voltage-gated channel Ion channels are pores in cell
membranes that allow the passage of ions in and out of
cells. There are two types, voltage-gated and chemically
gated channels. The opening and closing of the voltage-
dependent ion channels is regulated by voltage, the
electrical charge or potential difference between the
inside and outside of the membrane, while chemical
stimuli are responsible for opening and closing the
chemically gated channels. Neurons use these channels
to pass sodium and potassium ions through them.
voltaic cells Electrochemical cells in which sponta-
neous chemical reactions produce electricity. Common
household batteries are of this type. Also called gal-
vanic cells.
volume of activation (∆t-V) A quantity derived from
the pressure dependence of the RATE CONSTANTof a
reaction (mainly used for reactions in solution), defined
by the equation
∆-tV= –RT(∂lnk/∂p)Tproviding that the rate constants of all reactions
(except first-order reactions) are expressed in pressure-
independent concentration units, such as mol dm–3at a
fixed temperature and pressure. The volume of activa-
tion is interpreted, according to TRANSITION STATEthe-
ory, as the difference between the partial molar
volumes of the transition state (‡V) and the sums of the
partial volumes of the reactants at the same tempera-
ture and pressure, i.e.,
∆-tV= t-V– Σ(rVR)
where r is the order in the reactant R and VRits partial
molar volume.VSEPR(valence-shell electron-pair repulsion) A the-
ory that allows the prediction of shapes of simple poly-
atomic molecules by applying a set of rules.VSEPR 277