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Teacher: What is the formula for water? Student: H, I, J, K, L, M, N, O Teacher: That's not what I taught you. Student: But you said the formula for water was...H to O. Water cycling in ecosystems. What’s water potential?.

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Water cycling in ecosystems

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Water cycling in ecosystems

Teacher: What is the formula for water?Student: H, I, J, K, L, M, N, OTeacher: That's not what I taught you.Student: But you said the formula for water was...H to O.

Water cycling in ecosystems

What s water potential

What’s water potential?

  • Water potentials (ψ) are a way of measuring the free-energy (G) of water. Water will flow spontaneously from a high potential to a low potential, like a ball rolling down a hill.

  • What forces cause water to move?

    • Pressures

      • Gravity

      • Forces created by organisms (turgor pressure)

    • Osmotic gradients (concentration of solutes)

Water cycling in ecosystems

  • Water Cycle:

  • These forces also operate in the hydrologic (water) cycle.

  • Remember in the hydrologic cycle that water runs downhill (likewise it falls from the sky, to get into the sky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle)

What s the deal with water potential

What’s the deal with water potential?

  • Plants need water (as a solvent, chemical reactant in hydrolysis, for cell shape, to counteract lost water from transpiration-necessary for CO2 uptake and photosynthesis)

  • The water potential at any given point, ψ, is the combined effect of all the factors that make water move.

  • Water potential is usually negative so it moves from -1 to -4 (high to low).

What factor s affect water potential

What factor’s affect water potential

  • Its concentration (chemical potential): water will diffuse from a dilute solution to concentrated solution (osmosis).

  • Its pressure (mechanical potential): water will from from a high pressure system (hose-pipe) to a low pressure system (vacuum).

  • Its height (gravitational potential): water will flow downhill.

  • Its charge (electrical potential): water is uncharged, so we can ignore this.

  • These factors sum together to give the overall free energy of a mass of water.

Water cycling in ecosystems

Plant-available water:

Difference between field capacity and permanent

wilting point

  • Coarse FineParticle size

Components of water potential

Components of Water Potential

  • Pressure potential (ψp).

    • Difference between pressure of the water and that of the atmosphere. Most healthy cells exhibit a slightly + pressure potential due to the outward push of water (within vacuoles & cytoplasm) on the membrane or cell wall= turgor pressure

  • Osmotic pressure (solute potential) (ψs).

    • Affect of concentrations of solutes inside vs outside the cell that influence osmosis

  • Matric potential (in soil)

    • Affect of adhesion sticking water to soil surfaces. Measures how likely it is water becomes “unstuck”

  • Vapor potential

  • Gravitational potential

Pressure potential

Pressure Potential

  • Turgid cell (left) have a high positive pressure potential (the water presses out on the cell wall, and the wall presses back on the water with an equal but opposite force).

  • Cells that are flaccid or plasmolysed (right) have a pressure potential of zero.

  • We assume a pressure potential of zero when measuring water potential in our experiment.

Osmotic solute potential

Osmotic/ Solute Potential

  • ψs = − C R T

  • C, concentration of solutes (M).

  • R, Universal gas constant (8.314472 J K−1 mol−1).

  • T, absolute temperature (K).

    • Higher temp means more rapid molecular movt. More rapid osmosis.

  • Considered primary factor affecting our experiment, often critical in living systems,

  • Solute potential of pure water is 0

Matric potential

Matric Potential

  • Not used in our study b/c these interactions are more common in soils

  • Adhesive forces between water and soil particles influence the “readiness” of water to leave soil pores.

  • As soil becomes drier matric potentials increase & water is less likely to flow into roots.

Gravitational potential and vapor potential

Gravitational Potential and Vapor potential

  • Their effects are negligible at the cellular level within a small plant.

  • Gravitational potential explains why condensed water falls as rain or precipitation.

    • Higher up you go, the higher the gravitational potential

    • Can also be a factor in moving water up tall trees

  • Vapor potential -don’t worry about it, except that it becomes more important when explaining water movement from leaves--> atmosphere



  • In plant cells (and animals) equilibrium is achieved when water potential in/out of cell are same:

  • Concentration of solutes inside and out of cell are equal (facilitated largely by osmosis)

  • Pressures are same (of water inside and outside cell)

  • For working, growing plants most cells are in dynamic (every changing) equilibrium…in this way they can change with environmental conditions and move water and nutrients as needed.

Water cycling in ecosystems

Water moves along a pressure gradient

Transpiration is a major driving force

Transpiration is a major driving force

  • Dry warm air has low water potential & pulls water from cells with higher potential.

  • Water is lost under these conditions when pores on the leaf surface are open for gas exchange for photosynthesis

  • Water moves in continuous column from soil particles to leaf cells

  • Plant spends no energy in transporting water

    • Passive transport driven be transpiration

What about capillary action

What about Capillary Action?

  • Cohesive and adhesive forces of capillary action help HOLD water in plant tissues

  • Can only move water against gravity ~1/2 meter on their own.

  • Capillary action also explains how water is held in soils

What about vessel size

What about vessel size?

  • Transpiration causes the difference in water potentials necessary to move large quantities of water from root to leaf in large plants.

  • Flow rate increases in proportion to capillary radius (larger is faster)

    • 45 meters per hour vs. 6 per hour

    • Larger vessels = larger risk. Water columns break as they carry larger volumes. The drier the plant becomes the more frequent the breaks.

    • Most plant have large and small vessels (xylem)

Root pressure

Root Pressure

  • Differences in concentrations of salts in soil or sugars in roots can influence water flow.

  • In spring, sugar concentration is high in roots. This difference in concentration increases osmotic potential in cells.

  • Water can then “push” into the roots and fill empty vessels from last season or new growing tissue. The vessels must be full before cohesion and transpiration can cause a steady pull of water up the plant.

Water potential and water flow in plants in a nutshell

Water potential and water flow in plants in a nutshell

  • Water moves from one part of the plant to another down a water potential gradient. Different components of ψ are important at different stages.

  • Soil to roots: matric potential.

  • Roots to stems: pressure potential.

  • Stems to cells: osmotic potential.

  • Cells to stomata: vapor potential.

  • Where did our lab focus?

Water cycling in ecosystems

Precipitation (Yg)

Overland flow

(erosion, nutrients)




Water cycling in ecosystems


Evaporation from

leaf surfaces

Transpirational water lossto the atmosphere

Evaporation from


Water transport through the plant

Water uptake by

plant roots

Water cycling in ecosystems


Yt = - 30 MPa


Yt = - 1.5 MPa

Surface roots

Yt = - 1.1 MPa

Water in the soil-plant-atmosphere continuum J = L (Dyt/l)

What happens

at night?

Water moves along a gradient of decreasingwater potential

Stomata close – which term doesthis affect?

How would waterpotential gradientrespond?

Surface soil

water Yt = - 0.8 MPa

Relatable topics

Relatable topics

  • You should be able to use water potential in discussions regarding:

  • Photosynthesis and transpiration

  • The water cycle

  • The chemical importance of water to life on earth

  • Plant physiology

  • Cell homeostasis and water balance

  • Membrane function and cell wall function

  • Plant evolution and adaptations of higher (vascular plants)

  • Form and function of membranes and vacuoles

  • Kinetics, free energy change

  • Diffusion/ Osmosis

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