The problem of the desorption of a lipid molecule from a lipid vesicle (don
or) and its incorporation into another vesicle (acceptor) at high acceptor
concentrations, which has been investigated experimentally (Jones, J. D. an
d Thompson, T. E., 1990. Biochemistry, 29:1593-1600), is analyzed here from
a theoretical point of view, formulated in terms of the diffusion equation
with appropriate boundary conditions. The goal is to determine whether or
not the observed acceleration of the off-rate from a donor is caused by int
eraction with an acceptor vesicle at short range, or is simply the result o
f statistical effects due the proximity of the acceptor and its influence o
n the probability of the test lipid returning to the donor. We establish a
correspondence between the theoretical parameters and the experimental, the
rmodynamic and dynamic variables entering the problem. The solution shows t
hat, because of the extremely high Gibbs activation energy for desorption o
f a phospholipid, the process would always be first-order, even at very hig
h vesicle concentrations. This means that acceleration of the off-rate must
be due to donor-acceptor interactions at short distances, as proposed in t
he experimental work.