A ecisterna, plural cisternae, is a flattened sac, or compartment, inside what is known as the Golgi apparatus. The Golgi apparatus is found inside the cells of animals and plants, and is involved in modifying and sorting proteins, sugars and fats. A number of cisternae stacked together make up the Golgi, and molecules enter at one end before moving through the stack, undergoing changes aided by different sets of enzymes in each cisterna. Most of the modified molecules then leave the cisterna at the other end of the Golgi, nearest the cell membrane. Substances can be moved from there to the surface of the cell and released, a process which is especially important in secretory cells, such as those which line the gut and produce mucus.
Inside the cell, a structure called the rough endoplasmic reticulum manufactures proteins, fats and sugars, which then travel to the Golgi apparatus within tiny bubbles called vesicles. Once they arrive at the Golgi, the vesicles fuse with its membrane and the molecules are released into the first cisterna. The end of the stack of cisternae where molecules enter is called the cis-Golgi network, and the end near the cell membrane where molecules leave is known as the trans-Golgi network.
Usually, there are only about five cisternae in a Golgi stack, although the Golgi apparatus in algae can have up to five dozen. In animal cells, each on is linked to the others by tiny tubular connections to form a single unit. A Golgi apparatus is normally located near the nucleus of the cell and they can be present in varying numbers depending on the function of the cell.
As proteins, sugars and fats travel through the Golgi apparatus, the enzymes they meet inside each cisterna vary and are involved in carrying out different processes. Some parts of the molecules are trimmed away and other parts are added as they move through the various regions of the stack. It is not yet fully understood how the progression from one to the next occurs, but the most popular theory is that the cisternae themselves actually move. New cisternae are thought to form continually at the cis end of the Golgi stack, and as they mature they move up through the stack, passing their enzymes back to the newer cisternae.
When a cisterna finally reaches the trans end of the Golgi stack, many vesicles break off from it, heading for the cell membrane with their contents, and it ceases to exist. This is not a problem since other maturing cisternae are continually moving up to replace it. The vesicles which were produced fuse with the cell membrane, and the molecules inside are released from the cell.