Hormones are chemical signals that are released by cells in one part of the body that travel through the bloodstream to signal cells in another part of the body. Insulin is a hormone that is released by the pancreas that induces the
uptake of glucose molecules from the bloodstream into cells. In this way, insulin lowers the overall blood glucose
levels of the body. Osteoblasts and osteoclasts are two types of bone cells that play a role in regulating blood
glucose levels (Figure 1).
Binding of insulin to the insulin receptor on osteoblasts activates a signaling pathway that results in osteoblasts
releasing a molecule, , that binds to neighboring osteoclasts. In response, the osteoclasts release protons ( )
and create an area of lower outside the cell. This low activates osteocalcin, a protein secreted in an inactive
form by osteoblasts.
The gene encodes a protein that alters the structure of the insulin receptor on osteoblasts and interferes with
the binding of insulin to the receptor. A researcher created a group of osteoblasts with an mutation that
prevented the production of a functional product (mutant). The researcher then exposed the mutant strain and
a normal strain that expresses to glucose and compared the levels of insulin in the blood near the osteoblasts
(Figure 2).

25. Based on the information provided, which of the following best justifies the claim that osteocalcin is a
hormone?
A
The phosphorylation of the insulin receptor causes a response in osteoblast bone cells.

B
The osteoblasts in the bone secrete osteocalcin, which causes cells in the pancreas to change their activity.

C
The change in expression of changes the insulin receptor activity of the osteoblast.

D
The activation of the osteocalcin by a bone cell is dependent.