The<unk>  or<unk>  is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae. The lacey membranes of the<unk>  were first seen by Keith R. Porter, Albert Claude, and<unk> nest F. Fullam in 1945. These structures are responsible for several specialized functions: Protein translation, folding, and transport of proteins to be used in the cell membrane (e.g., transmembrane receptors and other integral membrane proteins), or to be secreted (exocytosed) from the cell (e.g., digestive enzymes); sequestration of calcium; and production and storage of glycogen, steroids, and other macromolecules. The<unk>  is part of the endomembrane system. The basic structure and composition of the<unk>  membrane is similar to the plasma membrane.
Endoplasmic reticulum
Golgi apparatus
Cytoskeleton
Lysosome
Mitochondrion
Nuclear envelope
Cytoplasm
T-tubule