Water Transport in Plants: Xylem (3)

Water Potential (Ψ)

The measure of the potential energy in water; describes the tendency of water to move from one area to another.

Ψ = Ψs + Ψp

The mathematical formula for calculating total water potential by summing solute and pressure potential.

Megapascal (MPa)

The standard unit of pressure used to measure water potential in plant biology.

Solute Potential (Ψs)

The component of water potential that decreases as more solutes are dissolved (also known as osmotic potential).

0 MPa (Pure Water)

The maximum possible value for solute potential, occurring in pure water at sea level.

Pressure Potential (Ψp)

The component of water potential representing physical pressure; can be positive (turgor) or negative (tension).

Tension

A negative pressure potential (Ψp < 0) often found in the xylem of transpiring plants.

Turgor Pressure

Positive pressure inside a plant cell pushing the plasma membrane against the cell wall.

Wall Pressure

The mechanical force exerted by the rigid cell wall that resists the expansion of the cell.

Hypotonic Solution

An external environment with a higher water potential than the cell, causing water to enter.

Hypertonic Solution

An external environment with a lower water potential than the cell, causing water to leave.

Isotonic Solution

An environment where water potential equals that of the cell; no net movement of water occurs.

Dynamic Equilibrium

The state where water moves in and out of the cell at equal rates, maintaining constant volume.

Plasmolysis

The shrinking of the protoplast away from the cell wall when a cell is placed in a hypertonic solution.

Flaccid

The state of a plant cell where pressure potential (Ψp) is zero, often leading to wilting.

Turgid

The state of a plant cell where Ψp is high enough to keep the plant upright and rigid.

Wilting

The visible result of loss of turgor pressure across a plant's tissues.

Proton Pump (H+-ATPase)

The primary active transport protein that establishes the electrochemical gradient in plants.

Membrane Potential

The voltage across the plasma membrane, usually negative inside the cell relative to the outside.

Symporter

A membrane protein that transports two different substances in the same direction (e.g., H+ and sucrose).

Antiporter

A membrane protein that moves two different substances in opposite directions across the membrane.

Secondary Active Transport

Using the energy of an established H+ gradient to move other solutes against their own gradients.

Aquaporins

Channel proteins that increase the permeability of the membrane to water for faster osmosis.

Tonoplast

The membrane of the central vacuole that regulates the movement of ions to maintain turgor.

Osmolarity

The concentration of a solution expressed as the total number of solute particles per liter.

Electrochemical Gradient

The combined effect of a concentration gradient and an electrical charge gradient.

Central Vacuole

The organelle that stores water and solutes, essential for maintaining high pressure potential.

Bulk Flow

The movement of water and solutes together due to a pressure gradient, rather than diffusion.

Diffusion

The net movement of particles from high to low concentration (inefficient over long distances).

Net Movement

The overall direction in which water flows, determined by the total water potential gradient.