Chloroplasts composition

Typically several different carotenoids occur in plant tissues containing this class of pigments. Carotenoids are accumulated in chloroplasts of all green plants as mixtures of a- and P-carotene, P-cryptoxanthin, lutein, zeaxanthin, violaxanthin, and neoxanthin. These pigments are found as complexes formed by noncovalent bonding with proteins. In green leaves, carotenoids are free, nonesterified, and their compositions depend on the plant and developmental conditions. In reproductive... 

Over-expression of bacterial phytoene synthase led to only modest increases in pigment accumulation (except in the case of chloroplast-contaiifing tissues). Attention turned to CrtI, one gene that might control flux through the entire four desaturation steps from phytoene to lycopene (discussed in Section 5.3.2.4). Only a modest increase in carotenoid content in tomatoes and a variety of changes in carotenoid composition including more P-carotene, accompanied by an overall decrease in total carotenoid content (no lycopene increase), resulted when CrtI was over-expressed under control of CaMV 35S. Apparently, the bacterial desaturase... 

Chang has made observations on the polysomes of pinto bean leaves exposed to ozone (at 0.35 ppm for 20-50 min). He found that the chloroplast polysomes were more susceptible to oxidation than was the cytoplasmic ribosomes. The sulfhydryl content of the chloroplast ribosomes was also much more susceptible to oxidation than was that of the cytoplasmic ribosomes. Finally, it was found that the effects of ozone on ribosome composition could be reproduced by p-mer-curicbenzoate. Chang s results imply that either ozone itself or a product of ozone oxidation passes from the cytoplasmic membrane to the interior of the chloroplast before having its effect. These results connect with a number of papers on the oxidation of sulfhydryl compounds by ozone. Tomlinson and Rich have reported decreases in leaf sulfhydryl groups after ozone exposure (at 1 ppm for 30-60 min). 

The I vaginalis hydrogenosomal presequences are generally short, ranging from 5 to 14 amino acid residues for those that have been proven experimentally, and up to 17 residues for the predicted presequences (Table 1). The presequences are enriched in the amino acid residues Ser (20%), Leu (14%), Arg (11%), Ala (8%), Phe (7%), Val (6%), Thr (6%) and Asn (5%). The other amino acids are significantly under-represented. Incidentally, or accidentally, the three amino acids most commonly found in these presequences, Ser, Leu and Arg, are the ones that are each encoded by six codons. This may have been relevant in the evolution of these presequences. The mitochondrial matrix N-terminal presequences are enriched in Arg (14%), Leu (12%), Ser (11%) and Ala (14%). On the other hand, chloroplast leader peptides have a different amino acid composition with 19% Ser and 9% Thr (von Heijne et al. 1989). Markedly under-represented in hydrogenosomal presequences are the acidic residues, as in the case of both mitochondrial and plastidic presequences (von Heijne et al. 1989). 

For studies of membrane composition, the first task is to isolate a selected membrane. When eukaryotic cells are subjected to mechanical shear, their plasma membranes are torn and fragmented, releasing cytoplasmic components and membrane-bounded organelles such as mitochondria, chloroplasts, lysosomes, and nuclei. Plasma membrane fragments and intact organelles can be isolated by centrifugal techniques described in Chapter 1 (see Fig. 1-8). 

Light-driven electron transfer in plant chloroplasts during photosynthesis is accomplished by multienzyme systems in the thylakoid membrane. Our current picture of photosynthetic mechanisms is a composite, drawn from studies of plant chloroplasts and a variety of bacteria and algae. Determination of the molecular structures of bacterial photosynthetic complexes (by x-ray crystallography) has given us a much improved understanding of the molecular events in photosynthesis in general. 

Generalized representations of the internal structures of animal and plant cells (eukaryotic cells). Cells are the fundamental units in all living systems, and they vary tremendously in size and shape. All cells are functionally separated from their environment by the plasma membrane that encloses the cytoplasm. Plant cells have two structures not found in animal cells a cellulose cell wall, exterior to the plasma membrane, and chloroplasts. The many different types of bacteria (prokaryotes) are all smaller than most plant and animal cells. Bacteria, like plant cells, have an exterior cell wall, but it differs greatly in chemical composition and structure from the cell wall in plants. Like all other cells, bacteria have a plasma membrane that functionally separates them from their environment. Some bacteria also have a second membrane, the outer membrane, which is exterior to the cell wall. 

J.P. Thomber, J.C. Stewart, M.W.C. Hatton and J.L. Bailey, Studies on the nature of chloroplast lamellae, II, Chemical composition and further physical properties of two chlorophyll-protein complexes, Biochemistry 6 (1967) 2006-2014. 

Chapman, D.J., J. De Felice, and J. Barber (1985). Characteristics of chloroplast thylokoid lipid composition associated with resistance to triazine herbicides. Planta, 166 280-285. 

Lehoczki, E., E. Polos, G. Laskay, and T. Farkas (1985). Chemical compositions and physical states of chloroplast lipids related to atra-zine resistance in Conyza canadensis L. Plant Sci., 42 19-24. 

Eukaryotes have various DNA molecules, arranged in linear fibers which are repeatedly coiled and folded to produce highly organised chromosomes, and a composite cytoplasm which is divided into distinct compartments and houses a variety of cell organelles (mitochondria, chloroplasts, lysosomes, the endoplasmic reticulum, etc.) the form of the cell is due to an internal cytoskeleton which is made of three different types of filaments (microtubules, microfilaments and intermediate filaments). 

DSC has been used to study the individual protein components of biological membranes of relatively simply protein composition and the interaction of several of these components with lipids and with other proteins. The red blood cell membrane, which has been most intensively studied, exhibits five discrete protein transitions, each of which has been assigned to a specific membrane protein. The response of each of these thermal transitions to variations in temperature and pH as well as to treatment with proteases, phospholipases, specific labelling reagents, and modifiers and inhibitors of selected membrane activities, has provided much useful information on the interactions and functions of these components in the intact erythrocyte membrane (46-49). Similar approaches have been applied to the bovine rod outer segment membrane (50) and to the spinach chloroplast thylakoid membrane (51). 

CF], the site of ATP synthesis, has a subunit composition a 3 P 3 y 8 e. The P subunits contain the catalytic sites, similar to the F] subunit of mitochondrial ATP synthase. Remarkably, P subunits of com chloroplast ATP synthase are more than 60% identical in amino acid sequence with those of human ATP synthase, despite the passage of approximately 1 billion years since the separation of the plant and animal kingdoms. 

Concurrently to the assimilation in leaves there is always CO2 production in respiration and photorespiration (the first occurs all the time and involves the mitochondria, the latter occurs only in the light and involves also the oxygenase activity of Rubisco in the chloroplasts). Any discrimination in the respiration process will be reflected in the isotopic composition of the CO2 produced and mixed into the leaf internal CO2 pool and its effect must, therefore, be subtracted from Ap. The combined effect of the two respiratory components can be subtracted from Equation (24) as (Farquhar and Lloyd, 1993 Farquhar et al., 1982)... 

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