What are organelles within a cell?

Organelles within a cell are defined as membrane-bound structures that perform specific functions essential for the cell's life processes. Each organelle has a unique role; for example, the nucleus houses genetic material and directs cellular activities, while mitochondria are involved in energy production through cellular respiration. The term "membrane-bound" is critical, as it indicates that these organelles are enclosed by lipid bilayers, creating distinct environments within the cell for various biochemical reactions.

The other options describe concepts that do not accurately represent the definition of organelles. For instance, cells that can perform photosynthesis refer to plant cells equipped with chloroplasts, but this does not encompass the broader definition of organelles, which includes various structures across different cell types. Similarly, small molecules that store energy might refer to substances like ATP or glucose, which are important for cellular metabolism but are not organelles themselves. Finally, all types of cells in an organism would include not just organelles but also the entire cell structure, making this choice too broad and imprecise to define organelles specifically. Thus, option A correctly encapsulates what organelles are and their functional significance within the cell.