JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Insulin production. Illustration of insulin prod- uction in a beta cell of the human pancreas. The cell is shown in section, with various organelles visible: mitochondria (which provide cells with energy) are green, and the nucleus (top right) is purple. The folded membrane below the nucleus is the endoplasmic reticulum, which divides the cell into compartments. Insulin made in the cell is packaged in secretory granules (blue balls in pink saucers). These migrate through the cell and are secreted directly into a blood capillary (bottom, containing red blood cells). Insulin is a hormone that controls blood sugar level by promoting gluc- ose uptake in the liver and muscles.
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