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Cell Structures
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Printed: October 12, 2015
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C HAPTER
Chapter 1. Cell Structures
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Cell Structures
Lesson Objectives
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Describe the structure and functions of the cell membrane.
Identify the parts and roles of the cytoplasm and cytoskeleton.
List organelles in eukaryotic cells and their special jobs.
Describe structures found in plant cells but not animal cells.
Lesson Vocabulary
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cell wall
vacuole
endoplasmic reticulum (ER)
Golgi apparatus
lysosome
mitochondrion (mitochondria, plural)
vacuole
Introduction
In some ways, a cell resembles a plastic bag full of Jell-O. Its basic structure is a cell membrane filled with cytoplasm.
The cytoplasm of a eukaryotic cell (one with a nucleus present) is like Jell-O containing mixed fruit. It also contains
a nucleus and other organelles.
Figure 1.1 shows the structures inside a typical eukaryotic cell. The model cell in the figure represents an animal
cell. Refer to the model as you read about the structures below.
FIGURE 1.1
Model of an animal cell
Cell Membrane
The cell membrane is like the bag holding the Jell-O. It encloses the cytoplasm of the cell. It forms a barrier between
the cytoplasm and the environment outside the cell. The function of the cell membrane is to protect and support the
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cell. It also controls what enters or leaves the cell. It allows only certain substances to pass through. It keeps other
substances inside or outside the cell.
Cytoplasm
Cytoplasm is everything inside the cell membrane (except the nucleus if there is one). The water in the cytoplasm
makes up about two-thirds of the cell’s weight.
Roles of Cytoplasm
Why does a cell have cytoplasm? Cytoplasm has several important functions. These include:
• suspending cell organelles.
• pushing against the cell membrane to help the cell keep its shape.
Eukaryotic cells contain a nucleus and several other types of organelles. These structures carry
out many vital cell functions.
Nucleus
The nucleus is the largest organelle in a eukaryotic cell. It contains most of the cell’s DNA. DNA, in turn, contains
the genetic code. This code “tells” the cell which proteins to make and when to make them. Besides DNA, the
nucleus contains a structure called a nucleolus. Its function is to form ribosomes.
The mitochondrion (mitochondria, plural) is an organelle that makes energy available to the cell.
It’s like the power plant of a cell.
Ribosomes
A ribosome is a small organelle where proteins are made. It’s like a factory in the cell. Ribosomes may be
found floating in the cytoplasm. Some ribosomes are located on the surface of another organelle, the endoplasmic
reticulum.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. It’s made of
folded membranes. Bits of membrane can pinch off to form tiny sacs called vesicles. The vesicles carry proteins or
lipids away from the ER.
There are two types of endoplasmic reticulum. They are called rough endoplasmic reticulum (RER) and smooth
endoplasmic reticulum (SER). Both types are shown in Figure 1.2.
Golgi Apparatus
The Golgi apparatus is a large organelle that sends proteins and lipids where they need to go. It’s like a post office.
It receives molecules from the endoplasmic reticulum. It packages and labels the molecules. Then it sends them
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Chapter 1. Cell Structures
FIGURE 1.2
RER and SER are located outside the cell
nucleus. The red dots on the RER are
ribosomes.
where they are needed. Some molecules are sent to different parts of the cell. Others are sent to the cell membrane
for transport out of the cell. Small bits of membrane pinch off the Golgi apparatus to enclose and transport the
proteins and lipids. You can see a Golgi apparatus at work in this animation:
http://www.johnkyrk.com/golgiAlone.html
Vacuoles
V acuoles are sac-like organelles. They store and transport materials in the cell. They are like movable storage
containers.
• Some vacuoles are used to isolate materials that are harmful to the cell.
• Other vacuoles are used to store needed substances such as water.
Lysosomes
A lysosome is an organelle that recycles unneeded molecules. It uses enzymes to break down the molecules into their
components. Then the components can be reused to make new molecules. Lysosomes are like recycling centers.
Special Structures in Plant Cells
All but one of the structures described above are found in plant cells as well as animal cells. Plant cells have
three additional structures that are not found in animals cells. These include a cell wall, large central vacuole, and
organelles called plastids. You can see these structures in the model of a plant cell in Figure 1.3. You can also see
them in the interactive plant cell at this link:
http://www.cellsalive.com/cells/cell_model.htm
Cell Wall
The cell wall is a rigid layer that surrounds the cell membrane of a plant cell. It’s made mainly of the complex
carbohydrate called cellulose. The cell wall supports and protects the cell. The cell wall isn’t solid like a brick wall.
It has tiny holes in it called pores. The pores let water, nutrients, and other substances move into and out of the cell.
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FIGURE 1.3
Model of a plant cell
Central Vacuole
Most plant cells have a large central vacuole. It can make up as much as 90 percent of a plant cell’s total volume.
The central vacuole is like a large storage container. It may store substances such as water, enzymes, and salts. It
may have other roles as well. For example, the central vacuole helps stems and leaves hold their shape. It may also
contain pigments that give flowers their colors.
Plastids
Plastids are organelles in plant cells that may have various jobs. The main types of plastids are chloroplasts,
chromoplasts, and leucoplasts.
• Chloroplasts contain chlorophyll. Chlorophyll is a green pigment. It gives plants their green color.
Photosynthesis takes place in chloroplasts. They capture sunlight and use its energy to make glucose.
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Chapter 1. Cell Structures
Lesson Summary
• The cell membrane encloses the cytoplasm and controls what enters and leaves the cell.
• The cytoplasm helps the cell keep its shape.
• Eukaryotic cells contain a nucleus and other organelles. They include the mitochondrion, endoplasmic reticulum, Golgi apparatus, vesicles, vacuoles, lysosomes, and—in animal cells—centrioles. Each type of organelle
has a special function.
• Plant cells have several structures not found in animal cells. They include a cell wall, large central vacuole,
and plastids such as chloroplasts.
Lesson Review Questions
1.
2.
3.
4.
Which organelle MUST be present in a eukaryotic cell?
Describe the composition of the cytoplasm and list its functions.
Identify three organelles in eukaryotic cells and state their roles.
Compare and contrast plant and animal cells.
References
1. User:Kelvinsong/Wikimeida Commons. Model of an animal cell . Public Domain
2. OpenStax College. Drawing of the endoplasmic reticulum . CC-BY 3.0
3. Mariana Ruiz Villarreal (User:LadyofHats/Wikimedia Commons). http://commons.wikimedia.org/wiki/File:P
lant_cell_structure_svg.svg . Public Domain
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