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120 Chapter 4 Cells: The Basic Units of Life
Eukaryotic Cells
Key Concept Eukaryotic cells have organelles
that perform important functions.
Even though most cells are small, cells are complex. A
eukaryotic cell has many parts that help the cell stay alive. Some
eukaryotic cells can be classified as plant cells or animal cells.
Compare the plant cell in Figure 1 with the animal cell in Figure 2
to see the differences between these two types of cells.
Cell Wall
Plant cells have an outermost structure called a cell wall.cell wall. A
cell wall is a rigid structure that gives support to a cell. The
cell walls of plants, fungi, archaea, and bacteria can be made
of different materials. For example, plants and algae have cell
walls made of a complex sugar called cellulose. Figure 1 shows
the cellulose fibers in the cell wall of a plant cell. Animal cells
do not have cell walls.
What is one characteristic that distinguishes plant cells from animal cells? 7.1.b
cell wallcell wall (SEL WAWL) a rigid structure that surrounds the cell membrane and provides support to the cell
Endoplasmic reticulum
Large central vacuole
Cytoplasm
Mitochondrion
Ribosome
Cell wall
Cell membrane
Cellulose fibers
Golgi complex
Chloroplast
Cytoskeleton
Nucleus
Figure 1 A Plant Cell
What You Will Learn
• Eukaryotic cells have many parts—
such as cell membranes, a nucleus, and ribosomes—in common.
• Plant cells and animal cells have
some cell parts that are different.
Why It Matters
Learning how organelles function helps you know how cells stay alive.
Vocabulary
- cell wall • mitochondrion
- cytoskeleton • chloroplast
- ribosome • Golgi complex
- endoplasmic • vesicle reticulum • lysosome
Graphic Organizer In your Science Journal, make a Comparison Table that compares the structure, function, location in the cell, and presence in animal and plant cells of all the organelles discussed in this section.
Quick Lab
Section 2 Eukaryotic Cells 121
Cell Membrane
All cells have a cell membrane made up of proteins and
lipids. The cell membrane is a protective barrier that encloses
a cell. It separates the cell’s contents from the cell’s environ-
ment. The cell membrane is the outermost structure in cells
that lack a cell wall. In cells that have a cell wall, the cell
membrane lies just inside the cell wall.
The cell membrane has two layers of phospholipids, shown
in Figure 2. A phospholipid is a type of lipid. Each phospholipid
has a hydrophobic, or “water fearing,” end and a hydrophilic, or
“water loving,” end. The “water fearing” ends are on the inside of
the cell membrane. The “water loving” ends form the outer part
of the membrane. This structure makes it difficult for materials
to pass through the membrane. Not allowing materials to pass
through is one way the cell membrane protects the cell.
Some materials, such as nutrients and wastes, must pass
through the cell membrane. These materials are able to pass
through passageways made of proteins. Nutrients move into
the cell—and wastes move out of the cell—through these pro-
tein passageways.
How does the cell membrane protect the cell? 7.1.a
Hydrophilic heads
Hydrophobic tails
Phospholipids
Cell membrane
Ribosomes
Figure 2 An Animal Cell
7.1.a Students know cells function similarly in all living organisms. 7.1.b Students know the characteristics that distinguish plant cells from animal cells, including chloroplasts and cell walls. 7.1.c Students know the nucleus is the repository for genetic information in plant and animal cells. 7.1.d Students know that mitochondria liberate energy for the work that cells do and that chloroplasts capture sunlight energy for photosynthesis.
Cell Diagrams
1. Draw an outline of a plant cell and an animal cell on separate pieces of paper. 2. As you read about eukary- otic cells, use colored pencils to add the correct cell parts to each cell. Label the cell parts. 3. Which cell parts are found in both plant cells and animal cells? 4. Which cell parts are found either in plant cells or in animals cells but not in both types of cells?
20 min
7.1.b 7.7.d
Mitochondrion
Golgi complex
Lysosome
Endoplasmic reticulum
Nucleus
Cytoplasm
Cytoskeleton
Section 2 Eukaryotic Cells 123
Ribosomes
Organelles that make proteins are called ribosomes.ribosomes. Ribo-
somes are the smallest organelles. And there are more ribosomes
than there are any other organelles in a cell. Some ribosomes
float freely in the cytoplasm. Others are attached to membranes
or the cytoskeleton. Unlike most organelles, ribosomes are not
covered by a membrane.
Ribosomes make proteins by assembling chains of amino
acids. An amino acid is any of about 20 different organic mol-
ecules that are used to make proteins. All cells need proteins
to live. Thus, all cells have ribosomes.
Endoplasmic Reticulum
Many chemical reactions take place in a cell. Many of these
reactions happen on or in the endoplasmic reticulum. The
endoplasmic reticulum,endoplasmic reticulum, or ER, is a system of folded membranes
in which proteins, lipids, and other materials are made. The
ER is shown in Figure 5.
The ER is part of the internal delivery system of the cell. Its
folded membrane contains many tubes and passageways. Sub-
stances move through the ER to different places in the cell.
The endoplasmic reticulum is either rough or smooth. The
part of the ER covered in ribosomes is rough ER. Rough ER is
usually found near the nucleus. Ribosomes on rough ER make
many of the cell’s proteins. The ER then delivers these proteins
throughout the cell. The ER that lacks ribosomes is smooth
ER. The functions of smooth ER include making lipids and
breaking down toxic materials that could damage the cell.
Rough ER
Smooth ER
Endoplasmic reticulum
Figure 5 The endoplasmic reticulum (ER) is a system of membranes. Rough ER is covered with ribosomes. Smooth ER does not have ribosomes.
Ribosomes
ribosomeribosome (RIE buh SOHM ) a cell organelle composed of RNA and pro- tein; the site of protein synthesis
endoplasmic reticulumendoplasmic reticulum ( EN doh PLAZ mik ri TIK yuh luhm) a system of membranes that is found in a cell’s cytoplasm and that assists in the production, processing, and transport of proteins and in the production of lipids
Smooth ER (^) Rough ER
124 Chapter 4 Cells: The Basic Units of Life
Mitochondria
A mitochondrionmitochondrion is the main power source of a cell. A
mitochondrion is the organelle in which sugar is broken
down to release energy. Mitochondria are covered by two
membranes, as shown in Figure 6. Energy released by mito-
chondria is stored in a substance called ATP ( a denosine
t ri p hosphate). The cell then uses ATP to do work. ATP
can be made at several places in a cell. But most of a
cell’s ATP is made on the inner membrane of the cell’s
mitochondria.
Most eukaryotic cells have mitochondria. Mitochondria
are the size of some bacteria. Like bacteria, mitochondria
have their own DNA, and mitochondria can divide within
a cell.
Why are mitochondria important for cells? 7.1.d
Chloroplasts
Animal cells cannot make their own food. Plant cells
are different. Some of them have chloroplasts.ChloroplastsChloroplasts
are organelles in which photosynthesis takes place. They
are found in plant, algae, and some prokaryotic cells.
Like mitochondria, chloroplasts have two membranes
and their own DNA. A chloroplast is shown in Figure 7.
Photosynthesis is the process by which cells, such as plant
cells, use sunlight, carbon dioxide, and water to make
sugar and oxygen.
Chloroplasts are green because they contain chloro-
phyll, a green pigment. Chlorophyll is found in an internal
membrane system within a chloroplast. Chlorophyll traps
the energy of sunlight. This energy is used to make sugar.
The sugar produced by photosynthesis is then used by
mitochondria to make ATP.
Figure 6 Mitochondria break down sugar and make ATP. ATP is produced on the inner membrane.
Figure 7 Chloroplasts harness and use the energy of the sun to make sugar. A green pigment—chlorophyll— captures the sun’s energy.
mitochondrionmitochondrion ( MIET oh KAHN dree uhn) in eukary- otic cells, the cell organelle that is surrounded by two membranes and that is the site of cellular respiration
chloroplastchloroplast (KLAWR uh PLAST ) an organelle found in plant and algae cells where photosynthesis occurs
Inner membrane
Outer membrane
Outer membrane
Inner membrane
Inner membrane Outer membrane
Inner membrane
Outer membrane
126 Chapter 4 Cells: The Basic Units of Life
Lysosomes
LysosomesLysosomes are vesicles found mainly in animal cells. Lyso-
somes contain digestive enzymes. They are responsible for diges-
tion inside a cell. Lysosomes destroy worn-out or damaged
organelles, get rid of waste materials, and engulf foreign invad-
ers. The foreign invaders are digested, and most of them are
no longer harmful to the cell.
When eukaryotic cells engulf particles, they enclose the par-
ticles in vesicles. Lysosomes, shown in blue in Figure 9, bump
into the vesicles, shown in purple, and pour enzymes into
them. These enzymes digest the particles in the vesicles.
Why are lysosomes important? 7.1.a
Vacuoles
A vacuole (VAK yoo OHL) is another type of vesicle found in
cells. In plant and fungal cells, some vacuoles act like lysosomes.
They store digestive enzymes and aid in digestion within the
cell. The large central vacuole in a plant cell stores water and
other liquids. Large central vacuoles that are full of water, such
as the one in Figure 9, help support the cell. Some plants wilt
when their large central vacuoles lose water. Some organelles
and their functions are shown in Table 1.
Figure 9 Lysosomes digest materials inside a cell. In plant cells, the large central vacuole stores water.
Lysosome
Large central vacuole
lysosomelysosome (LIE suh SOHM ) a cell organelle that contains digestive enzymes
Nucleus the organelle that contains the cell’s DNA
Chloroplast the organelle that uses sun- light, carbon dioxide, and water to make food
Ribosome the organelle upon which amino acids are hooked together to make proteins
Endoplasmic reticulum the organelle that makes lipids, breaks down toxic substances, and packages proteins for the Golgi complex
Mitochondrion the organelle that breaks down food molecules to make ATP
Lysosome the organelle that digests wastes, cell parts, and foreign invaders
Large central vacuole the organelle that stores water and other materials
Golgi complex the organelle that processes and transports materials within and out of the cell
Table 1 Organelles and Their Functions
Internet Resources
For a variety of links related to this
chapter, go to www.scilinks.org
Review
Topic: Eukaryotic Cells SciLinks code: HY
- Eukaryotic cells have organelles that perform functions that help cells remain alive.
- All cells have a cell membrane. Some cells have a cell wall. Some cells have a cytoskeleton.
- The nucleus of a eukaryotic cell contains the cell’s genetic material, DNA.
- Ribosomes are the organelles that make proteins. Ribosomes are not covered by a membrane.
- The endoplasmic reticulum (ER) and the Golgi complex make and process proteins before the proteins are transported to other parts of the cell or out of the cell.
- Mitochondria and chloroplasts are organelles that provide chemical energy for the cell.
- Lysosomes are organelles responsible for diges- tion within a cell. In plant cells, the large central vacuole stores cell materials and sometimes acts like a large lysosome.
- Plant cells have cell parts that are not found in animal cells. Plant cells have cell walls, chloro- plasts, and a large central vacuole.
1 Write an original definition for^ mitochondria,
nucleus, and cell wall.
2 Listing^ What are two functions of the cytoskel-
eton in animal cells?
3 Describing^ What is the function of the Golgi
complex? What is the function of the endo- plasmic reticulum?
4 Comparing^ Describe three ways in which
plant cells differ from animal cells.
5 Applying^ Every cell needs ribosomes.
Explain why.
INTERPRETING GRAPHICS Use the diagram
below to answer the next two questions.
b a
c
6 Identifying^ Is this a diagram of a plant cell or
an animal cell? Explain how you know.
7 Describing^ What is the function of the organ-
elle labeled “b”?
8 Predicting Consequences^ A certain virus
attacks the mitochondria in cells. What would happen to a cell if all of its mitochondria were destroyed?
9 Expressing Opinions^ Do you think that having
chloroplasts gives plant cells an advantage over animal cells? Support your opinion.
0 Making Calculations^ There are 11 foreign
invaders and 4 lysosomes in Cell A. If it takes each lysosome 1 h to digest 1 foreign invader, how long will it take to digest all of the foreign invaders?
q Making Inferences^ Amoebas are single-celled
eukaryotes. An amoeba moves by creating an extension of the cell. The cytoplasm from the rest of the cell flows into the extension. Given what you know about cell parts, determine which cell part inside of an amoeba is most likely used to make the extension.
Summary
7.1.a, 7.1.b, 7.1.c, 7.1.d
