Beta cell interior. Computer artwork showing the interior of an insulin-producing cell of the pancreas (a beta cell). Seen here is the nucleus (spherical structure at centre), the rod-shaped energy-producing mitochondria (around the outside) and the rough endoplasmic reticulum (with bobbles on), where proteins, such as insulin, are synthesised and stored. The insulin is seen as pink spheres, packaged in secretory granules. These granules migrate through the cell to release their insulin cargo into the bloodstream. Insulin is a hormone that controls blood sugar level by promoting glucose storage in the liver and muscles.