhesion helps counter gravity.

Xerophytes often have features such as a thick waxy cuticle to reduce evaporation and sunken stomata to trap moist air and reduce the water potential gradient.

Proton pumps in companion cells actively transport H+ ions out of the cell, creating a proton gradient. This gradient drives the cotransport of sucrose into the companion cells, and then into the phloem, against its concentration gradient.

Mass flow is the movement of assimilates down a hydrostatic pressure gradient in phloem. High concentration of assimilates at the source lowers water potential, causing water to enter and increase pressure. Lower concentration at the sink results in water leaving, decreasing pressure.