Membrane unstacking

The physical separation of PS II and PS I permits the chloroplasts to respond to changes in illumination. The relative amount of light absorbed by these two systems varies with the distribution of light harvesting complexes (LHCs) between the stacked and unstacked portion of the thylakoid membrane. 

PS II absorbs more light than PS I. As such PS I cannot take electrons as fast as PS II can supply, leaving plastoquinone in its reduced state. This reduced plastoquinone activates a protein kinase that phosphorylates the threonine (Thr) residue of the LHCs that in turn migrate to the unstacked portion of the thylakoid membrane where it binds to PS I. As a result, a large portion of incident light is funneled to PS I. 

ATP synthase, whose knobs also protrude into the stroma. A photosynthetic unit can also be defined chemically by the number of various types of molecules present in a chloroplast membrane for each four manganese atoms (Table 23-2). Separate units contain PSI and PSII. These reaction centers appear to have a different distribution within the thylakoids, the PSI units being located principally in the unstacked membranes and the PSII units in the grana stacks.255 259... 

Figure 19.33. Location of Photosynthesis Components. Photosynthetic assemblies are differentially distributed in the stacked (appressed) and unstacked (nonapressed) regions of thylakoid membranes. [After a drawing kindly provided by Dr. Jan M. Anderson and Dr. Bertil Andersson.]...

KR Miller and LA Staehelin (1976) Analysis of the thylakoid outer surface coupling factor is limited to unstacked membrane regions. J Cell Biol 68 30-47... 

EFj - exoplasmic fracture face of s.tacked membrane EFjj - exoplasmic fracture face of unstacked membrane... 

Chloroplasts have inner and outer membranes. A third membrane forms within the aqueous, enzyme-rich stroma into flattened sacs called thylakoids. A stack of thylakoids is called a granum. Unstacked, connecting thylakoid membrane is referred to as stroma lamellae, (a) An electron micrograph of a chloroplast, (b) a diagrammatic view of a chloroplast,... 

PSI are most abundant in the unstacked stromal lamellae. In contrast, PSII are located primarily in the stacked regions of thy-lakoid membrane. Cytochrome b6f is found in both areas of thylakoid membrane. The ATP synthase is found only in thylakoid membrane that is directly in contact with the stroma. 

The main problem is solving the Fourier series in Eq. 12 is again the indeterminany of the signs of i.e. the phase problem. An important aid to the correct assignment can be obtained from the continuous structure factors F determined from experiments with random, unstacked samples provided, of course, that the membrane structures are the same in both cases. This has been discussed in detail recently by Blaurock Many other methods have been described in the literature, most of them, however, have to rely on trial-and-error searches involving information from other sources... 

Rationalize the differences in the photosynthetic assemblies in the stacked and unstacked regions of the thylakoid membranes. 

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