Outer membrane chloroplast

Compound Myelin (bovine) Retinal rod Plasma membrane (human erythrocyte) Mitochondrial membranes E. (inner and outer membranes) Chloroplasts ... 

Like the Tom and Tim systems on mitochondrial outer and inner membranes, chloroplasts use the Toe and Tic systems on their outer and inner envelope membranes. Although there may not be a direct correspondence between both subunits, their functions for protein translocation appear quite similar. Thus, most of the sorting mechanisms within the envelope membranes are recognized as variations of the general sorting pathway to the stroma. 

Nobel and Wang reported that the permeability of the outer membranes of chloroplasts was increased by exposure to ozone (at 30 ppm for 5 min). They hypothesized that the effect was lipid oxidation. Freebaim showed that ozone inhibited respiratory activities of isolated mitochondria, and Lee found that the effect of ozone on oxidative phosphorylation was greater than on oxygen uptake. Mudd et reported that the metabolism of UDP-galactose by isolated... 

In photosynthetic eukaryotic cells, both the light-de-pendent and the carbon-assimilation reactions take place in the chloroplasts (Fig. 19-38), membrane-bounded intracellular organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, they are surrounded by two membranes, an outer membrane that is permeable to small molecules and ions, and an inner membrane that encloses the internal compartment. This compartment contains many flattened, membrane-surrounded vesicles or sacs, the thylakoids, usually arranged in stacks called grana (Fig. 19-38b). Embedded in the thylakoid membranes (commonly called lamellae) are the photosynthetic pigments and the enzyme complexes that carry out the light reactions and ATP synthesis. The stroma (the aqueous phase enclosed by the inner membrane) contains most of the enzymes required for the carbon-assimilation reactions. 

The photosynthetic process in green plants occurs in subcellular organelles called chloroplasts. These organelles resemble mitochondria they have two outer membranes and a folded inner membrane called the thy-lakoid. The apparatus for photosynthesis, including the chlorophyll reaction centers and electron carriers, is in the thylakoid membrane. The chemical reactions of the Calvin cycle take place in the stroma, the region around the thylakoid membrane. 

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. 

Cells must ensure that each newly synthesized protein is sorted to its correct location where it can carry out the appropriate function. This process is called protein targeting. In a eukaryotic cell, the protein may be destined to stay in the cytosol, for example an enzyme involved in glycolysis (see Topic J3). Alternatively it may need to be targeted to an organelle (such as a mitochondrion, lysosome, peroxisome, chloroplast or the nucleus) or be inserted into the plasma membrane or exported out of the cell. In bacteria such as E. coli, the protein may stay in the cytosol, be inserted into the plasma membrane or the outer membrane, be sent to the space between these two membranes (the periplasmic space) or be exported from the cell. In both prokaryotes and eukaryotes, if a protein is destined for the cytosol, it is made on free ribosomes in the cytosol and released directly into the cytosol. If it is destined for other final locations, specific protein-targeting mechanisms are involved. 

Chloroplasts are enclosed by two membranes. The outer membrane is freely permeable to small molecules (up to about 10 kDa) due to the presence of a porin and the inner membrane is the osmotic barrier and the site where specific transport occurs. The specificity of envelope permeability is strikingly highlighted by the contrast between Pi and PP the former being among the most rapidly translocated molecules and the latter among those to which the envelope is relatively impermeable. Carrier-mediated anion transport can be classified as ... 

Biological membranes show anisotropy, as their molecules are preferentially ordered in a definite direction in the plane of the membrane, and the coupling between chemical reactions (scalar) and diffusion flow (vectorial) can take place. Almost all outer and inner membranes of the cell have the ability to undergo active transport. Sodium and potassium pumps operate in almost all cells, especially nerve cells, while the active transport of calcium takes place in muscle cells. The proton pumps operate in mitochondrial membranes, chloroplasts, and the retina. 

At present, transmembrane /1-barrel proteins have been found exclusively in the outer membrane of gram-negative prokaryotes these membranes seem to lack a-helical proteins. Accordingly, a separation exists between a-proteins in all cytoplasmic membranes and /1-proteins in the specialized outer membranes. Following the endosymbiotic hypothesis, /1-proteins are also expected in the outer membranes of mitochondria and chloroplasts, butnone of these proteins has yet been structurally elucidated. Given the limited abundance of such membranes, the /1-proteins are likely to make up only a small, special class of membrane proteins. 

In 1937, Robin Hill succeeded in isolating chloroplasts from leaves which under suitable conditions still retained some form of photosynthetic activity. The material isolated by Hill actually consisted of thylakoid membranes which lacked the necessary enzymes for CO2 fixation as a result of the loss of the outer membrane envelope. When these chloroplasts were illuminated, no oxygen evolution could be observed. However, when an electron acceptor such as ferric oxalate, as was used by Hill initially, was added, oxygen evolution was observed upon illumination, accompanied by the reduction of the ferric oxalate to the ferrous form. Lurthermore, four equivalents of the oxidant were found to be photochemi-cally reduced for each mole of oxygen evolved. This reaction was called the Hill reaction. Later, ferricyanide and benzoquinone were commonly used as the oxidizing agents. 

Each chloroplast is bounded by an envelope of a highly permeable outer membrane and a nearly impermeable inner membrane, the two membranes being separated by a narrow, inter-membrane compartment [see Fig. 13 (C) and (C )]. The outer membrane allows small molecules to pass through, while the inner membrane presents a barrier which allows only certain metabolites or ATP to pass through with the help of special transport proteins embedded in the membrane. Enclosed by the inner membrane is the stroma, a concentrated solution containing the enzymes necessary for CO fixation, i.e., its conversion into carbohydrates. The stroma also contains the chloroplast s own DNA, RNA and ribosomes involved in the synthesis of proteins. This chloroplast stroma is analogous to the matrix in mitochondria. 

A newly synthesized protein in E. coli can stay in the cytoplasm or it can be sent to the plasma membrane, the outer membrane, the space between them, or the extracellular medium. Eukaryotic cells can direct proteins to internal sites such as lysosomes, mitochondria, chloroplasts, and the nucleus. How is sorting accomplished In eukaryotes, a key choice is made soon after the synthesis of a protein begins. The ultimate destination of a protein depends broadly on the location of the ribosome on which it is being synthesized. 

Chloroplasts have inner and outer membranes. A third membrane forms within the aqueous, enzyme-rich stroma into flattened sacs called thylakoids. A stack of thylakoids is called a granum. Unstacked, connecting thylakoid membrane is referred to as stroma lamellae, (a) An electron micrograph of a chloroplast, (b) a diagrammatic view of a chloroplast,... 

In plants, photosynthesis takes place in chloroplasts. Chloro-plasts possess three membranes. The outer membrane is highly permeable, whereas the inner membrane possesses a variety of carrier molecules that regulate molecular traffic into and out of the chloroplast. A third membrane, called the thylakoid membrane, forms an intricate series of flattened vesicles called grana. 

The porins are a class of transmembrane proteins whose structure differs radically from that of other integral proteins. Several types of porin are found in the outer membrane of gram-negative bacteria such as E. coli and in the outer membranes of mitochondria and chloroplasts. The outer membrane protects an intestinal bacterium from harmful agents (e.g., antibiotics, bile salts, and proteases) but permits the uptake and disposal of small hydrophilic molecules including nutrients and waste products. The porins in the outer membrane of an E. coli cell provide channels for the passage of disaccharides and other small molecules as well as phosphate. 

Chloroplasts are bounded by two membranes, which do not contain chlorophyll and do not participate directly In photosynthesis (Figure 8-30). As In mitochondria, the outer membrane of chloroplasts contains porins and thus Is permeable to metabolites of small molecular weight. The Inner membrane forms a permeability barrier that contains transport proteins for regulating the movement of metabolites into and out of the organelle. 

The light-capturing and ATP-generating reactions of photosynthesis occur in the thylakoid membrane located within chloroplasts. The permeable outer membrane and Inner membrane surrounding chloroplasts do not participate In photosynthesis (see Figure 8-30). 

Protein Import into the chloroplast stroma occurs through inner-membrane and outer-membrane translocation channels that are analogous in function to mitochondrial channels but composed of proteins unrelated in sequence to the corresponding mitochondrial proteins. 

As noted above, chloroplasts and other plastids are enriched in galactolipids (Fig. 1). They also contain a unique sulfolipid, sulfoquinovosyldiacylglycerol, whose head group is a modified galactose. The phospholipid components of plastids are less abundant. Phosphatidylglycerol, the most prominent phospholipid contributor to the thylakoid membrane system, comprises less than 10% of chloroplast glycerolipids, while plastidial phosphatidylcholine is limited primarily to the organelle s outer membrane. 

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