Brush border definition

One should also consider the glycocalyx, a carbohydrate-containing polymer network between the microvilli and the mucus gel coat. It probably consists mainly of oligosaccharide chains that are covalently linked to the lipids and proteins of the brush border membrane. The definite structure of the glycocalyx is not yet available, and nothing can be said about its possible importance as an absorption barrier. 

Only in brush-border membranes of some specialised cells and in erythrocyte membranes there seems to be an Mg-ATPase, which can be stimulated by anions, but only to a minor degree. Moreover, the properties of the enzyme in these membranes differ considerably from those of the activities in the microsomal and mitochondrial fractions of most other tissues. The anion sensitivity of the ATPase activity in the brush-border membranes of placenta and small intestine is a property of the alkaline phosphatase [37,41] and that in erythrocytes is part of the (Ca -b Mg )-ATPase activity [33,34]. Although this does not definitely exclude a role of the enzyme in anion transport, no valid arguments in favour of a role of this enzyme in anion or proton transport have been advanced. 

The second point about this example is that the flux is given per a projected area of intestine. Because of the brush border, we could argue that a more appropriate flux would be defined per vilH area. Either definition will work the only danger is that we do not specify which we are using. 

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