Handbook of Plant and Crop Physiology

PSI complexes are mainly situated in stromal lamellae [102]. Later it was found that two types of both
photosystems occur: grana-situated PSII (PSII) and PSI (PSI) and stroma lamellae–situated PSII
(PSII) and PSI (PSI) [103]. The two types differ not only in localization but also in structure and func-
tion. The type contains more chlorophyll, larger light-harvesting complexes, respectively, and is in-
volved in linear electron transport [104]. The stromal () photosystems have smaller (or lack of?) light-
harvesting complexes and carry out cyclic electron transport [104].
It has also been confirmed that cytochrome b 6 ƒ and ATP synthase are situated in both granal and stro-
mal lamellae [4,105]. Therefore, granal photosystems carry out the noncyclic photophosphorylation and
the stromal photosystems the cyclic type [5].
According to the latest models of the thylakoid membranes, 80% of the membrane is in the form of
grana and 20% consists of stroma lamellae [3,4]. Linear electron transport occurs in the grana, where
PSII, localized in the core domain of the grana, cooperates with PSIin the peripheral curved domain.
There is no sharp border between photosystems. This intermixing allows faster electron exchange via cy-
tochromeb 6 ƒ complexes localized in the peripheral domain. The ATP synthase is also situated within the
peripheral domains [4].
In each grana there is more chlorophyll associated (via LHCII) with PSII than PSI (ratio about 60:40)
[4]. However, LHCII is very functionally flexible. This flexibility is provided by a relatively complex as-
sembly of protein subunits and interactions between them, controlled by protonation, xanthophyll de-
epoxidation, and phosphorylation. On the other hand, LCHII and its assembly with other protein com-
plexes are vital for grana packing and grana stability [31].
The stroma lamellae contain PSI, cytochrome b 6 ƒ complexes, and ATP synthase and carry out
cyclic electron transport and photophosphorylation. The role of PSIIis unclear. Several alternatives
have been suggested: that PSIIpoises the cyclic electron flow around PSI, that PSIIis a precursor of
PSII, and that it is a stage in the repair cycle of PSII[4].

REFERENCES

1. J. Heslop-Harrison. Structure and morphogenesis of lamellar system in grana-containing chloroplasts. Mem-
   brane structure and lamellar architecture. Planta 60:243–260, 1963.


2. SW Thorne, JT Duniec. The physical principles of energy transduction in chloroplast thylakoid membranes. Q
   Rev Biophys 16:197–278, 1983.


3. P-O Arvidsson, C Sundby. A model for the topology of the chloroplast thylakoid membrane. Aust J Plant Phys-
   iol 26:687–694, 1999.


4. P-A Albertsson. The structure and function of the chloroplast photosynthetic membrane—a model for the do-
   main organization. Photosynth Res 4:141–149, 1995.


5. JM Anderson. Insights into the consequences of grana staring of thylakoid membranes in vascular plants: a per-
   sonal perspective. Aust J Plant Physiol 26:625–639, 1999.

PHOTOSYNTHETIC MEMBRANES IN HIGHER PLANTS 293

STROMAL

LAMELLAE

GRANA

Figure 6 Transmission electron micrograph of a Haberlea rhodopensis(Friv.) chloroplast with granal stack
and stromal lamellae (magnification 50,000).