- Water is transported through hollow, thick walled tubes called xylem vessels. The main force that pulls water through the xylem vessels in the stem of the plant is through transpiration.
- Transpiration is a passive process powered by the evaporation of water at the leaves by the sun
Movement of water out through Stomata
- The atmosphere that surrounds the stomata has a lower humidity compared to inside of the stomata.
- There is a water potential gradient from the air spaces through the stomata to the air.
- If the stomata are open, water vapour molecules diffuse out of the air in the plant out to the atmosphere
- Water lost by diffusion from the air space is replaced by water evaporating from the cell wall of the surrounding mesophyll cells.
- Changing the sizes of the stomatal pores, the plant is able to control the rate of transpiration
Movement of water across the cells of a leaf
- Water is lost through the mesophyll cells through evaporation from their cell walls
- This water loss creates an air space which is then replaced by water reaching the mesophyll cells from the xylem from both the cell wall and the cytoplasm
- The cytoplasmic route:
- Mesophyll cells lose water to the air spaces by evaporation from heat energy produced by the sun
- A lower water potential is formed and therefore water enters by osmosis from neighbouring cells
- The loss of water from these neighbouring cells lower their water potential which results in water being taken in through from their neighbour’s cell
- Water potential gradient is established that pull water from the xylem, across the leaf mesophyll and finally out into the atmosphere
Movement of water up the stem in the xylem
- Cohesion tension is the main driving force for the movement of water up a plant
- Water evaporation off of the mesophyll cells leading to transpiration
- Water molecules form hydrogen bonds between one another and hence tend to stick together; known as cohesion
- Water forms a continuous, unbroken column across the mesophyll cells and down the xylem; known as the transpiration pull
Evidence for cohesion tension theory
- The diameter of a tree trunk reduce during the night due to transpiration being at its lowest
- When the xylem vessels are broken, the air enters and the tree can no longer draw water up the tree, as the continuous water column has been broken
- Water does not leak out of a broken xylem vessel