Protein-based hormones such as insulin bind to cell-surface receptors. While they do not enter the cell, they cause changes in the cell’s metabolism. Specific cells in the pancreas secrete insulin, a hormone that regulates the concentration of glucose in the blood.

Skeletal muscles and the liver are targets for insulin. Insulin deficiency is responsible for Type I diabetes. In contrast, Type II diabetes, also known as adult-onset diabetes mellitus, is not the result of insulin deficiency but is rather due to insulin resistance. Cells with insulin resistance do not respond to increasing insulin levels by transporting glucose into cells. (For more about human diseases and biology, see the chapter “Biology and You.”)