Plant Physiology - Musiyenko M.M. 2001

Photosynthesis: Physiological, Biochemical, and Ecological Aspects
Other components of the thylakoid membrane

According to Electron Cell/15.html">Microscopy data, the thylakoid membrane features a mosaic Structure incorporating the discrete complexes listed above. The uneven distribution of Protein Complexes results in varying densities of charged groups on the surfaces of the thylakoid and stromal membranes. There are also distinct differences in the Lipid Composition of granal and stromal thylakoids. Neutral galactolipids predominate among the thylakoid Membrane Lipids, with monogalactosyldiacylglycerol (MGDG) accounting for up to 45%. In addition, sulfolipids comprise up to 12% and phosphatidylglycerols 10%. Phosphatidylcholine accounts for up to 5%, and plastoquinone for 2%. Thylakoid membranes are characterized by an exceptionally high degree of fatty acid chain saturation. Linoleic acid (C 18:3) makes up nearly 80% of these, ensuring membrane fluidity.

Electron transport is possible between individual rigidly fixed Components of the Electron Transport Chain (via tunneling-type electron transfer), although conformational changes within the complexes induced by shifts in pH, Temperature, and ionic COMPOSITION OF THE surrounding environment cannot be ruled out.

Furthermore, connection between spatially separated complexes is mediated by mobile electron carriers, specifically plastoquinone, plastocyanin, and ferredoxin.

Lateral movement of plastoquinone molecules occurs within the lipid phase of the membranes, whose thickness is comparable to the size of the plastoquinone molecule itself (Fig. 61).

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Fig. 61. Diagram of plastoquinone movement during its oxidation-reduction

The functioning of plastoquinone as an electron and proton carrier involves a relatively rapid (within 1–5 ms) reorientation of its quinoid moiety within the membrane. Specifically, its protonation—and consequently its reduction—occurs on the outer surface of the membrane, whereas oxidation and deprotonation take place on the inner thylakoid membrane surface. It is believed that the lateral diffusion of plastoquinone within the Hydrophobic core of the membranes establishes The connection between the spatially distant PS II and b₆—f complexes.

The second component of The electron transport chain, plastocyanin, diffuses within the thylakoid lumen. Plastocyanin is a protein with a Molecular Weight of 21,000, and each of its molecules contains 2 copper atoms. Its concentration in METABOLISM/14.html">Chloroplasts reaches up to 0.2% of the total chlorophyll content.

The distance between the two inner surfaces of the thylakoid membrane is 100–200 Å, which corresponds to the diameter of the plastocyanin molecule itself. The diffusion of plastoquinone and plastocyanin facilitates relatively rapid electron transfer.

Finally, the thylakoid membrane contains Three types of ferredoxins: a soluble ferredoxin with a Redox Potential of –0.43 V, containing a 2Fe–2S center, and two membrane-bound ferredoxins. They have potentials of –0.54 V and –0.59 V and are accordingly designated as ferredoxin 540 and ferredoxin 590 (Fd₅₄₀ and Fd₅₉₀). They contain

Fig. 62. Diagram of iron-sulfur center clusters

in their structure 4Fe–4S centers (Fig. 62). All ferredoxins are single-electron redox systems that do not transport protons. Ferredoxins play a crucial role in redistributing electrons between cyclic and non-Cyclic electron transport pathways.

Soluble ferredoxin can form a 1:1 complex with yet another component of the thylakoid membrane, ferredoxin-NADP⁺ oxidoreductase. The latter is a membrane-bound flavoprotein that drives the reduction of NADP⁺. This enzyme complex is thought to be loosely bound to the outer surface of the thylakoid membrane. Its reduction requires 2H⁺ and 2e^-, with protons supplied by the aqueous environment and electrons derived from reduced soluble ferredoxin molecules.

Thus, chloroplast thylakoids are capable of absorbing light and converting its energy owing to the presence of highly complex, plastic structures that can modulate their activity in response to the internal state of organisms and environmental changes.