Download The Cell Theory

Objectives
Explain the main ideas of the cell theory.
Describe how microscopes aid the study of cells.
Compare and contrast animal cells and plant cells.
Distinguish between prokaryotic and eukaryotic cells.
Key Terms
cell theory
micrograph
organelle
plasma membrane
nucleus
cytoplasm
cell wall
prokaryotic cell
eukaryotic cell
Cells are as basic to biology as atoms are to chemistry. All organisms are
made of cells. Organisms are either unicellular (single-celled), such as
most bacteria and protists, or multicellular (many-celled), such as plants,
animals, and most fungi. Because most cells cannot be seen without
magnification, people's understanding of cells and their importance is
relatively recent.
The Cell Theory
Human understanding of nature often follows the invention and
improvement of instruments that extend human senses. The development
of microscopes provided increasingly clear windows to the world of cells.
Light microscopes, the kind used in your classroom, were first developed
and used by scientists around 1600. In a light microscope, visible light
passes through an object, such as a thin slice of muscle tissue, and glass
lenses then enlarge the image and project it into the human eye or a
camera.
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In 1665, an English scientist named Robert Hooke observed
"compartments" in a thin slice of cork (oak bark) using a light
microscope. He named the compartments cells. Actually, Hooke was
observing the walls of dead plant cells. Many more observations by many
other scientists were needed to understand the importance of Hooke's
discovery. By 1700, Dutch scientist Anton van Leeuwenhoek (LAY vun
hook) had developed simple light microscopes with high-quality lenses to
observe tiny living organisms, such as those in pond water. He described
what he called "animalcules" in letters to Hooke and his colleagues.
For the next two centuries, scientists, using microscopes, found cells in
every organism they examined. By the mid-1800s, this evidence led to the
cell theory—the generalization that all living things are composed of
cells, and that cells are the basic unit of structure and function in living
things. Later, the cell theory was extended to include the concept that all
cells come from pre-existing cells.
Microscopes as Windows to Cells
Light microscopes (abbreviated LM) are useful for magnifying objects up
to about 1000 times their actual size. This type of microscope works for
viewing objects about the size of a bacterium or larger. But much of a
cell's structure is so small that even magnifying it 1000 times is not
enough to see it. Knowledge of cell structure took a giant leap forward as
biologists began using electron microscopes in the 1950s. Instead of
light, the electron microscope uses a beam of electrons. Certain electron
microscopes can magnify objects as much as a million (1,000,000) times,
enough to reveal details of the structures inside a cell.
Biologists use the scanning electron microscope (SEM) to study the
surface structures of cells. The transmission electron microscope (TEM)
is used to explore their internal structure. Specimens for both types of
electron microscopes must be killed and preserved before they can be
examined. For this reason, light microscopes are still useful for observing
living cells.
A photograph of the view through a microscope is called a micrograph.
Throughout your textbook, most micrographs have a notation alongside
the image that indicates the kind of microscope used to view the object
and its final magnification. For example, the notation "LM 200X"
indicates that the micrograph is an image made with a light microscope
and shown here at a magnification of 200X, or 200 times its actual size.
(You may notice that some light micrographs in your book have
magnifications listed of more than 1000X. That is because the
photographs have been further enlarged from the originals.)
As you tour the parts of a cell in this chapter, you will encounter
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comparisons to a scale model of a cell enlarged to the size of your
classroom. At this magnification, the "classroom cell" is over 300,000
times larger than a normal cell.
Figure 6-4
This diagram provides an overview of a generalized animal cell. Later in the
chapter, watch for miniature versions of the diagram with "you-are-here"
highlights. They will serve as road maps on your tour of cells.
An Overview of Animal and Plant Cells
Each part of a cell with a specific job to do is called an organelle,
meaning "mini-organ." Cutaway diagrams of a generalized animal cell
(Figure 6-4) and plant cell (Figure 6-5) show the organelles in each kind
of cell. For now, the cell parts labeled in the figures are just words and
structures, but these organelles will come to life as you take a closer look
at how each of them works, here and later in the chapter.
There are more similarities between animal and plant cells than there are
differences. Both kinds of cells have a thin outer covering, called the
plasma membrane, which defines the boundary of the cell and regulates
the traffic of chemicals between the cell and its surroundings. Each cell
also has a prominent nucleus (plural, nuclei), which houses the cell's
genetic material in the form of DNA. In the classroom-cell scale model,
the nucleus would be the size of a small car in the middle of your
classroom.
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Wednesday, October 19, 2011 9:13:34 AM CT
Figure 6-5
A plant cell has many of the same structures as an animal cell. Miniature
versions of this generalized plant cell diagram will appear in parts of the
chapter where its unique organelles are discussed.
The entire region of the cell between the nucleus and the plasma
membrane is called the cytoplasm (SYT oh plaz um), which consists of
various organelles suspended in a fluid. Many of these organelles are
enclosed by their own membranes. These membranes help to maintain
chemical environments inside the organelles that are different from the
environment of the rest of the cell.
If you compare Figures 6-4 and 6-5, you will see that there are a few key
differences in cell structure between plants and animals. One difference is
the presence of chloroplasts in some plant cells, but not in animal cells. A
chloroplast is the organelle in which photosynthesis occurs.
Photosynthesis converts light energy to the chemical energy stored in
molecules of sugars and other organic compounds. Also, a plant cell is
encased by a strong cell wall outside its plasma membrane. The cell wall
protects the plant cell and maintains its shape. Animal cells do not have
cell walls.
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Two Major Classes of Cells
There are two basic kinds of cells. One kind—a prokaryotic cell (pro KAR
ee oh tik)— lacks a nucleus and most other organelles. Bacteria and
another group of organisms called the archaea are prokaryotic cells.
Prokaryotic organisms appear earliest in Earth's fossil record. In contrast,
a eukaryotic cell (yoo KAR ee oh tik) has a nucleus surrounded by its
own membrane, and has other internal organelles bounded by membranes.
Protists, fungi, plants, and animals consist of eukaryotic cells. Organisms
with eukaryotic cells appeared later in Earth's history.
The major difference between these two main classes of cells is indicated
by their names. The word eukaryotic is from the Greek eu meaning "true,"
and karyon meaning "kernel." The kernel refers to the nucleus that
eukaryotic cells have and prokaryotic cells lack. In a eukaryotic cell, the
nucleus is the largest organelle. There are many other types of organelles
outside the nucleus, surrounded by membranes of their own.
A bacterium is an example of a prokaryotic cell (pro means "earlier
than"). Without a true nucleus and the organelles of eukaryotic cells,
prokaryotic cells are much simpler in structure (Figure 6-6). The DNA in
a prokaryotic cell is concentrated in an area called the nucleoid region,
which is not separated from the rest of the cell by a membrane, as is the
case in a eukaryotic cell. Most bacteria are 1 to 10 micrometers in
diameter, whereas eukaryotic cells are typically 10 to 100 micrometers in
diameter. You'll examine prokaryotic cells in more detail in Chapter 16.
Eukaryotic cells are the main focus of this chapter.
Figure 6-6
A cutaway diagram reveals the
structure of a generalized prokaryotic
cell.
Concept Check 6.1
1. What evidence led to the development of the cell theory?
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2. How do the various kinds of microscopes differ as tools in the study of
cells?
3. Identify two similarities and two differences between plant and animal
cells.
4. How is a eukaryotic cell different from a prokaryotic cell?
Copyright © 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights
reserved.
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Wednesday, October 19, 2011 9:13:34 AM CT