13
Structure and Function of Photosynthetic
Membranes in Higher Plants
Ilia D. Denev and Ivan N. Minkov
University of Plovdiv, Plovdiv, Bulgaria
281I. INTRODUCTION
The biological conversion of light quanta energy into chemical energy is known as photosynthesis. The
photosynthesis takes place in the chloroplasts. Electron micrographs (Figure 1) of chloroplasts of higher
plants revealed that they consist of a double membrane envelope enclosing a complex of inner membranes
known as thylakoids. The thylakoid membrane system of one chloroplast is believed to be formed from
one continuous membrane, which divides the inner chloroplast volume into two separate spaces: ex-
trathylakoid (stroma) and intrathylakoid (thylakoid lumen) [1–3]. The biochemical part of photosynthe-
sis takes place in the stroma, which contains all the enzymes of the CO 2 fixation pathway.
The thylakoids of higher plant chloroplasts are probably the most complexly organized of all bio-
logical membranes. Their main function is to capture light quanta and to drive series of redox reactions
that produce ATP and oxygen and reduce ferredoxin. The thylakoid membranes are 5–7 nm thick and con-
sist of a lipid bilayer where complexes of proteins, pigments, and some other minor components are sit-
uated. The predominant parts of thylakoid proteins and pigments are organized into intrinsic membrane-
spaning supramolecular complexes. Their transverse and lateral distribution is highly asymmetric, and
this is one of the basic features of photosynthetic membranes [4]. The main molecular supracomplexes in
thylakoid membranes are the complex photosystem I, the complex photosystem II, the cytochrome b 6 ƒ
complex, and ATP synthase [5].
II. COMPLEX PHOTOSYSTEM II
A. Supramolecular Organization of Photosystem II
Photosystem II (PSII) is a multisubunit membrane protein complex that catalyzes the light-driven oxida-
tion of water and reduction of plastoquinone. PSII contains about 25 polypeptides (Figure 2) [6]. The min-
imal PSII supramolecular complex, capable of carrying out stable charge separation, is the PSII reaction
center (RC) complex. The RC complex contains at least six proteins: the D1, D2, cytochrome b 559 (two
polypeptides), PsbI, and probably PsbL polypeptides [7]. These proteins bind several cofactors: tetra-Mn
cluster, nonheme Fe, two pheophytins and two quinones (QAand QB) per RC complex, six chlorophylls,
and two pheophytins [8]. Two of the RC chlorophylls make up the chlorophyll special pair (P680) and
two accessory chlorophylls, whose monomers presumably participate in electron transfer between the