 Dear students, in this topic, we shall discuss absorption by active transport and endocytosis. The epithelial cells of villi absorb many nutrients against their concentration gradient by active transport in which energy is utilized. The substances which are transported by active transport include most of the glucose molecules, amino acids, type aptides, tripe aptides, and many vitamins. The mechanism utilized for the active absorption of these substances involves a sodium core transport mechanism which supplies energy for active transport. In this mechanism, molecules to be transported bind to a specific transport protein which is present in the micro villi. This protein requires that sodium ions bind with it, then it will transport the molecule to be absorbed, the molecule of the nutrient to be absorbed. When sodium ions bind with a transport protein, then it has a tendency to move down its electrochemical gradient into the interior of the cell. Since it has a co-bound molecule, for example, amino acid, glucose, or peptide, then it also pulls the co-bound molecule. So both sodium ions and bound molecules are co-transported into the cell. This mechanism of active transport, which involves co-transport, is known as secondary active transport. Dear students, the transport proteins involved in co-transport are of five types which are present in the luminal membranes of epithelial cells. Each type of these transport proteins is specific for transporting certain groups of amino acids, peptides, and glucose. Dear students, endocytosis is also used by some absorptive cells to take up larger oligopeptides. For example, human milk has immunoglobulins. A human or other mammalian newborn, when it feeds on its mother's milk, then these immunoglobulins are not digested, but as such, intestine's epithelial cells of will-i in immunoglobulins are endocytosed as such, in larger oligopeptides, as such, cells. This process is called endocytosis. Certain substances are also absorbed by endocytosis.