Organelle are structures within cells that have specialized functions. Some of the key organelle in an animal cell are nucleus, mitochondria, golgi apparatus, cell membrane, endoplasmic recticulum, centrioles, etc.
The nucleus can be thought of as the center of all the operations, since this organelle controls the functions of pretty much all the other organelle. Cell membrane controls the traffic between the cell interior and the external environment. Since the cell is incapable of producing all the required molecules, some molecules have to be allowed entry into the cell. If all the molecules were allowed easy access, cell will not survive and hence this membrane is selectively permeable, that is acts as barrier to a large number of molecules. The endoplasmic recticulum has the primary function of folding of protein molecules and their transport to golgi apparatus.
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