Biology: Chapter 7: Transport in plants: Transpiration
Transpiration
- Transpiration: Mesophyll cells in leaf have many air spaces between them filled with water vapor - Walls of the mesophyll cells are wet, and the water evaporates into these air spaces.
- Air inside and outside of the leaf has direct contact through the stomata. If the water potential outside the cell is lower, then water diffuses out through the stomata.
- Some water is used for photosynthesis but most is lost through evaporation this way.
- When water evaporates out of the leaf more water is drawn to replace it, which comes from the xylem vessel.
- The water moves down a concentration gradient from cell to cell in two possible pathways:
- Apoplastic: Without entering the cells, water travels between the cell walls.
- Symplastic: Water travels cell to cell via plasmodesmata.
- Made from cells joined end to end.
- Vessel elements and tracheids: Involved in transport of water.
- Sclerenchyma fibers: Elongated cells with lignified walls to support plant.
- Dead at maturity.
- Vessel elements begins as a normal plant cell with lignin in its wall.
- As lignin builds around cell, cell and its contents die leaving a empty space inside the cell.
- Lumen: Empty space in the cell.
- Plasmodesmata of these original cells form gaps with no lignin in the cell walls. These form gaps in the xylem vessel wall.
- Pits: Gaps in the wall and throughout the xylem vessel. Made of freely permeable cellulose cell wall. Link with pits of neighboring cells so water can pass freely from one cell to the next.
- End walls break down and form a continuous tube.
- This continuous non-living tube is the xylem vessel and can be up to several meters long.
Transpiration
- When water leaves the xylem tube at the leaves it creates a lower hydrostatic pressure at the top of the tube than the bottom.
- The pressure causes the water to move up the tube in continuous columns.
- Strong lignified walls stop vessels from collapsing inwards due to pressure.
- Mass flow: Movement of water up the xylem vessels as a body of liquid, as a result of cohesion and adhesion.
- Cohesion: Attraction between water molecules.
- Adhesion: Attraction of water to cellulose and lignin in the xylem vessel.
- Small diameter and pits of the vessel help prevent air bubbles from forming, which would change the pressure and disrupt mass flow.
- Pits allow water to move out into neighboring living cells and prevent air locks.
