Chapter 4: Movement of Solutes and Water Across Cell Membranes

Vocabulary flashcards covering mechanisms of solute and water transport across plasma membranes, transport kinetics, osmolarity, endocytosis/exocytosis, and epithelial transport based on Vander's Human Physiology lecture notes.

Simple Diffusion

The movement of molecules from a region of higher concentration to a region of lower concentration, driven by random thermal motion and random collisions of molecules.

Diffusion Equilibrium

The state reached when solute molecules become uniformly distributed across a solution or boundary, resulting in equal movement in both directions and a net flux of zero ($\text{Net Flux} = 0$).

Flux

The amount of a substance that crosses a surface or membrane per unit of time.

Net Flux

The difference between the two one-way fluxes of a substance moving across a membrane.

Electrochemical Gradient

The combined dual force consisting of the concentration difference and the electrical difference (membrane potential) that determines the direction and magnitude of ion movement across a plasma membrane.

Channel Gating

The regulatory process of opening and closing ion channels in response to chemical, electrical, or mechanical stimuli.

Transporters

Integral membrane proteins that mediate the non-diffusional movement of ions, polar molecules, and large molecules across cell membranes by undergoing conformational changes.

Mediated Transport

The movement of substances across a cell membrane that requires the binding to and conformational change of transporter proteins.

Facilitated Diffusion

A type of mediated transport in which a solute moves across a membrane from higher to lower concentration using a transporter protein without consuming cellular energy ($\text{ATP}$).

Active Transport

A type of mediated transport that uses energy to move a substance uphill against its concentration gradient.

Primary Active Transport

Active transport that directly hydrolyzes $\text{ATP}$ as its direct energy source to pump solutes against their concentration gradient.

$\text{Na}^+/\text{K}^+\text{-ATPase}$ Pump

A primary active transporter found in every cell that hydrolyzes $\text{ATP}$ to pump $3\,\text{Na}^+$ ions out of the cell and $2\,\text{K}^+$ ions into the cell.

Secondary Active Transport

Active transport that uses the energy stored in an electrochemical gradient (often a $\text{Na}^+$ gradient) to move a molecule against its concentration gradient.

Cotransport

A form of secondary active transport in which the driving ion and the transported solute move across the membrane in the same direction (also called symport).

Countertransport

A form of secondary active transport in which the driving ion and the transported solute move across the membrane in opposite directions (also called antiport).

Osmosis

The net movement of water ($\text{H}_2\text{O}$) across a membrane from a region of lower solute concentration to a region of higher solute concentration.

Aquaporins

Membrane protein channels that mediate and facilitate the movement of water molecules across cell membranes.

Osmolarity

The total solute concentration of a solution, measured in osmoles per liter ($\text{Osm/L}$).

Osmol

A unit of measurement for osmotic concentration equal to $1\,\text{mol}$ of solute particles.

Semipermeable Membrane

A membrane that is permeable to water ($\text{H}_2\text{O}$), but impermeable to solute particles.

Osmotic Pressure

The pressure that must be applied to a solution to prevent the net flow of water into it across a semipermeable membrane by osmosis.

Nonpenetrating Solutes

Solutes that cannot cross the plasma membrane and therefore determine tonicity and changes in cell volume at equilibrium.

Isotonic Solution

A solution containing $300\,\text{mOsm/L}$ of nonpenetrating solutes, which causes no net change in cell volume.

Hypertonic Solution

A solution containing greater than $300\,\text{mOsm/L}$ of nonpenetrating solutes, causing water to leave the cell and the cell to shrink.

Hypotonic Solution

A solution containing less than $300\,\text{mOsm/L}$ of nonpenetrating solutes, causing water to enter the cell and the cell to swell.

Isoosmotic Solution

A solution containing $300\,\text{mOsm/L}$ of total solute, regardless of whether the solutes are penetrating or nonpenetrating.

Hyperosmotic Solution

A solution containing greater than $300\,\text{mOsm/L}$ of total solute, regardless of whether the solutes are penetrating or nonpenetrating.

Hypoosmotic Solution

A solution containing less than $300\,\text{mOsm/L}$ of total solute, regardless of whether the solutes are penetrating or nonpenetrating.

Endocytosis

The cellular process of transporting substances into the cell via membrane-bound vesicles formed by invagination of the plasma membrane.

Pinocytosis

A type of fluid endocytosis involving the non-specific uptake of extracellular fluid and dissolved solutes into vesicles.

Phagocytosis

A type of endocytosis in which a cell extends pseudopodia to engulf large particles or bacteria into a phagosome.

Receptor-Mediated Endocytosis

A specific type of endocytosis in which extracellular ligands bind to plasma membrane receptors, often in clathrin-coated pits, before being internalized.

Exocytosis

The process by which intracellular membrane-bound vesicles fuse with the plasma membrane to secrete contents into the extracellular fluid.

Apical Membrane

The surface of an epithelial cell plasma membrane facing a hollow tube, cavity, or fluid-filled lumen (also called the luminal membrane).

Basolateral Membrane

The surface of an epithelial cell plasma membrane adjacent to interstitial fluid and the blood vessel network (also called the serosal membrane).

Paracellular Pathway

The transport route across an epithelium in which substances diffuse between adjacent cells through tight junctions.

Transcellular Pathway

The transport route across an epithelium in which substances cross the plasma membrane into an epithelial cell, diffuse through cytosol, and exit across the opposite membrane.