Membrane of organelles

If bound first by albumin, heme circulates until it is transferred to hemopexin (52). In vitro in the absence of hemopexin, nonspecific cellular uptake of heme by diffusion is facile (55), but as expected, the presence of hemopexin greatly slows uptake (54), since receptor-mediated uptake is necessarily slower and of lower capacity than diffusion-limited uptake. There is currently no evidence that either receptors for albumin or membrane transporters for heme, like those in prokaryotes, are present in the plasma membrane of mammalian cells, although such transport proteins may be present in the membranes of organelles. 

Carbon tetrachloride causes centrilobular liver necrosis and steatosis after acute exposure, and liver cirrhosis, liver tumors, and kidney damage after chronic administration. The mechanism underlying the acute toxicity to the liver involves metabolic activation by cytochrome P-450 to yield a free radical (trichloromethyl free radical). This reacts with unsaturated fatty acids in the membranes of organelles and leads to toxic products of lipid peroxidation including malondialdehyde and hydroxynonenal. This results in hepatocyte necrosis and inhibition of various metabolic processes including protein synthesis. The latter leads to steatosis as a result of inhibition of the synthesis of lipoproteins required for triglyceride export. 

Rapid freezing (at — 196°C) was the treatment that better preserved the cellular and subcellular structures. In fact, plasmalemma retained its integrity and membranes of organelles remained distinguishable. Flowever,... 

MEMBRANE TRANSPORT Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be carefully regulated to meet each cell s metabolic needs. For example, a cell s plasma membrane regulates the entrance of nutrient molecules and the exit of waste products. Additionally, it regulates intracellular ion concentrations. Because lipid bilayers are generally impenetrable to ions and polar substances, specific transport components must be inserted into cellular membranes. Several examples of these structures, referred to as transport proteins or permeases, are discussed. 

Fibers of the cytoskeleton (green) provide structural support for the cell and its internal compartments. The internal membranes of organelles and vesicles possess more surface area than that of the plasma membrane but less area than that of the cytoskeleton, as schematically represented by the red, black, and green boxes. The enormous surface area of the cytoskeleton allows it to function as a scaffold on which cellular reactions can take place. 

The most important lipophilic antioxidant that acts in eucary-otic cells to protect unsaturated lipids against free radical damage is vitamin E, especially a-tocopherol. Along with P-carotene and coenzyme Q, it protects the structure and integrity of biomembranes, such as the cytoplasmic cell membrane (or plasmolema) and intracellular membranes of organelles (nucleus, mitochondria, lysosome and endoplasmic reticulum). It k ako employed in the protection of lipoproteins present in plasma. It is transported in the bloodstream by association with the lipid phase of low density Hpoprotein (LDL) particles (see Section 3.6.1). Each LDL particle contains six molecules of vitamin E. 

Necrosis is often initiated by damage to membranes, either the plasma membrane of the cell or the membranes of organelles, particularly mitochondria (Zimmerman, 1999). Cell membrane damage is often caused by membrane phospholipid peroxidation. Plasma membrane damage interferes wi ion regulation, calcium homeostasis, energy production, and decrease in the ability of that organelle to sequester calcium. Inhibition of protein synthesis is an alternative mechanism that may cause cell necrosis. Toxins that act in this way include phalloidin and related mushroom toxins, which inhibit the action of ribonucleic acid (RNA) polymerase, and therefore mRNA synthesis (Pineiro-Carrero and Pineiro, 2004). 

---