Membranes, cell chloroplast

The second group of membrane lipids are those that predominate in plant cells the galactolipids, in which one or two galactose residues are connected by a gly-cosidic linkage to C-3 of a 1,2-diacylglycerol (Fig. 10-10 see also Fig. 10-6). Galactolipids are localized in the thylakoid membranes (internal membranes) of chloroplasts they make up 70% to 80% of the total membrane lipids of a vascular plant. They are probably the most... 

Ferrochelatase (protoheme ferro-lyase)401 403 inserts Fe2+ into protoporphyrin IX to form heme. The enzyme is found firmly bound to the inner membrane of mitochondria of animal cells, chloroplasts of plants, and chromatophores of bacteria. While Fe2+ is apparently the only metallic ion ordinarily inserted into a porphyrin, the Zn2+ protoporphyrin chelate accumulates in substantial amounts in yeast, and Cu2+-heme complexes are known (p. 843). Ferrochelatase, whose activity is stimulated by Ca2+, appears to be inhibited by lead ions, a fact that may account for some of the acute toxicity of lead.404... 

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. 

The proteins involved with electron transfer vary in size and shape. Thus the internal membranes of chloroplasts and mitochondria are not uniform and regular. Proteins taking part in ion transport and cell wall synthesis can be embedded in the plasma membrane, and other proteins involved with transport occur in the tonoplast. Many globular proteins in membranes serve a structural role. 

Between photosystem II submembrane fractions which must be isolated and whole cells which must be cultivated, many immobilization works (28 on 45 references cited) employed chloroplasts and thylakoids intact chloroplasts, mixtures of chloroplasts and photosynthetic membranes, or specially, thylakoids alone. To obtain these photosynthetic membranes, the chloroplasts were subjected to osmotic shock by briefly placing them in a hypotonic medium. This procedure ruptures the chloroplast envelope and releases the stroma. The stripped chloroplasts were then returned to an isotonic medium. The interest of this photosynthetic material is justified by a direct contact between the reaction sites and the operation medium. The photosyntheric preparations were often obtained from spinach leaves. 

Chloroplasts are approximately oval saucers, 4-10 jj,m in diameter and about 1 (xm thick. Each has a double membrane (the chloroplast envelope) around it which separates it from the rest of the plant cell. Inside, the chloroplast contains stacks (known as grana) of green discs, called thylakoids, that look like miniature blocks of flats joined together by "walk-... 

Maeda, M., and G.A. Thompson, Jr., J. Cell Biol. 102 289-297 (1986). On the mechanism of rapid plasma membrane and chloroplast expansion in Dunaliella salina exposed to hypoosmotic shock. 

To further identify the regulatory mechanisms involved in vesicle-mediated lipid transfer, the different steps involved need to be characterized separatedly. This work concerns the first step the release of lipids, presumably as vesicles, from the lipid synthesizing membrane. Cell-free release of radiolabelled lipids from soybean ER was stimulated by palmitoyl-CoA and required cytosol. The release of radiolabelled lipids from chloroplast envelope was stimulated by ATP and GTP hydrolys, palmitoyl-CoA and required stroma. N-ethylmaleimide (NEM) had no effect on the lipid releases. The results point to similarities as well as differences between the regulation of galactolipid transfer from chloroplast envelope to the thylakoid and the regulation of vesicle trafficking among cytosolic membranes. 

In thermodynamics, one deals with closed and open systems, the difference between the two being that the latter involves the exchange of matter in addition to energy (heat and work), Clearly, a cell is an open system. Similarly, organelles such as chloroplasts and mitochondria are also open systems. Other energy transducing systems of interest are found in bacteria, in visual receptors. We shall mainly focus out attention on the thylakoid membrane of chloroplasts and the cristae membrane of rnito-chondria. It is particularly noteworthy that energy transduction and material transport in these two systems are coupled the products of photosynthesis are utilized as the reactants in respiration, and vice versa. 

Mitochondrial membranes. 2. Chloroplast membranes. 3. Bacterial cell walls. I. Tzagoloff, Alexander, 1937- [DNLM 1. Bacteria—Metabolism. 2. 

Plant cells contain a unique family of organelles, the plastids, of which the chloroplast is the prominent example. Chloroplasts have a double membrane envelope, an inner volume called the stroma, and an internal membrane system rich in thylakoid membranes, which enclose a third compartment, the thylakoid lumen. Chloroplasts are significantly larger than mitochondria. Other plastids are found in specialized structures such as fruits, flower petals, and roots and have specialized roles. 

---