Ch. 4 - Movement of Solutes and Water Across Cell Membranes

Simple diffusion

Movement of solutes down a concentration gradient without a transporter or ATP hydrolysis

Flux

Net flux

Difference between two one-way fluxes

Diffusion equilibrium

State during which diffusion fluxes in opposite directions are equal; net flux = zero

Fick’s 1st law of diffusion

Describes the rate of diffusion of a solute as a function of concentration gradient, area across which the solute diffuses, and other factors

Ion channels

Small passages in plasma membrane formed by integral membrane proteins and through which certain small-diameter molecules and ions can diffuse

Membrane potential

Voltage difference between inside and outside of cell

Electrochemical gradient

The driving force across a plasma membrane that dictates whether an ion will move into or out of a cell; established by both the concentration difference and the electrical charge difference between the cytosolic and extracellular surfaces of the membrane

Channel gating

Process of opening and closing ion channels

Ligand-gated ion channel

Membrane ion channels operated by the binding of specific molecules to channel proteins

Voltage-gated ion channels

Channels cell membrane ion channels opened or closed by changes in membrane potential

Mechanically-gated ion channels

Membrane ion channels that are opened or closed by deformation or stretch of the plasma membrane

Transporters

Integral membrane proteins that mediate passage of molecules through membrane; also called carrier

Mediated transport

Movement of molecules across membrane by binding to protein transporter; characterized by specificity, competition, and saturation; includes facilitated diffusion and active transport

Facilitated diffusion

System using a transporter to move molecules from high to low concentration across a membrane; energy not required

Active transport

Energy-requiring system that uses transporters to move ions or molecules across a membrane against an electrochemical difference

Primary active transport

Active transport in which chemical energy is transferred directly from ATP to transporter protein

Secondary active transport

Active transport in which energy released during transmembrane movement of one substance from higher to lower concentration is transferred to the simultaneous movement of another substance from lower to higher concentration

Na+/K+-ATPase pump

Primary active-transport protein that hydrolyzes ATP and releases energy used to transport sodium ions out of cell and potassium ions in

Cotransport

Form of secondary active transport in which net movement of actively transported substance and “downhill” movement of molecule supplying the energy are in the same direction

Countertransport

Form of secondary active transport in which net movement of actively transported molecule is in direction opposite “downhill” movement of molecule supplying the energy

Aquaporins

Protein membrane channels through which water can diffuse

Osmosis

Net diffusion of water across a selective barrier from region of higher water concentration (lower solute concentration) to region of lower water concentration (higher solute concentration)

Osmolarity

Total solute concentration of a solution; measure of water concentration in that the higher the solution osmolarity, the lower the water concentration

Osmol

1 mole of solute ions and molecules

Semipermeable membrane

Membrane permeable to some substances (usually water) but not to others (some solutes)

Osmotic pressure

Pressure that must be applied to a solution on one side of a membrane to prevent osmotic flow of water across the membrane from a compartment of pure water; a measure of the solution’s osmolarity

Nonpenetrating solutes

Dissolved substances that do not passively diffuse across a plasma membrane

Isotonic

Containing the same number of effectively nonpenetrating solute particles as normal extracellular fluid

Hypotonic solutions

Solutions containing a lower concentration of effectively nonpenetrating solute particles than normal (isotonic) extracellular fluid

Hypertonic solutions

Solutions containing a higher concentration of effectively membrane-impermeable solute particles than normal (isotonic) extracellular fluid

Penetrating solutes

Solute that can freely diffuse across the lipid bilayer of a plasma membrane

Isoosmotic

Having the same total solute concentration as extracellular fluid

Hypoosmotic

Having total solute concentration less than that of normal extracellular fluid

Hyperosmotic

Having total solute concentration greater than normal extracellular fluid

Endocytosis

Process in which plasma membrane folds into the cell forming small pockets that pinch off to produce intracellular, membrane-bound vesicles

Exocytosis

Process in which intracellular vesicle fuses with plasma membrane, the vesicle opens, and its contents are liberated into the extracellular fluid

Pinocytosis / fluid endocytosis

cell drinking; endocytosis when the vesicle encloses extracellular fluid or specific molecules in the extracellular fluid that have bound to proteins on the extracellular surface of the plasma membrane

Phagocytosis

cell eating; engulfment of particles by a cell

Receptor-mediated endocytosis

The specific uptake of ligands in the extracellular fluid by regions of the plasma membrane that invaginate and form intracellular vesicles

Phagosomes

Plasma-membrane-bound, intracellular sacs formed when a phagocyte engulfs a microbe

Receptors

(for messengers) protein either on cell surface, in the cytosol, or in the nucleus that binds a chemical messenger such as a hormone or neurotransmitter and mediates its actions

(in sensory system) specialized peripheral ending of afferent neuron, or separate cell intimately associated with it, that detects changes in some aspect of environment

Clathrin

A cytosolic protein that binds to regions of the plasma membrane and helps initiate receptor-mediated endocytosis

Clathrin-coated pit

Aggregation of ligand-bound receptors on a cell membrane that pinches off and is internalized into the cell

Apical membrane

The surface of an epithelial cell that faces a lumen, such as that of the intestines; also known as luminal membrane

Basolateral membrane

Sides of epithelial cell other than luminal surface; also called serosal or blood side of cell

Paracellular pathway

The space between adjacent cells of an epithelium through which some molecules diffuse as they cross the epithelium

Transcellular pathway

Crossing an epithelium by movement into an epithelial cell, diffusion through the cytosol of that cell, and exit across the opposite membrane