Dr. Universe
Energy comes
from many sources
Dear Dr. Universe:
When I eat an apple, how do I get energy?
-Andrew
Parma, Mich.
Sugar. At least that's the short answer.
Energy from the sun gets converted to sugar
through photosynthesis in the apple tree, and
you convert the sugar into energy when you eat
the apple. For the more complete answer, I
asked Ralph Yount for help.
Yount is a biochemist at Washington
State University. He studies how molecules
called actin and myosin in your muscles work
together to move you and how they get energy
from another molecule called ATP, or adenosine
triphosphate. So, quick quiz: What is the one
thing that all living things have in common?
ATP? Right!
That, says Yount, is because ATP is the
universal energy "currency." If sugar is your
savings account, ATP is the cash you get when
you withdraw money from the account.
Life requires a lot of that currency.
Every one of your cells contains a billion
molecules of ATP. When you eat that apple,
says Yount, your cells start changing the apple’s
sugar into carbon dioxide and water. Along the
way, the energy of the sugar is trapped in ATP.
Each ATP molecule has three linked
phosphate groups. There's enough energy in the
bonds connecting them to make the other bonds
life requires. Transferring energy is a matter of
taking and breaking chemical bonds. Bonds are
what hold atoms together to make molecules.
When high-energy bonds in certain molecules
are broken, energy can be transferred to other
molecules rather than being lost as heat. As is
often the case when bonds are broken.
Every time you walk or talk or think,
your cells are breaking ATP bonds to gain the
energy they contain.
Yount says life can actually get energy
from glucose (sugar) in two ways. The first is
without oxygen. This is called "glycolysis."
This is probably how very early organisms got
their energy. Today, "fermenters" such as yeast
get their energy through glycolysis. You also
use glycolysis to start breaking down sugar- and
when you need energy really quickly. The
problem with glycolysis is it is not very
efficient. It can capture only two ATPs from
each sugar molecule.
Much more efficient is respiration,
which requires oxygen and mitochondria.
(Respiration can capture 38 ATPs out of one
sugar molecule.) Way, way back in
evolutionary time, a type of bacterium got
trapped somehow in higher organisms and
adapted to live there permanently. This mutated
bacterium, called mitochondria, is what makes
ATP.
Every one of your cells contains
between 1,000 and 2,000 mitochondria. Yount
says that nearly a third of your heart is
mitochondria, because you need so much ATP
to pump your blood.
Even though there are similarities,
making ATP is not like burning the sugar in
wood. When wood burns, oxygen in the air
combines directly with the carbon and hydrogen
in its sugar molecules to produce carbon
dioxide, water, and heat.
Living cells work differently. They strip
electrons from the chemical bonds in sugar and
store them in a carrier molecule. Your
mitochondria use the electrons in the carrier
molecule to make ATP and to reduce oxygen to
water.
More than 95 percent of the oxygen you
breathe is used this way.
This process also gives heat, but 50
percent of the energy is trapped in ATP for the
cell to use later.
So, next time you eat an apple, think
about this: The energy it gives you started in the
sun, which sends its energy 93 million miles in
the form of energy packages called photons.
Chloroplasts in apple-leaf cells capture
that photon energy and store it in the bonds of
sugar molecules. The apple stores that sugar
which you munch.
Mitochondria in your cells break down
those bonds to make ATP, your energy cash in
hand. And nothing in life is free. Every time
you do anything-run, sing, even sleep-you pay
for it with that cash.
Dr. Universe/Energy
Answer these questions in complete sentences in your lab notebook.
1.
2.
3.
4.
Why is ATP like “cash?”
When you eat an apple, what do your cells do to the apple’s sugar?
Where is the energy located in the ATP molecules?
a) ATP can be produced in two ways. Which is more efficient?
b) Describe this process and tell how many ATP molecules are produced from each sugar
molecule.
c) Which way is least efficient?
d) Describe the process, including how many ATP molecules are produced.
5. a) Which organelles in our cells store ATP?
b) What is the origin of these organelles?
6. Explain how light energy transforms into chemical energy.
Organelle = Small, membrane-bound structures in cells