A vesicle is best defined as which of the following?

• A. The fluid inside a cell
• B. Part of the cell wall
• C. A type of ribosome
• D. A membrane bound sac that contains materials involved in transport of the cell

Answer: (D)

A vesicle is a small, membrane-bound sac that transports materials within the cell. It forms when a portion of a membrane buds off, then travels to a target site and fuses with another membrane to deliver its cargo. This encapsulated environment allows cargo like proteins, lipids, and other molecules to be moved around in a controlled way as part of the cell’s endomembrane system, such as from the endoplasmic reticulum to the Golgi, from the Golgi to the plasma membrane, or for storage and release in other organelles.

This definition fits because the fluid inside a cell is cytosol, not a vesicle; the cell wall is a rigid external layer in plant cells, not a transport sac; and a ribosome is a molecular machine for protein synthesis, not a membrane-bound transport sac.