living on solar power!

®
DEMYSTIFYING
EVERYDAY
CHEMISTRY
OCTOBER 2011
LIVING ON
SOLAR p.POWER!
8
Students Compete
to Build the Best Solar Home
in the Solar Decathlon,
p.10
Demystifiying Gross Stuff, p.12
How Artificial Sweeteners Work, p.15
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Technical Review
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®
Vol. 29, No. 3
OCTOBER 2011
DEPARTMENTS
Did You Know?
4
How dental braces straighten teeth; the amazing properties
of microfibers.
Open for Discussion: Lithium-Ion Batteries:
A Clean Source of Energy?
By Barbara Sitzman and Regis Goode
6
5
Lithium-ion batteries power computers, iPods, and hybrid cars.
But where does lithium come from? Are these batteries safe
for the environment?
5
By Gail Kay Haines
rstock
6
shutte
Sugar in the Blood Boosts Energy
istock
FEATURES
Where do you get the energy you need to read, think, or jog?
It all comes down to a sugar molecule, glucose, which you
produce from the digestion of food.
Harnessing Solar Power
By Michael Tinnesand
10
8
In the United States, the main source of energy is fossil fuels,
but solar energy is a promising alternative. How do we collect
this energy and use it to produce electricity?
Students Build Solar Homes
By Michael Tinnesand
10
Every other year, students compete to build the world’s best
solar homes. Check out what students have achieved in 2009
and what they will showcase this year at the Solar Decathlon
in Washington, D.C.
By Brian Rohrig
12
What’s behind acne, smelly breath, flatulence? Bacteria
and chemistry.
The Skinny on Sweeteners: How Do They Work? 15
SHUTTERSTOCK
Demystifying Gross Stuff
Stefano Paltera/U.S. Department of Energy Solar Decathlon
Check out the video
podcasts on acne and bad breath
at: www.acs.org/chemmatters
12
Spanish
translation
available
online!
By Christen Brownlee
Splenda, Equal, Sweet’N Low… Why are these artificial
sweeteners calorie-free sugar substitutes? Are they healthy?
Fighting Bacteria By Margaret Orford
Antibiotics are becoming less effective at killing bacteria as
experienced by Eric, 15, who recently had pneumonia and
realized it was caused by a resistant strain of bacteria.
17
TEACHERS!
find your complete:
teacher’s guide
for this issue at
www.acs.org/chemmatters
On the cover: University of maryland students work to install the green roof modules on the university of
maryland solar decathlon entry. courtesy of Jeff gipson/maryland.
chemmatters, OCTOBER 2011 3
DID YOU KNOW?...
DID YOU KNOW?...
Structure of Matter: How Dental
Braces Straighten Teeth
Polymers: The Amazing Properties
of Microfibers
M
M
4 Chemmatters, OCTOBER 2011
goes into the martensite phase,
forming a more pliable, bendable crystal structure. The atoms
stay attached to one another,
and the crystal deforms without
breaking. When
the material is
heated to 60 °C,
the atoms return
to the more rigid
austenite phase.
An orthodontist—a dentist
who specializes
in correcting
both photos from istock
any teens have braces to
straighten crooked teeth
or correct an overbite—the overlapping of the lower teeth by the
upper teeth. The metal used for
the wire in braces
is called memory
metal because it
“remembers” a
shape and returns
to it under the
right conditions.
Memory metal,
an alloy of nickel
and titanium,
was developed in
1965. Alloys are
mixtures of metal atoms that do
not react with one another but
exist side by side.
William J. Buehler, a scientist at a research facility from
the U.S. Navy called the Navy
Ordnance Laboratory, prepared
strips of a nickel-titanium alloy
that could be bent into folds
and then stretched back into
a straight piece. The alloy was
called Nitinol, for nickel, titanium, and the Navy Ordnance
Laboratory.
We know the common three
phases of matter: solid, liquid,
and gas. But Nitinol has two
solid phases called the austenite
and the martensite phases. As
the temperature rises, it changes
from one to the other. These
phases may look similar, but the
atomic structure in each phase is
actually different.
In the austenite phase, the
atoms of titanium are in the
center of a cube of nickel atoms,
forming a rigid and inflexible
structure. As the alloy cools, it
misalignment of teeth—shapes
the Nitinol wire to the shape
desired for your teeth. The Nitinol is heated to about 550 °C. It
goes into the austenite phase,
which keeps the original shape
of the teeth. Then, as it cools,
the wire moves into the martensite phase, so it becomes flexible
enough for the orthodontist to
place it in your mouth and attach
it to brackets.
The heat of your mouth makes
the wire go back to the austenite
phase and the original shape. As
the wire strains to move into the
original shape, it pulls your teeth
into the desired places—the
ones which will give you a beautiful smile of straight teeth.
—Roberta Baxter
www.acs.org/chemmatters
icrofibers are synthetic
fabrics that are exceptionally strong yet contain very
thin fibers—four times thinner than wool fiber and three
times thinner than cotton
fiber. They are used
for cleaning
purposes
because they
are soft and do
not scratch, and they
can easily remove dirt
and dust without using
chemicals.
Microfibers are also
used to make clothes
that feel soft and have
an amazing property:
They can wick away
moisture from your
skin in the summer and
keep you warm in the winter.
Microfibers wick moisture
away by absorbing as much as
seven times their weight in moisture. Cotton, for example, soaks
up water by absorbing it. By contrast, microfibers pull water away
from your skin to a drier part of
the fabric.
If you wear a T-shirt made with
microfibers, it will pull sweat
away to keep you cooler during
a workout and will pull rainwater
away from your skin if you get
caught in a downpour.
Microfibers are made of
polymers—long chain-like
molecules that consist of repeating units strung like beads on
a thread. They wick moisture
because water molecules pull on
each other, so as one molecule
moves, the nearby molecules
are pulled along. So, water
molecules that come from either
sweat or rain are taken away
from the wet sections of the fabric to the drier ones. This way,
your skin stays dry.
Microfibers can
also keep
you warm.
The small
diameter of
the fiber allows
shutterstock
air pockets to form in the fabric.
The trapped air provides insulation, keeping you warm. The
light weight of the fabric and
the insulation quality has made
microfibers a hit for jackets,
gloves, boots, and hats.
Microfibers are elastic, making them suitable for underwear.
They are also used in sheets,
blankets, and bed coverings.
Microfibers can be made into
an imitation leather called Ultrasuede that is cheaper and easier
to clean than suede leather,
which is made from the underside of the animal skin.
So, if you have never worn or
used fabric made with microfibers, you may want to give it
a try!
—Roberta Baxter
OPEN FOR DISCUSSION
By Barbara Sitzman
and Regis Goode
through the molten lithium
chloride to supply electrons to
Li ions, reducing them to lithium
metal and producing chlorine
gas: 2 LiCl(s) ➔ 2Li(s) + Cl2(g).
Unfortunately, the mining and
refining of lithium mars the landscape and produces wastes that
could damage the environment,
but since lithium atoms are light
and can produce more energy
per pound, less of the material is
needed.
B: Another problem is that
the majority of known lithium
deposits are outside the
United States. This may
have some serious political implications. Some
of the largest reserves are
located in Bolivia, Chile, Australia,
Argentina, and China. Does this
mean that we will be dependent
upon foreign sources? Will some
of these countries become the
next “Saudi Arabia of Lithium”?
To address these questions, U.S.
scientists and car manufacturers
are looking at deposits within the
United States.
R: Let’s look at how lithiumion batteries work. Compared to
other batteries, they can store
more energy for their size and
weight and operate at a higher
voltage and at a lower current
than regular AA batteries. One
Li cell can produce the same
voltage as multiple AA cells.
Lithium-ion batteries also have
a lower self-discharge rate, so
once they are charged they retain
their charge longer. This is called
having “high power density.”
A lithium-ion battery operates
using the process of oxidation-
reduction (Redox), in
which one substance
loses electrons (oxidation) to another
substance that gains
the electrons (reduction). By controlling the
flow of these electrons,
electricity is generated.
In a lithium-ion battery,
the cathode (the place
where reduction occurs)
is generally made of
lithium-cobalt oxide
(LiCoO2), and the anode (where
oxidation occurs) is coated
with graphite (carbon).
A “separator” between
the cathode and the
anode allows ions to pass
through. These components are
submerged in a solution that conducts a current called electrolyte.
While the battery charges,
the energy going in forces
lithium ions to move through
the electrolyte to the anode,
where they attach to the carbon.
When the battery is in use, the
lithium ions move back to the
cathode, stranding electrons on
the anode. These electrons must
take the long way back through
the circuit, creating an electrical
current.
B: It is important to remember
that the recharging of the batteries requires electricity produced
by burning coal in power plants.
This produces carbon dioxide
(CO2), a greenhouse gas, so
using batteries is not completely
emission-free.
R: I agree, but battery-operated
cars have no direct emissions.
Since electric cars are not burn-
© 2006 howstuffworks, inc.
Barbara: Hi, Regis. Do you
know what allows us to talk to
our friends, stay in touch with
the world, and save petroleum
resources? Well, it’s right there
on the periodic table, element
number 3.
Regis: You are right. Lithium
(Li) is the major component of
modern batteries. It powers our
computers, iPods, cameras,
electronic games, and hybrid
(gasoline-electric) cars. I wonder
if it is truly a clean energy source.
Let’s open this up for discussion.
B: The unique properties of
this element make it just what we
need to power tiny cell phones
and electric cars. Lithium, with
the lowest density of any solid
element, can pack three times
more energy per pound
than older batteries. What is
involved in
obtaining
the pure eleck
to
rs
ment?
te
ut
sh
R: Lithium, like
other alkali metals, is too reactive to exist as a
pure metal in nature. It is found
combined in various minerals
and in brine deposits that contain mostly salt. Lithium can be
extracted by evaporating salty
pools filled by rivers that have
washed over lithium-containing
rocks. As the water evaporates,
the lithium crystallizes as ionic
salts, usually lithium chloride and
lithium carbonate.
As with other very active metals, pure lithium can be produced
by electrolysis. In this process,
an electric current is passed
shutterstock
Lithium-Ion Batteries: A Clean Source of Energy?
ing gasoline, a fossil fuel, they
do not directly produce CO2. And
since they do not burn fossil
fuels, they are not consuming
a nonrenewable resource. This
helps reduce our dependence on
foreign oil.
B: Having said that, there are
always environmental issues
to consider, even in what is
described as clean energy. Mining and refining lithium certainly
have hidden environmental
issues, ranging from destroying
the landscape during the mining
process to producing emissions
during refining.
R: So, all energy has a cost. As
we look to the future and scientifically evaluate new technologies,
should we consider the actual
“cleanliness” of each energy
source? What do you think? Let
us know by contacting us at:
[email protected].
Barbara Sitzman and Regis Goode
are high school chemistry teachers
at Granada Hills Charter High School,
Granada Hills, Calif., and Ridge
View High School, Columbia, S.C.,
respectively.
Chemmatters, OCTOBER 2011 5
Sugar
in the
Blood Boosts
istock
By Gail Kay Haines
all figures by anthony fernandez
W
hoa! You skid into a parking
space at the far end of the lot,
with minutes to get to your firstperiod chemistry exam. Foot
speed, then brain surge required, full power.
Next stop: physical education, with time trials
for running the mile. Breakfast is fading, and
it’s another hour to lunch. How do you get the
energy for so much activity?
Energy is already cruising through your
bloodstream as you read, think, or run around
the track. One special sugar serves as nature’s
little battery pack—glucose. It is the primary
energy storage molecule used by all living
beings. People need about a teaspoon of
glucose every 15 minutes to keep the energy
going throughout their bodies.
Where do you get glucose? Short answer:
from plants. Plant cells take energy from the
sun, add water (H2O), pick up carbon dioxide (CO2) from the air, and produce glucose
(C6H12O6) and oxygen (O2).
6 CO2 + 6 H2O + energy (from the sun) à C6H12O6 + 6 O2
But how does your body get, store, and use
glucose to supply human energy? It’s a little
more complicated.
6 Chemmatters, OCTOBER 2011
A sweet source of
energy
CH2 OH
Glucose is a type of molecule called a carbohydrate. Carbohydrates are composed of
carbon, hydrogen, and oxygen and are found
in fruits, vegetables, dairy products, breads,
and sweets. Glucose (Fig. 1) is one of the
simplest forms of carbohydrate and the most
abundant carbohydrate in nature.
OH
CH2OH
H
OH
O
H
OH
H
H
OH
OH
H
Figure 1. Chemical structure of glucose
Glucose, which makes up about 0.1% of
our blood, represents the body’s simplest,
quickest source of energy. Glucose is also
present in common food products, such as
ordinary corn syrup—which is mainly glucose—and table sugar, in which glucose is
part of a molecule called sucrose (C12H22O11)
(Fig. 2).
Although glucose is the body’s favorite
food, we don’t usually eat it straight. Carbohydrates, which often contain glucose in
chemically bound forms, such as sucrose,
need to be broken down to make free glucose.
Proteins can be digested and used to make
glucose by a more complicated process, but
usually they are used by the body as building
www.acs.org/chemmatters
CH2OH
O
O
OH
O
OH
OH
CH2 OH
OH
Figure 2. Chemical structure of sucrose
materials, not for energy. And while fats are
broken down to produce energy, they are not
generally used directly to make glucose.
Regardless of the source of glucose, your
body can use glucose immediately or store it
in your liver for later use. The liver converts
unneeded glucose into a molecule called
glycogen (Fig. 3), which is later broken to
provide glucose.
So, when you are running from the parking
lot to your class or in between classes, your
body is using glucose from food you ate during breakfast or it is tapping glucose stored
in your liver. If your breakfast plays out, thank
last night’s dinner for your “A” on the chemistry exam! Some glucose spent the night as
“energy on tap.”
Energy from glucose
Inside your body, glucose molecules move
through your blood and travel to every tissue and organ. There, the glucose molecules
react with oxygen molecules coming from the
lungs—those that you breathe in from the
air—and, through a series of chemical reactions, these molecules produce energy as well
ATP + H2O à
ADP + HPO42– + energy
The ADP molecule is also a source of
energy. With help from enzymes present in
cells, it reacts with a water molecule to produce a still smaller molecule called adenosine
monophosphate (AMP), which contains one
phosphorus atom instead of two (Fig. 4c):
all photos from istock
Energy
With help from enzymes present in cells, an
ATP molecule reacts with a water molecule to
produce a smaller molecule than ATP called
adenosine diphosphate (ADP) (Fig. 4b), which
contains two phosphorus atoms instead of
three, along with a hydrogen phosphate ion
and energy:
ADP + H2O à AMP + HPO42– + energy
as carbon dioxide and water. That energy ultimately allows you to walk, talk, run, and think.
What happens in your body is essentially
the reverse of what happens in plants:
So, when you run around the track and
think your way through an exam on chemical
bonds, you use energy produced during these
two chemical reactions. Then, the body takes
energy from incoming glucose to reassemble
the ATP molecules, and the cycle starts again.
C6H12O6 + 6 O2 à
6 CO2 + 6 H2O + energy
Give me a straight
shot of glucose!
This equation actually summarizes a
sequence of more than 20 chemical reactions,
and the energy (right side of the equation) is
stored in small molecules called adenosine
triphosphate (ATP). This molecule (Fig. 4a)
is the most widely used in nature to store
energy. It contains a phosphate-ion “tail” in
which potential energy is stored.
If glucose is the favored
body and brain food, why not
live on sugar? Could we live
on sugar? The answer is a
qualified no. First, your body
requires all sorts of chemicals
from food in
addition to glucose—such as
proteins, healthy fats, fiber, vita(a)
mins, and antioxidants.
Also, only a healthy body can
process glucose properly. If your
heart, liver, and pancreas are not
working well, too much glucose
H CH
could lead to diabetes, a medi(b)
C
OH
O C
H
O
OH
cal condition in which the body
H
C
H
C
H
C
cannot make or use the glucoseH CH
OH
O C
C
OH
H
digesting hormone insulin.
O
OH
H
C
H
But recent research has
C
H
C
OH
O
shown that straight glucose may
CH OH
CH
CH OH
CH OH
sometimes be the way to go. A
H C O H
H C O H
H C O H
H C O H
H
H
H
H
study on the memory-enhancing
C OH H C
C OH H C
C OH H C
C OH H C
O
O
O
O
O
C C
C C
C C
C C
effect of glucose in college
H OH
H OH
H OH
H OH
students was conducted by
scientists at Swinburne UniverFigure 3. (a) Glycogen is made of a chain of glucose units, with
sity of Technology, Melbourne,
branches occurring every 8–10 units; (b) detailed chemical
structure of a few glucose units in the chain.
Australia. The researchers gave
2
2
2
2
2
2
H2N
O
O— P
O
P
O—
O
P
O—
O
N
N
O
O
O
N
N
O
—
OH OH
(a)
H2N
O
O
HO
P
O
O
—
P
N
N
O
N
N
O
O—
OH OH
(b)
H2N
O
HO
P
O
N
N
O
—
N
N
O
OH OH
(c)
Figure 4. Chemical structures of (a) ATP, (b) ADP;
and (c) AMP
drinks containing glucose to undergraduate
students and noticed that high doses of glucose improved their recall of words and large
numbers for a 2-hour period.
Now that you know how glucose is produced and used in your body, you may find
it easier to decide what to eat: healthy food
that will release glucose slowly or a “shot of
glucose” that will spike the amount of energy
in your body for a few hours. The choice is
yours, but don’t be surprised by how your
body reacts!
Selected references
Raloff, J. Brain Boosters: Some Nutritional
Supplements Provide Real Food for Thought.
Science News, Feb 26, 2011, 179 (5), pp
26–29: www.sciencenews.org/view/feature/
id/69708/title/Brain_​Boosters [accessed July
2011].
Quinn, E. What to Eat Before Exercise. About.
com: http://sportsmedicine.about.com/od/
sportsnutrition/a/EatForExercise.htm [accessed
July 2011].
Gail Kay Haines is a science writer and book
author from Olympia, Wash. Her most recent
ChemMatters article, “Is this Water Recycled
Sewage?” appeared in the February 2011 issue.
chemmatters, OCTOBER 2011 7
© 1998-2011 howstuffworks, inc.
The Utrik Atoll consists of 10 islands that are part
of the Marshall Islands, a nation of atolls and
islands in the middle of the Pacific Ocean.
Converting sunlight
into electricity
istock
By Michael Tinnesand
The Utrik leaders worked with Moana
Marine, LLC, a local alternative energy company, to install a solar panel power system
and two small wind-powered generators that
produced enough electricity to power the
island’s water system. Since then, the power
system has been providing an abundant supply of electricity to the island’s houses and has
allowed the opening of a new school, a new
community center, and an agriculture nursery
facility.
Utrik may be a model in miniature for the
fate that might await the rest of the world. In
the United States, the main source of energy
is fossil fuels—petroleum, natural gas, and
coal. Fossil fuels contribute 85% of the energy
that we need to light our homes, power
8 Chemmatters, OCTOBER 2011
our cars, and cook our meals. But we will
eventually run out of fossil fuels. Also, many
countries—including the United States—need
to import fossil fuels because they are found
only in certain parts of the world. Another
problem with fossil fuels is that burning them
releases carbon dioxide, a greenhouse gas
that contributes to global climate change.
A promising alternative to fossil fuels is
solar energy. The total human population on
Earth currently uses about 13 terawatts (1
terawatt equals one trillion watts) of energy.
This is only 0.01% of the 120,000 terawatts of
energy delivered by the sun to the surface of
the Earth. If we could only find a way to harness even a fraction of this solar energy, we
could solve our energy problems.
www.acs.org/chemmatters
To produce electricity from the sun, all that
is needed is a small device called a solar cell,
which converts solar energy directly into electricity. To power a house or a building, solar
cells are combined into modules and arrays to
form solar panels (Fig. 1).
Solar panels are “first cousins” to the chips
inside computers or cell phones. The technology used to make solar panels is similar to
making computer chips. Both use a class of
material called semiconductors—materials
that have a limited ability to conduct an electric
current.
Most semiconductors are made from crystalline silicon. In a pure silicon crystal, each
silicon atom is bonded to four other silicon
atoms, and each bond consists of a pair of
shared electrons. This is a stable configuration.
Electrons involved in these bonds move very
little and are restricted to the bonds.
Silicon does not conduct electricity because
its electrons do not move easily. Think about a
theater filled with people. If every seat is taken,
nobody can move or shift for a better seat.
Giff Johnson, The Marshall Islands Journal
P
eople have lived on Utrik Atoll for the past
4,000 years. It is a tropical paradise that is
part of the Republic of the Marshall Islands,
a nation of islands and atolls in the middle of the
Pacific Ocean. Utrik Atoll is a sanctuary for sea
turtles, birds, and many species of fish.
In recent times, global climate change brought
drought to the region, and fresh drinking water
was increasingly difficult to come by. What water
they could pump from wells relied on electricity from diesel generators, but with rising fuel
costs, this process was too expensive to sustain.
Utrik local government technician Beasa Beasa
(above right) uses a solar-powered device that
dispenses fresh, clean drinking water.
Electron
(a)
Missing electron
or “hole”
Gallium atom
Silicon atom
Silicon atom
Figure 2. (a) A gallium-doped p-type silicon semiconductor; (b) an arsenic-doped n-type silicon
semiconductor.
Sunlight
Cover glass
n
lectro
Hole
ctor e—
r E
ducto
icon
du
icon
e sem
p-typ
electricity from the electric grid—the
network of power lines used to
deliver
electricity to houses and builde—
ings—as usual. During a sunny day, the
solar panels produce electricity that is
used by the home, replacing electricity
from the grid. If too much solar energy
—
is
collected by the solar panels—that is,
e
if some of the solar energy is not used—it
goes into the grid and earns the homeowner money, paid by the electric company.
Perhaps the most desirable way of using
solar energy is for a home or building to be
totally independent of the power grid. Rather
than having a net-metered link to the power
grid, the solar home or building would exist
“off the grid.”
But what would happen during the night or
on cloudy days? On sunny days, excess solar
energy would be stored in large batteries and
used in the absence of sunlight. An alternative
is to use portable gas or diesel-powered generators to replace solar energy.
With continued improvement in technology
and lowered costs, chances are some day
the United States will follow the model of the
Utrik Atoll and put solar power to work in a
big way.
Transparent adhesive
Antireflective coating
Front contact
e sem
Back contact
Figure 3. Schematic diagram of one layer of
a solar cell, showing the n-type and p-type
semiconductors.
The second type of doped silicon is called
n-type (n is for “negative”). It is made by
including atoms that have one more electron
in their outer level than does silicon (Fig. 2b).
This additional electron is free to move. In our
theater analogy, it is like having one seat occupied by two people. Because this is uncomfortable, one of them would hop to another seat.
Extra electron
free to move
Arsenic atom
n-typ
Figure 1. Solar cells are arranged in the form
of a module, which are then combined to form
an array. A solar panel, such as the one shown
on the roof of this house, consists of a series of
arrays that are aligned next to each other.
Electron
(b)
Electron can
move into hole
shutterstock.com
all figures by anthony fernandez
This changes if silicon is “doped.” Doping
means intentionally adding a small amount of
another element, called a dopant, to silicon.
The first kind of doped silicon is called
p-type (p is for “positive”). It is produced by
introducing atoms—such as boron or gallium—that have one less electron in their outer
level than does silicon (Fig. 2a). This means
that instead of making four bonds of shared
pairs of electrons with other silicon atoms,
there is one open
“hole.” This is
similar to having a
few empty seats in
a theater. It makes
shifting from one
seat to another
much easier.
By placing a layer of p-doped silicon next
to a layer of n-doped silicon, we create what
is known in electronics as a p-n junction or
diode (Fig. 3). Diodes control electricity by
only allowing current to flow in one direction.
Imagine if our theater seats in the back with
two people per seat and empty seats near the
front. People would migrate from the back to
the front—and not in the other direction. This
is what happens in a p-n junction.
A solar cell is composed of many p-n junctions (Fig. 3). When a solar cell is exposed to
sunlight, the small particles that make up light,
called photons, enter the solar cell and knock
some of its electrons loose. When these electrons are in the border region—between the
n-type and p-type layers—they move from the
n-type to the p-type layer. Then, a metal wire
collects these electrons and returns them to
the back of the n-type layer through an external circuit, creating a flow of electricity.
Selected References
On and off the grid
Solar 101: How Solar Works, SolarWorld-USA:
http://www.solarworld-usa.com/solar-for-home/
solar-101/history-of-solar.aspx [accessed April
2011].
Ewing, R. A.; Pratt, D. Got Sun? Go Solar: Get Free
Renewable Energy to Power Your Grid-Tied
Home, PixyJack Press, Masonville, CO.
Walsh, B. Solar Power’s New Style, Time, June 12,
2008.
A major complication of solar energy is that
it can be collected only when it is sunny. To
solve this problem, most residential installations use a “net-metered” system. During the
night or on cloudy days, a homeowner uses
Michael Tinnesand is a science writer and education consultant who lives in Portland, Ore. His
latest ChemMatters article, “A Single Ignition: A
Cautionary Tale,” appeared in the April 2011 issue.
chemmatters, OCTOBER 2011 9
By Michael Tinnesand
E
Students
Build
Photos by Stefano Paltera/U.S. Department of Energy Solar Decathlon
very 4 years, the summer Olympics awards a gold medal for the
decathlon. The winner becomes
“the world’s greatest athlete.”
After facing 2 days of grueling athletic
competition in 10 events—which
includes sprint races, jumps, as well as
throwing a heavy disc and a long spear
called a javelin—the winners have proven
themselves to be “the world’s greatest.”
Every other year, another competition challenges college students to build the world’s
best solar homes. The students don’t have to
run, jump, or throw a disc, but they have to
show that their solar home can outdo other
homes in 10 contests. This international event,
called the Solar Decathlon, is organized by the
U.S. Department of Energy (DOE ) and is supported with in-kind donations from organizations such as Dow Corning, Lowe’s, M.C. Dean
Inc., Pepco, and Schneider Electric.
New this year is the sustaining level sponsorship from Dow Corning, a global leader
in silicones and silicon-based technology.
Dow Corning is sponsoring the educational
partnership of this year’s event by overseeing
the creation of educational resources that will
strengthen understanding of solar energy and
sustainability and of the importance of science,
r
a
l
o
S mes
o
H
technology, engineering, and mathematics.
“There has never been a more important
time to further develop viable, renewable,
clean, domestically generated energy
sources, and there is no better way to
achieve that goal than by challenging
great minds from universities all over the
world,” says Robert D. Hansen, President
and Chief Executive Officer of Dow Corning. “The students’ hard work is a testament to the endless possibilities attainable
through math and science education.”
The Solar Decathlon puts 20 teams of
college students from around the world
in head-to-head competition across the
10 contests (see sidebar). DOE helps
get things started by giving each school
a $100,000 grant. The completed house is
transported to the decathlon site in Washington, D.C., and reassembled for judging
and public viewing.
The houses must be
between 600 square
feet and 1,000 square
feet in size.
The contests are
both clever and
demanding. Some are
evaluated by a jury, as
in gymnastics or ice
skating. Other contests
are determined by specific measurements,
Team Germany’s solar house looked like a huge black box.
10 Chemmatters, OCTOBER 2011
www.acs.org/chemmatters
such as power consumption or total
energy produced. In the remaining
contests, points are awarded for the
satisfactory completion of a task.
In the most recent Decathlon,
which was held in 2009, the winner
was Team Germany, a group of students from the University of Darmstadt.
Their house looked like a huge black box that
intrigued the judges and the public, and drew
long lines of people seeking a look inside. One
of the reasons this solar house was so successful was that nearly every bit of its outside
Team Illinois poses in front of their house.
surface was coated with solar panels. Team
Germany placed first in the Net Metering and
Comfort Zone contests.
Many of the solar house designs have a
futuristic, spaceship look. But the team from
the University of Illinois at Urbana-Champaign took a different approach: They used
reclaimed barn boards to cover their house.
But underneath this traditional exterior was
cutting-edge construction. The house featured
12 inches of insulation in the wall, ceiling,
and floors, which allowed it to use 90% less
energy than a typical construction. The team
won three individual contests, including Hot
Water, Appliances, and Home Entertainment.
All this led to a second-place finish overall.
Team California, from Santa Clara University
and the California College of the Arts, finished
in third place with a house made of windows,
walls, and floors that collected, stored, and
distributed solar energy in the form of heat in
the winter and rejected solar heat in the summer. Team California placed first in the Architecture and Communications contests.
The 2011 contest entries are also packed
with innovative technology. An increasing
number of teams are incorporating phasechange materials, which can store and release
large amounts of energy. Heat is absorbed or
released when the material goes from liquid to
solid, solid to liquid, or other phase changes.
One of the homes that use phase-change
materials was designed by the team from
Appalachian State University, Boone, N.C. It is
made with interior walls that contain microscopic capsules filled with high-purity paraffin
wax. The wax is a phase-change material that
melts when enough energy is absorbed. Then,
as the house cools, the wax releases its heat
as it changes from liquid back to solid.
Another original design comes from Team
China. The team members, who are students
from Tongji University in Shanghai, designed
and assembled a Y-shaped house by using
Visitors toured Team California’s solar-powered
house on Oct. 11, 2009. Team California won first
place in the Architecture contest in 2009.
The 10 Contests
of 2009
Appliances: Using only solar power,
ensuring that a refrigerator stays
cold, a freezer keeps food frozen, and
clothes are washed and dried.
Engineering: Functionality and efficiency of basic systems of the house,
such as heating, ventilation, and air
conditioning.
Architecture: Look and style of the
house, including size and arrangement of the various rooms in the
house.
Home Entertainment: Ability to
hold two dinner parties and one
movie night for neighbors.
Comfort Zone: Inside temperature
and humidity (maximum score if
inside temperature between 71 °F
(22.2 °C) and 76 °F (24.4 °C) and
relative humidity below 60%).
Communications: Presence of
displays, Web sites, videos, or
photos that inform the public about
major features of the house and how
they work.
six recycled shipping containers. These cheap
building materials compensate for the cost
of solar cells, which cover the roof of the
house. The “Y Container” house—as it is
called—may score high in the “Affordability”
contest, which was introduced this year and
that focuses on the potential cost of the solar
houses.
Also, this year, all the competing houses
will be connected to an extension of the
electric grid—the network of power lines
that delivers electricity to homes and buildings. In addition to producing electricity
from solar energy, the houses will be challenged to release any electricity surplus that
they produce to the grid and, as a result, to
District of Columbia customers. With this
new challenge, the students participating
in the Solar Decathlon will push the limits
of what is possible with solar energy and
may pave the way for the solar house of the
future. The winners of this year’s decathlon
will clearly be the champions in this field,
Hot Water: Ability to deliver 15 gallons (56.8 liters) of hot water (110 °F
/43.3 °C) in 10 minutes or less.
Lighting Design: Presence of functional, energy-efficient, and aesthetically pleasing lighting systems
Market Viability: How attractive the
home might be for buyers.
Net Metering: How much energy the
house produces and consumes.
A computer-generated rendering of Team China’s
Y-shaped house
and only time will tell whether we can indeed
call them “the world’s greatest.”
Selected references
Collins, G. P. et al. Seven Radical Energy Solutions,
Scientific American, May 2011.
Solar Decathlon, U.S. Department of Energy:
http://www.solardecathlon.gov/contests.html
[accessed July 2011].
Michael Tinnesand is a science writer and education consultant who lives in Portland, Ore. His
latest ChemMatters article, “A Single Ignition: A
Cautionary Tale,” appeared in the April 2011 issue.
chemmatters, OCTOBER 2011 11
Courtesy of Team China
The 20 teams who participated in the 2010 Solar
Decathlon (distinguished by the color of their
shirts) spent 2 years designing and building houses
powered exclusively by the sun.
Demystifying
By Brian Rohrig
ere is some good news for you:
You can blame the sounds and
odors that come from your body
on bacteria. Yes, these little critters—which live on our skin, in our mouths,
and in our guts—are the ones responsible for
a lot of what is going on inside our bodies.
Since today’s society is quite obsessed with
cleanliness, we tend to be a little uptight about
all these bodily sounds and smells. But understanding the science behind what may appear
to be so gross may make it, well… less gross.
FIGURES BY anthony fernandez. PHOTOS FROM ISTOCK
ISTOCK
What’s on your nose?
Probably every teenager
has looked in the mirror
first thing in the morning
to discover a ginormous
zit staring back at them.
And it usually makes its
appearance at the worst
possible time, such as the
day of the prom.
Your skin is porous—it is filled with millions
and millions of tiny little holes, or pores. Hair
grows out of pores known as follicles. In the
skin, glands release an oily substance that is
pale yellow and that lubricates and protects
the skin (Fig. 1).
This oily substance, called sebum, is one of
the main causes of acne. Although it is essential to keep our skin soft and pliable and our
hair shiny, too much of it can be a problem.
Especially in teenagers, large levels of a sex
hormone, called testosterone, are produced,
12 Chemmatters, OCTOBER 2011
Hair
Skin surface
Sebum
Follicle
Sebaceous
gland
Figure 1. Sebaceous glands in the skin release
an oily substance called sebum that lubricates
and protects the skin.
up below the surface of the skin, and die.
These dead white blood cells, along with
dead skin cells and some bacteria, form a
white liquid known as pus. A pimple forms
when the excess sebum and dead skin cells
clog up and block the opening of the pore. This
type of pimple is called a whitehead (Fig. 2a).
Another type of pimple, called a blackhead
(Fig. 2b), appears when sebum and dead skin
cells clog the pore but not the opening, as in a
whitehead. While the pore is clogged, its surface remains open. A blackhead appears black
because melanin in the dead skin cells reacts
with oxygen from the air, which changes the
melanin’s color from brown to black.
If the infection worsens, a painful cyst may
develop under the skin. A cyst is a fluid-filled
sac that is the most severe type of acne. It can
cause permanent scarring.
A majority of teenagers have acne, some
worse than others. Washing your face with
soap and water several times each day is a
good way to minimize acne.
which causes the skin to release a lot of
sebum, too. Sometimes, this excess sebum
can clog up the pores.
Adding to the mix are dead skin cells. About
30,000 skin cells are shed every minute! A lot
of these skin cells are shed inside the pores
themselves.
A blocked skin pore also contains bacteria.
They feed off the dead skin cells and the
clogged sebum within the pores and produce
toxins that damage the lining of the pores. As
these bacteria grow and multiply, they invade the area surSkin surface
Skin surface
rounding the pore, which can
Blackhead
Postule
lead to a bacterial infection.
Enlargement
Enlargement
of follicle
A blocked pore initially turns of follicle
opening
opening
red because blood rushes to
Sebaceous
Sebaceous
gland
the site, which is one of the
gland
ways our body responds to an
Follicle
Follicle
infection. Then, white blood
cells—a type of blood cell
(a)
(b)
responsible for fighting infecFigure 2. Two types of pimples: (a) a whitehead; and (b) a blackhead
tion—destroy bacteria, build
www.acs.org/chemmatters
Soap is an amphiphilic molecule: One end
of the molecule binds to water molecules,
while the other end binds to fat molecules,
such as the ones present in sebum. These
ends are called “water-loving” and “oil-loving,” respectively—hence the word “amphiphilic” (from the Greek “amphis,” meaning
“both” and “philia,” meaning “love”).
The reason some molecules bind to water
and others to oil—and sometimes to both—
is due to one key difference between water
and oil, called polarity. Water is polar; oil is
nonpolar.
A polar molecule contains regions of partial
positive and negative charge that are due to
the uneven distribution of electrical charge. In
the case of a water molecule, the oxygen end
has a greater concentration of electrons than
the two hydrogen ends because oxygen tends
to attract shared electrons while hydrogen
tends to lose them. So, even though the water
molecule is electrically neutral, it contains a
partial negative charge on the oxygen end and
a partial positive charge on each of the hydrogen ends (Fig. 3).
A nonpolar molecule has an even distribution of electric charge, so it has no regions
of partial positive and negative charges. This
is the case with an oil molecule, in which the
electrons are shared equally between the molecule’s atoms.
Polar molecules bind to other polar molecules or to ions. Nonpolar molecules bind
to other nonpolar molecules. Polar and nonpolar molecules cannot bind with each other
because one has a partial electric charge but
not the other.
A common component of soap is sodium
stearate [CH3–(CH2)16–COONa]. When this
molecule dissolves in water, it produces a
stearate anion (C17 H35COO–) and a sodium
cation (Na+). The stearate anion is amphiphilic: It has a negatively charged end and a long,
nonpolar tail (Fig. 4).
The negatively charged end binds to the
electrically positive regions of water molecules, and the long tail binds to sebum
molecules. Then, the soap molecules group
together to form tiny spheres called micelles
CH3
(CH2)16
C
O—
Nonpolar tail
Figure 4. When dissolved in water, sodium
stearate—the soap that most of us use—
decomposes into an anion (top) and a cation
(Na+) (not shown). The anion has a polar head
that binds to water and a nonpolar tail that binds
to the skin’s oily substance. It is this anion that
helps clean pimples by washing away the skin’s
oily substance.
sive problem: bad
breath. Your mouth is
actually teeming with
about 10 billion bacteria that take in food
and excrete waste.
Bacterial waste is in
the form of gas, which
can be pretty smelly.
Bad breath is caused
by the combined
waste products of
these bacteria.
... bacteria
While you sleep,
multiply like
your body stops producing saliva, which
crazy at
contains antibacterial
night, and
compounds. In the
your breath
absence of this cleanscan smell
ing liquid, bacteria
Smelly breath
pretty bad in
multiply like crazy at
the morning.
If you wake up and are obsessing over
night, and your breath
that zit on the end of your nose, you may
can smell pretty bad in
not even notice a potentially more offenthe morning.
During the day, your body produces up to a
liter and a half of saliva, which keeps bacteria
Nonpolar
δ–
Region
in check and washes away food particles
O
+
δ
+
H
H
H
δ
that may be lodged in your teeth. If you don’t
δ+ δ+
O
CO H O
OOC
δ–
–
δ
brush your teeth before going to bed, the bac+
O
δ
δ+
H
H
H
COO H
teria have a lot more food particles to munch
+
+
δ
δ OOC
O
–
δ
on at night, leading to a lot of bacterial waste.
δ–
O
One of the best ways you can prevent
OILY
COO δ+ δ+
H
H
SUBSTANCES
H
H
breath odor—and strengthen your teeth—is
δ+ δ+ OOC
O
by using mouthwash, which kills the bacteria
δ–
–
δ
in your mouth. One key ingredient in many
O
COO +
δ
H
δ+
H + δ+ OOC
H
mouthwashes is fluoride, which is known to
H
δ
–
O
δ
COO +
strengthen tooth enamel (Fig. 6).
–
δ
δ
O H
C
δ+
+ OO
H
H
Polar
Fluoride (F–) is the ionic form of fluorine. It
H+ δ
δ
O
Region
–
δ
forms when a fluorine atom gains an electron.
Fluoride does not exist by itself, but it can be
Figure 5. After binding to oily substances from the
skin, sodium stearate anions pack together to form
found in compounds, such as sodium fluoride
a sphere called a micelle (top). The negatively
(NaF), which is present in many toothpastes
charged ends are in contact with water while the
long tails trap the oily substances inside the micelle. and mouthwashes. When this compound is
(Fig. 5). A micelle consists on the
outside of the negatively charged ends
attached to water molecules and on the
inside of the nonpolar tails attached to
sebum molecules. So, the sebum molecules are essentially stuck inside the
micelles and can be washed away.
But too much washing can dry out the
skin, causing the skin to produce more
sebum. Also, once acne has developed,
no amount of washing will remove it.
At that point, acne is due to a bacterial
infection, and only antibacterial agents
can treat it. Antibacterial substances
destroy or prevent the growth of bacteria.
They are available in the form of overthe-counter creams in pharmacies or are
prescribed by a dermatologist.
ISTOCK
H
δ+
O
Mike ciesielski
H
δ+
Polar head
–
–
–
–
–
–
–
anthony fernandez
δO
Figure 3. In a water
molecule, the electrons are
shared unevenly between
the oxygen end and the two
hydrogen ends, creating a
partial negative charge on
the oxygen end and a partial
negative charge on the
hydrogen ends.
–
–
–
chemmatters, OCTOBER 2011 13
Marie dauhenheimer
Ca5(PO4)3(OH) à 5 Ca2+ + 3 PO43– + OH–
A certain amount of demineralization is
normal. But it is also normal for the reverse
process, remineralization, to occur:
5 Ca2+ + 3 PO43– + OH– à Ca5(PO4)3(OH)
If too much bacterial acid is produced,
demineralization can outstrip mineralization,
leading to a cavity. How does this happen?
When acids are present in a solution, they
dissolve to produce hydrogen ions (H+). In the
mouth, as bacteria produce acids, the amount
of hydrogen ions builds up. These ions combine with the hydroxide ions produced during
demineralization to form water:
PHOTOS.COM
H+ + OH– à H2O
But hydroxide ions are essential to remineralization, so their neutralization by hydrogen
ions causes remineralization to slow down. The
hydroxyapatite on the surface of the teeth keeps
dissolving, ultimately leading to tooth decay.
Fluoride ions present in mouthwashes help
the enamel to remineralize. They accumulate
on the surface of the enamel, thus creating
a barrier that prevents bacterial acids from
reaching the enamel. Also, the fluoride ions
attract calcium ions, ultimately changing hydroxyapatite into fluoroapatite
[Ca5(PO4)3F], which is stronger than the
original hydroxyapatite.
Bad breath can be caused by many different
14 Chemmatters, OCTOBER 2011
CH2OH
CH2OH
gases, but two of
the most common
C
O
C
H
H
ones are hydrogen
H
H
sulfide (H2S) and
O C
C
C
H
OH
OH
methyl mercaptan
C
C
C
H
(CH3SH)—both
sulfur-containing
H
H
OH
compounds. Other
Figure 7. Chemical structure of cellulose
gases that lead to
bad breath are indole
(C8H7N) and skatole (C9H9N), the two gases
primarily responsible for the smell of feces.
CH2OH
O
C
www.acs.org/chemmatters
C
O
C
H
C
H
OH
H
C
C
H
OH
H
OH
Passing gas
Eating a lot of fiber can have an undesirable
side effect: the production of large amounts
of intestinal gas. When this gas is released
by the body, it is known as flatulence. The
gas itself is known as flatus. “Passing gas” is
actually a good way to describe this process.
People pass gas 14 times per day, on average.
This gas is produced by bacteria in the colon.
Fiber is made of
a substance called
cellulose (Fig. 7).
Cellulose belongs to
a group of materials
called carbohydrates
that are composed of
carbon, hydrogen,
and oxygen and are
made of a series
of repeating small
molecules. In the
People pass
case of cellulose,
gas
14 times
the repeating
per day, on
small molecule is
glucose (C6H12O6)
average.
(Fig. 8).
In the colon,
bacteria break down cellulose, so if undigested food enters the colon, there is more for
the bacteria to feed on. And when you have
a lot of bacteria, you have a lot of their waste
products in the form of gas. Foods high in
fiber—such as fruits, vegetables,
and beans—tend to produce a
lot of flatulence.
Some indigestible sugars
can have the same effect.
For instance, lactose
in milk, which is
a carbohydrate
molecule (Fig.
9) formed from
glucose and galac-
H
O
C
O
H
CH2OH
H
OH
O
H
OH
H
H
OH
OH
H
Figure 8. Chemical structure of glucose
CH2OH
O
HO
H
OH
H
H
OH
CH2OH
H
O
O OH
H
OH
H
H
OH
Figure 9. Chemical structure of lactose
SHUTTERSTOCK
dissolved in water,
the fluoride ions
are free to move.
Fluoride ions
Dentin
prevent tooth
decay by strengthening the enamel.
The primary comNerve
pound found in
tooth enamel is a
strong, insoluble
Figure 6. Enamel is the
mineral called
hard white sustance that
covers a tooth.
hydroxyapatite
[Ca5(PO4)3(OH)].
Hydroxyapatite contains positive ions (Ca2+)
and negative ions (PO43– and OH–), which are
attracted to each other to form the crystalline
structure of hydroxyapatite.
The bacteria present on our teeth produce
acids that cause hydroxyapatite to break
apart—a process called demineralization:
Enamel
tose molecules, is sometimes not broken
down completely. So, dairy products can
produce a lot of flatulence, especially if a
person is lactose intolerant.
Once the basic chemical reactions of
the body are understood, the “gross”
things of your body won’t seem all that
gross. On a molecular level, no one compound is grosser than any other. It’s all
just chemistry!
Check out the video podcasts on acne and
bad breath at: www.acs.org/chemmatters
Selected references
Masoff, J. Oh, Yuck! The Encyclopedia of
Everything Nasty, Workman Publishing: New
York, 2000.
Bad Breath: Unusual Causes of Halitosis. Health
Tree: http://www.healthtree.com/articles/halitosis/bad-breath-causes/ [accessed July 2011].
What Causes Acne? Skin Care Physicians, April
14, 2010: http://www.skincarephysicians.com/
acnenet/acne.html [accessed July 2011].
Brian Rohrig teaches chemistry at Jonathan Alder
High School in Plain City (near Columbus), Ohio.
His most recent ChemMatters article, “Myths:
Chemistry Tells the Truth,” appeared in the
December 2010 issue.
.COM
PHOTOS
How Do They Work?
By Christen Brownlee
Better than sugar?
First, let’s look at table sugar. It belongs
to a family of molecules called carbohydrates that are found in fruits,
vegetables, dairy products, breads,
and sweets. Carbohydrates are made of
many repeating units that are composed
of carbon, hydrogen, and oxygen.
Table sugar, or sucrose, is made of two
units. These two units, called glucose and
fructose, are combined to produce sucrose,
as follows:
CH2OH
H
HO
O
H
OH
H
H
OH
HOCH2
H
H
CH2OH
OH
Glucose
HO
HO
HO
OH
H
Fructose
CH2OH
H
H
O
O
H
OH
H
H
OH
HOCH2
H
H
H
O
HO
O
CH2OH
OH
Sucrose
H
+ H2O
Carbohydrates are an
excellent fuel for the body
because they are packed
full of energy. They are
broken apart first in
your mouth and then
in your small intestine.
The resulting molecules enter
the bloodstream and travel to cells,
where they are used to release energy.
But sucrose can have two negative health
effects. First, when we eat food or drink beverages that contain sucrose, bacteria that live
in our mouths also use sucrose as an energy
source and produce acid that contribute to
tooth decay.
Second, when we eat or drink too much
sucrose, the amount of insulin in our blood
spikes. Insulin is a hormone that regulates the
amount of sugar in our blood. Over time, too
much insulin in the blood can lead to diabetes,
a medical condition characterized by unusually
high blood sugar levels.
Chemists have been trying to find alternatives to sugar since 1878—that’s the year
that an American chemist named Constantin
Fahlberg discovered saccharin, the first artificial sweetener currently known by the brand
names Sweet’N Low and SugarTwin.
Saccharin is actually sodium 3-oxobenzisosulfonazole (C6H4SO2CONNa) (Fig. 1), a molecule that has little in common with sucrose
but is much sweeter than sucrose. Also,
the digestive system does not break it apart
to derive energy the same way it does with
sugar. Instead, saccharin dissolves into the
bloodstream and is flushed out of the body
in urine. Saccharin is now used to sweeten
countless products, including drinks, candies,
biscuits, and medicines.
O
N Na
—
S
+
Figure 1. Chemical
structure of saccharin, an artificial sweetener
O O
Mike ciesielski
A
ccording to the U.S. Department of Agriculture, Americans consume an average of
156 pounds of sugar each year. That’s a little more than 31 of the five-pound bags
you might see in the baking goods aisle in the grocery store! We all know that eating
too much sugar can cause tooth decay, weight gain, and type-2 diabetes, but is there
a way to indulge your sweet tooth and still avoid sugar? Yes. Food and beverages
labeled “diet” taste sweet yet don’t contain sugar—thanks to artificial sweeteners.
Why do artificial sweeteners have no calories? Could they be bad for your health? Let’s compare the chemistry of sugar and artificial sweeteners to find out.
chemmatters, OCTOBER 2011 15
Sweet aminos
CH2OH
CH2CI O
O
Cl
HO
OH
O
CH2Cl
OH
OH
Figure 4. The chemical structure of sucralose is
similar to the structure of sucrose.
... studies could not
find evidence that
saccharin causes
cancer in humans. It is
now used in food and
drinks all over
the world.
but like saccharin, sucralose has no calories.
It is washed out of the body without being
digested. Sucralose is 600 times as sweet as
sucrose, about three times as sweet as aspartame, and twice as sweet as saccharin.
Table 1 summarizes the relative sweetness
of common artificial sweeteners compared to
sucrose.
Sweet Substance
Brand Name
Relative Sweetness
Sucrose
none
1
Mike ciesielski
Not all artificial sweeteners look like saccharin. Aspartame, known by the brand names
NutraSweet and Equal, is the primary sweetener in most diet sodas. It is a combination of
amino acids, the building blocks of proteins—
organic compounds found in meat, eggs, milk,
and legumes. A protein is a molecule made of
a chain of repeating units of amino acids.
The structures of two amino acids, aspartic
acid and phenylalanine, are shown in Fig. 2.
Aspartame (Fig. 3) consists of a combination
of these two amino acids.
While saccharin tastes sweet, it also has a
lingering bitter and metallic taste that some
people can detect. That makes it a good
choice for sweetening tea and coffee, which
have their own bitter taste, but not necessarily
a good one for candies and soft drinks, which
are known to be sweet. Aspartame does not
have a bitter taste, which makes it a better
choice for a wide variety of sweet foods and
drinks.
Unlike other artificial sweeteners, aspartame
is metabolized in the body, so aspartame is
higher in calories. But aspartame is 180 times
sweeter than sugar, so it can be used in small
quantities and, as a result, does not generate
as many calories as sucrose.
Another popular artificial sweetener is
sucralose (brand name: Splenda). Its chemical
structure is similar to that of sucrose (Fig. 4),
anthony fernandez
of people with
brain tumors
Glucose
none
0.7
increased over
Fructose
none
1.3–1.8
the years after
Saccharin
Sweet’N Low, SugarTwin
300
aspartame was
Aspartame
NutraSweet, Equal
200
introduced on
Sucralose
Splenda
600
the market.
Further studTable 1. Relative sweetness of artificial sweeteners compared to sucrose
ies, however,
revealed that brain cancer started to rise 8
Any risks to human
years before aspartame was made publicly
health?
available. No other studies have since shown
a correlation between aspartame use and
Over the years, concerns have been raised
cancer.
that several artificial sweeteners may cause
Many other artificial sweeteners have been
health problems. In theory, artitested. None of these tests has provided clear
ficial sweeteners should be safe
O
OH
evidence of an association with cancer in
because they pass easily through
C
humans. So, avoiding too much sugar or artithe digestive system without being
CH2
CH2
ficial sweeteners might be beneficial to health
processed.
But
scientific
tests
OH
OH
H
H
and be just what the doctor ordered!
were needed to confirm that artifiN
N
C
C
C
C
H
H
cial sweeteners were indeed safe.
O
O
H
H
In 1977, rats that were fed saccharin developed bladder cancer.
Figure 2. Examples of two amino acids: (a) aspartic acid
Selected references
and (b) phenylalanine. Like all other amino acids, aspartic
The rats, however, had to eat an
Artificial Sweeteners: Understanding These and
acid and phenylalanine consist of three parts that bind to a
amount of saccharin comparable
Other Sugar Substitutes, Mayo Clinic: Nutrition
central carbon: an amino group (–NH2), a carboxyl group (–
and Healthy Eating, Oct 9, 2010: http://www.
to a human drinking hundreds of
COOH), and a side chain (middle) that varies depending on
mayoclinic.com/health/artificial-sweeteners/
the amino acid.
cans of soda each day. As a result,
MY00073 [accessed July 2011].
Congress required that all food containing
Suddath, C. Are Artificial Sweeteners Really That
saccharin display the following label: “Use of
Bad for You? Time, Oct. 20, 2009: http://www.
Phenylalanine residue
time.com/time/health/article/0,8599,
this product may be hazardous to your health.
1931116,00.html [accessed July 2011].
This product contains saccharin, which has
Aspartic acid residue
Gilman, V. What’s That Stuff: Artificial Sweeteners,
been determined to cause cancer in laboraChemical & Engineering News, June 21, 2004,
O
82 (25), p 43: http://pubs.acs.org/cen/whatstuff/
tory animals.” Subsequent studies could not
Methanol
stuff/8225sweeteners.html [accessed July 2011].
HO
OCH3
find evidence that saccharin causes cancer in
residue
N
humans. It is now used in food and drinks all
H
O
NH2
O
over the world.
Christen Brownlee is a science writer in Baltimore,
In 1996, studies suggested that aspartame
Figure 3. Chemical structure of aspartame, an
Md. Her latest ChemMatters article, “Sweet but
may cause brain tumors because the number
artificial sweetener
Good for You?” appeared in the April 2011 issue.
16 Chemmatters, OCTOBER 2011
www.acs.org/chemmatters
ric, 15, recently developed a
high fever and a bad cough.
His mother took him to the
hospital, where he was diagnosed with pneumonia, which is an inflammation of the lungs.
In Eric’s case, the pneumonia was the result of
a bacterial infection. He was sent home with a
prescription of an antibiotic called amoxicillin.
Antibiotics are drugs that kill bacteria or prevent them from growing in the body.
Three days later, Eric’s fever went away,
but he was still coughing and had difficulty
breathing. Another visit to the doctor showed
that he was actually infected with a strain of
bacteria that was resistant to amoxicillin. This
time, the doctor prescribed a more powerful
antibiotic, called Augmentin, and asked Eric to
take it for a week.
Luckily, by the end of the week, Eric recovered. The second antibiotic was powerful
enough to stop the infection. But for many
Centers for Disease Control and Prevention’s Public
Health Image Library
By Margaret Orford
The bacterium Streptococcus pneumoniae, a
common cause of pneumonia, imaged by an
electron microscope
people who are infected with antibiotic-resistant bacteria, finding the right antibiotic is not
easy. Antibiotics are proving less effective in
curing diseases such as malaria, tuberculosis,
and “staph” infections that occur in hospitals
and health care facilities.
How do bacteria become resistant to antibiotics, and how can we develop better drugs?
Let’s look at what happened to the bacteria
that infected Eric and how the two antibiotics
worked in his lungs.
istock
Powerful antibiotics
Pneumonia is mostly caused by bacteria,
viruses, or parasites. Eric was infected by bacteria, which he inhaled from the air, and the
bacteria then entered his lungs. The bacterial
infection triggered an immune response that
caused tiny air sacs in the lungs called alveoli
to fill with fluid, which caused difficulties in
breathing and coughing. If Eric hadn’t taken
his medication, the fluid buildup could have
impaired breathing enough to cause death.
The use of antibiotics, which started in the
early 1940s, has saved millions of lives from
infectious diseases. Penicillin is the most
prescribed type of antibiotic. It has often been
described as a miracle drug because it cured
not only minor wounds that became infected
but also diseases such as strep throat, some
sexually transmitted diseases, and an eye
inflammation contracted by babies at birth.
Penicillin is now used to cure a large number
of bacterial diseases.
Penicillin is not as successful now because,
over the years, many strains of bacteria have
become resistant to antibiotics. These bacteria, sometimes called “superbugs,” now
contribute to the reemergence of diseases that
were well controlled in the second part of the
20th century.
There are different types of penicillin medications, but they all do the same thing: They
stop bacteria from multiplying by preventing
them from forming the cell walls that surround daughter cells. When a bacterium
divides, it forms two daughter cells, each
surrounded with a cell wall. But penicillin
prevents the bacterium from making these cell
walls, so the daughter cells do not form, and
the bacterium cannot spread.
The bacterium that most commonly causes
pneumonia, Streptococcus pneumoniae, is
actually surrounded by two types of layers: a
flexible cell membrane and a tough, rigid cell
wall. Penicillin interferes with a molecule that
helps build the bacterial cell wall.
chemmatters, OCTOBER 2011 17
Three characters
The two antibiotics that Eric took are actually penicillin-based medications, which were
intended to prevent the bacteria in his lungs
from forming new cell walls. The first drug he
took worked with limited success; the second
drug was successful.
To better understand how these drugs
worked in Eric’s lungs, let’s consider the three
“characters” in this story: penicillin, along
with the two molecules involved in building
the bacterial cell wall, called peptidoglycan
and transpeptidase.
Let’s start with the first character, penicillin.
It is a relatively small molecule that contains
a square ring in the middle in which carbon
atoms form 90-degree angles (Fig. 1). Carbon
atoms usually form 109-degree angles when
forming single bonds, as in a molecule of
cyclohexane (Fig. 2a) and 120-degree angles
when forming a double bond with oxygen,
as in a molecule of carboxylic acid (Fig. 2b).
The square ring creates a strain on the carbon
atoms that makes penicillin highly reactive
because it wants to relieve that strain.
There are different types of penicillin (Fig.
3), depending on the chemical nature of the
R group (top left of the molecule shown in
Fig. 1).
H
N
R
O
H
S
CH3
N
CH3
O
COOH
Figure 1. Chemical structure of penicillin. The
four-sided beta lactam ring is circled in orange.
FIGURES BY anthony fernandez
Peptidoglycan, the second character, is
a bigger molecule (Fig. 4). It is the building
block of the cell wall of Streptococcus bacteria. To make the bacterial cell wall, peptidoglycan molecules are stitched together by their
dangling peptide chains, as shown in Fig. 5.
The third character, transpeptidase, is
an enzyme—a type of protein that speeds
up chemical reactions without undergoing
changes itself. Transpeptidase helps peptidoglycan units bind with one another.
Stopping bacterial
infection
The role of penicillin is to prevent the peptidoglycan molecules from getting together, so
the bacterial cell wall cannot be built.
18 Chemmatters, OCTOBER 2011
(a)
H
H
H
C
H
H
C H
C
H
(b)
H
C
H
C
C
H
O
H
R
C
OH
H
Figure 2. (a) Single-bonded carbon atoms in a
cyclohexane molecule form 109-degree angles;
(b) carbon atoms that are double-bonded to
oxygen in a carboxylic acid form 120-degree
angles.
First, let’s look at how two peptidoglycan
molecules come together. They do so by
binding to transpeptidase. The two molecules
attach to a part of transpeptidase called its
active site, and they are joined together in the
active site.
To understand how this works, imagine
using a soldering iron to weld two pieces of
metal together. You take two pieces of metal
and apply them to the soldering iron, which
melts their endings and fuse them together.
SimilarIy, transpeptidase acts like a soldering
iron that “fuses” the two transpeptidase molecules at their endings.
Penicillin prevents this from happening by
binding to the transpeptidase’s active site. As
mentioned earlier, the penicillin molecule (Fig.
2) contains a square ring in which carbon
atoms form 90-degree angles instead of the
more natural 109-degree angles. As soon as
the penicillin molecule reacts with transpeptidase, the square ring is broken. This relieves
the ring strain and locks penicillin in the active
site of transpeptidase (Fig. 6).
The end result is that transpeptidase is
blocked and cannot do its job anymore. When
penicillin molecules block transpeptidase molecules, the building blocks of the cell wall are
disconnected. The bacteria can no longer form
cell walls for daughter cells, so they cannot
infect the lungs anymore.
This is what happened to Eric as he was
taking amoxicillin, the first penicillin drug. But
bacteria in his lungs were still able to spread.
How come?
beta-lactam ring. In this form, penicillin is no
longer able to bond to transpeptidase.
It took another penicillin-based medication—Augmentin—to fight Eric’s infection.
Augmentin contains a mixture of amoxicillin
and another substance called clavulanic acid
that looks like a penicillin molecule (Fig. 7).
Clavulanic acid binds to beta-lactamase but
not to transpeptidase. This way, the betalactamase molecules are all blocked, and they
can’t bind to amoxicillin. Amoxicillin was able
to do its job of blocking transpeptidase and
stop bacterial infection.
In Eric’s case, Augmentin was all it took
to stop the infection. But pneumonia can be
caused by other types of antibiotic-resistant
bacteria. People infected with these bacteria
need to take stronger antibiotics. Also, people
who are hospitalized with pneumonia are at
risk for contracting a resistant strain of bacteria that causes skin infections and can kill
them within three days.
These bacteria, called methicillin-resistant
Staphylococcus aureus (MRSA), are resistant
to a type of penicillin called methicillin, but
they can be defeated with another type of
penicillin. The reason people with pneumonia
can die so quickly from an MRSA infection is
that their immune RsystemHN is in aHweakened
S
CH
condition, so they need to be diagnosed and
treated faster than patients
infected
only with
O
N
CH
O Disease Control
MRSA. Still, the Centers for
3
3
COOH
NH2
www.acs.org/chemmatters
S
O
CH3
N
CH3
O
HO
COOH
(a)
H
H
N
S
O
CH3
N
CH3
O
COOH
(b)
HO
O
H
N
Fighting back
Although amoxicillin prevented some of
the bacteria from spreading, most of them
were unaffected by it. The reason is that these
bacteria contained an enzyme called betalactamase that caused a reaction between
water and penicillin that opened the penicillin’s
H
H
N
S
(c)
O
H
S
CH3
N
CH3
O
COOH
Figure 3. Examples of penicillin molecules: (a)
amoxicillin; (b) penicillin G; and (c) ticarcillin.
Note that all these molecules have the common
structure shown in Fig. 1.
CH3
H
C=O
NH
CH2OH
H
H
O
O
H O
OH H
O H
H
O
O
H
H
H NH
CH2OH
HC CH3
C O
C O CH
H
OH
O
N
H
O
COOH
3
L-Alanine
HC (CH3)3COOH
C
O
NH
D-Glutamate
HC
C
Mesodiaminopimelate
tim vickers/wikipedia
NH
NH3
(CH3)3CHCOOH
O
NH
Figure 6. When penicillin (white molecule
in the middle) binds to the bacterial protein
transpeptidase (big yellow, red, and blue
molecule), it does not let go and effectively stops the bacterial infection.
HC CH3
C
O
NH
D-Alanine
HC
Bond broken when two
peptidoglycon molecules bind
with each other
C
D-Alanine
CH3
O
NH
HC
C
CH3
O
OH
and Prevention, Atlanta, Ga., estimate that
MRSA kills more Americans annually than
AIDS.
Almost every disease-causing bacterium
has become resistant to antibiotics. Pneumonia, malaria, tuberculosis, the flu, and
even ear infections, are harder to treat.
Most of the time, these types of infections
are cured by using antibiotics that are different from commonly
prescribed medications.
During the past few
L-Ala
L-Ala
L-Ala
L-Ala
D-Glu
D-Glu
D-Glu
D-Glu
decades, the number of
m-DAP
m-DAP
m-DAP
m-DAP
D-Ala
D-Ala
D-Ala
D-Ala
D-Ala
D-Ala
D-Ala
m-DAP
m-DAP
m-DAP
new antibiotics has been
D-Glu
D-Glu
D-Glu
L-Ala
L-Ala
L-Ala
declining while the number of resistant bacteria
has been increasing. One
L-Ala
L-Ala
L-Ala
L-Ala
D-Glu
D-Glu
D-Glu
D-Glu
of the main reasons for
m-DAP
m-DAP
m-DAP
m-DAP
D-Ala
D-Ala
D-Ala
D-Ala
D-Ala
D-Ala
D-Ala
D-Ala
this increase in resistant
m-DAP
m-DAP
m-DAP
m-DAP
D-Glu
D-Glu
D-Glu
D-Glu
L-Ala
L-Ala
L-Ala
L-Ala
bacteria is that people
either misuse or overuse
antibiotics.
L-Ala
L-Ala
L-Ala
For example, some
D-Glu
D-Glu
D-Glu
m-DAP
m-DAP
m-DAP
D-Ala
D-Ala
D-Ala
people discontinue their
prescription medication,
so all of the bacteria that
Figure 5. Chemical structure of the peptidoglycan layer. The orange and
blue squares represent the polysaccharide molecule shown in Fig. 4,
caused their infection may
and the gray-shaded area represents the peptide molecule shown in Fig.
not have been completely
4. Two successive peptidoglycan units are stitched together by linking
eliminated. This happens
m-diaminopimelic acid (m-DAP) of one unit with D-alanine (D-Ala) of
another.
when a patient starts a pre-
anthony fernandez
Figure 4. A peptidoglycan molecule is made
of two types of molecules: a polysaccharide
(horizontal molecule on top, made of two
simple sugars: N-acetylglucosamine and
N-acetylmuramic acid) and a peptide
molecule (arranged vertically and made of
five amino acids: L-alanine, D-glutamic acid,
m-diaminopimelic acid, and D-alanine (twice)).
The D-alanine at the bottom is cleaved when two
peptidoglycan molecules are joined together.
Figure 7. Clavulanic acid has a square ring
similar to that of a penicillin molecule (Fig. 1).
scription of antibiotics and, after a few days,
feels better. At that point, the bacteria have
been exposed to the antibiotic, but while some
have been defeated, others are trying to adapt.
If the patient stops taking the antibiotic before
the end date of the prescription, the bacteria
that have not been defeated may become
resistant to that antibiotic.
Some people use antibiotics for conditions
or diseases that are not caused by bacteria,
such as the common cold, which is caused
by a virus. Bacteria present in small amounts
in the body might be exposed to this antibiotic and may develop resistance
to it, even though they would not
cause an infection because of their
small number.
Antibiotics can mean the difference
between life and death when a person contracts a bacterial infection. Thanks to antibiotics, millions of lives have been saved for the
past 60 years, but this success story may
end soon. Unless new antibiotics are discovered in the near future, infectious diseases
will claim many lives, becoming untreatable
again, as in the days before we had access to
antibiotic treatment.
Selected references
Dance, A. New Ways to Fight Bacteria. The Los
Angeles Times, Sept 27, 2010: http://www.
latimes.com/health/la-he-in-the-works-antibi.
tics-20100927,0,4618631.story [accessed July
2011].
How Do Antibiotics Work? HowStuffWorks.com:
http://health.howstuffworks.com/medicine/
medication/question88.htm [accessed July
2011].
Wassenaar, T. M. Antibiotics. The Virtual Museum
of Bacteria, Jan 24, 2009:
http://www.bacteriamuseum.org/cms/How-WeFight-Bacteria/antibiotics.html [accessed July
2011].
Margaret Orford is a high school mathematics and
science teacher and a science writer who lives in
Nepean, Ontario, Canada. This is her first article in
ChemMatters.
chemmatters, OCTOBER 2011 19
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Celebrate National Chemistry Week
October 16-22, 2011 Theme: “Chemistry — Our Health, Our Future!”
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When it comes to the topic of health, the science of chemistry can be
emphasized in many ways. From the vitamins and minerals we take, to
the foods we chose to eat, and to the medicines that help keep us well,
many chemicals serve to improve our lives.
Celebrate these aspects of chemistry that keep us healthy now and in the future!
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