Biology Unit 2 HelpWatch
Sorry if this is in the wrong section!
I am wanting some help for BIOL2, can someone explain to me how water moves through a plant? Im really confused and the text book isn't helping me.
Need to know how Ions, Active Transport, Cohesion, Cohesion Tension, Apoplastic and Simplastic pathways are all linked in.....
- Study Helper
The Symplastic Pathway is via cytoplasm of the cells in the root. Water is absorbed into
the root hair cells by osmosis, since the cells have a lower water potential that the water in the soil.
Water then diffuses from the epidermis through the root to the xylem down the water potential gradient.
The cytoplasms of all the cells in the root are connected by plasmodesmata through holes in the cell
walls, so there are no further membranes to cross until the water reaches the xylem, increasing transport speed.
The Apoplastic Pathway is via the cell walls between cells. The cell walls are thick and very
open, so water can simply diffuse through cell walls down the water potential gradient. However the apoplast pathway stops at the endodermis because of the waterproof casparian strip, which seals the cell walls. At this point water has to cross the cell membrane by osmosis and enter the symplastic route. This allows the plant to have some control over the uptake of water into the xylem. Around 90% of water transport through the root uses the apoplast pathway, as the available volume is greater.
The uptake of water by osmosis actually produces a force that pushes water up the xylem. This force is
called root pressure. This helps push the water a few centimetres up short and young
stems, but is nowhere near enough pressure to force water up a long stem or a tree. Root pressure is the
cause of guttation, sometimes seen on wet mornings, when drops of water are forced out of the ends of
The xylem vessels form continuous pipes from the roots to the leaves. Water can move up through these
pipes at a rate of 8m h-1 (2 mm s-1), and can reach a height of over 100m. Since the xylem vessels are dead, open tubes water moves by mass flow. The driving force for the movement is transpiration in the leaves. This causes low pressure in the leaves, so water is sucked up the stem to replace the lost water. The column of water in the xylem vessels is therefore under tension (a stretching force). Water has a high tensile strength due to the tendency of water molecules to stick together by hydrogen bonding (cohesion), so the water column does not break under the tension force. This mechanism of pulling water up a stem is sometimes called the cohesion-tension mechanism. The very strong lignin walls of the xylem vessels stops them collapsing under the suction pressure, but in fact the xylem vessels (and even whole stems and trunks) do shrink slightly during the day when transpiration is maximum.