Explain how the model in the image supports this statement: the body carries out critical life functions through systems of specialized cells.

Indeed, steroid hormones have an important role in the well-being of the individual through specialized cells (having specific receptors). Take the example of vitamin D.

Vitamin D is involved in calcium and phosphorus absorption by the intestines, as well as in their reabsorption by the kidneys. It is involved also in bone mineralization of the skeleton and joints, as well as on muscle tone.

The mechanism of action of the hormone is identical to the scheme:

Its carrier is vitamin D-binding protein

The cell may be an intestinal cell, bone cells or renal cells.

The vitamin acts on the DNA is activating / inhibiting certain genes encoding proteins so as to increase the level of calcium in blood.