Their detailed structure has been revealed by electron microscope. Each chloroplast has two unit membranes made up of lipoprotein. Inside the membranes is the proteinaceous ground substance called stroma . Stroma contains a variety of particles, osmiophilic droplets, ribosomes, strands of DNA and RNA, dissolved salts, enzymes. Embedded in the stroma are many membranes, running parallel to each other throughout the length of the chloroplast. These are called lamellae, Each lamella is made up of two unit membranes. At some intervals the lamellae have 10-100 rounded flat sac like structures stacked one above the other. Such rounded flat structure is called thylakoid.
One stack of thylakoids is called granum. Different grana are connected with grana of other lamella with the help of tubular connections called stroma lamellae or frets. Each chloroplast contains 40-60 grana. Membrane of grana lamellae are made up of proteins