115
CHAPTER FOUR
Cellular Metabolism
proteins such as enzymes are produced, the second major topic
considers how information in the building block sequences of
DNA instructs the cell to assemble amino acids into proteins.
4.2
METABOLIC PROCESSES
Metabolic reactions and pathways are of two types. In
anab-
olism
(a
˘h-nab
o-liz
-e
˘m), larger molecules are constructed
from smaller ones, requiring input of energy. In
catabolism
(ka
˘-tab
o-liz
-e
˘m), larger molecules are broken down into
smaller ones, releasing energy.
Anabolism
Anabolism provides all the materials required for cellular
growth and repair. For example, a type of anabolic process
called
dehydration synthesis
(de
hi-dra
shun sin
the-sis)
joins many simple sugar molecules (monosaccharides) to form
larger molecules of glycogen. When a runner consumes pasta
the night before a race, digestion breaks down the complex car-
bohydrates in the prerace meal to monosaccharides. These are
absorbed into the bloodstream, which carries the energy-rich
molecules to body cells. Here, dehydration synthesis joins the
monosaccharides to form glycogen, which stores energy that
the runner may not need until later, as the ± nish line nears.
When monosaccharide units join, an —OH (hydroxyl group)
from one monosaccharide molecule and an —H (hydrogen
atom) from an —OH group of another are removed. As the
—H and —OH react to produce a water molecule, the mono-
saccharides are joined by a shared oxygen atom, as
f gure 4.1
shows (read from left to right). As the process repeats, the
molecular chain extends, forming a polysaccharide.
Glycerol and fatty acid molecules also join by dehydration
synthesis in fat (adipose tissue) cells to form fat molecules. In
4.1
INTRODUCTION
In every human cell, even in the most sedentary individual,
thousands of chemical reactions essential to life take place
every second. Special types of proteins called
enzymes
con-
trol the rate of each reaction. The sum total of chemical reac-
tions in the cell constitutes
metabolism.
Many metabolic reactions occur one after the other in
a linked fashion, in which the products of one reaction are
starting materials for the next. These reactions form path-
ways and cycles that may intersect where they share inter-
mediate compounds, each step catalyzed by an enzyme.
Metabolism in its entirety is complex. Individual pathways
of metabolism reveal how cells function—in essence, how
chemistry underlies biology.
Metabolic reactions and pathways can be subgrouped.
Intermediary
metabolism
refers to the processes that obtain, release, and use
energy. Another way to classify metabolic reactions is by their neces-
sity.
Primary metabolites
are products of metabolism essential to
survival.
Secondary metabolites
are not essential to survival, but may
provide an advantage or enhancement. Secondary metabolites are
best studied in plants, where they usually help to defend against
predators because they are toxins. Some of our most successful drugs
are plant secondary metabolites. The vinca alkaloids, for example,
protect the rosy Madagascar periwinkle that produces them by sick-
ening animals that eat the vegetation, but we use these biochemicals
to treat cancer. Their eF
ect is to destabilize microtubule formation.
This chapter covers two complex and related subjects. The
± rst is how metabolic pathways supply energy to a cell. Then,
as an illustration of how cellular energy is used, and also of how
clean water back±
red when workers unwittingly penetrated a layer of sediment
naturally rich in arsenic. The chemical has since been leaching into the water in
at least 2 million wells in Bangladesh, reaching levels ±
fty times the safety limit
set by the World Health Organization. When eF
ects on health began to appear
years later, the people thought arsenicosis was contagious. AF
ected individuals
not only suF
ered pain, but were shunned.
Arsenic damages the body by binding to bonds between sulfur atoms in
proteins. It affects metabolism by impairing an enzyme that transports the
breakdown products of glucose into mitochondria, where energy is extracted.
The cell runs out of energy.
Today UNICE² is helping the people of India and Bangladesh to avoid arse-
nic poisoning. Workers are diagnosing and treating arsenicosis and providing
tanks to collect and store rainwater. A vast education campaign has softened
the stigma of arsenicosis. Although cases will continue to appear for a few more
decades, the use of alternate water sources has ±
nally slowed the progression of
this public health problem.
D
isrupting the body’s ability to extract energy from nutrients
can drastically aF
ect health. Arsenic is a chemical element
that, if present in the body in excess, shuts down metabo-
lism. It can do so suddenly or gradually.
Given in one large dose, arsenic poisoning causes
chest pain, vomiting, diarrhea, shock, coma, and death. In contrast, many
small doses cause dark skin lesions that feel as if they are burning, numb
hands and feet, and eventually skin cancer. Such gradual poisoning, called
arsenicosis, may occur from contact with pesticides or environmental pollut-
ants. The world’s largest outbreak of arsenicosis, however, is due to a natural
exposure.
When the World Bank and UNICE² began tapping into aquifers in India
and Bangladesh in the late 1960s, they were trying to supply clean water to
areas ravaged by sewage and industrial waste released from rivers subject to
cycles of ³
oods and droughts. Millions of people had already perished from
diarrheal diseases due to the poor sanitation. But digging wells to provide
ARSENIC POISONING SHUTS DOWN METABOLISM
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