Advanced Placement (AP) Biology Practice Exam

Which of the following organelles is NOT membrane-bound?

1. Nucleus

2. Ribosome

3. Lysosome

4. Chloroplast

The correct answer is: Ribosome

Ribosomes are unique cellular structures that differ from other organelles in that they are not enclosed by a membrane. They are composed of ribosomal RNA (rRNA) and proteins, forming two subunits that come together to facilitate the process of translation, whereby proteins are synthesized from amino acids based on messenger RNA (mRNA) instructions. In contrast, organelles such as the nucleus, lysosome, and chloroplast are surrounded by membranes. The nucleus is encased in a double membrane known as the nuclear envelope, which separates the genetic material from the cytoplasm. Lysosomes, which contain digestive enzymes, are also membrane-bound, maintaining a specific internal environment necessary for their function. Chloroplasts, found in plant cells and some algae, are surrounded by a double membrane and are involved in photosynthesis. Understanding the structural characteristics of ribosomes is crucial for grasping cell biology concepts, particularly regarding the differences in how various cellular components are organized and function within the cell.