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Chapter 6: A Tour of the Cell
Name_______________________Period___________
Concept 6.1 Biologists use microscopes and the tools of biochemistry to study cells
1. The study of cells has been limited by their small size, and so they were not seen and described
until 1665, when Robert Hooke first looked at dead cells from an oak tree. His contemporary,
Anton van Leeuwenhoek, crafted lenses and with the improvements in optical aids, a new world
was opened. Magnification and resolving power limit what can be seen. Explain the difference.
Magnification is the ratio of an object’s image size to its real size.
Resolution is a measure of the clarity of the image; it is the minimum distance two points can be
separated and still be distinguished as two points.
2. The development of electron microscopes has further opened our window on the cell and its
organelles. What is considered a major disadvantage of electron microscopes?
The methods used to prepare the specimen kill the cells.
3. Study the electron micrographs in your text. Describe the different types of images obtained from:
scanning electron microscopy (SEM): Answers may vary, but should describe the 3-D
component of the specimen image.
transmission electron microscopy (TEM)
Answers may vary, but should mention that this type of microscopy profiles a thin section of a
specimen, resulting in various views of the cells prepared.
4. In cell fractionation, whole cells are broken up in a blender, and this slurry is centrifuged several
times. Each time, smaller and smaller cell parts are isolated. This will isolate different organelles
and allow study of their biochemical activities. Which organelles are the smallest ones isolated in
this procedure?
Ribosomes
Concept 6.2 Eukaryotic cells have internal membranes that compartmentalize their functions
5. Which two domains consist of prokaryotic cells?
Bacteria and Archaea
6. A major difference between prokaryotic and eukaryotic cells is the location of their DNA.
Describe this difference.
In a eukaryotic cell, most of the DNA is in an organelle called the nucleus, which is bounded by a
double membrane. In a prokaryotic cell, the DNA is concentrated in a region that is not
membrane enclosed, called a nucleoid.
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7. On the sketch of a prokaryotic cell, label each of these features and give its function or
description.
See page 98 in your text for the labeled figure.
cell wall: rigid structure outside the plasma membrane
plasma membrane: membrane enclosing the cytoplasm
bacterial chromosome: carries genes in the form of DNA
nucleoid: region where the cell’s DNA is located (not enclosed by a membrane)
cytoplasm: interior of cell
flagella: locomotion organelles of some bacteria
8. Why are cells so small? Explain the relationship of surface area to volume.
Cells are small because a high surface-to-volume ratio facilitates the exchange of materials
between a cell and its environment. As a cell (or any other object) increases in size, its volume
grows proportionally more than its surface area. (Area is proportional to a linear dimension
cubed.) Thus, a smaller object has a greater ratio of surface area to volume.
9. What are microvilli? How do these structures relate to the function of intestinal cells?
Microvilli are long, thin projections from the cell surface, which increase surface area without an
appreciable increase in volume. A sufficiently high ratio of surface area to volume is especially
important in cells that exchange a lot of materials with their surroundings, such as intestinal cells.
Concept 6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by
the ribosomes
10. In the following figure, label the nuclear envelope, nuclear pores, and pore complex.
See page 103 of your text for the labeled figure.
11. Describe the nuclear envelope. How many layers is it? What connects the layers?
The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm. The
nuclear envelope is a double membrane, meaning that there are two lipid bilayers. The nuclear
lamina, a netlike array of protein filaments, connects the layers of the nuclear envelope.
12. What is the nuclear lamina? Nuclear matrix?
The nuclear lamina is the netlike array of protein filaments that maintains the shape of the
nucleus by mechanically supporting the nuclear envelope.
The nuclear matrix is a framework of protein fibers extending throughout the nuclear interior.
The nuclear matrix and nuclear lamina may help organize the genetic material so it functions
efficiently.
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13. Found within the nucleus are the chromosomes. They are made of chromatin. What are the two
components of chromatin? When do the thin chromatin fibers condense to become distinct
chromosomes?
Chromatin is composed of proteins and DNA. Chromatin fibers condense to become distinct
chromosomes as a cell prepares to divide.
14. When are the nucleoli visible? What are assembled here?
Nucleoli are visible in a nondividing nucleus and in cells active in protein synthesis.
Within the nucleoli, proteins imported from the cytoplasm are assembled with rRNA into large
and small subunits of ribosomes.
15. What is the function of ribosomes? What are their two components?
Ribosomes are the cellular components that carry out protein synthesis. Their two components
are a large subunit and a small subunit.
16. Ribosomes in any type of organism are all the same, but we distinguish between two types of
ribosomes based on where they are found and the destination of the protein product made.
Complete this chart to demonstrate this concept.
Type of Ribosome Location Product
Free ribosomes Suspended in the cytosol Proteins that function within the
cytosol
Bound ribosomes Attached to the outside of the Proteins for insertion into
endoplasmic reticulum or membranes
nuclear envelope
Concept 6.4 The endomembrane system regulates protein traffic and performs metabolic functions
in the cell
17. List all the structures of the endomembrane system.
Nuclear envelope
Endoplasmic reticulum
Golgi apparatus
Lysosomes
Vesicles
Vacuoles
Plasma membrane
18. The endoplasmic reticulum (ER) makes up more than half the total membrane system in many
eukaryotic cells. Use this sketch to explain the lumen, transport vesicles, and the difference
between smooth and rough ER.
See page 104 of your text for the labeled figure.
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The ER lumen is the cavity, or cisternal space. Because the ER membrane is continuous within the
nuclear envelope, the space between the two membranes of the envelope is continuous with the lumen
of the ER.
Transport vesicles bud off from a region of the rough ER called transitional ER and travel to the Golgi
apparatus and other destinations.
Smooth ER is so named because its outer surface lacks ribosomes.
Rough ER is studded with ribosomes on the outer surface of the membrane and thus appears rough
through the electron microscope.
19. List and describe three major functions of the smooth ER.
1. Synthesis of lipids: Enzymes of the smooth ER are important in the synthesis of lipids, including
oils, phospholipids, and steroids.
2. Detoxification of drugs and poisons: Detoxification usually involves adding hydroxyl groups to
drug molecules, making them more soluble and easier to flush from the body.
3. Storage of calcium ions: In muscle cells, the smooth ER membrane pumps calcium ions from the
cytosol into the ER lumen.
20. Why does alcohol abuse increase tolerance to other drugs such as barbiturates?
Barbiturates, alcohol, and many other drugs induce the proliferation of smooth ER and its
associated detoxification enzymes, thus increasing the rate of detoxification. This, in turn,
increases the tolerance to drugs, meaning that higher doses are required to achieve a particular
effect, such as sedation.
21. The rough ER is studded with ribosomes. As proteins are synthesized, they are threaded into the
lumen of the rough ER. Some of these proteins have carbohydrates attached to them in the ER to
form glycoproteins. What does the ER then do with these secretory proteins?
After secretory proteins are formed, the ER membrane keeps them separate from proteins that
are produced by free ribosomes and that will remain in the cytosol. Secretory proteins depart
from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized
region called transitional ER.
22. Besides packaging secretory proteins into transport vesicles, what is another major function of
the rough ER?
The rough ER grows membrane proteins and phospholipids for the cell by adding them to its
own membrane. The ER membrane expands, and portions of it are transferred in the form of
transport vesicles to other components of the endomembrane system.
23. The transport vesicles formed from the rough ER fuse with the Golgi apparatus.
Use this sketch to label the cisternae of the Golgi apparatus, and its cis and trans faces.
Describe what happens to a transport vesicle and its contents when it arrives at the Golgi
apparatus.
See page 106 of your text for the labeled figure.
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24. What is a lysosome? What do they contain? What is the pH range inside a lysosome?
A lysosome is a membranous sac of hydrolytic enzymes that an animal cell uses to digest
(hydrolyze) macromolecules. The pH range inside a lysosome is acidic.
25. One function of lysosomes is intracellular digestion of particles engulfed by phagocytosis.
Describe this process of digestion. What human cells carry out phagocytosis?
Amoebas and many other protists eat by engulfing smaller organisms or food particles, a process
called phagocytosis. The food vacuole formed in this way then fuses with a lysosome, whose
enzymes digest the food. Digestion products, including simple sugars, amino acids, and other
monomers, pass into the cytosol and become nutrients for the cell.
Some of the human cells that carry out phagocytosis are macrophages, a type of white blood cell
that helps defend the body by engulfing and destroying bacteria and other invaders.
26. A second function of lysosomes is to recycle cellular components in a process called
autophagy. Describe this process.
During autophagy, a damaged organelle or small amount of cytosol becomes surrounded by a
double membrane, and a lysosome fuses with the outer membrane of this vesicle. The lysosomal
enzymes dismantle the enclosed material, and the organic monomers are returned to the cytosol
for reuse. With the help of the lysosomes, the cell community renews itself. A human liver cell,
for example, recycles half of its macromolecules each week.
27. What happens in Tay-Sachs disease? Explain the role of the lysosomes in Tay-Sachs.
In Tay-Sachs disease, a lipid-digesting enzyme is missing or inactive, and the brain becomes
impaired by an accumulation of lipids in the cells. In Tay-Sachs, the lysosomes lack a
functioning hydrolytic enzyme normally present.
28. There are many types of vacuoles. Briefly describe:
food vacuoles: Food vacuoles are formed by phagocytosis.
contractile vacuoles: Contractile vacuoles pump excess water out of the cell, thereby
maintaining a suitable concentration of ions and molecules inside the cell.
central vacuoles in plants: Central vacuoles in plants develop by the coalescence of smaller
vacuoles, contained in mature plant cells. Solution inside the central vacuole, called cell sap, is
the plant cell’s main repository of inorganic ions, including potassium and chloride. The central
vacuole plays a major role in the growth of plant cells, which enlarge as the vacuole absorbs
water, enabling the cell to become larger with a minimal investment in new cytoplasm.
(give at least three functions/materials stored here)
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29. Use this figure to explain how the elements of the endomembrane system function together to
secrete a protein and to digest a cellular component. Label as you explain.
See page 108 in your text for the labeled figure.
Nuclear envelope is connected to rough ER, which is also continuous with smooth ER.
Membranes and proteins produced by the ER flow in the form of transport vesicles to the Golgi
apparatus. Golgi apparatus pinches off transport vesicles and other vesicles that give rise to
lysosomes, other types of specialized vesicles, and vacuoles. Lysosome is available for fusion
with another vesicle for digestion. Transport vesicle carries proteins to plasma membrane for
secretion. Plasma membrane expands by fusion of vesicles; proteins are secreted from cell.
Concept 6.5 Mitochondria and chloroplasts change energy from one form to another
30. What is an endosymbiont?
An endosymbiont is a cell living within another cell.
31. What is the endosymbiont theory? Summarize three lines of evidence that support the model of
endosymbiosis.
The endosymbiont theory states that an early ancestor of eukaryotic cells engulfed an oxygenusing
nonphotosynthetic prokaryotic cell, and over the course of evolution, the host cell and its
endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion. At least
one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of
eukaryotic cells that contain chloroplasts.
Three lines of evidence that support the model of endosymbiosis:
1. Rather than being bound by a single membrane, like organelles of the endomembrane
system, mitochondria and typical chloroplasts have two membranes surrounding them.
2. Like prokaryotes, mitochondria and chloroplasts contain ribosomes, as well as circular DNA
molecules attached to their inner membranes.
3. Also consistent with their probable evolutionary origins as cells, mitochondria and
chloroplasts are autonomous organelles that grow and reproduce within cells.
32. Mitochondria and chloroplasts are not considered part of the endomembrane system, although
they are enclosed by membranes. Sketch a mitochondrion here and label its outer membrane,
inner membrane, inner membrane space, cristae, matrix, and ribosomes.
See page 110 of your text for the labeled figure.
33. Now sketch a chloroplast and label its outer membrane, inner membrane, inner membrane space,
thylakoids, granum, and stroma. Notice that the mitochondrion has two membrane
compartments, while the chloroplast has three compartments.
See page 111 of your text for the labeled figure.
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34. What is the function of the mitochondria?
Mitochondria are the sites of cellular respiration, the metabolic process that uses oxygen to
generate ATP by extracting energy from sugars, fats, and other fuels.
35. What is the function of the chloroplasts?
Chloroplasts are sites of photosynthesis. These organelles convert solar energy to chemical
energy by absorbing sunlight and using it to drive synthesis of organic compounds such as sugars
from carbon dioxide and water.
36. Recall the relationship of structure to function. Why is the inner membrane of the mitochondria
highly folded? What role do all the individual thylakoid membranes serve? (Notice that you
will have the same answer for both questions.)
As highly folded surfaces, the cristae give the inner mitochondrial membrane a large surface area,
thus enhancing the productivity of cellular respiration. As in mitochondria, thylakoid membranes
serve to increase the surface area and thus the function of the chloroplasts.
37. Explain the important role played by peroxisomes.
Peroxisomes contain enzymes that remove hydrogen atoms from various substrates and transfer
them to oxygen, thus producing hydrogen peroxide as a by-product.
SUMMARY
On these diagrams of plant and animal cells, label each organelle and give a brief statement of its
function.
See pages 100–101 of your text for the labeled figures and a brief statement of each organelle’s
function.
Concept 6.6 The cytoskeleton is a network of fibers that organizes structures and activities in the cell
38. What is the cytoskeleton?
The cytoskeleton is a network of fibers extending throughout the cytoplasm.
39. What are the three roles of the cytoskeleton?
1. Maintenance of cell shape
2. Mechanical support
3. Cell motility (movement) both of the cell as a whole and more limited movement of parts of the
cell
40. There are three main types of fibers that make up the cytoskeleton. Name them.
Microtubules, Microfilaments, Intermediate Filaments
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41. Microtubules are hollow rods made of a globular protein called tubulin. Each tubulin protein is a
dimer made of two subunits. These are easily assembled and disassembled. What are four
functions of microtubules?
1. Maintenance of cell shape
2. Cell motility
3. Chromosome movement in cell division
4. Organelle movement
42. Animal cells have a centrosome that contains a pair of centrioles. Plant cells do not have
centrioles. What is another name for centrosomes? What is believed to be the role of centrioles?
Another name for centrosome is “microtubule-organizing center.” The centrioles function as
compression-resisting girders of the cytoskeleton.
43. Describe the organization of microtubules in a centriole. Make a sketch here that shows this
arrangement in cross section.
See page 114 of your text for the labeled figure.
The two centrioles are at right angles to each other, and each is made up of nine sets of three
microtubules.
44. Cilia and flagella are also composed of microtubules. The arrangement of microtubules is said to
be “9 + 2.” Make a cross-sectional sketch of a cilium here. (See Figure 6.24b in your text.)
See page 115 of your text for the labeled figure.
45. Compare and contrast cilia and flagella.
Cilia and flagella are both microtubule-containing extensions that project from some cells. Cilia
and flagella share a common structure, each having a group of microtubules sheathed in an
extension of the plasma membrane. Flagella and cilia differ in their beating patterns. A flagellum
has an undulating motion that generates force in the same direction as the flagellum’s axis, like the
tail of a fish. In contrast, cilia work more like oars, with alternating power and recovery strokes
generating force in a direction perpendicular to the cilium’s axis.
46. How do motor proteins called dyneins cause movement of cilia? What is the role of ATP in this
movement? This figure might help you explain.
See page 116 of your text for the labeled figure.
Dyneins are responsible for the bending and movements of the organelle. A dynein molecule
performs a complex cycle of movements caused by changes in the shape of the protein, with ATP
providing the energy for these changes.
47. Microfilaments are solid, and they are built from a double chain of actin. Study Figure 6.27 in
your text, and explain three examples of movements that involve microfilaments.
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1. Myosin motors in muscle cell contraction: The “walking” of myosin projections (the so-called
heads) drives the parallel myosin and actin filaments past each other so that the actin filaments
approach each other in the middle. This shortens the muscle cell. Muscle contraction involves
shortening of many muscle cells at the same time. See also Figure 6.27a on page 117.
2. Amoeboid movement: Interaction of actin filaments with myosin causes contraction of the cell,
pulling the cell’s trailing end forward. See also Figure 6.27b on page 117.
3. Cytoplasmic streaming in plant cells: A layer of cytoplasm cycles around the cell, moving over
a carpet of parallel actin filaments. Myosin motors attached to organelles in the fluid cytosol may
drive the streaming by interacting with the actin. See also Figure 6.27c on page 117.
48. What are the motor proteins that move the microfilaments? Myosin
49. Intermediate filaments are bigger than microfilaments but smaller than microtubules. They are
more permanent fixtures of cells. Give two functions of intermediate filaments.
Possible answers include:
1. Maintenance of cell shape (tension-bearing elements)
2. Anchorage of nucleus and certain other organelles
3. Formation of nuclear lamina
Concept 6.7 Extracellular components and connections between cells help coordinate cellular
activities
50. What are three functions of the cell wall?
1. Protects the plant cell
2. Maintains its shape
3. Prevents excessive uptake of water
51. What is the composition of the cell wall?
Microfibrils made of the polysaccharide cellulose are synthesized by an enzyme called cellulose
synthase and secreted to the extracellular space, where they become embedded in a matrix of
other polysaccharides and proteins.
52. What is the relatively thin and flexible wall secreted first by a plant cell?
Primary cell wall
53. What is the middle lamella? Where is it found? What material is it made of?
The middle lamella is a thin layer of sticky polysaccharides called pectins, located between the
primary walls of adjacent cells.
54. Explain the deposition of a secondary cell wall.
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The secondary wall, often deposited in several laminated layers, has a strong and durable matrix
that affords the cell protection and support.
55. On this sketch, label the primary cell wall, secondary cell wall, middle lamella, cytosol,
plasma membrane, central vacuole, and plasmodesmata.
See page 119 of your text for the labeled figure.
56. Animal cells do not have cell walls, but they do have an extracellular matrix (ECM). On this
figure, label the elements indicated, and give the role of each.
See page 120 of your text for the labeled figure.
57. What are the intercellular junctions between plant cells? What can pass through them?
Plasmodesmata are the intercellular junctions between plant cells. Cytosol passes through the
plasmodesmata and joins the internal chemical environments of adjacent cells.
58. Animals cells do not have plasmodesmata. This figure shows the three types of intercellular
junctions seen in animal cells. Label each type and summarize its role.
See page 121 of your text for the labeled figure.
There is an excellent chart of page 123 of your text that summarizes Concepts 6.3–6.5. Be sure
study it, and answer the three questions there.
Testing Your Understanding Answers
Now you should be ready to test your knowledge. Place your answers here:
1. b 2. d 3. b 4. e 5. a 6. d 7. c
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