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 paraplasm or inclusions.
Based on the level of the membrane around each organelle, the cellular organelles have been separated into two types such as a single membrane-bound organelle, and double membrane-bound organelle.
Eukaryotic cells are well-studied cells found in the plants (from algae to angiosperms) and in the animals (from Protozoa to mammals). Though, the cells of eukaryotic organisms have different size, shape, and physiology; all the cells are generally composed of plasma membrane, cytoplasm and cellular organelles, such as mitochondria, endoplasmic reticulum, ribosomes, Golgi apparatus (Golgi complex), peroxisomes, lysosomes, cytoskeleton, centrosome, and a true nucleus.
Cytoplasm of prokaryotic cell lacks in 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 present 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 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 sub-cellular organelles and other particles.
What is an organelle made up of ?
The membrane of the organelle is composed of both protein and lipid molecules. The proportion of these two molecules various in different organelle membranes. The membrane of the organelle, which is metabolically higher active, that contains more protein than lipid. For instant, Mitochondria and chloroplast show higher activity and have a higher proportion of proteins and more granular appearance than those are metabolically less active e.g. myelin sheath of the central nervous system.