What is transcytosis primarily responsible for?

• A. Transporting small ions across cell membranes
• B. Transporting macromolecules across epithelial cells
• C. Creating energy in cells
• D. Facilitating cell division

Answer: (B)

Transcytosis is a cellular process that plays a crucial role in the transport of large macromolecules across the barrier of epithelial cells. This mechanism involves the endocytosis of substances into a cell, followed by their transport across the cell and subsequent exocytosis on the opposite side. It allows for the selective uptake of proteins, antibodies, and other large molecules that cannot simply diffuse across the cell membrane.

This process is particularly important in areas where barriers, like the epithelial lining of blood vessels and the gastrointestinal tract, serve as selective filters for substances entering or leaving the bloodstream. For example, transcytosis is responsible for the movement of immunoglobulins from maternal milk into the bloodstream of neonates.

In contrast, transporting small ions across cell membranes typically occurs through ion channels and pumps rather than via transcytosis. The energy creation in cells is primarily linked with processes like cellular respiration and the functioning of mitochondria, and cell division involves complex processes such as mitosis, neither of which relate to the function of transcytosis. Thus, the primary role of transcytosis is accurately described as the transport of macromolecules across epithelial cells.