water potential

Examples of water potential in the following topics:

• Water and Solute Potential

  • Water potential is the measure of potential energy in water and drives the movement of water through plants.
  • Water potential is critical for moving water to leaves so that photosynthesis can take place.
  • Water potential is a measure of the potential energy in water, or the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature).
  • When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential.
  • Solutes reduce water potential (resulting in a negative Ψw) by consuming some of the potential energy available in the water.

• Pressure, Gravity, and Matric Potential

  • Water potential is affected by factors such as pressure, gravity, and matric potentials.
  • This increases water potential between the water in the the petiole (base of the leaf) and in the leaf, thereby encouraging water to flow from the petiole into the leaf.
  • The force of gravity pulls water downwards to the soil, which reduces the total amount of potential energy in the water in the plant (Ψtotal).
  • The binding of water to a matrix always removes or consumes potential energy from the system.
  • When (a) total water potential (Ψtotal) is lower outside the cells than inside, water moves out of the cells and the plant wilts.

• Transportation of Photosynthates in the Phloem

  • The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient.
  • The high percentage of sugar decreases Ψs, which decreases the total water potential, causing water to move by osmosis from the adjacent xylem into the phloem tubes.
  • This flow of water increases water pressure inside the phloem, causing the bulk flow of phloem sap from source to sink.
  • This reduces the water potential, which causes water to enter the phloem from the xylem.
  • Transpiration causes water to return to the leaves through the xylem vessels.

• Movement of Water and Minerals in the Xylem

  • Transpiration aids in the movement of water and minerals in the xylem, but it must be controlled in order to prevent water loss.
  • It is the main driver of water movement in the xylem.
  • Water from the roots is pulled up by this tension.
  • The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere.
  • Evaporation from the mesophyll cells produces a negative water potential gradient that causes water to move upwards from the roots through the xylem.

• Types of Energy

  • The various types of energy include kinetic, potential, and chemical energy.
  • Other examples of potential energy include the energy of water held behind a dam or a person about to skydive out of an airplane.
  • This type of potential energy is called chemical energy, and like all potential energy, it can be used to do work.
  • Water behind a dam has potential energy.
  • Moving water, such as in a waterfall or a rapidly flowing river, has kinetic energy.

• Introduction to Osmoregulation

  • Doctors typically recommend drinking eight to ten glasses of water a day.
  • A cell immersed in plain water tends to swell as water diffuses in from the hypotonic or "low salt" solution .
  • Complex multicellular animals exchange water and nutrients with the environment by consuming food and water, and by excreting sweat, urine, and feces.
  • When disease or injury damage the mechanisms that regulate osmotic pressure, toxic waste or water may accumulate, with potentially dire consequences.
  • In a hypotonic environment, cells tend to swell due to intake of water.

• Water’s Cohesive and Adhesive Properties

  • When a small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser (the mass per unit volume) than the water.
  • Adhesion is observed when water "climbs" up the tube placed in a glass of water: notice that the water appears to be higher on the sides of the tube than in the middle.
  • Without these properties of water, plants would be unable to receive the water and the dissolved minerals they require.
  • In another example, insects such as the water strider use the surface tension of water to stay afloat on the surface layer of water and even mate there.
  • Water's cohesive and adhesive properties allow this water strider (Gerris sp.) to stay afloat.

• Water’s Solvent Properties

  • Water's polarity makes it an excellent solvent for other polar molecules and ions.
  • The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules.
  • When ionic compounds are added to water, individual ions interact with the polar regions of the water molecules during the dissociation process, disrupting their ionic bonds.
  • Water is a poor solvent, however, for hydrophobic molecules such as lipids.
  • This change in the hydrogen-bonding pattern of the water solvent causes the system's overall entropy to greatly decrease, as the molecules become more ordered than in liquid water.

• Water’s Polarity

  • While there is no net charge to a water molecule, the polarity of water creates a slightly positive charge on hydrogen and a slightly negative charge on oxygen, contributing to water's properties of attraction.
  • As a result of water's polarity, each water molecule attracts other water molecules because of the opposite charges between them, forming hydrogen bonds.
  • A polar substance that interacts readily with or dissolves in water is referred to as hydrophilic (hydro- = "water"; -philic = "loving").
  • Oil and water do not mix.
  • As this macro image of oil and water shows, oil does not dissolve in water but forms droplets instead.

• The Water (Hydrologic) Cycle

  • More than half of the human body is made up of water, while human cells are more than 70 percent water.
  • However, when examining the stores of water on earth, 97.5 percent of it is non-potable salt water.
  • Of the remaining water, 99 percent is locked underground as water or as ice.
  • This leads to the evaporation (water to water vapor) of liquid surface water and the sublimation (ice to water vapor) of frozen water, which deposits large amounts of water vapor into the atmosphere.
  • Only 2.5 percent of water on earth is fresh water.