In a review article published in the March 2003 issue of Trends in Plant Science (vol. 8:117-122), László Mustárdy and Gyz Garab at the Hungarian Academy of Sciences, provide an up-to-date computer model of the chloroplast granum. The model has been prepared with the help of an electron micrographic study of serial sections through granum-stroma assemblies. With the help of these micrographs, they were able to shed more light into the functioning of the multi-lamellar membrane system of granum.
Description of the internal structure of a chloroplast: The inner membranes of chloroplasts are called thylakoids. They carry out all light-harvesting and energy-transducing functions. The aqueous medium in which thylakoids are spread out are called the stroma. Mature thylakoid membranes differentiate into granum and stroma regions. Each granum appears to be composed of 10-20 layers of cylindrical structures pressed together. In reality, grana are a part of thylakoid membrane system constituting a continuous membrane with a single enclosed space called the thylakoid lumen. In other words, the lumen represents a single anastomosing chamber contributed by a continuous thylakoid network.
Two distinct types of membrane domains: The thylakoid network, consists of two distinct types of membrane domains , the stacked grana thylakoids and the unstacked stroma thylakoid which interconnect the grana stacks through the chloroplast stroma. Grana are interconnected by lamellae of several hundred nm in length. In order to understand the functions of different components of chloroplasts, the authors point out that it is important to make a critical study of the structure of the granum–stroma assembly of thylakoid membranes as a structural unit, and their interconnections.
Frequently used Folded-membrane model:The authors describe the frequently used folded-membrane model, which assumes that the granum originates by forming a ‘fork’ from a layer ‘above’ and a layer ‘below’ the stacked pair of membranes. Changes in the intensity of chlorophyll a fluorescence, induced by cation changes, supports the above model. Furthermore, the model also offers an explanation as to why fully reversible folding occurs in the form of a continuous membrane into multiple granum stacks. Scientists working on this system considered the changes to be due to de-stacking and re-stacking of thylakoids. However, this model loses its ground, if destacking is carried out completely. In such a situation, re-addition of cations only partially restores re-stacking of membranes.
Current model: In the current computer model, the multiple helices of stroma lamellae, comprising thylakoid membranes are shown to be wound in a right-handed fashion all around the cylindrical grana. The multiple helices form a contiguous system. The computer model assumes that the granum discs have eight connecting slits towards the stroma thylakoids and that the helices and the granum stacks have the same width.
Citing the work of Jan M. Anderson and Keith N. Boardman published in 1970s, the authors emphasize that the division of thylakoids into granum and stroma membrane regions are related to both structural and functional differences. The stroma thylakoid region contain Photosystem I (PSI) and that of grana Photosystem II (PSII). While chlorophyll a/b light-harvesting complex (LHCII) is located in the stacked membranes (grana), the components of LHCI are found in the stroma thylakoids. The spatial separation of the two light harvesting complexes, LHCI and LHCII, is probably necessary in order to stop unregulated excitation energy flow between the two photosystems. In this context, some questions remain to be answered as to how the two photosystems work without grana in the primitive chlorophyll-b-containing green alga, Mantoniella squamata.
The authors discuss the structural complexities of LHCII by referring to the chlorina-f2 barley mutants. These mutants lacks chlorophyll b and are deficient in LHCII. The mutants require unusually high concentrations of magnesium ion, and the (normally) minor component, Lhcb5, appears to stabilize the granal structure.
Strength of Stacking: The authors point out that phosphorylation of LHCII and several other phosphoproteins determine the strength of stacking. In this connection, they also explain the causes of the lack of complete re-formation of grana after they are totally unstacked. By suitable experiments it has been shown that thylakoid bilayers tolerate only small deformations. The stroma thylakoids, which wind around the granum stacks have several functions and the two thylakoid systems form a continuous structure by the fusion of two (or more) stroma membranes at their edges. Another important feature of the two systems is that the ratio of their surface areas is ~ 1:1 which varies during unstacking in the margin.
Concluding remarks. In their concluding remarks, the authors address several questions emphasizing the need of further research in order to unravel the structural and functional details of grana. Among many unsolved questions, some important ones are: (a) working out the details of the role that grana and the LHCII-containing structures play in regulating energy dissipation in excess light; (b) behavior of grana as special optical units in the context that the properties of grana are also influenced by the macroorganization of chromophores and other geometrical factors; (c) determination whether variation in the number of grana layers in the sun- and shade plants plays important regulatory roles in the light-harvesting capabilities inside the granum and/or laterally between grana and stroma. It is still an open question as to why stroma thylakoids around the granum are right-handed helices. The authors suggest that small-angle X-ray and neutron- scattering techniques need to be applied more thoroughly in order to study ultrastructural changes that occur on small time scale. The authors hope that a combined attack by advanced molecular tools aided by the study with most modern electron microscopy techniques will shed light on the granum–stroma contiguous thylakoid membrane system, their ability for self-assembly and structural elasticity.