An organelle is a specialized subunit within a cell that has a specific function and is usually separately enclosed within its own lipid membrane. Organelles are unique to eukaryotic cells, which include animals, plants, fungi, and protists. Some examples of organelles include the nucleus (which contains genetic material), mitochondria (which produce energy through cellular respiration), chloroplasts (which carry out photosynthesis in plant cells), endoplasmic reticulum (which helps to synthesize proteins and lipids), and Golgi apparatus (which packages and transports molecules within the cell).