What are organelles?

Organelles are defined as membrane-enclosed structures within a cell that perform specific functions necessary for the cell's survival and proper operation. These structures allow for compartmentalization of various biochemical processes, enabling the cell to maintain a complex internal environment that supports life.

For instance, organelles like mitochondria are involved in energy production, while the endoplasmic reticulum is crucial for protein and lipid synthesis. The presence of membranes around organelles helps to isolate these processes from the cytoplasm, ensuring that conditions can be optimized for each specific function—something that is critical in larger, more complex cells, especially eukaryotic cells.

The other options do not accurately describe organelles. Small cells within larger cells describes something different, such as symbiotic relationships or certain types of cellular structures like some endosymbionts, which is not the definition of organelles. Components of the cell wall typically refer to structural elements found in plant cells (like cellulose) or bacterial cells, but not to organelles. Lastly, water-filled spaces in the cytoplasm could refer to vacuoles or other structures, but they do not encompass the full definition and functionality of what organelles are.