Which of the following is NOT considered an organelle?

The cell membrane is not classified as an organelle because it does not fit the criteria typically assigned to organelles, which are defined as specialized structures within a cell that perform distinct processes. Organelles are membrane-bound compartments within the cytoplasm or other specialized structures that carry out specific functions vital to the cell's life and activities.

In contrast, the cell membrane is a boundary that encloses the entire cell, providing structure and protecting the internal components from the external environment. It plays a crucial role in regulating what enters and exits the cell but is not a distinct functional entity or compartment within the cell like ribosomes, the nucleus, or mitochondria.

Ribosomes are essential for protein synthesis; the nucleus houses genetic material and orchestrates cell activities; and mitochondria are responsible for energy production. Each of these organelles is fundamental to specific cellular processes, underscoring the distinction between them and the cell membrane.