Mechanisms of absorption
The nutrients presented for absorption at the mucosal brushborder vary \videly in molecular size, solubility, and other properties. Due to the unique nature of the lipoprotein membrane ofthe microvilli, several mechanisms are necessary to ensure passage into the cell of all nutrients regardless of their size and solubility. Also, as was pointed out, the nutrients encounter several different barriers while passing from the luminal to the serosal side (lamina propria) of the epithelial cell and finally into the blood or lymph vessels. Therefore, more than one mechanism is likely to be involved in the total absorption process for anyone nutrient. When a solute passes "downhill" from a higher to a lower concentration, it is said to move along its concentration gradient; the process does not require expenditure of energy and is known as passive transport. In the same manner osmotic pressure differences and electrical gradients across the membranes also can determine the direction of the movement of water and ions, respectively, by passive mechanisms.
During intestinal absorption many nutrients are transported "uphill," against the gradient; this can be accomplished only by expendi ture of energy, and the process is known as active transport.
Several nutrients are knovm to be absorbed bv both active and passive mechanisms. Furthermore, it is likely that in active absorption only one step, such as entrance into or exit from the mucosal cell, requires energy.