Epithelial cells with important transport functions such as absorption and secretion often bear on their free surfaces.

microvilli

Explanation:

Microvilli are projections, extensions of the plasma membrane, found in epithelial cells and absorption cells, such as intestinal cells that aid in nutrient absorption. In other words microvilli are folds in the plasma membrane at the surface of the intestinal cells that increase the contact surface and thereby increase nutrient absorption.

Many researchers have observed that epithelial cells with important transport functions, such as absorption and secretion, often carry microvilli on their free surfaces.

littering streams           is the answer