Transport of Solutes & Water (CH 5)

Vocabulary flashcards covering key concepts from CH 5: transport of solutes and water, including osmolality/osmolarity, fluid compartments, transport mechanisms, and epithelial transport.

Osmolality

Concentration of osmotically active particles per kilogram of water; units: Osm/kg; tends to be roughly 290 mOsm/kg in body fluids and is similar across compartments.

Osmolarity

Osmoles of solute per liter of solution (Osm/L); includes all solutes, regardless of membrane permeability.

Tonicity

Effective osmolality of a solution as it affects water movement across a membrane; depends on non-permeant solutes; determines cell volume (isotonic, hypertonic, hypotonic).

Isosmotic

Two solutions with equal osmolality; no net water movement across a membrane.

Hypoosmotic

Solution with lower osmolality than another; water tends to enter cells, causing swelling.

Hyperosmotic

Solution with higher osmolality than another; water tends to exit cells, causing shrinking.

Intracellular Fluid (ICF)

Fluid inside body cells; about 25 L in a 42 L total body water; contains high K+ and low Na+ relative to extracellular fluid.

Extracellular Fluid (ECF)

Fluid outside cells; about 17 L; includes plasma, interstitial fluid, and transcellular fluid.

Interstitial Fluid

Part of ECF that bathes cells in the extracellular space; involved in exchange with plasma.

Transcellular Fluid

Specialized extracellular fluid in spaces such as CSF, synovial fluid, urine; part of the ECF.

Donnan Equilibrium

Distribution of ions near a semi-permeable membrane due to impermeant intracellular proteins; affects membrane potential and ionic distribution.

Gibbs–Donan Effect

Explanation of how charged particles distribute near a semi-permeable membrane, influencing osmotic and electrical balance.

Nernst Equation

Equation that gives the equilibrium (rest) membrane potential for an ion across a membrane based on ion concentrations and valence.

Equilibrium (Nernst) Potential

Membrane potential at which there is no net electrochemical flux of a given ion.

Na+/K+-ATPase

Primary active transporter that uses ATP to pump 3 Na+ out and 2 K+ in; maintains Na+ and K+ gradients and cellular osmotic balance.

Primary Active Transport

Transport powered directly by ATP hydrolysis moving solutes uphill against their gradient (e.g., Na+/K+-ATPase, Ca2+-ATPase, H+-ATPases).

Secondary Active Transport

Transport powered by gradients created by primary pumps (e.g., Na+-driven glucose cotransport, Na+/Ca2+ exchanger, Na+-H+ exchanger).

Na+/Ca2+ Exchanger (NCX)

Secondary active transporter using the Na+ gradient to move Ca2+ out of cells or Ca2+ in, depending on conditions.

Na+-H+ Exchanger (NHE)

Secondary active transporter exchanging Na+ for H+ to regulate intracellular pH and cell volume.

Cl-/HCO3- Exchanger

Exchanger moving Cl- and HCO3- across membranes; helps regulate pH and acid-base balance.

CFTR

Cystic fibrosis transmembrane conductance regulator; Cl- channel; part of the ABC transporter family; mutations cause cystic fibrosis.

ABC Transporters

Family of ATP-binding cassette transporters; can function as pumps or channels; often hydrolyze ATP to move substrates.

Facilitated Diffusion

Transport down a concentration gradient mediated by membrane proteins; does not require direct energy input.

Pores

Always-open, non-selective pathways in membranes (e.g., water channels); allow rapid translocation of small molecules.

Channels

Gated diffusion pathways that can open/close; highly selective and allow rapid ion movement when open.

Carriers

Transport proteins that undergo conformational changes to move substrates; highly saturable (Michaelis-Menten kinetics).

GLUT Transporters

Family of glucose transporters that mediate Na+-independent facilitated diffusion of glucose across membranes.

Km and Jmax

Km: substrate concentration at half-maximal transport; Jmax: maximum rate of transport when all carriers are saturated.

Na+/K+-ATPase Cycle (E1/E2)

Sequential pump cycle: 3 Na+ in from cytosol, ATP→ADP, Na+ occluded, extracellular release, 2 K+ in, dephosphorylation, returns to start.

Transcellular Fluid

Fluid that moves through epithelial cells (secretion/absorption) rather than between cells.

Epithelial Transport

Movement of solutes and water across epithelia via apical channels, basolateral pumps, and paracellular pathways.

Water Transport through Epithelium

Water follows ions through cells and tight junctions; paracellular and transcellular routes contribute to absorption/secretion.

Osmotic Equilibrium

State where osmotic forces balance across compartments; maintained by pumps and Donnan effects; water movement adjusts to maintain balance.

Regulatory Volume Increase (RVI)

Rapid cellular response to hypertonic environments: ions are accumulated to draw water back in and restore volume.

Regulatory Volume Decrease (RVD)

Rapid cellular response to hypotonic environments: ions are expelled to draw water out and shrink cell back to normal size.

Long-term Regulation of Cell Volume

In hyperosmolality, cells accumulate relatively impermeant solutes (sorbitol, inositol, betaine, taurine) to restore volume over hours to days.

Isotonic Saline

0.9% NaCl solution; ~290 mOsm; increases extracellular fluid volume without changing intracellular volume.

Isotonic vs Permeant Solutes (Urea Effect)

Permeant solutes (like urea) can alter tonicity over time; initial water movement may occur until osmotic equilibrium is reestablished.