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Lesson 4: What is the role of
microorganisms?
Microorganisms
Microorganisms are everywhere. They
are in the air, soil, and water. Some
microorganisms are even on your skin and
inside your body. Most microorganisms are
harmless. Many are very helpful to other
living things.
The number of microorganisms in the
world is larger than the number of animals
and plants. There can be more than ten
thousand protists in a liter of pond or
ocean water. Most protists have only one
cell. Algae are protists that make their own
food, like plants. Algae are producers. They
are the main producers in most food webs
in oceans, lakes, and rivers. Some protists
eat other microorganisms. This helps clean
the water supply.
Other Helpful Microorganisms
Use with pp. 106–109
Penicillium molds are blue-green fungi. In
1928, Alexander Fleming discovered that
Penicillium mold makes an antibiotic. An
antibiotic is a medicine that kills diseasecausing bacteria.
Microorganisms at Work
The soil is full of insects and
microorganisms. One liter-sized container
of dirt can contain a trillion bacteria,
thousands of worms, and millions of
protists. Some bacteria help fertilize the
soil. These bacteria change the element
nitrogen into a different compound. Plants
use this compound to grow. They also use
this compound to make proteins. All living
things need proteins.
Observing Microorganisms
Microorganisms are too small for you to
see with just your eyes. You need to use a
microscope.
You can make a mixture where
microorganisms will grow. Then you can
observe them. Take some hay or dried
grass. Mix it with equal amounts of pond
water and distilled water. Keep the mixture
in the dark at room temperature. After a
few days, microorganisms will grow.
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Most microorganisms are helpful, not
harmful. Bacteria in your intestines help
you digest food. These helpful bacteria
also protect you from harmful bacteria.
Herbivores like cows and buffalo have
bacteria in their stomachs. These bacteria
help the animals digest grass and other
plants.
There are bacteria in dairy products
like yogurt, sour cream, and cheese.
Cheesemakers put the fungus Penicillium
candidum into some cheeses. The fungus
makes the cheese taste good.
Lesson 4 Summary
22 Chapter 3, Lesson 4 Summary
Intervention Study Guide
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Lesson 4 Questions
Use with pp. 106–109
Lesson 4 Questions
1. What are the main producers of food in most ecosystems in lakes, rivers, and
oceans?
2. You have bacteria in your intestines. What do these bacteria do to help you?
3. What is an antibiotic?
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4. How do bacteria in the soil help fertilize the soil for plants?
Intervention Study Guide
Chapter 3, Lesson 4 Questions 23