Module 13: Water movement

What do plants require to perform photosynthesis?

CO2, H2O, and light within photosynthetic organs (leaves and stems).

How do plants move resources without a mechanical pump?

They utilize simple physical processes such as transpiration and movement of solutes to move water and nutrients

What is water potential (Ψ)?

The difference in potential energy between pure water and water in a system.

What are the components of water potential in plants?

Osmotic potential (from dissolved solutes)(Ψπ), pressure potential (from turgor and transpiration)(ΨP), matric potential (cohesive and adhesive forces of water)(Ψm), and gravitational potential (Ψg).

What is transpiration?

The loss of water vapor through the stomata of leaves.

What is turgor?

Pressure inside a plant cell

What is the formula for calculating water potential?

Ψ = Ψπ + ΨP + Ψm + Ψg. Adding up the water potentials that have an effect on plants

What drives water movement in plants?

Water moves from areas of higher to lower water potential. The gradient allows water to move from the soil through the plant and into the atmosphere.

What is the main driver of water movement in plants?

Tension created via transpiration.

What is the order of water potential from soil to atmosphere?

Ψsoil > Ψroot > Ψstem > Ψleaf > Ψatmosphere.

What are the five major forces that move water in plants?

Diffusion, osmosis, capillary forces, hydrostatic pressure, and gravity.

What is diffusion?

The movement of molecules in fluids from areas of higher to lower concentrations.

What is osmosis?

The diffusion of water across a differentially permeable membrane. In cells it creates hydrostatic pressure (turgor).

What are cohesion and adhesion in relation to water?

Cohesion is water molecules sticking to each other, while adhesion is water molecules sticking to hydrophilic molecules.

How do capillary forces affect water movement?

They enable water to be pulled up through narrow tubes due to cohesion and adhesion.

What is negative hydrostatic pressure?

large tension created by cohesion and adhesion that can flatten water and pull it into small spaces, such as between cells or soil particles

What role does transpiration play in water movement?

Transpiration creates a tension that drives water movement from the roots to the leaves.

Why are leaves nearly saturated with water vapor?

Intercellular air spaces in the leaves are near equilibrium with the solution in the cellulose

Where does water vapor in the leaf diffuse to?

To the atmosphere via an open stomata

How does the boundary layer affect transpiration?

A larger boundary layer results in a gentler gradient of water vapor and slower rate of diffusion. This prevents excessive diffusion.

Why is wind problematic?

It causes the boundary layer on leaves to be thinner and increases transpiration ( water loss)

What adaptations do plants have to prevent water loss due to wind?

Dense trichomes and stomatal crypts (sunken stomata).

How does light influence stomatal opening?

Light increases solute concentration in guard cell vacuoles ( causes starch to convert to malic acid and proton pumps to expel protons, creating a gradient across the membrane), leading to water influx and turgor pressure.

What happens to intercellular water vapor when it diffuses out of the leaf?

It reduces the water potential of intercellular spaces, increasing tension and drawing water from surrounding areas. Once tension (water potential) is transferred to cells near vascular tissue, water will move in from the xylem

How does water move from xylem to mesophyll in leaves?

Hydrogen bonds pull the water column due to cohesion, while adhesion transfers tension to the xylem walls ( walls are too rigid to move)

What type of cells can support greater tension?

Tracheids

What happens once water has been lost from the xylem in the leaves?

It reduces the water potential of xylem in the roots, since the xylem is connected to the apoplast of the stele ( central region inside endodermis). Water loss from the stele is replaced with water from the cortex, which is replaced with water from the soil

Apoplast

In plants, the continuum of cell walls plus the extracellular spaces.

Stele

The vascular tissue of a stem or root.

What is the role of the Casparian strip in root transport?

It requires water to cross two plasma membranes to reach the xylem.

What happens as water is drawn into roots?

The hydrostatic tension in soil around the root increases drawing in water from soil particles further away

What happens when soil is too dry for plants?

The hydrostatic pressure becomes too large for plants to obtain water, leading to permanent wilting.

What is the significance of xylem pressure and cavitation?

Xylem pressure must be lower than the soil water potential for effective water transport.

What do xylem vulnerability curves (VCs) indicate?

They quantify an organ's resistance to xylem cavitation based on pressure exposure.