how xylem are able send the water upwards which was observed by root hairs explain it with an experiment
Answers
Explanation:
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Answer:
Root pressure pushes water up
Capillary action draws water up within the xylem
Cohesion-tension pulls water up the xylem
We’ll consider each of these in turn.
Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil. Water moves into the roots from the soil by osmosis, due to the low solute potential in the roots (lower Ψs in roots than in soil). This intake of water in the roots increases Ψp in the root xylem, driving water up. In extreme circumstances, root pressure results in guttation, or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not strong enough to move water up the height of a tall tree.
Capillary action or capillarity is the tendency of a liquid to move up against gravity when confined within a narrow tube (capillary). Capillarity occurs due to three properties of water:
Surface tension, which occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water.
Adhesion, which is molecular attraction between “unlike” molecules. In the case of xylem, adhesion occurs between water molecules and the molecules of the xylem cell walls.
Cohesion, which is molecular attraction between “like” molecules. In water, cohesion occurs due to hydrogen bonding between water molecules.
On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree.
This video provides an overview of the important properties of water that facilitate this movement:
Water potential is a measure of the potential energy in water, specifically, water movement between two systems. Water potential can be defined as the difference in potential energy between any given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψpure H2O) is designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are expressed relative to Ψpure H2O.
The water potential measurement combines the effects of solute concentration (s) and pressure (p):
Ψsystem = Ψs + Ψp
where Ψs = solute potential, and Ψp = pressure potential. Addition of more solutes will decrease the water potential, and removal of solutes will increase the water potential. Addition of pressure will increase the water potential, and removal of pressure (creation of a vacuum) will decrease the water potential.
Water always moves from a region of high water potential to an area of low water potential, until it equilibrates the water potential of the system. At equilibrium, there is no difference in water potential on either side of the system (the difference in water potentials is zero). In order for water to move through the plant from the soil to the air (a process called transpiration), Ψsoil must be > Ψroot > Ψstem > Ψleaf > Ψatmosphere.
Let’s consider solute and pressure potential in the context of plant cells:
Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water, because solutes reduce water potential to a negative Ψs. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content. Because of this difference in water potential, water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential. Plant cells can metabolically manipulate Ψs by adding or removing solute molecules.
Negative water potential continues to drive movement once water (and minerals) are inside the root; Ψ of the soil is much higher than Ψ or the root, and Ψ of the cortex (ground tissue) is much higher than Ψ of the stele (location of the root vascular tissue). Once water has been absorbed by a root hair, it moves through the ground tissue through one of three possible routes before entering the plant’s xylem:
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