and built up when glucose is abundant.
Starches
are
polysaccharides of glucose that are found mostly in
plants such as corn, wheat, potatoes, and barley. The
body digests starches to glucose as another energy
source.
Cellulose
is a polysaccharide found in plant cell
walls (Figure 2.10e). Although humans cannot digest
cellulose, it does provide bulk (roughage, or fiber) that
helps move feces through the large intestine. Unlike
simple sugars, polysaccharides usually are not soluble
in water and do not taste sweet. Examples include corn
starch and potato starch.
Lipids Store Energy and Comprise Cell
Membranes and Hormones
Like carbohydrates, lipids (LIP-ids;
lip- =
“fat”) contain
carbon, hydrogen, and oxygen. Unlike carbohydrates,
they do not have a 2:1 ratio of hydrogen to oxygen. The
proportion of oxygen atoms in lipids is usually smaller
than in carbohydrates, so there are fewer polar covalent
bonds. As a result, most lipids are hydrophobic (insoluble
in water), but they are
fat soluble
(dissolve in fat). The di-
verse lipid family includes:
Triglycerides (tn-GLI-cer-ides;
tri-
= “three”), fats and
oils that store chemical energy
Fatty acids
Phospholipids, lipids that contain phosphorus
Steroids
Fat-soluble vitamins, such as vitamins A, D, E, and K
A triglyceride consists of two types of building blocks,
a single glycerol molecule that forms the backbone and
three fatty acid molecules, one attached to each carbon
of glycerol by dehydration reactions (Figure 2.11 a). The
fatty acid chains of a triglyceride may be saturated (only
single covalent bonds), monounsaturated (one double
covalent bond), or polyunsaturated (more than one dou-
ble covalent bond). Triglycerides that consist mainly of
saturated fatty acids are solid at room temperature and
occur mostly in meats (especially red meats) and non-
skim dairy products (whole milk, cheese, and butter).
They also occur in a few tropical plants, such as cocoa,
palm, and coconut. Diets that contain large amounts of
saturated fats are associated with disorders such as heart
disease and colorectal cancer. Monounsaturated fats are
found in olive oil, peanut oil, canola oil, and most nuts;
they are thought to decrease the risk of heart disease.
Polyunsaturated fats are found in corn oil, safflower oil,
sunflower oil, soybean oil, and fatty fish (salmon, tuna,
and mackerel); they are also believed to decrease the
risk of heart disease. However, when products such as
margarine and vegetable shortening are made from poly-
unsaturated fats, compounds called
trans
fatty acids are
produced. Like saturated fats,
trans
fatty acids increase
the risk of cardiovascular disease.
Like triglycerides, phospholipids have a glycerol back-
bone and two fatty acids attached to the first two carbons
(Figure 2.11 b) . Attached to the third carbon is a phosphate
group (PO43-) that links a small negatively charged group to
the glycerol backbone. If you look at Figure 2.11b, you can
see that the molecule is divided into two parts: the non-polar
fatty acids form the hydrophobic “tails” of a phospholipid,
and the polar phosphate group and charged group form the
hydrophilic “head.” Phospholipids line up tail-to-tail in a dou-
ble row called a lipid bilayer to make up much of the mem-
branes that surrounds each cell or internal parts of cells.
The structure of steroids, with their four rings of car-
bon atoms, differs considerably from triglycerides and phos-
pholipids. Cholesterol (Figure 2.11c), which has received
a lot of negative press because of its role in heart disease, is
an important steroid needed for membrane structure; it is
the steroid from which body cells make other steroids. For
example, cells in the ovaries of females make the female
sex hormones estrogens, which regulate sexual functions.
Other steroids include the following:
Testosterone
(the main male sex hormone), which regulates
sexual functions
Cortisol,
which is necessary for maintaining normal
blood sugar levels
Vitamin D,
which is necessary for bone growth
Proteins Have Many Functions
Proteins are large molecules that contain carbon, hy-
drogen, oxygen, nitrogen, and sometimes sulfur. Proteins
are classified as fibrous proteins (structural proteins) or
globular proteins (mobile, spherical proteins with many
functions). Proteins have a more complex structure than
carbohydrates or lipids. Among their many roles in the
body, proteins are largely responsible for the structure of
body cells. Some proteins, called
enzymes,
speed up partic-
ular chemical reactions, while others are responsible for
contraction of muscles. Proteins called
antibodies
help de-
fend the body against invading microbes, and hormones
are protein messengers.
Proteins are made of building blocks called amino ac-
ids (a-ME-no). All amino acids have an
amino group
(—NH2)
at one end and a
carboxyl group
(—COOH) at the other end.
Each of the 20 different amino acids has a different
side
chain
called an R group. The amino group from one amino
36 CHAPTER 2
Introductory Chemistry
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