Eukaryote cells intracellular membranes

The examples mentioned above exclusively apply to eukaryotic cells. In prokaryotic cells, intracellular membranes are the exception. However,... 

Eukaryotic cells have evolved a complex, intracellular membrane organization. This organization is partially achieved by compartmentalization of cellular processes within specialized membrane-bounded organelles. Each organelle has a unique protein and lipid composition. This internal membrane system allows cells to perform two essential functions to sort and deliver fully processed membrane proteins, lipids and carbohydrates to specific intracellular compartments, the plasma membrane and the cell exterior, and to uptake macromolecules from the cell exterior (reviewed in [1,2]). Both processes are highly developed in cells of the nervous system, playing critical roles in the function and even survival of neurons and glia. 

In addition to their function as a permeability barrier to the extracellular environment, membranes also fulfil important tasks inside most eukaryotic cells and in some bacteria. One crucial role is the separation of different cell compartments. A few examples of intracellular membranes may illustrate the large variety of membrane functions ... 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

Homeostatic mechanisms also allow animals to control their intracellular pH very strictly. In humans for example, blood pH (usually taken as a reliable but indirect measure of cellular pH) is 7.4 0.04. At 37 °C cytosolic pH is actually slightly lower at about 7.0 but different compartments within the eukaryotic cells may have quite different pH, for example, lysosomes have an internal pH of about 5 the inside of a mitochondrion is more alkaline than the outside whilst the inside of a phagosome in a white blood cell is more acidic than its surrounding cytosol, both situations arising due to proton pumping across a membrane. 

Substrate availability to the cell is affected by the supply of raw materials from the environment. The plasma membranes of cells incorporate special and often specific transport proteins (translocases) or pores that permit the entry of substrates into the cell interior. Furthermore pathways in eukaryotic cells are often compartmentalized within cytoplasmic organelles by intracellular membranes. Thus we find particular pathways associated with the mitochondria, the lysosomes, the peroxisomes, the endoplasmic reticulum for example. Substrate utilization is limited therefore by its localization at the site of need within the cell and a particular substrate will be effectively concentrated within a particular organelle. The existence of membrane transport mechanisms is crucial in substrate delivery to, and availability at, the site of use. 

During the last ten years, it has become apparent that calcium-dependent papain-like peptidases called calpains (EC 3.4.22.17) represent an important intracellular nonlysosomal enzyme system [35][36], These enzymes show limited proteolytic activity at neutral pH and are present in virtually every eukaryotic cell type. They have been found to function in specific proteolytic events that alter intracellular metabolism and structure, rather than in general turnover of intracellular proteins. Calpains are composed of two nonidentical subunits, each of which contains functional calcium-binding sites. Two types of calpains, i.e., /i-calpain and m-calpain (formerly calpain I and calpain II, respectively), have been identified that differ in their Ca2+ requirement for activation. The activity of calpains is regulated by intracellular Ca2+ levels. At elevated cytoplasmic calcium concentrations, the precursor procal-pain associates with the inner surface of the cell membrane. This interaction seems to trigger autoproteolysis of procalpain, and active calpain is released into the cytoplasm [37]. 

Much of industrial chemistry takes place in organic solvents, or involves apolar compounds. Biocatalysis, in contrast, typically involves aqueous environments. Nevertheless, enzymes and microorganisms do in fact encounter apolar environments in Nature. Every cell is surrounded by at least one cell membrane, and more complex eukaryotic cells contain large amounts of intracellular membrane systems. These membranes consist of lipid bilayers into which many proteins are inserted present estimates, based on genomic information, are that about one-third of all proteins are membrane proteins, many of which are so-called intrinsic proteins that are intimately threaded through the apolar bilayer. These proteins are essentially dissolved in, and function partly within, an apolar phase. 

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. 

Active transport of a solute against a concentration gradient also can be driven by a flow of an ion down its concentration gradient. Table 17.6 lists some of the active-transport systems that operate in this way. In some cases, the ion moves across the membrane in the opposite direction to the primary substrate (antiport) in others, the two species move in the same direction (symport). Many eukaryotic cells take up neutral amino acids by coupling this uptake to the inward movement of Na+ (see fig. 17.26c). As we discussed previously, Na+ influx is downhill thermodynamically because the Na+-K+ pump keeps the intracellular concentration of Na+ lower than the extracellular concentration and sets up a favorable electric potential difference across the membrane. Another example is the /3-galactosidc transport system of E. coli, which couples uptake of lactose to the inward flow of protons (see fig. 17.26Proton influx is downhill because electron-transfer reactions (or,... 

Gene delivery into eukaryotic cells is commonly performed for research purposes as well as in gene therapy procedures. Cellular membranes do not spontaneously take up ectopic nucleic acid because of the polar nature of the phospholipid bilayer [1] which functions as a natural barrier that prevents entry of most water-soluble molecules such as nucleic acids. In studies of gene or protein function and regulation, manipulation of the intracellular expression level is a fundamental approach. For this reason, multiple methods for delivery of nucleic acids through membranes using chemical, physical, or biological systems have been established in the last 40 years. 

Cell fractionation studies of five strains of cyanobacteria indicate that MAAs are located primarily (>90%) within the cytoplasm and not the cell sheaths, walls, or membranes.132 Extracellular placement of MAAs does occur in some cyanobacterial species that posses cellular sheath layers.134135 Extracellular MAAs are covalently bonded to oligosaccharide molecules embedded in the cyanobacterial sheath matrix and provide substantial protection to prevent photobleaching of chlorophyll within the cell. Intracellular or extracellular distributions of MAAs in eukaryotic cells have not been investigated. Based on the high MAA concentrations of Phaeocystis antarctica colonies, it has been suggested that MAAs are associated with the extracellular mucopolysaccharide matrix of the colony.125 This may be a more common phenomenon than currently recognized, and future research efforts will be necessary to further document extracellular occurrence of MAAs in cyanobacteria and algae. 

This simple calculation indicates that if H+ were passively distributed across the plasma membrane, and for a mean membrane potential of-60 mV, intracellular pH values (pHi) would be expected to be 1 pH unit lower than the external pH values (pHo). Numerous experiments performed on a large variety of cells indicate, however, that pHi values are usually close to pHo values, or slightly below (Roos and Boron, 1981). This implies that H+ ions are not passively distributed across the plasma membrane and that H+ pumps drive H+ out of equilibrium. Different H+ pumps have been identified in eukaryotic cells an ubiquitous Na+/H+ exchanger, and pumps that have a more restricted cellular distribution. These are (H+)ATPases,... 

The bulk constituent of cells is water (H20). The cell membrane or plasma membrane (PM) that encloses the living cell is basically composed of a phospholipid bilayer, a 0.01 micrometre ( xm) (10 nm) thick bimolecular layer of hydrophobic (or water repelling) fatty molecules. In eukaryotes (organisms having a nucleus) there is a phospholipid bilayer PM enclosing the cell. Similar membranes bound specialized intracellular organelles, namely the endoplasmic reticulum (ER), ER-associated Golgi vesicles, lysosomes, vacuoles, peroxisomes, nucleus and mitochondria (and, additionally, the chloroplasts in plant cells). 

Over the past 30 years, the study of intracellular protein sorting has grown to a large and diversified field. A host of different sorting pathways have been found in both prokaryotic and eukaryotic cells, and most of these can handle both soluble and membrane-bound proteins. 

---