Water Uptake

In Plants

1. Water is absorbed in the roots through osmosis
2. Water travels to the xylem through aplast (between cells) and symplast (through cytoplasm) pathways, reaching the xylem vessels in the middle of the root
3. The endodermis cells have a casparian strip between the cells, preventing substances from being able to pass through to the xylem without going through the cells (symplast pathway). This helps prevent toxins and other harmful substances from getting into the xylem and killing / damaging the plant
4. In some plants, the endodermis secretes mineral ions into the vascular tissue, reducing the water potential. This causes more water to move into the xylem, creating an upward pressure (root pressure), which can push water up a few meters.
5. Xylem vessels are continuous tubes of dead cells reaching from the roots to the leaves. They have ‘pits’ (small holes) in their exterior surface, which allow water molecules to move between xylem vessels, preventing a blockage from killing part of the plant
6. As xylem vessels are completely sealed, they do not allow air bubbles to form in the column of water. This means that, as a water molecule is removed at the top in the leaves, a water molecule is ‘sucked’ into the vessel at the bottom. This allows tall plants to have water above a few meters (adhesion-tension theory).
7. Water is lost in the leaves of the plant, during transpiration and respiration, mainly through stomata. During the day, stomata are open, however when water is not being used by photosynthesis, they close at night.