Ribosomes are amembranous ribonucleoprotein particles. These are the site of protein synthesis. Nucleolus is also amembranous and contain large amount of RNA and some DNA. The main function is synthesis of ribosomal RNA. Nucleus (control centre of cell), chloroplast (photosynthetic organelle), mitochondria (power house of cell), endoplasmic reticulum and Golgi body are surrounded by double layered membrane. Lysosomes are spherical tiny sacs bounded by a single membrane. These are reservoirs of hydrolytic enzymes used for controlling intracellular digestion of macromolecules.