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What are Cells?
Have you ever wondered how people are similar to bacteria? It may seem
like a silly question. After all, humans and bacteria are very different in size
and complexity. Yet scientists have learned that we also have much in
common with our microscopic companions.
Scientists classify all organisms into groups based on their external
characteristics. For example, some plants produce fruits with seeds, but
other plants do not. Scientists also use internal characteristics to classify
organisms. For example, some animals have backbones, while others do
not. Can you think of some other external or internal characteristics that
scientists can use to classify organisms?
organism: a living thing
How is this
bacterium
similar to a
human?
The cell is the basic unit of life.
One of the most important internal characteristics that scientists use to classify organisms is the
cell. All organisms are made up of one or more cells. A cell is the basic unit of life; it is surrounded
by a cell membrane that keeps the cell intact. Inside all eukaryotic cells are specialized structures
called organelles that carry out specific functions inside the cell. Organelles are suspended in a
thick, gel-like fluid called cytoplasm.
All cells also have genetic material called DNA, which contains instructions for making new
organisms and for carrying out all functions that keep a cell alive. In some cells, DNA is packaged
inside a membrane in an organelle called a nucleus. In other cells, it floats freely in the cytoplasm.
All of these different types of cells are found in the human body. Can you identify where you find
these cells in the body?
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What are Cells?
What evidence for the existence of cells did scientists first find? Answers to this question come
from analyzing the events that led up to the formation of cell theory and the laboratory evidence
found to support cell theory.
The invention of the microscope led to the discovery of cells.
Prior to cell theory, in the 1600s, Antony van Leeuwenhoek used a
single lens microscope to see the first unicellular organisms which
he called "animalcules." Later that century, experimental scientist
Robert Hooke used the term "cell" for the box-like structures he
observed when viewing the tissue of a cork through a lens (image
on the right). In the 1670s, van Leeuwenhoek discovered bacteria
and protozoa.
A unified cell theory was
finalized in 1855 by German
scientists, Schneider,
Schwann, and Virchow, in
which they stated: all living
things are composed of one
or more cells; the cell is the
basic unit of life; and all cells
come from pre-existing cells.
Evidence for Cell Theory
•  Evidence for the cell as the basic unit of life is supported by microscope observations.
•  Evidence for the cell being the smallest unit of life is supported by experiments that have shown
the structures within the cell cannot exist independently. The entire cell is needed to support cell
function.
•  Evidence for the cells coming from existing cells is supported by early experiments by Francisco
Redi which showed that maggots did not appear spontaneously. Fly larvae (maggots) did not
appear on meat in a screened container, but did on an unscreened jar where flies could lay their
eggs . Louis Pasteur repeated similar experiments in curved neck containers to better control air
access to the sample. He also found the air contained tiny particles that contaminated the meat.
His work led to the pasteurization of milk to protect it from bacteria. Scientists have imaged
actual cell division (mitosis and meiosis).
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What are Cells?
All living organisms are composed of one or more cells.
When you think about an organism, you might think of
something very familiar, such as people, cats, or trees. These
organisms are complex; they are made up of a great number
of different kinds of cells. Scientists estimate that the average
adult human has somewhere between 10 and 100 trillion cells
in his or her body!
Cells come in many different sizes and types, and they are
very different from each other in their shapes and functions.
The diagram on the previous page shows examples of
different types of cells in the human body.
Euglena are single-cell
organisms that live in fresh and
salt water.
Not all organisms are complex. Some are very simple. In fact, some organisms are made up of
only one cell. Take a look at this euglena. A euglena is an organism made up of a single cell.
Unlike humans, it does not have specialized organs, such as a brain or stomach. However, it can
move through its environment using its whip-like flagellum. It even has a primitive eye called an
eyespot for sensing light levels. All this in a single cell!
There are two main types of cells: prokaryotic and eukaryotic.
All organisms are made up of cells. However, scientists separate cells into two categories:
prokaryotic and eukaryotic. Examples of prokaryotic cells include bacteria. Eukaryotic cells
include the cells of plants, animals, and fungi.
Scientists think prokaryotic cells were the first cells on Earth. The earliest records of prokaryotic
cells date to around 4 to 3.8 billion years ago. Prokaryotic cells have the basic structures common
to all cells. These structures include a plasma membrane surrounding cytoplasm. However,
prokaryotic cells do not have membrane-enclosed organelles, such as mitochondria or a nucleus.
Prokaryotic cells do contain ribosomes, but there is debate as to whether or not a ribosome counts
as a type of organelle. Therefore, the question of whether or not prokaryotic cells contain any
organelles is still up for debate.
Eukaryotic cells are more complex. Similar to prokaryotic cells, eukaryotic cells have a cell
membrane, cytoplasm, and DNA. However, they have something that prokaryotic cells do not.
Eukaryotic cells have organelles surrounded by membranes. This includes mitochondria and a
nucleus, where DNA is stored.
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What are Cells?
A prokaryotic cell (left) has a cell membrane, cytoplasm,
and DNA. A eukaryotic cell (right) also has these features.
Eukaryotic cells also have membrane-enclosed organelles
such a s mitochondria and a nucleus.
Prokaryotic cells were the first cells to evolve on Earth. However, this does not mean they
disappeared when the eukaryotic cells evolved 1.5 billion years ago. Bacteria are prokaryotic cells
that are very much still alive today. In fact, thousands of species may live in one spoonful of soil.
Prokaryotic and eukaryotic cells store DNA in different ways.
Both eukaryotic and prokaryotic cells have DNA, the “blueprint” of an
organism. In eukaryotic cells, the DNA is neatly organized inside a nuclear
membrane. The combination of nuclear membrane and DNA is called the
nucleus. Each eukaryotic cell has just one nucleus. The DNA inside the
nucleus is organized into units called chromosomes, which are linear and
can be seen under a microscope when the cell divides.
Prokaryotic cells are less organized than eukaryotic cells. They lack a
nuclear membrane around their DNA. Instead, their DNA floats in the
cytoplasm. The DNA of a prokaryotic cell is all contained within a single
circular chromosome.
Eukaryotic cells
organize their
DNA into
chromosomes.
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What are Cells?
Does the following picture show a prokaryotic or eukaryotic cell? Why do you think this?
Prokaryotic and eukaryotic cells have other
important similarities and differences.
Both prokaryotic and eukaryotic cells have other
things in common. Both have ribosomes in
their cytoplasm. Ribosomes are responsible for
making proteins in the cytoplasm. The
ribosomes in eukaryotic cells are bigger and
more complex than those in prokaryotic cells.
However, they have the same function of
making proteins.
Prokaryotic cells tend to be much smaller than
eukaryotic cells. On average, eukaryotic cells
are about 10 times larger than prokaryotic cells.
Eukaryotic cells have much greater diversity in
shape and size than prokaryotic cells.
Organisms with prokaryotic cells are so small
they can be seen only through a microscope.
You also need a microscope to s e e eukaryotic
cells. However, many organisms with eukaryotic
cells are large enough to s e e without a
microscope.
These pictures represent some of
the bacteria that commonly infect
humans. Can you identify the cells
that are spherical (round), rod-shaped,
and spiral?
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What are Cells?
Discover Science: How did organelles become established in eukaryotes?
Scientists have an interesting theory to explain how organelles came to be present in eukaryotic
cells. They theorize that prokaryotes were present on Earth long before eukaryotes. Lacking food,
some prokaryotes lost their cell walls. Their flexible membranes began to fold and create several
internal membranes and a nucleus. These primitive eukaryotic cells began engulfing or taking in
smaller prokaryotes as shown in the diagram below.
However, scientists think some of these events did not result in the larger cell digesting the smaller
cell. Instead, the smaller cell may have provided some advantage to the larger cell. For example, if
the smaller cell were able to carry out photosynthesis, it could provide energy from this process for
the larger cell. In return, the larger cell provided protection for the smaller cell. This mutually
beneficial relationship is known as symbiosis. The theory about the origin of organelles is known
as endosymbiotic theory. The word endosymbiotic is used because the root word endo- refers to
the engulfing process, and symbiotic refers to the relationship that led to organelle development.
According to the theory, over many years, the two symbiotic cells became a more complex,
eukaryotic cell.
Both prokaryotic and eukaryotic cells can be single-celled organisms. However, there are no
multicellular prokaryotes. Only eukaryotes can be multicellular.
Eukaryotic cells come in all sorts of shapes and sizes. Prokaryotic cells have just three basic
shapes: rod, spherical, and spiral. The shapes of the cell help scientists identify prokaryotes using
a microscope.
Career Corner: Knowing the different types of cells can save lives.
When a person is infected with a bacterium, it is important to know the identity of the infectious
agent. Antibiotic drugs can be specific for particular organisms. If a doctor does not know which
organism is causing an illness, the doctor may not be able to treat the patient.
Doctors will often take a small sample of an infected area. For example, a patient with symptoms
for strep throat may be given a throat swab. The swab is then cultured to grow any microorganisms
present in the patient. When there are enough microorganisms growing in the culture, the doctor
may be able to identify which species is causing the illness. Then an appropriate treatment can be
prescribed.
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What are Cells?
What Do You Know?
Scientists classify cells as prokaryotic or eukaryotic. The table below has a list of cell structures.
For each structure, circle the cell type(s) where you would find this cell structure.
Cell Structure
Cell Type
Mitochondria
Prokaryotic
Eukaryotic
Both prokaryotic and eukaryotic
Ribosomes
Prokaryotic
Eukaryotic
Both prokaryotic and eukaryotic
Nucleus
Prokaryotic
Eukaryotic
Both prokaryotic and eukaryotic
DNA
Prokaryotic
Eukaryotic
Both prokaryotic and eukaryotic
Cell membrane
Prokaryotic
Eukaryotic
Both prokaryotic and eukaryotic
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What are Cells?
Prokaryotic And Eukaryotic Cells In Your Neighborhood
Children remember information best when they are able to associate new information with familiar
topics. Take your students for a walk in their neighborhood. Take turns playing “I Spy” to identify
organisms you find. These may include animals, plants, and fungi. As you play the game, identify
each organism as prokaryotic or eukaryotic. (All of the organisms that you “spy” will be eukaryotic,
as prokaryotic cells can only be seen with a microscope.) Be careful not to touch or otherwise
disturb any organisms that you observe.
Here are some questions to discuss with your child:
•  Why did you find only eukaryotic organisms on your walk? (Prokaryotic cells can only be seen
with a microscope.)
•  Where might you expect to find prokaryotic organisms? (Answers might include in the soil or in
the water.)
•  Have you ever been sick because of an infection by a prokaryotic organism? (Answers will
vary. Keep in mind that bacteria are prokaryotes, whereas viruses are not. Children are often
familiar with strep throat, which is caused by bacterial infection, and them common cold and flu,
which are caused by viral infections.)
Your child might be tempted to classify organisms based on whether they can see them with their
unaided eyes or with a microscope only. It is important to stress that eukaryotic and prokaryotic
cells are not classified on the basis of whether a microscope is necessary to observe them.
Instead, the classification of eukaryotic and prokaryotic cells is based primarily on whether the
cell’s DNA is organized in a nucleus (eukaryotic), or whether it floats in the cytoplasm (prokaryotic).
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