انرژی ها نو

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three

elements. (Elements are the basic building blocks that all
things are made of.) Oxygen combines with hydrogen to form
water. Water makes up, on average, about 60 percent of body
weight. Scientists refer to water as H2O. This series of letters
and numbers is called a chemical formula. The formula has a
specific meaning. It tells us that water has two parts hydrogen
(H) for every one part oxygen (O).
Carbon combines easily with many other elements. These
combinations are called compounds. Millions of compounds
contain carbon. Many of them also contain hydrogen and
oxygen. These three elements are all found in ethanol. The
chemical formula for ethanol is C2H5OH.

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How Plants Store Energy

All living things get energy from the Sun, but simply collecting
energy is not enough. Living things also need a way to store
energy for later use. Carbon compounds are a key part of this
process. They act like a storage battery. They hold energy until
it is needed.
At the heart of the carbon cycle is a process called

photosynthesis

. The first part of the word (photo) come from
the Greek word meaning “light.” The second part (synthesis)
comes from a word meaning “to combine” or “to make.”
In photosynthesis, green plants and algae take energy from
sunlight. They combine this energy with water (H2O) and

carbon dioxide (CO2). CO2 is a gas found in the air

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Sugars and Starches

During photosynthesis, plants produce two substances. The
fi rst substance is oxygen. It goes back into the air. The second
substance is

glucose. Glucose is a simple sugar. It contains
carbon, hydrogen, and oxygen. The word glucose comes from an
old Greek word meaning “sweet.” This form of sugar is found in
fruits, honey, and sugarcane. It is also found in the bloodstream.
Glucose is a storehouse of energy. Animals and plants rely on
it as an energy source. Animals get glucose by feeding on the
plants that produce it. Glucose unites with other elements
to make complex sugars and starches. Starches form a major
part of the human diet. Rice, potatoes, bread, and noodles all
contain starches

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Together, sugars and starches are called

carbohydrates.
Carbohydrates are compounds that contain carbon, hydrogen,
and oxygen. The first part of the word (carbo) means that the
substance has carbon. The second part (hydrates) means that
the hydrogen and oxygen occur in the same proportion as in
water. That is, carbohydrates have two parts hydrogen for every
one part oxygen.

Energy to Burn

All living things need energy to grow. Animals also use energy
to move and keep warm. How do animals unlock the energy
stored in carbohydrates? The answer is a process that is called

respiration

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During respiration, animals breathe in oxygen from the
air. Oxygen acts like the “on” switch in a flashlight. It turns
the energy stored in carbohydrates into usable power. When
oxygen is combined with glucose, the glucose breaks down.
This process releases energy along with H

2O and CO2.
Photosynthesis and respiration are like two sides of the same
coin. In photosynthesis, green plants take CO2 out of the air and
give back oxygen. In respiration, animals take oxygen out of the
air and give back CO2. The carbon cycle has been going on like
this for hundreds of millions of years.

Adding Fuel to the Fire

People’s actions affect the
carbon cycle. For example,
people dig or pump fossil fuels
out of the ground. Mining and
burning fossil fuels releases
carbon that has been locked
up in these fuels for many
millions of years. This carbon
enters the atmosphere as
carbon dioxide. CO

2 is a
greenhouse gas. The buildup
of CO2 in the atmosphere
contributes to global warming.

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As the use of fossil fuels has grown, so has the amount of
CO

2 entering the air. In 1980, burning fossil fuels released 18.5
billion metric tons of CO2. By 2006, more than 29 billion metric
tons was released. China burns a great deal of coal in power
plants. Between 1980 and 2006, China’s output of CO2 jumped
from 1.5 billion metric tons to 6.0 billion metric tons.

Biofuels and Global Warming

Because biofuels come from living things, they, too, contain
carbon. This presents a problem. When biofuels are burned,
is carbon released into the
atmosphere? If so, do biofuels
contribute to global warming?
Some scientists say no. They
believe that burning biofuels
instead of fossil fuels can actually
reduce the threat of global
warming. The explanation for this
is based on the carbon cycle.
Biofuel crops use CO

2 as they
grow. Biofuels release CO2 as they
burn. As long as people continue
to plant biofuel crops year after
year, some or all of the CO2 that
is released will be absorbed by
the growing plants. The result
is that less CO2 remains in the
atmosphere. The more CO2 that
biofuel crops absorb, the less stays
in the air to cause global warming.

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Scientists use the word

biomass

to describe plant matter and animal
wastes that can serve as fuel. Biomass
comes in many different forms. These include
trees, crops (such as corn and sugarcane), grasses,
algae and other plants that grow in water, animal wastes
(manure), human wastes (sewage), and garbage.
Some forms of biomass can be burned directly. This is true
of dry wood branches and twigs. Other forms of biomass need
processing before they can be used as fuel. For example, freshpicked
corn cannot be burned in a car engine. Instead, the
starch in the corn must be converted to sugar, and this sugar
must be turned into ethanol. The ethanol can then be used as
fuel in an engine.
Biomass that must be processed is called a

feedstock.
A feedstock serves as raw material. The fuel is the fi nished
product. For example, corn, sugarcane, and grasses can all be
used as feedstocks for making ethanol.

A Renewable Resource

A major benefi t of using biomass to create energy is that it is
renewable. Forests are cut down, but they can be replanted.
Crops are harvested, but new crops can be grown. People and
animals produce waste daily. People cart a steady supply of
garbage to waste dumps everywhere. The world may run out of

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fossil fuels—but as long as the Sun shines, rain falls, and plants
grow, the world will not run out of biomass.
Another major benefit of biomass is that every nation has
its own supply. In Brazil and India, for example, the climate
favors sugarcane growing. These countries use sugarcane as a
feedstock for making ethanol. Sugarcane does not grow well
in most of the United States, but corn does. So U.S. ethanol
producers use corn as their feedstock.
This gives biofuels another big advantage over fossil fuels.
Every country can grow crops that fit its biomass needs. Every
country produces wastes and garbage that can serve as a

feedstock to produce energy. In contrast, fossil fuels are not

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produced in every country. For example, three countries—
China, the United States, and Russia—together hold about 60
percent of the world’s coal. Most other countries have very little
coal or none at all. When they need coal, countries that do not
have it must pay a high price to buy it from those that do. The
situation is similar with the other fossil fuels—oil and natural
gas. Biomass, on the other hand, is produced everywhere.

Turning Waste into Energy

In some countries, people use animal manure as a feedstock.

People on farms collect the manure, mix it with water, and put

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the mixture in an airtight container. This container is called a

digester

. When the manure decays (breaks down), it produces

biogas

, which can be used for fuel. Biogas from cow manure is
about 60 percent methane gas. Methane (CH4) is a compound
of carbon and hydrogen. It burns cleanly and can be used for
lighting, cooking, heating, and making electricity. Methane is
also the main ingredient in natural gas.
Digesters have been used since the 1950s in Kenya, a

country in east Africa. The manure from two cows can supply