Transpiration and cohesion - tension theory

https://www.youtube.com/watch?v=fKbi1AJJzxU

Job of waxy cuticles

Reduce water loss from surface of leaf by evaporation

Whats transpiration

The loss of water vapour from a plants surface, throught the stomata by evaporation and diffusion

What does transpiration provide plants with water for

Cooling - loss of water by evaporation

Photosynthesis

Turgor pressure- support provides hydrosatic skeleton, roots enavles plants to force way through firm materials

Movement of minerals

What is root pressure

Water is pushed up the xylem by hydrostatic pressure

Mineral salts are pumped into the xylem vessels in the root by the endodermal cells which lowers water potential in xylem

Water moves in from the surrounding cells by osmosis which raises the hydrostatic pressure so pushing water up the xylem

Whats capilarity

Water rises up narrow tubes due to the adhesive forces between the water molecules and wall of tube

Water rises high in narrower tubes, xylem vessels are very narrow

Whats cohesion - tension theory

The sun provides energy to pull the water up by providing energy for evaporation

Water moves up the xylem by mass flow from higher pressure in roots to lower pressure in leaves

The column of water does not break due to cohesive forces. H bonds between water molecules

Water also formes H bonds with carbohydrates in the walls of the xylem (adhesion)
The theoretical max height water can move this way is around 150m. the tallest trees are around 115 m

Explain cohesion tension hypothesis for movement of water up the xylem vessels

Transpiration : Water evaporates from the spongy mesophyll cells and diffuses into the atmosphere

Lower water potential in leaf cells, water moves from down the water potential gradeint

Water moves across root from soil down the water concentration gradient, via apoplast and symplast pathways