HUMAN BIOLOGY

hoW do hormones interact With target

cells?

• Hormones interact with receptors that span a target cell’s plasma
  membrane or are in its nucleus. The hormone stimulates or halts
  the synthesis of proteins or the activity of enzymes in the cell.


• Most steroid hormones interact with a target cell’s DNA after
  they enter the nucleus or bind a receptor in the cell’s cytoplasm.


• Nonsteroid hormones bind to receptors in a target cell’s plasma
  membrane. This binding activates an enzyme system. Often a
  second messenger relays the signal to the cell’s interior, where
  the full response unfolds.

taKe-home message

An activated enzyme launches a cascade of reactions by

converting ATP to cyclic AMP. Molecules of cyclic AMP are

signals for the cell to activate molecules of another enzyme.

These act on still other enzymes, and so forth, until a final

reaction converts stored glycogen in the cell to glucose.

Soon a huge number of molecules are taking part in the

cell’s final response to the hormone.

A slightly different example is a muscle cell that has

receptors for insulin, a protein hormone. When insulin

binds to the receptor, one result is that transport proteins

insert themselves into the plasma membrane so that the

cell can take up glucose faster. The signal also activates

enzymes that catalyze reactions allowing the cell to store

glucose not needed right away for its metabolism.

4

3

Glucagon binds with a

receptor. Binding activates

an enzyme that catalyzes the

formation of cyclic AMP from

ATP inside the cell.

Cyclic AMP

activates another

enzyme in the cell.

The enzyme activated by

cyclic AMP activates another

enzyme, which in turn activates

another kind that catalyzes the

breakdown of glycogen to its

glucose monomers.

The enzyme

activated by cyclic

AMP also halts

glycogen synthesis.

A peptide hormone molecule

diffuses from blood into tissue

fluid bathing the plasma

membrane of a cell. This example

shows the hormone glucagon

acting on a liver cell.

A steroid hormone

molecule passes from

blood into tissue fluid

bathing a target cell.

The hormone

diffuses through the

cytoplasm and nuclear

envelope. It binds with its

receptor in the nucleus.

Being

lipid-soluble,

the hormone

easily diffuses

across the

cell’s plasma

membrane.

The resulting

mRNA moves into

the cytoplasm and

is transcribed into

a protein.

ATP

cyclic AMP +Pi

receptor

change in cell

activity

hormone–

receptor

complex

glucagon receptor at

target cell’s plasma

membrane

1

2

5

A specific

gene responds

to the hormone

receptor complex.

4

1

2

3

5

Figure 15.2 Animated! Different types of hormones cause change in a target cell by different mechanisms. Part a shows an example
of a mechanism by which a steroid hormone triggers changes in a target cell’s activities. Part b is an example of how a peptide hormone
triggers changes in the activity of a target cell. In this example, the hormone is glucagon. Cyclic AMP, a type of second messenger, relays
the hormone’s signal inside the cell. (© Cengage Learning)

a b

the endOCrIne system 289

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