62
UNIT ONE
absorbed into the bloodstream. Plant starch is another type of
polysaccharide. Starch molecules consist of highly branched
chains of glucose molecules connected differently than in cel-
lulose. Humans easily digest starch.
Animals, including humans, synthesize a polysaccharide
similar to starch called
glycogen,
stored in the liver and skel-
etal muscles. Its molecules also are branched chains of sugar
units; each branch consists of up to a dozen glucose units.
Lipids
Lipids
(lip
idz) are a group of organic chemicals that are
insoluble in water but soluble in organic solvents, such
as ether and chloroform. Lipids include a number of com-
pounds, such as fats, phospholipids, and steroids, that have
vital functions in cells and are important constituents of cell
membranes (see chapter 3, p. 79). The most common lip-
ids are the
fats,
primarily used to supply energy for cellular
activities. Fat molecules can supply more energy gram for
gram than can carbohydrate molecules.
Like carbohydrates, fat molecules are composed of car-
bon, hydrogen, and oxygen atoms. However, fats have a
much smaller proportion of oxygen than do carbohydrates.
The formula for the fat
tristearin,
C
57
H
110
O
6
, illustrates these
characteristic proportions.
Carbohydrates are water-soluble molecules that include
atoms of carbon, hydrogen, and oxygen. These molecules
usually have twice as many hydrogen as oxygen atoms,
the same ratio of hydrogen to oxygen as in water mole-
cules (H
2
O). This ratio is easy to see in the molecular for-
mulas of the carbohydrates glucose (C
6
H
12
O
6
) and sucrose
(C
12
H
22
O
11
).
Carbohydrates are classified by size. Simple car-
bohydrates, or
sugars,
include the
monosaccharides
(mon
o-sak
ah-rı¯dz) (single sugars) and
disaccharides
(di-sak
ah-rı¯dz) (double sugars). A monosaccharide may
include from three to seven carbon atoms, in a straight chain
or a ring
(f g. 2.11)
. Monosaccharides include the ± ve-car-
bon sugars ribose and deoxyribose, as well as the six-carbon
sugars glucose, dextrose (a form of glucose), fructose, and
galactose
(f g. 2.12
a
)
. Disaccharides consist of two 6-carbon
units (± g. 2.12
b
). Sucrose (table sugar) and lactose (milk
sugar) are disaccharides.
Complex carbohydrates, also called
polysaccharides
(pol
e-sak
ah-rı¯dz), are built of simple carbohydrates (± g. 2.12
c
).
Cellulose is a polysaccharide abundant in plants. It is made of
many bonded glucose molecules. Humans cannot digest cel-
lulose. It is considered to be dietary ± ber, passing through
the gastrointestinal tract without being broken down and
TABLE
2.6
|
Inorganic Substances Common in Cells
Substance
Symbol or Formula
Functions
I. Inorganic Molecules
Water
H
2
O
Major component of body F
uids (chapter 21, p. 811); medium in which most biochemical reactions
occur; transports various chemical substances (chapter 14, p. 536); helps regulate body temperature
(chapter 6, p. 182)
Oxygen
O
2
Used in release of energy from glucose molecules (chapter 4, p. 120)
Carbon dioxide
CO
2
Waste product that results from metabolism (chapter 4, p. 122); reacts with water to form carbonic
acid (chapter 19, p. 766)
II. Inorganic Ions
Bicarbonate ions
HCO
3
Help maintain acid-base balance (chapter 21, p. 821)
Calcium ions
Ca
+2
Necessary for bone development (chapter 7, p. 204); muscle contraction (chapter 9, p. 291), and blood
clotting (chapter 14, ±
g. 14.19)
Carbonate ions
CO
3
–2
Component of bone tissue (chapter 7, p. 204)
Chloride ions
Cl
Help maintain water balance (chapter 21, p. 818)
Hydrogen ions
H
+
pH of the internal environment (chapters 19, pp. 757–758, and 766)
Magnesium ions
Mg
+2
Component of bone tissue (chapter 7, p. 204); required for certain metabolic processes
(chapter 18, p. 720)
Phosphate ions
PO
4
–3
Required for synthesis of ATP, nucleic acids, and other vital substances (chapter 4, p. 125); component
of bone tissue (chapter 7, p. 204); help maintain polarization of cell membranes (chapter 10, p. 366)
Potassium ions
K
+
Required for polarization of cell membranes (chapter 10, p. 366)
Sodium ions
Na
+
Required for polarization of cell membranes (chapter 10, p. 366); help maintain water balance
(chapter 21, p. 818)
Sulfate ions
SO
4
–2
Help maintain polarization of cell membranes (chapter 10, p. 366) and acid-base balance
(chapter 21, p. 818)
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