Structure, Composition, Functional Organization and Dynamic Properties of Thylakoid Membranes

Abstract

Chloroplasts are semi-autonomous organelles comprised of two envelope membranes, an aqueous matrix known as stroma, and internal membranes called thylakoids. All of the light-harvesting and energy-transducing functions are located in the thylakoids, which form a physically continuous membrane system that encloses an aqueous compartment, the thylakoid lumen. With few exceptions thylakoids are differentiated into stacked grana and non-stacked stroma membrane regions. A model of the three-dimensional relationship between grana and stroma thylakoids is presented. The membrane continuum is formed by a lipid bilayer that contains unique types of lipids. The principal functions of thylakoids are the trapping of light energy and the transduction of this energy into the chemical energy forms, ATP and NADPH. During this process, water is oxidized and oxygen is released. These functions are performed by five large protein complexes: Photosystem I with bound antennae, Photosystem II with bound antennae, light-harvesting complex II, cytochrome b 6 f, and ATP synthase. The roles of these complexes in photosynthetic electron transport and ATP synthesis are discussed. The differentiation of thylakoids into grana and stroma membrane regions is a morphological reflection of an underlying non-random distribution of the five complexes between the two types of membrane domains. The most prominent effect of membrane stacking is the physical segregation of most Photosystem II to stacked grana membranes, and of most Photosystem I to unstacked stroma membranes. The evolutionary roots and the functional implications of this non-random organization of thylakoid membrane components are discussed in some detail. The final section of this chapter describes how thylakoid membranes adapt to long-term and short-term changes in the light-environment. Long-term light changes cause alterations in the ratios of the different types of protein complexes in turn to optimize the use of available light energy. In contrast, short-term light changes modulate the organization of membrane components and serve primarily to protect the photosystems and only secondarily to optimize the turnover of the electron transport chain.