The grana margins of plant thylakoid membranes

Plant thylakoid membranes contain three structurally distinct domains: the planar appressed membranes of the grana; the planar non-appressed stroma thylakoids; and the highly curved, non-appressed margins of the grana. Evidence is presented to suggest that the grana margins form a significant structural domain, which has hitherto been neglected. If indeed the grana margins contain some of the cytochrome b/f complex, photosystem (PS) I complex and ATP synthase, they form a third functional domain of the laterally heterogeneous continuous thylakoid membrane network. The consequences of grana margins containing complexes are explored with respect to linear electron transport under light-saturating and light-limiting conditions, non-cyclic vs cyclic photophorylation, and the regulation of light energy distribution to both PS I and PS II.     

Deletion of the chloroplast-localized AtTerC gene product in Arabidopsis thaliana leads to loss of the thylakoid membrane and to seedling lethality

Early seedling development in plants depends on the biogenesis of chloroplasts from proplastids, accompanied by the formation of thylakoid membranes. An Arabidopsis thaliana gene, AtTerC , whose gene product shares sequence similarity with bacterial tellurite resistance C (TerC), is shown to be involved in a critical step required for the normal organization of prothylakoids and transition into mature thylakoid stacks. The AtTerC gene encodes an integral membrane protein, which contains eight putative transmembrane helices, localized in the thylakoid of the chloroplast, as shown by localization of an AtTerC–GFP fusion product in protoplasts and by immunoblot analysis of subfractions of chloroplasts. T-DNA insertional mutation of AtTerC resulted in a pigment-deficient and seedling-lethal phenotype under normal light conditions. Transmission electron microscopic analysis revealed that mutant etioplasts had normal prolamellar bodies (PLBs), although the prothylakoids had ring-like shapes surrounding the PLBs. In addition, the ultrastructures of mutant chloroplasts lacked thylakoids, did not have grana stacks, and showed numerous globular structures of varying sizes. Also, the accumulation of thylakoid membrane proteins was severely defective in this mutant. These results suggest that the AtTerC protein plays a crucial role in prothylakoid membrane biogenesis and thylakoid formation in early chloroplast development.     

Green tissue within the haustorium of the dwarf mistletoe Korthalsella (Viscaceae). An ultrastructural comparison between chloroplasts of sucker and aerial stem tissues

Summary Korthalsella (Viscaceae) is a dwarf mistletoe attached to its host branch by a single haustorium. Plants are leafless with flattened or cylindrical stems that function in photosynthesis. When a fresh haustorium is cut the sucker within the host appears bright green. Transmission electron microscopy reveals that this greening is due to chloroplasts, but that their organization differs from those of the aerial stem. The three representatives of Korthalsella endemic to New Zealand were the main species investigated. In the stem, chloroplasts have short stacks of cylindrical grana interconnected by stroma thylakoids typical of normal chloroplasts. Sucker chloroplasts have a more variable organization, with most containing extensive granal stacks and poorly differentiated stroma thylakoids. These granal thylakoids exhibit extensive partitions formed by appression of adjacent membranes. Some sucker plastids also approach etioplasts in having a prominent prolamellar body from which radiate thylakoids with short partitions. Sucker chloroplasts usually contain a few large starch grains, plastoglobuli, and sometimes also a stroma centre. The extensive granal thylakoids in sucker chloroplasts of Korthalsella resemble that found in certain shade plants and tissue grown under low light conditions. Sucker chloroplasts probably have a low level of photosynthesis. This activity might provide a local source of osmotically active material used to assist transport between host and parasite.     

The structural and functional domains of plant thylakoid membranes

In plants, the stacking of part of the photosynthetic thylakoid membrane generates two main subcompartments: the stacked grana core and unstacked stroma lamellae. However, a third distinct domain, the grana margin, has been postulated but its structural and functional identity remains elusive. Here, an optimized thylakoid fragmentation procedure combined with detailed ultrastructural, biochemical, and functional analyses reveals the distinct composition of grana margins. It is enriched with lipids, cytochrome b₆f complex, and ATPase while depleted in photosystems and light‐harvesting complexes. A quantitative method is introduced that is based on Blue Native Polyacrylamide Gel Electrophoresis (BN‐PAGE) and dot immunoblotting for quantifying various photosystem II (PSII) assembly forms in different thylakoid subcompartments. The results indicate that the grana margin functions as a degradation and disassembly zone for photodamaged PSII. In contrast, the stacked grana core region contains fully assembled and functional PSII holocomplexes. The stroma lamellae, finally, contain monomeric PSII as well as a significant fraction of dimeric holocomplexes that identify this membrane area as the PSII repair zone. This structural organization and the heterogeneous PSII distribution support the idea that the stacking of thylakoid membranes leads to a division of labor that establishes distinct membrane areas with specific functions.     

Molecular crowding and order in photosynthetic membranes

The integrity and maintenance of the photosynthetic apparatus in thylakoid membranes of higher plants requires lateral mobility of their components between stacked grana thylakoids and unstacked stroma lamellae. Computer simulations based on realistic protein densities suggest serious problems for lateral protein and plastoquinone diffusion especially in grana membranes, owing to strong retardation by protein complexes. It has been suggested that three structural features of grana thylakoids ensure efficient lateral transport: the organization of protein complexes into supercomplexes; the arrangement of supercomplexes into structured assemblies, which facilitates diffusion process in crowded membranes; the limitation of the diameter of grana discs to less than approximately 500 nm, which keeps diffusion times short enough to support regulation of light harvesting and repair of photodamaged photosystem II.     

Thylakoid membrane remodeling during state transitions in Arabidopsis

Adaptability of oxygenic photosynthetic organisms to fluctuations in light spectral composition and intensity is conferred by state transitions, short-term regulatory processes that enable the photosynthetic apparatus to rapidly adjust to variations in light quality. In green algae and higher plants, these processes are accompanied by reversible structural rearrangements in the thylakoid membranes. We studied these structural changes in the thylakoid membranes of Arabidopsis thaliana chloroplasts using atomic force microscopy, scanning and transmission electron microscopy, and confocal imaging. Based on our results and on the recently determined three-dimensional structure of higher-plant thylakoids trapped in one of the two major light-adapted states, we propose a model for the transitions in membrane architecture. The model suggests that reorganization of the membranes involves fission and fusion events that occur at the interface between the appressed (granal) and nonappressed (stroma lamellar) domains of the thylakoid membranes. Vertical and lateral displacements of the grana layers presumably follow these localized events, eventually leading to macroscopic rearrangements of the entire membrane network.     

Insights into the complex 3-D architecture of thylakoid membranes in the unicellular cyanobacterium Cyanothece sp. ATCC 51142

In cyanobacteria and chloroplasts, thylakoids are the complex internal membrane system where the light reactions of oxygenic photosynthesis occur. In plant chloroplasts, thylakoids are differentiated into a highly interconnected system of stacked grana and unstacked stroma membranes. In contrast, in cyanobacteria, the evolutionary progenitors of chloroplasts, thylakoids do not routinely form stacked and unstacked regions, and the architecture of the thylakoid membrane systems is only now being described in detail in these organisms. We used electron tomography to examine the thylakoid membrane systems in one cyanobacterium, Cyanothece sp. ATCC 51142. Our data showed that thylakoids form a complicated branched network with a rudimentary quasi-helical architecture in this organism. A well accepted helical model of grana-stroma architecture of plant thylakoids describes an organization in which stroma thylakoids wind around stacked granum in right-handed spirals. Here we present data showing that the simplified helical architecture in Cyanothece 51142 is lefthanded in nature. We propose a model comparing the thylakoid membranes in plants and this cyanobacterium in which the system in Cyanothece 51142 is composed of non-stacked membranes linked by fret-like connections to other membrane components of the system in a limited left-handed arrangement.Key words: cyanobacteria, Cyanothece 51142, thylakoid membrane, electron tomography, chloroplast     

Light regulation of photosynthetic membrane structure,organization, and function

The light environment during plant growth determines the structural and functional properties of higher plant chloroplasts, thus revealing a dynamically regulated developmental system. Pisum sativum plants growing under intermittent illumination showed chloroplasts with fully functional photosystem (PS) II and PSI reaction centers that lacked the peripheral chlorophyll (Chi) a/b and Chl a light-harvesting complexes (LHC), respectively. The results suggest a light flux differential threshold regulation in the biosynthesis of the photosystem core and peripheral antenna complexes. Sun-adapted species and plants growing under far-red-depleted illumination showed grana stacks composed of few (3–5) thylakoids connected with long intergrana (stroma) thylakoids. They had a PSII/PSI reaction center ratio in the range 1.3–1.9. Shade-adapted species and plants growing under far-red-enrichcd illumination showed large grana stacks composed of several thylakoids, often extending across the entire chloroplast body, and short intergrana stroma thylakoids. They had a higher PSII/PSI reaction center ratio, in the range of 2.2–4.0. Thus, the relative extent of grana and stroma thylakoid formation corresponds with the relative amounts of PSII and PSI in the chloroplast, respectively. The structural and functional adaptation of the photosynthetic membrane system in response to the quality of illumination involves mainly a control on the rate of PSII and PSI complex biosynthesis.     

Three-dimensional architecture of grana and stroma thylakoids of higher plants as determined by electron tomography

We have investigated the three-dimensional (3D) architecture of the thylakoid membranes of Arabidopsis (Arabidopsis thaliana), tobacco (Nicotiana tabacum), and spinach (Spinacia oleracea) with a resolution of approximately 7 nm by electron tomography of high-pressure-frozen/freeze-substituted intact chloroplasts. Higher-plant thylakoids are differentiated into two interconnected and functionally distinct domains, the photosystem II/light-harvesting complex II-enriched stacked grana thylakoids and the photosystem I/ATP synthase-enriched, nonstacked stroma thylakoids. The grana thylakoids are organized in the form of cylindrical stacks and are connected to the stroma thylakoids via tubular junctions. Our data confirm that the stroma thylakoids are wound around the grana stacks in the form of multiple, right-handed helices at an angle of 20° to 25° as postulated by a helical thylakoid model. The junctional connections between the grana and stroma thylakoids all have a slit-like architecture, but their size varies tremendously from approximately 15 × 30 nm to approximately 15 × 435 nm, which is approximately 5 times larger than seen in chemically fixed thylakoids. The variable slit length results in less periodicity in grana/stroma thylakoid organization than proposed in the original helical model. The stroma thylakoids also exhibit considerable architectural variability, which is dependent, in part, on the number and the orientation of adjacent grana stacks to which they are connected. Whereas some stroma thylakoids form solid, sheet-like bridges between adjacent grana, others exhibit a branching geometry with small, more tubular sheet domains also connecting adjacent, parallel stroma thylakoids. We postulate that the tremendous variability in size of the junctional slits may reflect a novel, active role of junctional slits in the regulation of photosynthetic function. In particular, by controlling the size of junctional slits, plants could regulate the flow of ions and membrane molecules between grana and stroma thylakoid membrane domains.     

Dynamic flexibility in the structure and function of photosystem II in higher plant thylakoid membranes: the grana enigma

Grana are not essential for photosynthesis, yet they are ubiquitous in higher plants and in the recently evolved Charaphyta algae; hence grana role and its need is still an intriguing enigma. This article discusses how the grana provide integrated and multifaceted functional advantages, by facilitating mechanisms that fine-tune the dynamics of the photosynthetic apparatus, with particular implications for photosystem II (PSII). This dynamic flexibility of photosynthetic membranes is advantageous in plants responding to ever-changing environmental conditions, from darkness or limiting light to saturating light and sustained or intermittent high light. The thylakoid dynamics are brought about by structural and organizational changes at the level of the overall height and number of granal stacks per chloroplast, molecular dynamics within the membrane itself, the partition gap between appressed membranes within stacks, the aqueous lumen encased by the continuous thylakoid membrane network, and even the stroma bathing the thylakoids. The structural and organizational changes of grana stacks in turn are driven by physicochemical forces, including entropy, at work in the chloroplast. In response to light, attractive van der Waals interactions and screening of electrostatic repulsion between appressed grana thylakoids across the partition gap and most probably direct protein interactions across the granal lumen (PSII extrinsic proteins OEEp-OEEp, particularly PsbQ-PsbQ) contribute to the integrity of grana stacks. We propose that both the light-induced contraction of the partition gap and the granal lumen elicit maximisation of entropy in the chloroplast stroma, thereby enhancing carbon fixation and chloroplast protein synthesizing capacity. This spatiotemporal dynamic flexibility in the structure and function of active and inactive PSIIs within grana stacks in higher plant chloroplasts is vital for the optimization of photosynthesis under a wide range of environmental and developmental conditions.     

Comparative ultrastructural properties of pallisade tissue and spongy tissue chloroplasts from normal and mutant leaves of Pisum sativum L.*

Abstract. The ultrastructure of chloroplasts from palisade and spongy tissue was studied in order to analyse the adaptation of chloroplasts to the light gradient within the bifacial leaves of pea. Chloroplasts of two nuclear gene mutants of Pisum sativum (chlorotica-29 and chlorophyll b-less 130A), grown under normal light conditions, were compared with the wild type (WT) garden-pea cv. ‘Dippes Gelbe Viktoria’. The differentiation of the thylakoid membrane system of plastids from normal pea leaves exhibited nearly the same degree of grana formation in palisade and in spongy tissue. Using morphometrical measurements, only a slight increase in grana stacking capacity was found in chloroplasts of spongy tissue. In contrast, chloroplasts of mutant leaves differed in grana development in palisade and spongy tissue, respectively. Their thylakoid systems appeared to be disorganized and not developed as much as in chloroplasts from normal pea leaves. Grana contained fewer lamellae per granum, the number of grana per chloroplast section was reduced and the length of appressed thylakoid regions was decreased. Nevertheless, chloroplasts of the mutants were always differentiated into grana and stroma thylakoids. The structural changes observed and the reduction of the total chlorophyll content correlated with alterations in the polypeptide composition of thylakoid membrane preparations from mutant chloroplasts. In sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), polypeptide bands with a relative molecular mass of 27 and 26 kilodalton (kD) were markedly reduced in mutant chloroplasts. These two polypeptides represented the major apoproteins of the light harvesting chlorophyll a/b complex from photosystem II (LHC-II) as inferred from a comparison with the electrophoretic mobility of polypeptides isolated from the LHC-II.     

Dark-adapted spinach thylakoid protein heterogeneity offers insights into the photosystem II repair cycle

In higher plants, thylakoid membrane protein complexes show lateral heterogeneity in their distribution: photosystem (PS) II complexes are mostly located in grana stacks, whereas PSI and adenosine triphosphate (ATP) synthase are mostly found in the stroma-exposed thylakoids. However, recent research has revealed strong dynamics in distribution of photosystems and their light harvesting antenna along the thylakoid membrane. Here, the dark-adapted spinach (Spinacia oleracea L.) thylakoid network was mechanically fragmented and the composition of distinct PSII-related proteins in various thylakoid subdomains was analyzed in order to get more insights into the composition and localization of various PSII subcomplexes and auxiliary proteins during the PSII repair cycle. Most of the PSII subunits followed rather equal distribution with roughly 70% of the proteins located collectively in the grana thylakoids and grana margins; however, the low molecular mass subunits PsbW and PsbX as well as the PsbS proteins were found to be more exclusively located in grana thylakoids. The auxiliary proteins assisting in repair cycle of PSII were mostly located in stroma-exposed thylakoids, with the exception of THYLAKOID LUMEN PROTEIN OF 18.3 (TLP18.3), which was more evenly distributed between the grana and stroma thylakoids. The TL29 protein was present exclusively in grana thylakoids. Intriguingly, PROTON GRADIENT REGULATION5 (PGR5) was found to be distributed quite evenly between grana and stroma thylakoids, whereas PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1) was highly enriched in the stroma thylakoids and practically missing from the grana cores. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

Significance of structural variation in thylakoid membranes in maintaining functional photosystems during reproductive growth

Structural variation in the stroma‐grana (SG) arrangement of the thylakoid membranes, such as changes in the thickness of the grana stacks and in the ratio between grana and inter‐grana thylakoid, is often observed. Broadly, such alterations are considered acclimation to changes in growth and the environment. However, the relation of thylakoid morphology to plant growth and photosynthesis remains obscure. Here, we report changes in the thylakoid during leaf development under a fixed light condition. Histological studies on the chloroplasts of fresh green Arabidopsis leaves have shown that characteristically shaped thylakoid membranes lacking the inter‐grana region, referred to hereafter as isolated‐grana (IG), occurred adjacent to highly ordered, large grana layers. This morphology was restored to conventional SG thylakoid membranes with the removal of bolting stems from reproductive plants. Statistical analysis showed a negative correlation between the incidences of IG‐type chloroplasts in mesophyll cells and the rates of leaf growth. Fluorescence parameters calculated from pulse‐amplitude modulated fluorometry measurements and CO₂ assimilation data showed that the IG thylakoids had a photosynthetic ability that was equivalent to that of the SG thylakoids under moderate light. However, clear differences were observed in the chlorophyll a/b ratio. The IG thylakoids were apparently an acclimated phenotype to the internal condition of source leaves. The idea is supported by the fact that the life span of the IG thylakoids increased significantly in the later developing leaves. In conclusion, the heterogeneous state of thylakoid membranes is likely important in maintaining photosynthesis during the reproductive phase of growth.     

Electron tomography of plant thylakoid membranes

For more than half a century, electron microscopy has been a main tool for investigating the complex ultrastructure and organization of chloroplast thylakoid membranes, but, even today, the three-dimensional relationship between stroma and grana thylakoids, and the arrangement of the membrane protein complexes within them are not fully understood. Electron cryo-tomography (cryo-ET) is a powerful new technique for visualizing cellular structures, especially membranes, in three dimensions. By this technique, large membrane protein complexes, such as the photosystem II supercomplex or the chloroplast ATP synthase, can be visualized directly in the thylakoid membrane at molecular (4-5 nm) resolution. This short review compares recent advances by cryo-ET of plant thylakoid membranes with earlier results obtained by conventional electron microscopy.     

The structure and function of the chloroplast photosynthetic membrane — a model for the domain organization

Recent work on the domain organization of the thylakoid is reviewed and a model for the thylakoid of higher plants is presented. According to this model the thylakoid membrane is divided into three main domains: the stroma lamellae, the grana margins and the grana core (partitions). These have different biochemical compositions and have specialized functions. Linear electron transport occurs in the grana while cyclic electron transport is restricted to the stroma lamellae. This model is based on the following results and considerations. (1) There is no good candidate for a long-range mobile redox carrier between PS II in the grana and PS I in the stroma lamellae. The lateral diffusion of plastoquinone and plastocyanin is severely restricted by macromolecular crowding in the membrane and the lumen respectively. (2) There is an excess of 14±18% chlorophyll associated with PS I over that of PS II. This excess is assumed to be localized in the stroma lamellae where PS I drives cyclic electron transport. (3) For several plant species, the stroma lamellae account for 20±3% of the thylakoid membrane and the grana (including the appressed regions, margins and end membranes) for the remaining 80%. The amount of stroma lamellae (20%) corresponds to the excess (14–18%) of chlorophyll associated with PS I. (4) The model predicts a quantum requirement of about 10 quanta per oxygen molecule evolved, which is in good agreement with experimentally observed values. (5) There are at least two pools of each of the following components: PS I, PS II, cytochrome bf complex, plastocyanin, ATP synthase and plastoquinone. One pool is in the grana and the other in the stroma compartments. So far, it has been demonstrated that the PS I, PS II and cytochrome bf complexes each differ in their respective pools.Abbreviations PS I and PS II Photosystem I and II - P 700 reaction center of PS I - LHC II light-harvesting complex II     

Towards elucidation of dynamic structural changes of plant thylakoid architecture

Long-term acclimation of shade versus sun plants modulates the composition, function and structural organization of the architecture of the thylakoid membrane network. Significantly, these changes in the macroscopic structural organization of shade and sun plant chloroplasts during long-term acclimation are also mimicked following rapid transitions in irradiance: reversible ultrastructural changes in the entire thylakoid membrane network increase the number of grana per chloroplast, but decrease the number of stacked thylakoids per granum in seconds to minutes in leaves. It is proposed that these dynamic changes depend on reversible macro-reorganization of some light-harvesting complex IIb and photosystem II supracomplexes within the plant thylakoid network owing to differential phosphorylation cycles and other biochemical changes known to ensure flexibility in photosynthetic function in vivo. Some lingering grana enigmas remain: elucidation of the mechanisms involved in the dynamic architecture of the thylakoid membrane network under fluctuating irradiance and its implications for function merit extensive further studies.     

Light-induced structural changes of thylakoids in normal and carotenoid deficient chloroplasts of maize

Summary Changes of membrane thickness and loculi were studied after red (650 nm) and far-red (707 nm) light in thylakoids of maize with different stacking and pigment compositions.The most intensive shrinkage of thylakoid membranes occurred in grana and under red light. Membranes of stroma thylakoids responded more to far-red light. Bundle sheath thylakoid membranes did not change in thickness. Loculi decreased in all types of thylakoids under both, red and far-red light. Thylakoids obtained from a -carotenic mutant exhibited a contrasting response: swelling under red light followed by photodestruction. Changes under far-red light were similar to that of normal stroma thylakoids.The data on normal chloroplasts show that the light induced shrinkage of membranes and the decrease of loculi are coupled to a different degree in various kinds of thylakoids; that the thylakoid flattening can be correlated with the Photosystem content of the membranes; and that two kinds of single thylakoids (stroma lamellae and bundle sheath lamellae) are different in molecular structure and function.Data on carotenoid deficient chloroplasts indicate a photooxidative destruction of the thylakoids by Photosystem 2 that occurs in the absence of normal carotenoids.     

New insights in thylakoid membrane organization

High-pressure freezing (HPF) in combination with freeze substitution (FS) was used to analyse changes in the structure of barley chloroplasts during the daily change of light and darkness. In contrast to conventional treatment of samples, HPF-FS revealed substantial differences in chloroplast shape, volume and ultrastructure in the light period and during darkness. While chloroplasts have an ellipsoidal shape in the light, they have an enlarged and round form during the dark period. Samples collected in the light show the typical differentiation of stroma and grana thylakoids as observed by conventional ultrastructural analyses. In chloroplasts of samples collected during the dark period, thylakoids were swollen and grana stacks to a large extent were disintegrated. Similar changes occurred when leaves in the light were treated with the uncoupler gramicidin. The results suggest that the light-dependent changes in thylakoid membrane organization are related to the light-dependent changes in the ionic milieu of the thylakoid lumen and the stroma.     

Correlation between spatial (3D) structure of pea and bean thylakoid membranes and arrangement of chlorophyll-protein complexes

ABSTRACT: BACKGROUND: The thylakoid system in plant chloroplasts is organized into two distinct domains: granaarranged in stacks of appressed membranes and non-appressed membranes consisting ofstroma thylakoids and margins of granal stacks. It is argued that the reason for thedevelopment of appressed membranes in plants is that their photosynthetic apparatus need tocope with and survive ever-changing environmental conditions. It is not known however,why different plant species have different arrangements of grana within their chloroplasts. Itis important to elucidate whether a different arrangement and distribution of appressed andnon-appressed thylakoids in chloroplasts are linked with different qualitative and/orquantitative organization of chlorophyll-protein (CP) complexes in the thylakoid membranesand whether this arrangement influences the photosynthetic efficiency. RESULTS: Our results from TEM and in situ CLSM strongly indicate the existence of differentarrangements of pea and bean thylakoid membranes. In pea, larger appressed thylakoids areregularly arranged within chloroplasts as uniformly distributed red fluorescent bodies, whileirregular appressed thylakoid membranes within bean chloroplasts correspond to smaller andless distinguished fluorescent areas in CLSM images. 3D models of pea chloroplasts show adistinct spatial separation of stacked thylakoids from stromal spaces whereas spatial divisionof stroma and thylakoid areas in bean chloroplasts are more complex. Structural differencesinfluenced the PSII photochemistry, however without significant changes in photosyntheticefficiency. Qualitative and quantitative analysis of chlorophyll-protein complexes as well asspectroscopic investigations indicated a similar proportion between PSI and PSII corecomplexes in pea and bean thylakoids, but higher abundance of LHCII antenna in pea ones.Furthermore, distinct differences in size and arrangements of LHCII-PSII and LHCI-PSIsupercomplexes between species are suggested. CONCLUSIONS: Based on proteomic and spectroscopic investigations we postulate that the differences in thechloroplast structure between the analyzed species are a consequence of quantitativeproportions between the individual CP complexes and its arrangement inside membranes.Such a structure of membranes induced the formation of large stacked domains in pea, orsmaller heterogeneous regions in bean thylakoids. Presented 3D models of chloroplasts showed that stacked areas are noticeably irregular with variable thickness, merging with eachother and not always parallel to each other.     

Structural and functional self-organization of Photosystem II in grana thylakoids

The biogenesis of the well-ordered macromolecular protein arrangement of photosystem (PS)II and light harvesting complex (LHC)II in grana thylakoid membranes is poorly understood and elusive. In this study we examine the capability of self organization of this arrangement by comparing the PSII distribution and antenna organization in isolated untreated stacked thylakoids with restacked membranes after unstacking. The PS II distribution was deduced from freeze-fracture electron microscopy. Furthermore, changes in the antenna organization and in the oligomerization state of photosystem II were monitored by chlorophyll a fluorescence parameters and size analysis of exoplasmatic fracture face particles. Low-salt induced unstacking leads to a randomization and intermixing of the protein complexes. In contrast, macromolecular PSII arrangement as well as antenna organization in thylakoids after restacking by restoring the original solvent composition is virtually identical to stacked control membranes. This indicates that the supramolecular protein arrangement in grana thylakoids is a self-organized process.