Human Physiology, 14th edition (2016)

40 Chapter 2

testes and ovaries (collectively called the gonads ) secrete sex

steroids, which include estradiol and progesterone from the

ovaries and testosterone from the testes. The adrenal cortex

secretes the corticosteroids, including hydrocortisone and

aldosterone, as well as weak androgens (including dehydro-

epiandrosterone, or DHEA). Cholesterol is also an important

component of cell membranes, and serves as the precursor

molecule for bile salts and vitamin D 3.

Prostaglandins

Prostaglandins are a type of fatty acid with a cyclic hydro-

carbon group. Their name is derived from their original dis-

covery in the semen as a secretion of the prostate. However,

we now know that they are produced in almost all organs

where they serve a variety of regulatory functions. Pros-

taglandins are implicated in the regulation of blood vessel

diameter, ovulation, uterine contraction during labor, inflam-

mation reactions, blood clotting, and many other functions.

Structural formulas for different types of prostaglandins are

shown in figure 2.25.

Steroids

In terms of structure, steroids differ considerably from tri-
glycerides or phospholipids, yet steroids are still included in
the lipid category of molecules because they are nonpolar and
insoluble in water. All steroid molecules have the same basic
structure: three six-carbon rings joined to one five-carbon ring
( fig. 2.24 ). However, different kinds of steroids have differ-
ent functional groups attached to this basic structure, and they
vary in the number and position of the double covalent bonds
between the carbon atoms in the rings.
Cholesterol is an important molecule in the body because
it serves as the precursor (parent molecule) for the steroid
hormones produced by the gonads and adrenal cortex. The

Figure 2.25 Structural formulas for various

prostaglandins. Prostaglandins are a family of regulatory

compounds derived from a membrane lipid known as

arachidonic acid.

O

COOH

OH OH

Prostaglandin E 1

COOH

OH OH

Prostaglandin F 1

O

COOH

OH OH

Prostaglandin E 2

OH

COOH

OH OH

Prostaglandin F 2

OH

Figure 2.24 Cholesterol and some of the steroid
hormones derived from cholesterol. The steroid hormones
are secreted by the gonads and the adrenal cortex. The carbon
atoms in cholesterol are indicated by the numbers.

C 27

CH 2 OH

CH 3

CH 3

CO

OH

Cholesterol

C 21

Cortisol

(hydrocortisone)

1

2

3

HO

HO

O

CH 3

CH 3

OH

C 19

O Testosterone

CH 3

CH 3

OH

C 18

HO Estradiol

4

9

11

12

13 1716

14 15

10

19

18

21 22

20 23 25

27

24 26

8

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