Light photosynthesis

Natural populations of Trichodesmium which are often found in the upper layers of the euphotic zone (see Chapter 16 by Karl et al, this volume), appear to be adapted to high light with a relatively shallow compensation depth (typically 100-200 imol quanta s ) for photosynthesis (Carpenter, 1983a,b LaRoche and Breitbarth, 2005). Several early studies considered the light-photosynthesis relationships o Trichodesmium (e.g., Lewis et al, 1988 Li et al, 1980). Half saturation (4) constants for photosynthesis are reported to be about 300 pmol quanta m s (based on results from four studies, LaRoche and Breitbarth, 2005). See LaRoche and Breitbarth (2005) for a recent comprehensive summary of observed physiological and photosynthetic parameters for Trichodesmium. 

Berthon J. F. and Morel A. (1992) Validation of a spectral light-photosynthesis model and use of the model in conjunction with remotely sensed pigment observations. Limnol. Oceanogr. 37(4), 781—796. 

J.T.O. Kirk (1994). Light Photosynthesis in Aquatic Ecosystems. Cambridge University Press, New York. 

Light (photosynthesis) or compounds with high potential energy (respiration)... 

A chemical reaction (often a decomposition) caused by exposure to light. Photosynthesis... 

Weber S, Ohmes E, Thurnauer M C, Norris J R and Kothe G 1995 Light-generated nuclear quantum beats a signature of photosynthesis Proc. Natl Acad. Sc/. USA 92 7789-93... 

The by-product CO2 is released to the atmosphere. In plant metaboHsm, carbon dioxide from the air is taken into the leaves of the plant. Using energy from light, carbon dioxide reacts with water ia the presence of enzymes to produce sugar. This reaction, photosynthesis, is the reverse of the above reaction. 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

Bacterial Photosynthesis A light-dependent, anaerobic mode of metabolism. Carbon dioxide is reduced to glucose, which is used for both biosynthesis and energy production. Depending on the hydrogen source used to reduce COj, both photolithotrophic and photoorganotrophic reactions exist in bacteria. 

Chloroplasts are the site of photosynthesis, the reactions by which light energy is converted to metabolically useful chemical energy in the form of ATP. These reactions occur on the thylakoid membranes. The formation of carbohydrate from CO9 takes place in the stroma. Oxygen is evolved during photosynthesis. Chloroplasts are the primary source of energy in the light. 

Photosynthesis Consists of Both Light Reactions and Dark Reactions... 

FIGURE 22.4 The light-dependent and light-independent reactions of photosynthesis. Light reactions are associated with the thylakoid membranes, and light-independent reactions are associated with the stroma. 

The existence of two separate but interacting photosystems in photosynthetic eukaryotes was demonstrated through analysis of the photochemical action spectrum of photosynthesis, in which the oxygen-evolving capacity as a function of light wavelength was determined (Figure 22.10). 

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