American Academy of Professional Coders (AAPC) Practice Exam

Which structure forms the boundary of a cell?

1. Nucleus

2. Cytoplasm

3. Cell membrane

4. Chromosomes

The correct answer is: Cell membrane

The cell membrane serves as the boundary of a cell, defining its shape and protecting its internal components. This semi-permeable structure allows the cell to maintain homeostasis by regulating the entry and exit of substances, such as ions, nutrients, and waste products. The cell membrane is primarily composed of a phospholipid bilayer, along with proteins, cholesterol, and carbohydrates that contribute to its various functions, including signal transduction and cell recognition. In contrast, the nucleus, while crucial for housing genetic material and controlling cellular activities, is located within the cell and is not directly involved in enclosing the cell itself. The cytoplasm is the gel-like substance inside the cell membrane that contains organelles, but it does not form the boundary. Chromosomes, which are structures made of DNA, are found within the nucleus and are responsible for genetic information but also do not play a role in forming the cell's boundary.