Photoinhibition is a state of physiological stress that occurs in all oxygen evolving photosynthetic organisms exposed to light. The primary damage occurs within the reaction center of Photosystem II (PS II). While irreversible photoinduced damage to PS II occurs at all light intensities, the efficiency of photosynthetic electron transfer decreases markedly only when the rate of damage exceeds the rate of its repair, which requires de novo PS II protein synthesis. Photoinhibition has been studied for over a century using a large variety of biochemical, biophysical and genetic methodologies. The discovery of the light induced turnover of a protein, encoded by the plastid psbA gene (the D1 protein), later identified as one of the photochemical reaction center II proteins, has led to the elucidation of the underlying mechanism of photoinhibition and to a deeper understanding of the PS II ‘life cycle.’

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Photoinhibition – a historical perspective

Physiology and cytological chemistry blue-green algae.

Structure and function of gas vacuoles.

Publisher Summary In higher plants, the process of photosynthesis occurs within specific membrane bounded organelles called “chloroplasts.” All the chloroplasts exhibit three major structural regions— namely, highly organized internal sac-like flat compressed vesicles called “thylakoids,” an amorphous background rich in soluble proteins called “stroma,” and a pair of outer membranes known as the “envelope.” The envelope essentially renders functional and structural integrity to the chloroplast. This chapter discusses the structure, isolation, chemical composition, and origin of the higher plant chloroplast envelope. The chapter examines the multiple functions of this important membranous system involved in the regulation of the inflow of raw materials for photosynthesis and the outflow of photosynthetic products. The chloroplast envelope of higher plants is a permanent structure and consists of two morphologically and topologically distinct membranes separated by a region about 10–20 nm thick, which appears electron-translucent. The structure of both envelope membranes is consistent with the lipid-globular protein mosaic model of membrane structure as proposed by Singer and Nicolson.

Structure and Function of the Plastid Envelope

All chemical and physical properties of cells have a structural basis. Thus, a complete understanding of photosynthetic membrane function requires a detailed knowledge of the structural organization of such membranes. During the last 15 years significant progress has been made in the comprehensive analysis of the photochemical reactions catalyzed by chlorophyll, in the identification of the molecules associated with these processes, and in the elucidation of the three-dimensional arrangement of these molecules in photosynthetic membranes. As all of these fields have progressed, a noticeable convergence of ideas has occurred, blurring the artificial boundaries between different areas of research, and bringing us closer to a true understanding of how light energy is converted into chemical energy forms. At the same time, interest in “nontradi-tional” photosynthetic organisms has begun to increase, partly out of general curiosity, and partly because such organisms may possess unique features that allow for better probing of basic photosynthetic mechanisms. Thus, two of the main goals of this review are to provide an overview of the structural diversity of photosynthetic membrane systems, and to highlight how architectural parameters help define functional parameters of photosynthetic membranes.

T. Bisalputra

Papers

Possible respiratory sites in a blue-green alga Nostoc sphaericum as demonstrated by potassium tellurite and tetranitro-blue tetrazolium reduction.

Subunits in chloroplast membranes of Scenedesmus quadricauda.