What forms the outer boundary of a cell?

The outer boundary of a cell is formed by the plasma membrane. This semi-permeable membrane is crucial for maintaining the integrity of the cell by providing a barrier that separates the internal environment from the external surroundings. It regulates the movement of substances in and out of the cell, allowing essential nutrients to enter while keeping waste products and harmful substances out.

The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates, which contribute to its functionality and fluidity. This structure ensures that the cell can interact with its environment while maintaining homeostasis.

In contrast, cytoplasm refers to the jelly-like substance within the cell that houses organelles but does not serve as a boundary. The nucleus is an organelle that contains genetic material and is located within the cell, not on its boundary. A cell wall, found in plant cells and some bacteria, provides additional structure and protection but is not present in animal cells. Therefore, the plasma membrane is the correct answer as it serves as the defining outer layer for all types of cells.