Coated pits, defined

The folate receptor facilitates the cellular uptake of folate and 5-methyltetrahydrofolate via receptor-mediated endocytosis at caveolae (caveolae are plasma membrane invaginations distinct from the classical clathrin-coated pits) (6). It has been hypothesized that the folate receptor is functionally coupled to an anion transporter to mediate cytosolic folate delivery by a process defined as potocytosis (6). More recent studies suggest that folate receptor endocytosis also occurs at clathrin-coated pits (7). Studies by Low and coworkers at Purdue University have shown that folate conjugates are also taken up by the folate receptor (8-10), but not by the reduced folate carrier. Figure 2 illustrates an endocytic pathway of the type envisioned for folate conjugates. Unfortunately, the subcellular transport pathway of the folate conjugates has been only partially characterized and may well be affected by the properties of the molecule attached to folate. 

In prerecorded CD-ROM disks, the information is stamped as low reflectivity pits on a high reflectivity background. The disk has a reflectivity of 70% (achieved byusing an aluminum layer), whereas the pits have a reflectivity of 30% (these reflectivity specifications have been defined in the Red Book). The CD-ROM drive uses this difference in reflectivity to sense the information stamped on the disk. To be compatible with CD-ROM readers, a CD-R disk must also use this reflectivity difference when recording data. To accomplish this, a CD-R disk is coated with an organic polymer that can change its local reflectivity permanently upon sufficient heating by a laser spot. The structure of a CD-R disk is shown in Fig. 14.19. 

Mass loss tests are a common means of testing many metals in atmospheric and submerged exposure. For steel in concrete they are destructive to the concrete and difficult to perform. Laboratory specimens typically have a portion of the embedded bar coated or taped to define a specific test area. These materials are often hard to remove and could gain or lose mass. Furthermore, at the initial stages of corrosion most of the damage is in pits which have relatively low mass loss relative to that lost in the formation of the passive film [12], and thus mass loss is not necessarily a good indicator of corrosion activity. 

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