Prokaryote membranes

Cserzo, M., Wallin, E., Simon, I., von Heijne, G., and Elofsson, A. (1997). Prediction of transmembrane cc-helices in prokaryotic membrane proteins the dense alignment surface method. Protein Eng. 10, 673-676. 

The lipid composition of membranes can vary in the same manner. Major components of prokaryotic membrane lipids are phospholipids and gly-colipids eukaryotic membrane lipids typically contain these two classes and, in addition, sterols such as cholesterol, and sphingolipids. Table III-3 gives the lipid content of some membranes from various sources. Table III-3 does not list some of the unusual and less abundant lipids found in membranes, such as lipamino acids, glycolipids, and phos-phatides containing unusual fatty acid residues. 

Thus far, the interactions of phospholipid head groups have been considered, because the model was applied toward rationalizing the membranolysis of eukaryotic cells such as erythrocytes, and PLs are the primary constituents of eukaryotic cell membranes. A reasonable question to ask at this time is whether the above results are relevant to prokaryotic membranes. Although PLs constitute a smaller proportion of the prokaryotic cell wall and cell membrane, the other constituent molecules such as liopolysaccharides and teichoic acids, are also amphiphilic. The general structure of a hydrophilic portion attached to a hydrophobic tail is common... 

Eukaryotic as well as most prokaryotic membranes are bllayers of phospholipid and protein. Each monolayer, which is about 2.1 nm thick (19,20), is believed to contain the sterol in a nonhomogeneous distribution, and at least in some cases sterol can move between the monolayers. This process is called "flip-flop". Sterol has been found both in the mitochondria (21) and the plasma membrane of cerevlslae (13.18), and the ability to support the growth of anaerobic yeast presumably Is associated with Its membranous function. 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Prokaryotic cells have only a single membrane, the plasma membrane or cell membrane. Because they have no other membranes, prokaryotic cells contain no nucleus or organelles. Nevertheless, they possess a distinct nuclear area where a single circular chromosome is localized, and some have an internal membranous structure called a mesosome that is derived from and continuous with the cell membrane. Reactions of cellular respiration are localized on these membranes. In photosynthetic prokaryotes such as the cyanobacteria,... 

Eukaryotic ceils possess a discrete, membrane-bounded nucleus, the repository of the cell s genetic material, which is distributed among a few or many chromosomes. During ceil division, equivalent copies of this genetic material must be passed to both daughter ceils through duplication and orderly partitioning of the chromosomes by the process known as mitosis. Like prokaryotic... 

Without consulting chapter figures, sketch the characteristic prokaryotic and eukaryotic cell types and label their pertinent organelle and membrane systems. 

Even the plasma membranes of prokaryotic cells (bacteria) are complex (Figure 9.1). With no intracellular organelles to divide and organize the work, bacteria carry out processes either at the plasma membrane or in the cyto-... 

Although the interior of a prokaryotic cell is not subdivided into compartments by internal membranes, the cell still shows some segregation of metabolism. For example, certain metabolic pathways, such as phospholipid synthesis and oxidative phosphorylation, are localized in the plasma membrane. Also, protein biosynthesis is carried out on ribosomes. 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

Fig. 3-7 A thin section through a prokaryotic cell. Note that the nuclear material (N) is not bound by a membrane, but is free in the cytoplasm. Mitochondria and other intracytoplasmic structures are absent. (Reprinted with permission from J. J. Cardamone, Jr., Univ. of Pittsburgh/Biological Photo Service.)...

Cyanobacteria, prokaryotic algae that perform oxygenic photosynthesis, respond to a decrease in ambient growth temperature by desaturating the fatty acids of membrane lipids to compensate for the decrease in the molecular motion of the membrane lipids at low temperatures. During low-temperature acclimation of cyanobacterial cells, the desaturation of fatty acids occurs without de novo synthesis of fatty acids [110, 111]. All known cyanobacterial desaturases are intrinsic membrane proteins that act on acyl-Hpid substrates. 

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