Photosynthesis
Imagine if you were hungry, and all you had to do
to eat was step into the sunshine, dig your toes
into the soil, and breathe. Wouldn’t that make life
easy if you could make your own food?
That is basically the process green plants use to
get the nutrition they need to grow. Green plants
make their own food with the air, water, and
sunshine,
How do green plants make their own food?
Plants are able to produce their own food through
photosynthesis: the process
the process of photosynthesis. The name means
used by plants to change light
“made from light.” Water is absorbed by the roots
energy into chemical energy
and carried up through the stems. Carbon dioxide
for food
is taken in through openings in the leaves. Sunlight
is absorbed by the leaves where the green material chlorophyll helps all the
materials turn the radiant (light) energy into chemical energy. which is stored as a
sugar for the plant to use. Oxygen is produced as a waste product and released from
the leaves back into the air. Animals cannot use radiant (light) energy in this way.
They must obtain energy from the environment in the form of food. Animals have to
eat in order to survive.
Photosynthesis
Radiant energy from the Sun
Chemical energy in glucose molecules
Molecules
Plants are called producers because they can make their own food. Plants use the
chemical energy to grow and reproduce. Some plants, like fruits and vegetables,
store the chemical energy. When you eat an apple or an ear of corn, the stored
chemical energy is passed along to you. Any organism that eats plants is called a
consumer because they can’t make their own food and must consume a producer.
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Photosynthesis
We can observe photosynthesis using a simple experiment.
Photosynthesis is a chemical process. Because this process takes place at the level
of molecules and atoms, we cannot observe it directly. However, we can observe the
results of photosynthesis.
Joseph Priestley was one of the first scientists to observe a
product of photosynthesis. He ran an experiment in which he
placed a burning candle and a mouse under a glass dome. After
a few minutes, the flame went out. Priestley showed that the
gases in this dome could not keep the mouse alive. However, if
the experiment was repeated with a plant present under the
dome, the mouse remained alive. What change had the plant
made to the dome?
The plant replaced the oxygen that had been used up by the burning candle. Oxygen
is a colorless, odorless gas that humans cannot observe directly. However, its
presence is required for animals such as mice to live. Priestley demonstrated that
the plant could produce oxygen. Oxygen is one of the products of photosynthesis.
The inputs and outputs of photosynthesis are shown in
the diagram at right. Carbon dioxide in the air enters a
plant through its leaves. Water in the soil enters
through a plant’s roots. Oxygen exits from the leaves.
Glucose can be used immediately or stays behind to
serve as the plant’s food.
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Photosynthesis
Photosynthesis is necessary for life on Earth.
Humans require oxygen to survive. Many other animals also require oxygen.
However, Earth’s atmosphere has not always contained oxygen gas. During its first
2.3 billion years of existence, Earth had an atmosphere that contained water vapor,
nitrogen, carbon dioxide, sulfur dioxide, and hydrogen sulfide. What changed that led
to oxygen gas becoming part of the atmosphere?
A scientist takes a
sample of
cyanobacteria from
the surface of a lake.
When cyanobacteria
populations grow too
quickly in a water
body, they can harm
other organisms in the
ecosystem. This
situation is called an
algal bloom.
The answer is that microorganisms capable of
photosynthesis arose on the planet. These
microorganisms are called cyanobacteria. Cyanobacteria
live in both freshwater and saltwater environments.
Millions and millions of these tiny creatures produced
enough oxygen to change the composition of Earth’s
atmosphere. Oxygen became established in the
atmosphere as a result of photosynthesis. Since this shift,
many organisms have evolved to depend on oxygen.
Photosynthesis is necessary to continue supplying
oxygen for these organisms. In addition, all life forms
need photosynthesis as the mechanism for capturing and
using radiant energy. Plants benefit from this directly
because they use photosynthesis to make the glucose
molecules that are their food sources. Animals benefit
indirectly because they, too, must have sources of food.
Animals either eat plants to obtain chemical energy in the
form of glucose or they eat other animals that ate plants.
Energy moves from the Sun to plants to animals.
Photosynthesis is necessary for that energy flow.
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Photosynthesis
Looking to the Future: Can humans
make use of photosynthesis to
produce fuels?
This scientist is
studying the
production of
organic compounds
by cyanobacteria. If
successful, she will
be able to use
photosynthesis to
produce fuels for
cars, planes, and
trains.
The same microorganisms that created
our oxygenated atmosphere could be
used to develop new fuels. Think about it.
Food is a kind of fuel that we use to run
our bodies. If photosynthesis can produce
the kind of fuel that our bodies use, it
should be able to produce the kinds of
fuels that vehicles such as cars or
airplanes use, too.
These fuels would not contain glucose, but they would contain other carboncontaining compounds. Such biofuels have been produced from corn, sugarcane,
soybeans, and sunflowers. Making biofuel from cyanobacteria is not hard to do;
however, the process is expensive. Right now, it costs too much to develop biofuel
from the strains of cyanobacteria available. Each cell produces a very small amount
of the compounds that serve as the raw material for biofuel production. It is also
costly to provide the large quantities of water needed to grow the microorganisms. In
all, the cost of making the biofuel is greater than the price at which the fuel can be
sold. Much research is being done to find ways to bring costs down so that the
process is economical.
What Do You Know?
Read the following statements on the next page. Fill in each blank with the correct
word from the following list.
Sun
photosynthesis
food
radiant (light) energy
chemical energy
water
glucose
carbon dioxide
oxygen
producer
consumers
hydroponics
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Photosynthesis
1.  The process green plants use to make food is ___________________________.
2.  Leaves take in ______________________________ from the air.
3.  The green material in plants is called __________________________________.
4.  An organism that can make its own food is called a ______________________.
5.  Growing plants without soil in nutrient-rich water is called __________________.
6.  The chemical reactions through photosynthesis produce __________________.
7.  All organisms need glucose or a source of _______________________ to carry
out basic life functions.
8.  Animals obtain glucose from ________________________________________.
9.  The reactants of photosynthesis are _______________________________ and
__________________________________.
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Photosynthesis
A Hydroponic Garden at Home
To help your student learn more about the environment, discuss having a hydroponic
garden at your home. Hydroponics have been around since the 1600s and involve
water, air, and natural sunlight.
Here are some questions to discuss with your
student:
•  Should the hydroponic garden be in the house or
outside on a patio or deck?
•  What direction should the garden face?
•  What equipment will be needed?
•  What should we grow in the garden?
hydroponics:
growing plants in nutrientrich water without soil or in
gravel or pebbles
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