A change in transport expressed per unit weight reflects only a change in functional activity independent of a change in mass. 2.2 Intestinal sugar and amino acid transporters . These nutrients are initially absorbed by carriers in the brushborder membrane facing the lumen and are subsequently transported from intracellular compartments across the basolateral membrane by a different set of carriers. Both transport rates can be determined experimentally by radioisotope tracer techniques. Nutrient transport via the transcellular path entails crossing the brushborder and basolateral membranes in series, that via the paracellular path need only cross the tight junction.
Paracellular transport accounts for only a small fraction of total intestinal nutrient transport.