HUMAN BIOLOGY

nonsteroid hormone
Hormone derived from
an amine, a peptide, or
a protein.

second messenger
Molecule that relays a
hormone signal inside a
target cell.

steroid hormone
Hormone derived from
cholesterol.

types of hormones and their signals

n There are two basic categories of hormones—steroids and

nonsteroids that consist of amino acids.

n    Links to Steroids 2.10, Amino acids 2.11, Proteins

of the plasma membrane 3.4

Hormones come in several chemical forms

Hormones vary in their chemical structure, which affects

how they function. Steroid hormones are lipids derived

from cholesterol. Amino acids or

chains of them are the raw material

of nonsteroid hormones. In this

group are amine hormones (modi-

fied amino acids), peptide hormones

(short amino-acid chains), and pro-

tein hormones (longer amino-acid

chains). Table 15.2 lists some exam-

ples of each.

Regardless of their chemical

makeup, hormones affect cell activi-

ties by binding to protein receptors

of target cells. The signal is then converted into a form that

can work in the cell. Then the cell’s activity changes:

Some hormones cause a target cell to take in more of a

substance, such as glucose. Other hormones stimulate or

inhibit the target cell in ways that alter the rate at which it

makes new proteins or modifies existing proteins or other

structures in the cytoplasm. Sometimes a hormone may

even change a cell’s shape.

It’s important to keep in mind that only cells with recep-

tors for a given hormone will respond to it. For example,

many types of cells have receptors for the hormone cortisol,

so it has widespread effects in the body. If only a few types

signal

received

signal

converted

cell

responds

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of cells have receptors for a particular hormone, its effects

in the body will be limited to tissues and organs where

those types of cells are present.

Steroid hormones interact with cell DNA

Steroid hormones are produced by cells in the adrenal

glands and in the primary reproductive organs—ovaries

and testes. Estrogen made in the ovaries and testosterone

made in the testes are good examples.

Figure 15.2A illustrates how a steroid hormone may act.

Being lipid-soluble, it may diffuse directly across the lipid

bilayer of a target cell’s plasma membrane. Once inside the

cytoplasm, the hormone molecule usually moves into the

nucleus and binds to a receptor. In some cases it binds to a

receptor in the cytoplasm, and then the hormone–receptor

complex enters the nucleus. There the complex interacts

with a particular gene—a segment of the cell’s DNA. Genes

carry the instructions for making proteins. By turning

genes on or off, steroid hormones turn protein-making

machinery on or off. This change in a target cell’s activity

is the response to the hormone signal.

Some steroid hormones act in another way. They bind

receptors on cell membranes and change the membrane

properties in ways that affect the target cell’s function.

Thyroid hormones are not chemically the same as steroid

hormones, but they behave the same. So does vitamin D.

Vitamin D meets the definition of a hormone because it is

made in the skin and ultimately arrives via the bloodstream

in the GI tract, where it acts on target cells in ways that

increase the absorption of calcium. En route “raw” vitamin D

(called cholecalciferol) is activated by steps that occur in the

liver and kidneys. Like steroid hormones, activated vitamin

D and thyroid hormones bind with receptors in the nucleus,

so we can consider them as part of this group.

nonsteroid hormones act indirectly,

by way of second messengers

Nonsteroid hormones don’t enter a target cell. Their chemi-

cal makeup makes them water-soluble, and this property

means they can’t cross a target cell’s lipid-rich plasma

membrane. Instead, when this type of hormone binds to

receptors in the plasma membrane, the binding sets in

motion a series of reactions that activate enzymes. These

reactions lead to the target cell’s response.

For instance, consider a liver cell that has receptors for

glucagon, a peptide hormone. As sketched in Figure 15.2B,

this type of receptor spans the plasma membrane and

extends into the cytoplasm. When a receptor binds glucagon,

the cell produces a second messenger. This is a molecule

called cyclic AMP (cyclic adenosine mono phosphate) that

forms in the cytoplasm and relays the incoming hormonal

signal onward. (The hormone itself is the “first messenger.”)

15.2

Steroid Sex hormones (estrogens, progesterone,

hormones testosterone), aldosterone, cortisol

Steroidlike: Vitamin D, thyroid hormones

Amines Melatonin, epinephrine, norepinephrine,

thyroid hormone (thyroxine, triiodothyronine)

Peptides Oxytocin, antidiuretic hormone, calcitonin,

parathyroid hormone

Proteins Growth hormone (somatotropin), insulin,

prolactin, follicle-stimulating hormone,

luteinizing hormone

Table 15.2 Categories of Hormones

and a Few Examples

288 Chapter 15

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