Water potential
The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure.
Pressure potential
A component of water potential that consists of the physical pressure on a solution, which can be positive, zero, or negative.
Solute potential
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.
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 model
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.
Transpiration
The evaporative loss of water from a plant.
Stomata
A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant.
Guard cells
The two cells that flank the stomatal pore and regulate the opening and closing of the pore.
Resting potential
The membrane potential characteristic of a nonconducting excitable cell, with the inside of the cell more negative than the outside.
Sodium potassium pump
A transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell.
Equilibrium potential
The magnitude of a cell's membrane voltage at equilibrium
Depolarization
A change in a cell's membrane potential such that the inside of the membrane is made less negative relative to the outside.
Action potential
An electrical signal that propagates (travels) along the membrane of a neuron or other excitable cell as a nongraded (all-or-none) depolarization.
facilitated diffusion
The passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure.
osmoregulation
Regulation of solute concentrations and water balance by a cell or organism
Pinocytosis
A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes
Phagocytosis
A type of endocytosis where large particulate substances or small organisms are taken up by the cell
Receptor-mediated endocytosis
The movement of specific molecules into a cell by the infolding of vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances
selective permeability
A property of biological membranes that allows them to regulate the passage of substances across them
transport protein
A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane
Turgid
Swollen or distended, as in plant cells.
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
proton pump
An active transport protein in a cell membrane that uses ATP to transport hydrogen ions out of a cell against their concentration gradient, generating a membrane potential in the process
peripheral protein
A protein loosely bound to the surface of a membrane or to part of an integral protein and not embedded in the lipid bilayer
osmosis
The diffusion of free water across a selectively permeable membrane
passive transport
The diffusion of a substance across a biological membrane with no expenditure of energy
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.
fluid mosaic model
The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.
membrane potential
The difference in electrical charge (voltage) across a cell's plasma membrane due to the differential distribution of ions.
hypertonic
Referring to a solution that, when surrounding a cell, will cause the cell to lose water
hypotonic
Referring to a solution that, when surrounding a cell, will cause the cell to take up water
ion channel
A transmembrane protein channel that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient.
Ligand
A molecule that binds specifically to another molecule, usually a larger one
Isotonic
Referring to a solution that, when surrounding a cell, causes no net movement of water into or out of the cell
integral protein
A transmembrane protein with hydrophobic regions that extend into and often completely span the hydrophobic interior of the membrane and with hydrophilic regions in contact with the aqueous solution on one or both sides of the membrane (or lining the channel in the case of a channel protein).
gated channel
A transmembrane protein channel that opens or closes in response to a particular stimulus
glycolipid
A lipid with one or more covalently attached carbohydrates
Glycoprotein
A protein with one or more covalently attached carbohydrates
active transport
The movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy
Amphipathic
Having both a hydrophilic region and a hydrophobic region
aquaporin
A channel protein in a cellular membrane that specifically facilitates osmosis, the diffusion of free water across the membrane
concentration gradient
A region along which the density of a chemical substance increases or decreases
cotransport
The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.
diffusion
The random thermal motion of particles of liquids, gases, or solids. The net movement of a substance from a region where it is more concentrated to a region where it is less concentrated.
electrochemical gradient
The diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force).
electrogenic pump
An active transport protein that generates voltage across a membrane while pumping ions
exocytosis
The cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane
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