Glossary of Physiology Terms

Net flux

Net flux represents the amount of substance moved in or out of the cell. It is the mathematical difference between influx and efflux.

Net flux = Influx − Efflux

Similar to influx and efflux, net flux is reported as a rate. It is the net amount of substance that moves through a given area of the plasma membrane per unit time.

Flux
Influx
Efflux
Unidirectional flux

Electrochemical driving force

V_DF

When an ion is not at its electrochemical equilibrium, an electrochemical driving force (V_DF) acts on the ion, causing the net movement of the ion across the membrane down its own electrochemical gradient.

The electrochemical driving force is generally expressed in millivolts and is calculated according the following equation:

V_DF = V_m − V_eq

where V_DF is the electrochemical driving force, V_m is the membrane potential, and V_eq is the equilibrium potential.

Membrane potential
Equilibrium potential
Electrochemical gradient

Resting Membrane Potential - Electrochemical Driving Force Acting on Ions
Electrochemical Driving Force Calculator

Electrogenic pump

Electrogenic pumps are primary active transporters that hydrolyze ATP and use the energy released from ATP hydrolysis to transport ions across biological membranes leading to the translocation of net charge across the membrane.

For example, the Na⁺/K⁺ ATPase (sodium pump) is an electrogenic pump because during every transport cycle, it transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell. This leads to the movement of one net positive charge out of the cell making this process electrogenic.

Electrogenic

Electrogenic

An electrogenic transport process is one that leads to the translocation of net charge across the membrane. For example, ion channels such as Na⁺, K⁺, Ca²⁺, and Cl⁻ channels are electrogenic.

The Na⁺/K⁺ ATPase is electrogenic because for every ATP molecule hydrolyzed, 3 Na⁺ ions are transported out of the cell and 2 K⁺ ions are transported into the cell (leading to the translocation of one net positive charge out of the cell).

Many secondary active transporters are also electrogenic. For example, the Na⁺/glucose cotransporter (found in the small intestine and kidney proximal tubules), transports 2 Na⁺ ions and 1 glucose molecule into the cell across the plasma membrane (leading to the translocation of two net positive charges into the cell per transport cycle).

Equilibrium potential

V_eq. or E_eq.

Refers to the membrane potential at which there is no net movement of an ion across the plasma membrane into or out of the cell.

Resting Membrane Potential - Establishment of the Membrane Potential
Resting Membrane Potential - Nernst Equilibrium Potential
Nernst Potential Calculator

Glutamate

Glutamate (Glu, E) is one of the standard twenty (20) amino acids used by cells to synthesize peptides, polypeptides, and proteins. It has a molecular weight of 147.13 g/mol. Its side chain has a pK_a of 4.07 and, therefore, glutamate has a net negative charge at physiological pH.

In the nervous system, glutamate is an excitatory amino acid neurotransmitter. In fact, glutamate is the most abundant excitatory neurotransmitter in the nervous system. Glutamate is a classical neurotransmitter. Its action is exerted via the activation of glutamate receptors (GluR), some of which are ligand-gated ion channels (ionotropic receptors), and some are G protein coupled receptors (GPCRs, metabotropic receptors). At glutamatergic synapses, the action of glutamate is terminated by glutamate transporters (EAAT, excitatory amino acid transporter), which transport glutamate from the extracellular space in synaptic and extrasynaptic regions into neurons and glia.

Impermeant

Not permeant. Incapable of passing through or penetrating. Impermeant refers to the inability of a substance (e.g., ion or molecule) to cross (i.e., permeate or penetrate) a biological membrane or channel pore. For example, it can be said that ions are membrane impermeant.

See also permeant.

Permeant
Permeability
Permeable
Impermeable

Lipid Bilayer Permeability

Nernst equation

An equation used to calculate the equilibrium potential (V_eq.) of an ion. The equilibrium potential for an ion is also referred to as the Nernst potential for that ion. It is the membrane potential at which no net movement of the ion in question occurs across the membrane.



where V_eq. is the equilibrium potential, R is the universal gas constant, T is the temperature in Kelvin, z is the valence of the ionic species, F is the Faraday's constant, and [X]_o and [X]_i are the extracellular and intracellular, respectively, concentrations of the ion in question.

Resting Membrane Potential - Nernst Equilibrium Potential
Derivation of the Nernst Equation

Permeant

Permeant refers to the ability of a substance (e.g., ion or molecule) to cross (i.e., permeate or penetrate) a biological membrane or channel pore. For example, it can be said that fat-soluble molecules are membrane permeant.

See also impermeant.

Impermeant
Permeability
Permeable
Impermeable

Lipid Bilayer Permeability

Unidirectional flux

The rate of movement of a substance across an interface in only one, and not the opposite, direction (i.e., flux in only one direction). For example, when referring to the plasma membrane of cells, we can think of unidirectional flux of a substance ino the cell (referred to as influx), as well as unidirectional flux of the substance out of the cell (referred to as efflux). The difference between two unidirectional fluxes is referred to as net flux, which is the net amount that moves into or out of the cell.

Flux
Influx
Efflux
Net flux