Water Transport in Plants
Water enters a plant through its root hair cells. Water has to get from the soil to the root and into the xylem. The root is covered in root hairs which increases the surface area speeding up water uptake. Water moves from a high water potential (soil) to a low water potential (root). This maintains a gradient.
Water travels through the apoplast pathway (non living parts of the cells) through the cell walls. However when the water gets to the endodermis its path is blocked by a waxy strip called the casparian strip so the water has to take the symplast pathway. The symplast pathway (living parts of the cells) goes through the cytoplasm which is good as it has to go through the cell membrane which controls what enters and leaves the cell. The cytoplasm of neighbouring cells are connected via plasmodesmata. Once past the barrier water moves into the xylem.
Cohesion and tension help water move up to the leaves. Water transpires from the leaves which creates a tension and pulls more water into the leaves. Water molecules are cohesive so when some are pulled up the rest follow so all the water in the xylem moves up. Adhesion is when the xylem also sticks to the water molecules and gets pulled in slightly. Root pressure also helps water move upwards. When water is transported into the xylem from the roots it creates a pressure that pushes more water up.
Transpiration is the evaporation of water from a plants surface. Water evaporates from the cell walls and accumulates in the spaces between the cells in the leaf. When the stomata open it moves out of the leaf down the concentration gradient.
Four main factors affect transpiration rate
- Light - the lighter it is the faster the transpiration rate, stomata open in the light
- Temperature - the higher the temperature the faster the transpiration rate. The water molecules will have more kinetic energy and increases the concentration outside the leaves.
- Humidity - the lower the humidity the faster the transpiration rate as there will be a higher concentration gradient.
- Wind - the windier it is the faster the transpiration rate lots of air movement blows the water molecules away which increases the concentration gradient.