Febs Letters Volume 105, Number 1-2./Volume 106, Number 1-2. [antikvár]

Volume 105, number 1 I-liBS LETTERS September 1979 Hypothesis AN EXPLANATION OF THE PROTON UPTAKE OF CHLOROPLAST MEMBRANES IN TERMS OF ASYMMETRY OF THE SURFACE CHARGES J. T. DUNIEC and S. W. THORNE CSIRO, Division of Plant Industry, PO Box 1600, Canberra City, ACT 2601, Australia Received 17 May 1979 1. Introduction In spite of large amounts of experimental data concerning active transport phenomena across biological membranes, there is a lack of knowledge of prmciples by which the active transport operates. Quite often there is a tendency to use terms such as 'ATP-driven sodium pump' or 'light-driven hydrogen pump' in order to name sophisticated lipid-protein complexes which appear to push specific ions against their electrochemical gradients. One can hardly be satisfied with the acceptance of the name of a device as a principle of its operation, hence it seems important to seek the reasons for these phenomena. The model we present here is an attempt to give a physical basis to the events associated with the ionic transport, especially througli the energy transducing membranes. In the simplest words, the postulated model can be summarized as follows: 1. In equilibrium the enclosed volume and the outside solution have the same electric potential and the same concentration of ions, save for the diffuse charge layers adjacent to the membrane (fig.la). 2. There is a difference in surface charge densities between inside and outside face of the membrane. In equiUbrium this difference creates an internal electric field in the membrane. 3. On absorption of light, or due to oxido—reduction reactions, current carriers (e.g., electrons moved into an excited state by light) are created in the membrane and a current flows in the direction of the field leading to a decrease in the potential difference between the two faces of the membrane. 4. The movement of permeant ions, and other recom- bination processes in response to the decrease of the potential difference tend to restore the original equihbrium field. A steady state can be reached when all these fluxes are minimal. In general, the steady state depends on the permeability of the outside solution inside solution I o; o, o,= a. Fig.l. Potential profiles across the thylakoid membrane: o,, Oj, surface charge densities; surface potentials at the external and internal side of the membrane, respectively. (a) In the dark equilibrium lo, KloJ hence IF, KIK, I. (b) In the light-induced steady state a, = a, hence K, = F,. Elsevier !North-Holland Biomedical Press 1