Major Storage Compartments

The inputs of nitrogen to wetlands include biological N2 fixation, and point and nonpoint loads from external sources (Eigure 8.2). Examples include atmospheric nitrogen deposition, agricultural and urban runoff, application of fertilizers to rice paddies, stormwater runoff carrying nutrients and 

Plant biomass Microbial biomass Soil organic N Sell pore water Exchangeable N Clay fixed NH4-N 

FIGURE 8.2 Major inputs and outputs of nitrogen in wetlands. 

Particulate organic nitrogen (detritus, soil, and water) Microbial biomass nitrogen (MBN) (detritus, soil, and water) Dissolved organic nitrogen (DON) (detritus, soil, and water) Inorganic forms of nitrogen 

Gaseous end products (atmosphere, detritus, soil, and water) 

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Liver and adipose tissue are the major storage compartments for CACs. Ratios between liver and adipose tissue distribution are species dependent and follow the order rodents > birds > humans > fish.30 In marine mammals this ratio is determined by their relatively large adipose tissue compartment, with large storage capacities. CAC metabolites may bind to specific target proteins, e.g. in blood, kidney or lung. 

The bioavailability of an element in the diet is determined as the fraction of element that has been absorbed in the intestine, retained by the body, and utilized for physiological function, including deposition into storage tissues [18]. The actual element utilization of an administered label, however, is difficult to measure directly. Major functional or storage compartments, such as liver, muscle tissue, and bone, are difficult to access. They require tissue biopsies, which can be taken with minimal associated health risks, but can hardly be considered a routine sampling approach for research purposes in healthy volunteers. The only exception is iron, for which two-thirds of body iron circulates as hemoglobin in blood, which can be easily sampled. This has led to the development of two different approaches to assess iron absorption and bioavailability in the human body. 

Once in the serum, aluminium can be transported bound to transferrin, and also to albumin and low-molecular ligands such as citrate. However, the transferrrin-aluminium complex will be able to enter cells via the transferrin-transferrin-receptor pathway (see Chapter 8). Within the acidic environment of the endosome, we assume that aluminium would be released from transferrin, but how it exits from this compartment remains unknown. Once in the cytosol of the cell, aluminium is unlikely to be readily incorporated into the iron storage protein ferritin, since this requires redox cycling between Fe2+ and Fe3+ (see Chapter 19). Studies of the subcellular distribution of aluminium in various cell lines and animal models have shown that the majority accumulates in the mitochondria, where it can interfere with calcium homeostasis. Once in the circulation, there seems little doubt that aluminium can cross the blood-brain barrier. 

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