Human Physiology, 14th edition (2016)

Chemical Composition of the Body 33

Carbohydrates and lipids are similar in many ways. Both

groups of molecules consist primarily of the atoms carbon,

hydrogen, and oxygen, and both serve as major sources of

energy in the body (accounting for most of the calories con-

sumed in food). Carbohydrates and lipids differ, however, in

some important aspects of their chemical structures and physi-

cal properties. Such differences significantly affect the func-

tions of these molecules in the body.

Carbohydrates

Carbohydrates are organic molecules that contain car-

bon, hydrogen, and oxygen in the ratio described by their

name— carbo (carbon) and hydrate (water, H 2 O). The gen-

eral formula for a carbohydrate molecule is thus C n H 2 n O n ;

the molecule contains twice as many hydrogen atoms as car-

bon or oxygen atoms (the number of each is indicated by the

subscript n ).

Monosaccharides, Disaccharides, and

Polysaccharides

Carbohydrates include simple sugars, or monosaccharides,

and longer molecules that contain a number of monosaccha-

rides joined together. The suffix - ose denotes a sugar mol-

ecule; the term hexose, for example, refers to a six-carbon

monosaccharide with the formula C 6 H 12 O 6. This formula

is adequate for some purposes, but it does not distinguish

between related hexose sugars, which are structural isomers

of each other. The structural isomers glucose, galactose,

and fructose, for example, are monosaccharides that have

the same ratio of atoms arranged in slightly different ways

( fig. 2.14 ).

2.2 Carbohydrates and Lipids

Carbohydrates are a class of organic molecules that includes

monosaccharides, disaccharides, and polysaccharides. All

of these molecules are based on a characteristic ratio of

carbon, hydrogen, and oxygen atoms. Lipids constitute a

category of diverse organic molecules that share the physi-

cal property of being nonpolar, and thus insoluble in water.

with such molecules in a stereo-specific way in chemical reac-
tions—cannot combine with the “wrong” stereoisomer. The
enzymes of all cells (human and others) can combine only with
L-amino acids and D-sugars, for example. The opposite stereo-
isomers (D-amino acids and L-sugars) cannot be used by any
enzyme in metabolism.

CLINICAL APPLICATION

In the mid-twentieth century, the drug thalidomide was

widely used by pregnant women to treat morning sickness

until scientists realized that it caused birth defects; it was

banned in 1961. These teratogenic effects were produced

by only one of its enantiomers, but because tho enantio-

mers rapidly interconvert in the body, no form of thalido-

mide is safe for pregnant women. However, thalidomide

has a number of beneficial effects that make it useful for

the treatment of a variety of cancers, particularly multiple

myeloma, as well as other diseases, including AIDS and

leprosy.

Clinical Investigation CLUES

Brian was concerned that the interconvorsion of thalido-

mide enantiomers (to form a racemic mixture of the two)

in his body would have negative effects.

• What are the enantiomers of monosaccharides and
  amino acids?


• What danger does thalidomide pose, and would this
  affect Brian?

| CHECKPOINTS

1. List the components of an atom and explain how
   they are organized. Explain why different atoms are
   able to form characteristic numbers of chemical
   bonds.


2. Describe the nature of nonpolar and polar covalent
   bonds, ionic bonds, and hydrogen bonds. Why are
   ions and polar molecules soluble in water?
   3a. Define the terms acidic, basic, acid, and base. Also
   define pH and describe the relationship between pH
   and the H^1 concentration of a solution.
   3b. Using chemical equations, explain how bicarbonate
   ion and carbonic acid function as a buffer pair.


3. Explain how carbon atoms can bond with each other
   and with atoms of hydrogen, oxygen, and nitrogen.

LEARNING OUTCOMES

After studying this section, you should be able to:

5. Identify the different types of carbohydrates and
   lipids, and give examples of each type.


6. Explain how dehydration synthesis and hydrolysis
   reactions occur in carbohydrates and triglycerides.


7. Describe the nature of phospholipids and
   prostaglandins.