Directions:
1. Before Reading:
a. Look up the definition of “bacteria”.
Bacteria b. Answer the Predict question below.
Predict: Where do you think bacteria might be found?
2. Read the article closely.
3. Answer the Making Sense questions at the end.
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Cells, Organisms, & Living Things
Are All Cells the Same?
You have observed different kinds of cells in class. Scientists divide cells into two groups based
on their characteristics: prokaryotes and eukaryotes. Most of the cells you’ve seen are
eukaryotes. Like plant cells and the cells in your body, all eukaryotic cells have a nucleus and
cell membrane. But there are some cells, like bacteria, that do not have a nucleus or any other
structures inside the cell. Scientists call these prokaryotes. Both types of cells are living things.
What Makes Something a Living Thing?
There are several criteria something must meet in order to be considered a living thing. All living things are made of one or
more cells, grow, reproduce, absorb nutrients (eat), and respond to stimuli (react to their
environment).
What about viruses? Viruses are very small particles made up of DNA and proteins. Most viruses
are not harmful to humans, but others can make us extremely sick. Viruses are not made up of
cells, and they do not have the structures an organism needs to live or reproduce on its own. In
order to function, viruses must be inside of a living cell because they cannot make or store
energy, therefore, viruses are not living things.
What Does It Mean to Be an Organism?
One thing that can be confusing when learning about cells is that some cells are considered organisms while other cells are
just cells. All cells are living because they meet all of the characteristics of living things; however, not all cells are organisms.
In order for a living thing to be considered an organism, it must be able to do those things on its own.
Cells that are able to live on their own are called single-celled organisms. Single-celled means the entire organism is just one
cell. It can do all the things living things do just by interacting with its
environment. Organisms can also be multicellular. Multicellular organisms
are made of many cells. YOU are a multicellular organism made of many
different types of cells—cheek cells, heart cells, bone cells, and others. The
cells in your body are living things, but not organisms. They do a number of
things that organisms do, but they cannot be a living thing on their own.
They cannot get what they need unless they are part of a larger organism.
Where Are Single-Celled Organisms Found?
There are many species of single-celled organisms. The most well known are bacteria. Two more examples are algae and
single-celled fungi. Single-celled creatures come in a wide range of shapes, sizes, and colors and are found almost
everywhere. They are in the soil, water, the air, and even in the plaque on your teeth. In fact, there are bacteria living inside
and all over your body! Pick up one teaspoon of soil and you are holding over
1,000,000,000 bacteria, about 120,000 fungi, and 25,000 algae.
Single-Celled Organisms Can Be Harmful
Did you ever accidentally drink spoiled milk or smell a rotting apple? Food spoils
when it gets old, or if it isn’t kept at the proper
temperature. When food spoils, it is usually because
single-celled organisms have started to grow. When milk
is old, or is left out of the refrigerator for too long, harmful bacteria can grow. Food poisoning is
due to eating food containing harmful bacteria.
Single-celled organisms may also infect food as it is being grown. During the 1840s, a plant disease
caused by a single-celled organism ruined potato crops all over Europe. In Ireland, where potatoes were the main crop,
about 1 million people starved to death when the potato crop was ruined.
Single-Celled Organisms Can Be Helpful
Although plants usually get the credit for making the oxygen we breathe, most of Earth’s oxygen is actually produced by
plankton--single-celled organisms that live in the oceans. These organisms make their own food using energy from sunlight,
just like plants do. As they do, they give off oxygen. Without them, people might not have enough oxygen to survive.
Single-celled creatures are involved in another important environmental process—recycling.
Single- celled organisms in the soil feed on things that were once living, like dead leaves, twigs,
and scraps of food. As they feed, these decomposers break down the organic materials, giving off
minerals that plants need in order to grow. Today, many people use this process to recycle
garbage into compost. Adding compost to gardens makes healthy soil by returning nutrients to
the soil that are used by plants as they grow.
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Making Sense
1. What characteristics make something a living thing?
2. Are single-celled organisms living things? Why or why not?
3. Why aren’t human body cells considered organisms, even though they are living things?
4. What do you think the cells in your body need from other parts of the body in order to stay alive?
5. How are single-celled organisms and multicellular organisms alike and different?
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Similarities
Differences
(What do they have in common?)
(How are they different?)
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