process of transpiration

define what transpiration is

The main force that pulls water through the xylem vessel in the stem of a plant is the evaporation of water from the leaf

how does transpiration occur is simple format

• energy from the sun

• causes water to evaporate from spongy cells and diffuse out of the leaves

• which decreases the water potential in the leaf

• so water is sucked up xylem by mass flow

• which decreases the water potential in the root xylem

• so water diffuses through the root cells to the xylem

• which decreases the water potential in the root epidermis cells

• so water diffuses into root hair cells from soil by osmosis

movement of water out through stomata

the humidity of the atmosphere is usually less than the air spaces next to the stomata. As a result there is a water potential gradient from the air spaces through the stomata to the air. Provided the stomata are open water vapour molecules diffuse out of the air space into the surrounding air. water lost by diffusion from the air space is replace by water evaporating from the cell walls of the surrounding mesophyll cells. by changing the size of the stomatal pores plants can control their rate of transpiration

movement of water across the cell of a leaf

mesophyll cells loose water to the air space by evaporation due to heat supplied by the sun

these cells now have a lower water potential and so water enters by osmosis from neighbouring cells

the loss of water from these neighbouring cells lowers their water potential

they in turn take water from their neighbours by osmosis

movement up the stem up the xylem

water evaporates from mesophyll cells due to heat from the sun leading to transpiration

water molecules form hydrogen bonds between one another and hence tend to stick together this is known as cohesion

water forms a continuous unbroken column across the mesophyll cells and down the xylem