Biological concentration cells

Biological Corrosion The metabohc activity of microorganisms can either directly or indirectly cause deterioration of a metal by corrosion processes. Such activity can (1) produce a corrosive environment, (2) create electrolytic-concentration cells on the metal surface, (3) alter the resistance of surface films, (4) have an influence on the rate of anodic or cathodic reaction, and (5) alter the environment composition. 

The use of dispersants is highly recommended in systems containing silt, sand, oil, grease, biological material, and/or other foreign material. Not only does increased dispersion generally increase the effectiveness of chemical inhibition, it also prevents nucleation of oxygen concentration cells beneath foulants. 

Almost all cooling water system deposits are waterborne. It would be impossible to list each deposit specifically, but general categorization is possible. Deposits are precipitates, transported particulate, biological materials, and a variety of contaminants such as grease, oil, process chemicals, and silt. Associated corrosion is fundamentally related to whether deposits are innately aggressive or simply serve as an occluding medium beneath which concentration cells develop. An American... 

Passive corrosion caused by chemically inert substances is the same whether the substance is living or dead. The substance acts as an occluding medium, changes heat conduction, and/or influences flow. Concentration cell corrosion, increased corrosion reaction kinetics, and erosion-corrosion can he caused by biological masses whose metabolic processes do not materially influence corrosion processes. Among these masses are slime layers. 

Slime is a network of secreted strands (extracellular polymers) intermixed with bacteria, water, gases, and extraneous matter. Slime layers occlude surfaces—the biological mat tends to form on and stick to surfaces. Surface shielding is further accelerated by the gathering of dirt, silt, sand, and other materials into the layer. Slime layers produce a stagnant zone next to surfaces that retards convective oxygen transport and increases diffusion distances. These properties naturally promote oxygen concentration cell formation. 

Shells, clams, wood fragments, and other biological materials can also produce concentration cell corrosion. Additionally, fragments can lodge in heat exchanger inlets, locally increasing turbulence and erosion-corrosion. If deposits are massive, turbulence, air separation, and associated erosion-corrosion can occur downstream (see Case History 11.5). 

As mentioned earlier, there is an inverse relationship between water volumes and oxygen concentration in soil. As soils dry, conditions become more aerobic and oxygen diffusion rates become higher. The wet-dry or anaerobic-aerobic alternation, either temporal or spatial, leads to higher corrosion rates than would be obtained within a constant environment. Oxygen-concentration-cell formation is enhanced. This same fluctuation in water and air relations also leads to greater variation in biological activity within the soil. 

Cells. We finally come to what are the direct building blocks of biological materials cells. Cells are assemblies of molecules enclosed within a plasma membrane that carry out specific functions. The human body contains over 10 " cells, all of which take in nutrients, oxidize fuels, and excrete waste products. Despite their varied functions, all cells have a similar internal organization. We will concentrate on this internal organization for now and will leave the topics of cell reproduction, energy production, and related concepts to the molecular biologist. 

Pitting corrosion is a general term that can be considered a visible sign of the results of concentration cell corrosion and of further induced-corrosion processes such as when chloride attack occurs. Although pits can also occur with acid corrosion, etc., under-deposit corrosion, of course, can also involve direct metal surface attack, from, say, biologically induced corrosion (but that is discussed separately). 

Dietary biotin bound to avidin (Section 11.6) is unavailable, but intravenously administered avidin-biotin is biologically active. Cells in culture are not inhibited by the addibon of avidin to the culmre medium, and can take up the avidin-biotin complex by pinocytosis followed by lysosomal hydrolysis, releasing free biotin. Unlike other B vitamins, for which concentrative uptake into tissues is achieved by facilitated diffusion, followed by metabolic trapping, the incorporation of biotin into enzymes is slow and cannot be considered part of the uptake process. 

The surface finish of the component also has an impact on the mode and severity of the corrosion that can occur. Rough surfaces or tight crevices can facilitate the formation of concentration cells. Surface cleanliness can also be an issue with deposits or films acting as initiation sites. Biological growths can behave as deposits, or can change the underlying surface chemistry to promote corrosion. 

Surfactants mnst be chosen with dne care, as their strong interfacial activity can elicit adverse reactions in biological systems. Cell membrane constituents (phospholipids, cholesterol, etc.) can be solubilized at supermicellar concentrations, impairing the integrity of membranes. This disruption of the membrane enhances the permeability of drng substances and other snbstances present in the extracellular fluid. At low concentrations such alterations are reversible and membranes recover rapidly, but higher concentrations... 

A 5—500 pi aliquot of the aqueous sample is precisely selected (Fig. 1.3a) and introduced into the flow manifold. This is especially relevant for the analysis of biological fluids, cell tissues, blood sera, dew and other volume-limited samples, as well as for in vivo assays. Moreover, sampling strategies relying on mini-probes become more practical. In spite of the low sample volume, reliable results are obtained even for very low analyte concentrations. 

A biological cell can be compared to a concentration cell for the purpose of calculating its membrane potential. Membrane potential is the electrical potential that exists across the membrane of various kinds of cells, including muscle cells and nerve cells. It is responsible for the propagation of nerve impulses and heart beat. A membrane potential is established whenever there are unequal concentrations of the same type of ion in the interior and exterior of a cell. For example, the concentrations of ions in the interior and exterior of a nerve cell are 400 mM and 15 mM, respectively. Treating the situation as a concentration cell and applying the Nemst equation, we can write... 

The recticulated vitrous carbon is cut into 2mm x I mm X 0.5 mm. A copper connective wire (0.3 mm) with thermoplastic insulation is attached to the RVC by silver epoxy. The silver epoxy is allowed to dry overnight in a vacuum oven (70 C) and is then insulated by nonconductive epoxy. The polymeric film is deposited from a degased solution of monomeric porphyrin using the same procedure as described for the preparation of a single-fiber sensor. After film formation, the electrode is dried in a vacuum oven at 40 C and then dip-coated in 1% Nafion in alcohol. The biological cells suspended in 3 ml culture medium are placed dropwise onto the sensor (sterilized by UV light) located in a 60 x 15 mm tissue culture dish. The concentrated cell suspension on the sen.sor is incubated for approxi-... 

The process of osmosis is important in biological systems. Cell walls often act as semipermeable membranes. Do you ever eat pickles Cucumbers are soaked in a brine solution in order to make pickles. The concentration of the solution inside the cucumber is less than the concentration of the brine solution, so water migrates through the cell walls into the brine, causing the cucumber to shrink. 

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