Osmosis

The diffusion of free water across a selectively permeable membrane.

Water potential

(Ψ) The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure.

Megapascal (MPa)

A unit of pressure equivalent to about 10 atmospheres of pressure.

Solute potential

(Ψ_S) A component of water potential that is proportional to the molarity of a solution and that measures the effect of solutes on the direction of water movement; also called osmotic potential, it can be either zero or negative.

Pressure potential

(Ψ_P) A component of water potential that consists of the physical pressure on a solution, which can be positive, zero, or negative.

Protoplast

The living part of a plant cell, which also includes the plasma membrane.

Turgor pressure

The force directed against a plant cell wall after the influx of water and swelling of the cell due to osmosis.

Flaccid

Limp. Lacking turgor (stiffness or firmness), as in a plant cell in surroundings where there is a tendency for water to leave the cell. (A walled cell becomes flaccid if it has a higher water potential than its surroundings, resulting in the loss of water.)

Plasmolysis

A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall; occurs when the cell loses water to a hypertonic environment.

Turgid

Swollen or distended, as in plant cells. (A walled cell becomes turgid if it has a lower water potential than its surroundings, resulting in entry of water.)

Wilting

The drooping of leaves and stems as a result of plant cells becoming flaccid.

Aquaporins

A channel protein in the plasma membrane of a plant, animal, or microorganism cell that specifically facilitates osmosis, the diffusion of free water across the membrane.

Bulk flow

The movement of a fluid due to a difference in pressure between two locations.

Endodermis

In plant roots, the innermost layer of the cortex that surrounds the vascular cylinder.

Casparian strip

A water-impermeable ring of wax in the endodermal cells of plants that blocks the passive flow of water and solutes into the stele by way of cell walls.

Xylem sap

The dilute solution of water and dissolved minerals carried through vessels and tracheids.

Transpiration

The evaporative loss of water from a plant.

Root pressure

Pressure exerted in the roots of plants as the result of osmosis, causing exudation from cut stems and guttation of water from leaves.

Guttation

The exudation of water droplets from leaves, caused by root pressure in certain plants.

Cohesion-tension hypothesis

The leading explanation of the ascent of xylem sap. It states that transpiration exerts pull on xylem sap, putting the sap under negative pressure or tension, and that the cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots.

Circadian rhythyms

A physiological cycle of about 24 hours that persists even in the absence of external cues.

abscisic acid (ABA)

A plant hormone that slows growth, often antagonizing the actions of growth hormones. Two of its many effects are to promote seed dormancy and facilitate drought tolerance.

Xerophytes

A plant adapted to an arid climate.

translocation

The transport of organic nutrients in the phloem of vascular plants.

phloem sap

The sugar-rich solution carried through a plant’s sieve tubes.

sugar sink

A plant organ that is a net consumer or storer of sugar. Growing roots, shoot tips, stems, and fruits are examples of sugar sinks supplied by phloem.

sugar source

A plant organ in which sugar is being produced by either photosynthesis or the breakdown of starch. Mature leaves are the primary sugar sources of plants.