Glycocalyx unstirred water’ layer

The glycocalyx and the mucus layer make up the structure of the unstirred water layer (UWL) [73]. The thickness of the UWL is estimated to be 30-100 pm in vivo, consistent with very efficient stirring effects [74]. In isolated tissue (in the absence of stirring), the mucus layer is 300-700 pm thick [73]. The pH in the unstirred water layer is 5.2-6.2, and might be regulated independently of the luminal pH (Section 2.3). The mucus layer may play a role in regulating the epithelial cell surface pH [73]. 

The aqueous boundary layer or the unstirred water layer (UWL) is a more or less stagnant layer, about 30-100 pm in thickness, composed of water, mucus, and glycocalyx adjacent to the intestinal wall that is created by incomplete mixing of the lumenal contents near the intestinal mucosal surface. The glycocalyx is made up of sulfated mucopolysaccharides, whereas mucus is composed of glycoproteins (mucin), enzymes. 

In summary, a lipid molecule on its route from the luminal bulk phase into the intracellular compartment of an enterocyte has to overcome two unstirred water layers and one plasma membrane of lipid bilayer structure. The unstirred water layer on the luminal side partly coincides with the mucus gel and the glycocalyx relatively little is known of the importance of these diffusional barriers. 

Figure 1. Solute transfer across an idealised eukaryote epithelium. The solute must move from the bulk solution (e.g. the external environment, or a body fluid) into an unstirred layer comprising water/mucus secretions, prior to binding to membrane-spanning carrier proteins (and the glycocalyx) which enable solute import. Solutes may then move across the cell by diffusion, or via specific cytosolic carriers, prior to export from the cell. Thus the overall process involves 1. Adsorption 2. Import 3. Solute transfer 4. Export. Some electrolytes may move between the cells (paracellular) by diffusion. The driving force for transport is often an energy-requiring pump (primary transport) located on the basolateral or serosal membrane (blood side), such as an ATPase. Outward electrochemical gradients for other solutes (X+) may drive import of the required solute (M+, metal ion) at the mucosal membrane by an antiporter (AP). Alternatively, the movement of X+ down its electrochemical gradient could enable M+ transport in the same direction across the membrane on a symporter (SP). A, diffusive anion such as chloride. Kl-6 refers to the equilibrium constants for each step in the metal transfer process, Kn indicates that there may be more than one intracellular compartment involved in storage. See the text for details...

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