The macroscopic electric polarization resulting from the primary charg
e separation in reaction centers of photosynthetic membranes is observ
ed in media containing chloroplasts, photosynthetic bacteria, and orie
nted membrane fragments. Photovoltages elicited by a short nonsaturati
ng flash, resulting from the so-called ''light-gradient'' effect in ch
loroplast suspensions, were measured almost 20 years ago by Fowler and
Kok [1] and Witt and Zickler [2]. Because of the antiparallel orienta
tion of reaction centers in the opposing thylakoid membranes and the s
hadowing of the ''lower'' membrane by the ''upper'' one, a net dipole
moment appears, which is a source of a small potential difference. It
was thought that the polarity of this potential difference could be de
duced from the known position of electron carriers in the photosynthet
ic reaction center. However, the observed polarity of the signal induc
ed at some flash wavelengths was opposite to that predicted by this cl
assical model assumption. In addition, the measured photovoltage ampli
tudes were not quantitatively related to experimental parameters. Here
, it is shown that the light propagation and interference in pigmented
multilayers of the sample are responsible for the light-gradient effe
ct. A model calculation is carried out for a pair of membranes, simula
ting stroma lamellae of chloroplasts. A wavelength-dependent light dis
tribution and photovoltage, as well as the polarity, is thus predicted
. For low intensities, the amplitude is found to be proportional to th
e intensity of the incoming light, to the optical density, and to the
reciprocal of the dielectric constant of the sample. When the membrane
s contain no chromophores or when the absorption coefficient is low, t
he predicted polarity is opposite to that expected from the original m
acroscopic picture. The model is tested with a set of experimental pho
tovoltage data obtained at different wavelengths.