Pancreas cell. Coloured transmission electron micrograph (TEM) of part of an acinar (exocrine) pancreatic cell. The endoplasmic reticulum (red) controls the cell's secretion of zymogens. Zymogens are inactive precursors of digestive enzymes. The enzymes are secreted in vesicles known as zymogen granules (green) and are activated using proteolysis in the small intestine to aid the breakdown of carbohydrates, fats and proteins. The cell nucleus is seen center left. Magnification: x11, 000 when printed 10 centimetres wide.