An organelle is a specialized structure suspended in the cytoplasm of the eukaryotic cells and contributes to specific cellular function.
The cytoplasmic matrix contains both living and non-living structures. Living structures are called organelles and non-living structures are called periplasm or inclusions.
Based on the level of the membrane around each organelle, the cellular organelles have been separated into two types such as single membrane-bound organelles, and double membrane-bound organelles.
Eukaryotic cells especially plant (from algae to angiosperms) and animal cells (from Protozoa to mammals) are studied for analyzing the membrane-bound organelles. Though the cells of eukaryotic organisms have different size, shape, and physiology; all the cells are generally composed of the plasma membrane, cytoplasm, and cellular organelles, such as mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus (Golgi complex), peroxisomes, lysosomes, cytoskeleton, centrosome, and a true nucleus.
The cytoplasm of prokaryotic cells lacks well-defined cellular organelles such as endoplasmic reticulum, Golgi apparatus, mitochondria, centrioles, etc.
In eukaryotic cells, nuclear contents, such as DNA, RNA, nucleoproteins, and nucleolus remain separated from the cytoplasm by thin perforated nuclear membranes.
Organelles, which are structurally different are mainly found in unicellular organisms perform a similar function to organs present in multicellular organisms. Interestingly, Sigle cell-eukaryotic organism, protozoans have contractile vacuole, that performs extraction of fluid wastes from the cell and eliminates them from the organism, which is a similar function as kidney does in higher organisms.
In 1974 the biology scientists A. Claude, C. de Duve, and G. Palade got Nobel Prize for the isolation and characterization of subcellular organelles and other particles.
What is an organelle made of?
The membrane of the organelle is composed of both protein and lipid molecules. The proportion of these two molecules various in different membranes of the organelles. The membrane of the organelle is metabolically more active and contains more proteins than lipids. For instance, mitochondria and chloroplast show higher activity and have a higher proportion of proteins and more granular appearance than those who are metabolically less active e.g. myelin sheath of the central nervous system.