Advanced Placement (AP) Biology Practice Exam

Which structure is directly connected to the nuclear membrane?

1. Plasma membrane

2. Endoplasmic reticulum

3. Golgi apparatus

4. Mitochondria

The correct answer is: Endoplasmic reticulum

The endoplasmic reticulum (ER) is directly connected to the nuclear membrane, making it the correct answer. This connection facilitates the smooth transfer of molecules between the nucleus and the ER. The outer membrane of the nuclear envelope is continuous with the membrane of the rough endoplasmic reticulum, which is involved in protein synthesis and processing. This association allows ribosomes on the rough ER to synthesize proteins that may be secreted from the cell or used in the cell membrane, all while remaining in close proximity to the genetic material housed within the nucleus. In contrast, the plasma membrane serves as a boundary for the entire cell and is not physically connected to the nuclear envelope. The Golgi apparatus, while involved in modifying and packaging proteins, is located further away from the nuclear membrane as it receives proteins from the ER. Mitochondria are the energy-producing organelles and do not share a direct connection with the nuclear structure. Thus, the endoplasmic reticulum's unique role and structural continuity with the nuclear membrane highlight its essential function in cellular processes.