Explain how water moves up a plant via transpiration?
Plants transpire by the Cohesion Tension theory.
- Water diffuse out of the stomata, this creates tension in the xylem.
- Water is consequently pulled upwards as it replaces the water that is lost. It travels in a continuous "line" up the xylem due to the cohesion within water, this meaning the hydrogen bonds that form between each water molecule.
Additionally, water experiences adhesion to the xylem apoplast walls, this aids it's movement upwards.
- At the roots, their is root pressure, this is caused by the active transport of mineral ions into the root cells which results in water following and diffusing into the root by osmosis down a water potential gradient. Providing a plentiful supply of water to ensure a continuous flow.
What is transpiration?
Transpiration is the loss of water vapour from a plant, mainly through the stomata of the leaves.
How does water move up through a plant?
The cells around the xylem vessels in the root uses active transport to absorb mineral salts up a concentration gradient into the plant. This lowers the water potential in the xylem vessels. Water therefore passes from the living cells into the xylem vessels by osmosis and flows upwards
Water tends to move up inside very narrow tubes due to the interactions between water molecules and the surfaces of the the tube. If the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container wall act to propel the liquid.
When water leaves the plant by transpiration, it creates a negative pressure ( suction ) on the water to replace the lost amount of water. It is like your typical straw when you suck on it. This negative pressure on the water pulls the entire column of water in the xylem vessel. This negative pressure due to transpiration is known as "transpiration pull". It is the main force in drawing water and mineral salts up through the plant.
Hence, these 3 factors work together hand in hand to move water up a plant.
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