Glucagon and insulin are antagonistic hormones because their actions have opposite effects on the body. When the blood glucose level is high, insulin acts to lower it; when the blood glucose level is low, glucagon raises glucose levels, says the Division of Biology and Medicine at Brown University.
Both hormones originate in the pancreas, but insulin is made in the beta cells of the pancreas and glucagon is produced by the alpha cells, according to Brown University. Glucagon is only released if blood sugar is low; on the other hand, at least a small amount of insulin is secreted into the bloodstream at all times.
Insulin and glucagon work together to maintain homeostasis in blood sugar levels. When the body detects low levels of blood sugar, the pancreas secretes glucagon into the bloodstream. The hormone acts on liver cells to break down glycogen into glucose. In addition, glucagon can stimulate the liver to make new glucose molecules from raw material such as amino acids, as stated by the Biomedical Hypertexts at Colorado State University.
The beta cells of the pancreas contain channels in their membranes that can detect glucose, according Kimball's Biology Pages. When a spike in glucose is found, insulin is secreted, causing skeletal muscle and liver and muscle cells to take up glucose and convert it into glycogen. Insulin also arrests the breakdown of glycogen and the synthesis of glucose.