Chloroplasts occur in a variety of shapes and sizes, such as corkscrewlike ribbons or bracelet-shapes found in certain green algae. The chloroplasts of higher plants, however, tend to be shaped somewhat like two frisbees glued together along their edges, and when they are sliced in median section they resemble the outline of a football. Chloroplasts may be from 2 to 10 micrometers in diameter, and each is bounded by an envelope consisting of two delicate unit membranes . The outer membrane apparently is derived from endoplasmic reticulum whereas the inner membrane is believed to have orginated from the cell membrane of a blue-green bacterium. Within is a colorless, fluid, enzyme-containing matrix, called the stroma. Grana (singular: granum), which are stacks of coin-shaped double membranes called thylakoids are suspended in the stroma. The membranes of the thylakoids contain green chlorophyll and other pigments. Theses "coin-stacks" of grana, are vital to life as we know it on our planet today, for it is within the thylakoids that the first steps of the all-important process of photosynthesis occurs.