CAP5 - transpiration and cohesion tension theory

The loss of water from the stomata by evaporation.

Light intensity, wind speed, humidity temperature

Posiitve correlation - more light means more stomata open to let co2 in for photosyntsis = larger sa for evaporation

Negative corelation more water vapour in the air will increase the water potewntial outisde of the leaf, reducing the water potential gradient between air and leaf

posiitve correlation - more wind will blow away humid air containing water vapour, maintaining wp gradiengt

• positive correlation more heat energy = more kinetic energy, molecules move fatsre so quicker evaporatipn

• water molecules are polar (S+/-) • can therefore form hydrogen bonds with other water molecules • thiscreates cohesion with water molecules - they stick togther and travel up the xylem as a continuous column of wtaer

when water sticks to other molecules by forming hydrogen bonds - sticks to the xylem walls column pulling upwards causes xylem to become longer and narrower, increasing the sa for water to stick to, moves up the tube faster and easier

water evaporates from the surface of mesophyll cells into the air spaces in the leaf and then diffuses out of the leaf via stomata - transpiration

• as water is lost the WP of the mesophyll cells decreases, • water is drawn in from neighbouring cells by osmosis, down a wp gradient • this creates a pull on the water column in the xylem vessels • this pull creates tension in the xylem, drawing water up from the roots

• water molecules stick together due to cohesion formed by hydrogen bonding, so water travels up the xylem as a continuous column

at the same time, adhesion between molecules of water and the lignified walls of the xylem helps support tis column and prevent it from collapsing under tension