Thursday 30 April 2020
Describe the route taken by water from the soil up to the evaporating surface of a plant
Water is drawn into the root hair cells by osmosis; due to the presence
of dissolved substances in the cell sap of root hairs, the concentration of
cell sap is greater than that of the surrounding solution in the soil/concentration
gradient; this exerts a higher osmotic pressure, thus drawing the water
molecules across the cell wall and cell membrane into the root hair cells; more
water drawn into the root hair cells dilutes the cell sap; making it less
concentrated than that in the adjacent cortex cell of the root; due to osmotic
gradient, water moves from the adjacent cells to the next by osmosis; until it
enters the xylem vessels located in the center of the root; the xylem vessels
of the root then conduct the water up into the xylem vessels in the stem into
the leaves; there is a force in the roots which pushes water up the stem; this
force is known as root pressure; and can be considerably high in some plants;
energy from the endodermal cells of the root is responsible for driving this
force; in the xylem vessels, water would rise up by capillarity; to some extent
because the vessels are narrower and there is
a high attractive force between the water molecules and the cell walls;
the cohesive; and adhesive forces are important in the maintenance of a
continuous and uninterrupted water column in the xylem vessels up the tree to
the leaves; water vaporizes from the spongy mesophyll cells; their cell sap
becomes concentrated than the adjacent cells. This increases the osmotic
pressure of the spongy mesophyll cells; as a result of this, water flows into
the cell from other surrounding cell, which in turn takes in water from xylem
vessels within the leaf veins; this creates a pull/suction force that pulls a
stream of water from xylem vessels in the stem and roots. This force, known as
transpiration pull; helps in maintaining a continuous column of water from the
roots to the leaves; water flows from the midrib into leaf veins from where it
enters leaf cells; from the mesophyll cells, it enters the airspaces; then the
substomatal air chambers; from where it evaporates through the stomata; to the
atmosphere;
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