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Chapter 2
Cells
CHAPTER OVERVIEW
Chapter 2 explores the organizational hierarchy of eukaryotic cells and
investigates the relationship between cellular function and genetic
abnormalities in humans. Cytoplasmic organelles are unique to
eukaryotic cells and function as compartments that sequester enzymes
involved in related biochemical reactions (division of labor) and allow
unique microenvironments to be established. At the molecular level,
organelles are formed by the aggregation of macromolecules such as
proteins, lipids, carbohydrates, and nucleic acids. The interaction
between the cell membrane and the cytoskeleton determines cellular
architecture. Defects in cytoskeletal components can result in a number
of severe disorders. Embryonic development of multicellular organisms
requires cell growth and division (mitosis), as well as cell death
(apoptosis). Stem cells have a crucial role in the formation of
specialized (differentiated) cells during embryonic development and in
the repair and replacement of tissues to maintain health of the
organism. Stem cells persist in many adult tissues and have the
potential to replace injured or diseased tissue. Researchers are
investigating uses of stem cells to replace or rejuvenate injured or
diseased tissue.
.
CHAPTER OUTLINE
2.1 Introducing Cells
1. Understanding how the trillions of cells in the human body function and interact
reveals how they contribute to health and disease.
2. Cells comprising the body are termed somatic cells.
3. Somatic cells are diploid and contain two copies of the genome.
4. Gametes, sperm and egg cells, are haploid.
5. Stem cells, which are diploid, can reproduce themselves and differentiate for repair
and replacement of specialized cells.
2.2 Cell Components
1. All cells must maintain basic functions for growth and reproduction, responding to the
environment, and utilizing energy.
2. Specialized cells in humans and other multicellular organisms perform additional
functions related to their role within tissues or organs.
3. There are three broad varieties of cells  Eubacteria (the more common forms of
bacteria), Archaea (the extreme bacteria), and Eukaryotes (higher cells)  based on
cellular complexity.
4. The Archaea and Eubacteria are similar in that they are single-celled organisms, but
they differ in certain features of their RNA and membranes. These cells lack nuclei
and other organelles and therefore are categorized as prokaryotes.
5. Eukaryotic cells are complex, with abundant and diverse organelles that
compartmentalize biochemical reactions. Human cells are therefore eukaryotic.
Chemical Constituents
1. Cells are constructed from numerous small molecules and macromolecules.
2. The building blocks of cells include carbohydrates (simple sugars and
polysaccharides), amino acids and proteins, lipids (fats and oils), and nucleic acids
(nucleotides, DNA, RNA).
3. Enzymes are proteins that catalyze the multitude of biochemical reactions that occur
in the cell.
4. The study of nucleic acids is key to the field of genetics.
Organelles
1. Organelles represent the compartments (and unique microenvironments) in the cell
and are involved in a variety of functions (division of labor).
2. The nucleus (the storehouse of the majority of DNA in the cell) has a double
membrane and nuclear pores, which allow macromolecular traffic in and out of the
nucleus.
3. The outer boundary of the cell is termed the plasma membrane.
4. The cytoplasm is the term for the portion of the cell outside the nuclear membranes
and inside the plasma membrane. The cytoplasm contains numerous other
organelles, macromolecules and small molecules.
5. The rough endoplasmic reticulum (ER), smooth ER, and Golgi apparatus function as
a membrane network for the synthesis of proteins and lipids that are targeted for
delivery to the plasma membrane, organelles, or for secretion.
6. Protein secretions bud off the ER in vesicles and travel to the Golgi apparatus for
further processing. Lipids are exported from the ER without a vesicle.
7. Lysosomes contain enzymes that degrade cellular debris. This is termed autophagy.
8. Peroxisomes house enzymes that detoxify certain substances, break down lipids,
and synthesize bile acids.
9. A mitochondrion has a double membrane whose inner folds carry enzymes that
catalyze reactions that extract energy from nutrients.
The Plasma Membrane
1. The plasma membrane surrounds the cell and regulates which molecules enter and
leave.
2. Biological membranes are composed of phospholipids which have hydrophobic and
hydrophylic portions. Because of this, phospholipids spontaneously form bilayers.
3. Phospholipid bilayers are the primary structural components of biologic membranes.
4. Proteins, glycoproteins and glycolipids residing in the cell membrane function as
enzymes, signal transduction receptors, transport proteins, and cell adhesion
proteins.
The Cytoskeleton
1. The cytoskeleton gives a cell its specific architecture.
2. The major cytoskeleton components include microtubules (tubulin), microfilaments
(actin), and intermediate filaments (a family of closely related proteins).
3. Microtubules also serve at the foundation of cilia and flagella.
4. Spherocytosis is a heredity defect in the cytoskeleton of the red blood cell. An
abnormal cytoskeletal protein, ankyrin, lies beneath the cell membrane and causes
the red blood cells to balloon out, blocking narrow blood vessels in organs.
2.3 Cell Division and Death
1. Mitosis, the process by which chromosomes duplicate and divide, cytokinesis, the
process by which the cell divides, and apoptosis, programmed cell death, work
together to maintain health and balance within the body and enable it to grow or
repair itself.
The Cell Cycle
1. The cell cycle consists of interphase, when a cell is not dividing, and mitosis.
2. During mitosis chromosomes are duplicated.
3. The chromosome sets move to daughter cells which separate during a process
termed cytokinesis.
4. This duplication and division maintains the proper ratio between a cell and its
chromosomes.
5. Gametes with one set of chromosomes are produced through the process of meiosis
and cytokinesis.
6. Interphase is an active stage within a cell. During interphase, proteins, lipids, and
carbohydrates are produced in the G1 phase; DNA and proteins are made during S
phase; and more proteins are produced in G2.
7. Replicated chromosomes have two sister chromatids attached at their centromeres.
8. Non-dividing cells may become arrested during interphase and enter a quiescent
phase (G0).
9. In mitotic prophase, replicated chromosomes condense, a spindle forms, and the
nuclear membrane breaks down.
10. In metaphase, chromosomes align down the center of the cell (equator or
metaphase plate).
11. In anaphase, centromeres part, one chromatid from each pair is pulled to opposite
ends of the cell.
12. In telophase, the cell pinches in the middle (cytokinesis), and the two new cells
separate.
13. The cell cycle is tightly controlled and regulated at several “checkpoints.”
14. A cellular clock that limits the number of divisions is based on shrinking telomeres.
15. Crowding, hormones, and growth factors are extracellular influences on mitosis.
16. Within cells, kinases and cyclins activate the genes whose products carry out
mitosis.
Apoptosis
1. Mitosis (cell division) and apoptosis (cell death) are continuous processes that are
both initiated by signals in the extracellular environment.
2. The balance between cell division and death maintains tissues in growth,
development, and repair.
3. In prenatal development, coordination of these processes sculpts body form. After
birth, mitosis and apoptosis protect and maintain the body.
4. Disruption of the balance between cell division and cell death can lead to cancer or
other disorders.
2.4 Cell-Cell Interactions
Signal Transduction
1. In signal transduction, receptors get information from extracellular first messengers
and trigger the release of second messengers inside the cell.
2. The second messengers cause a reaction within the cell.
3. Neurofibromatosis type I (NF1) is caused by faulty signal transduction. Nerve cells
beneath the skin inappropriately transmit a growth factor signal, triggering cell
division and a tumor forms.
Cellular Adhesion
1. Cell adhesion is a precise sequence of interactions between cell surface proteins
that join cells.
2. In inflammation, cell adhesion molecules (CAMs) guide white blood cells to injury
sites.
3. Leukocyte-adhesion deficiency and cancer are disorders that can result from
abnormal cell adhesion.
2.5 Stem Cells
Cell Lineages
1. Stem cells are unique non-specialized cells that retain the potential to differentiate
and enable a tissue to grow or repair itself.
2. A fertilized egg is totipotent, capable of producing any cell type.
3. Later in development, pluripotent stem cells give rise to progenitor cells that are
committed to a particular pathway.
Stem cells persist in many adult tissues and have the potential to replace injured or
diseased tissue.
4. Researchers are investigating uses of stem cells to replace or rejuvenate injured or
diseased tissue
IDEAS FOR CLASSROOM DISCUSSION
1. Have students research the “Elephant Man” disease. Use the historical figure, John
Merrick, to stimulate discussion about signal transduction and genetic disease.
Neurofibromatosis type 1, NF1, (MIM Number 162200) is a rare genetic disorder that
is inherited as an autosomal dominant trait. NF1 is often incorrectly referred to as
the “the elephant man disease.” John Merrick (the Elephant Man), the famous patient
of Sir Frederick Treves, did not have NF1 as commonly believed, but was afflicted by
the rare genetic disorder known as Proteus Syndrome (MIM Number 176920) which
was not discovered until a century after his death. Proteus Syndrome is a condition
involving atypical growth of the bones, skin, head and a variety of other symptoms.
An analysis of John Merrick’s skeleton at the Royal London Hospital supports the
diagnosis. More information is available through the Proteus Syndrome Foundation
<http://www.proteus-syndrome.org.uk/>.
2. Have students make a graphic organizer of the concepts presented in this chapter.
Have them present their graphic organizers to the class in a classroom discussion.
Ask them what they understand and what they need to know more about. Ask them
to describe what they can do when they need to learn more about a topic.
INTERNET RESOURCES AND ACTIVITIES
1. Online Mendelian Inheritance in Man (MIM), Johns Hopkins University, Baltimore,
MD. MIM is a searchable database and a source for information on the various
genetic disorders discussed in the text (i.e., familial hypercholesterolemia, LeschNyhan Syndrome, and Maple syrup urine diseases). This database contains
information about the family histories, clinical descriptions, pattern of inheritance, and
molecular information about many diseases and traits in humans. The MIM access
numbers are listed in the text when a genetic disease or condition is introduced.
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM>
2. National Organization for Rare Disorders, Inc. (NORD). New Fairfield, CT. NORD is
a non-profit organization and is not government funded. The mission of NORD is to
educate the world about rare disorders, such as those discussed in chapter 2. The
NORD website has three different databases that allow users to obtain information
about rare diseases, foundations and organizations that support groups and assist
affected individuals, and an orphan drug database with over 900 listings provided by
the FDA. < http://www.rarediseases.org>
3. “Cell Biology Problem Sets and Tutorials.” 2004. The Biology Project, University of
Arizona. This site contains problem sets and tutorials for exploring many aspects of
cell biology. Problem sets and tutorials are available for studying cells as the
fundamental unit of life, cell cycle and mitosis, meiosis, and the differences between
cells and viruses. <http://www.biology.arizona.edu>
ANSWERS TO REVIEW QUESTIONS
1. a.
b.
c.
d.
e.
f.
g.
4
6
2
1
7
3
5
2. Compartmentalization separates biochemicals that could harm certain cell
constituents. It also organizes the cell so it can function more efficiently.
3. a. Tubulin forms microtubules and actin forms microfilaments, which comprise the
cytoskeleton.
b. Caspases carry out apoptosis.
c. Changing levels of cyclins and kinases regulate the cell cycle.
d. Checkpoint proteins provide choices during the cell cycle.
e. Cellular adhesion molecules allow certain cell types to stick to each other.
4. Hormones, growth factors, cyclins and kinases
5. Specialized cells express different subsets of all the genes that are present in all cell
types, except for red blood cells.
6. a. A bacterial cell is usually small and lacks a nucleus and other organelles. A
eukaryotic cell contains membrane-bound organelles, including a nucleus, that
compartmentalize biochemical reactions.
b. During interphase, cellular components are replicated. During mitosis, the cell
divides, distributing its contents into two daughter cells.
c. Mitosis increases cell number. Apoptosis eliminates cells.
d. Rough ER is a labyrinth of membranous tubules, studded with ribosomes that
synthesize protein. Smooth ER is the site of lipid synthesis.
e. Microtubules are tubules of tubulin and microfilaments are rods of actin. Both
form the cytoskeleton.
f. A stem cell has greater developmental potential than a progenitor cell.
g. A totipotent cell can differentiate as any cell type; a pluripotent cell’s fates are
more restricted.
7. Intermediate filaments are similar to microtubules and microfilaments in that they are
all composed of protein subunits. They differ in size and protein composition.
Microfilaments are the smallest and composed mainly of actin. Microtubules are the
largest, and largely tubulin. Intermediate filaments are intermediate in size and there
are several types, each assembled from distinct proteins.
8. The plasma membrane is the scaffold that holds many of the molecules that
intercept incoming signals and consort in ways that amplify and/or spread the
message.
9. Embryonic stem (ES) cells are pluripotent cells from the inner cell mass of a very
early stage embryo. Induced pluripotent stem (iPS) cells are pluripotent cells created
from non-pluripotent somatic cells through genetic manipulation. Adult stem cells are
found throughout the body from embryonic development onward. They can
reproduce themselves and differentiate for repair and replacement of specialized
cells. Pros and Cons: ES cells are naturally pluripotent and self-renewing. However,
use of human embryonic stem cells is more controversial than that of iPS cells or
adult stem cells and countries vary in their policies regarding such research. The full
potential of iPS and adult stem cells is not yet known. Adult stem cells are
multipotent rather than pluripotent. Potential risks involve rejection of implanted
somatic cells and induction of cancer.
ANSWERS TO APPLIED QUESTIONS
1. a.
b.
c.
d.
e.
Lack of cell adhesion can speed the migration of cancer cells.
Impaired signal transduction can block a message to cease dividing.
Blocking apoptosis can cause excess mitosis, and an abnormal growth.
Lack of cell cycle control can lead to too many mitoses.
If telomerase is abnormal, a cell might not cease to divide when it normally
would.
2. Stem cells maintain their populations because mitosis produces a daughter cell that
differentiates, as well as one that remains a stem cell
3. A cell in an embryo would not be in G0 because it has to divide frequently to support
the tremendous growth rate.
4. Mitochondria
5. Peroxisome
6. A sodium channel regulates the movement of sodium into and out of cells. It is found
in the plasma membrane.
7. Signals from outside the cell interact with receptors embedded in the cell membrane,
and the cell membrane’s interior face contacts the cytoskeleton.
8.
Treatment of severe burns or baldness
9. Answers vary depending on disease and treatment selected
10. a. No
b. Research showing that topical delivery induces intracellular change within the
skin
WEB ACTIVITIES
1. Answers vary with website selected
2. a. Belgium
b. Norway
c. Austria
3. Ciliopathy
CASE STUDIES AND RESEARCH RESULTS
1. Nucleus and lysosomes.
2. A study of male air traffic controllers
FORENSICS FOCUS
1. DNA profiling
ADDITIONAL QUESTIONS
1. Severe childhood autosomal recessive muscular dystrophy, prevalent in North Africa,
affects both sexes and is caused by a deficiency of a dystrophin-associated
glycoprotein called adhalin. Explain how mutations in two different genes  those for
dystrophin and adhalin  can cause the same symptoms of muscle wasting.
2. Why wouldn’t you expect to see adults with mutations in the telomerase gene that
affect phenotype?
3. A mouse has the gene encoding a particular enzyme inactivated or "knocked out"
and is a model of the human genetic disease Zellweger syndrome. Which organelle
do cells of these mice lack?
4. Achondroplasia is the most common form of inherited dwarfism. The causative gene
encodes a protein receptor on surfaces of bone and cartilage cells that normally
binds a growth factor. Explain how an abnormal form of this gene might cause
dwarfism.
5. In the novel The Cobra Event, by Richard Preston, a genetically engineered deadly
virus is used as a weapon. The virus inserts into the human genome and disrupts
production of an enzyme, causing an inborn error of metabolism leading to self
mutilation by uncontrollable biting of the lips, fingers, and shoulders. Which genetic
disease mentioned in the chapter does this sound like?
6. Match the disorder to the organelle, biochemical, or structure that is abnormal.
a. Tay Sachs disease
1. peroxisome
b. muscle weakness
2. nucleic acid recycling
c. Zellweger syndrome
3. mitochondria
d. cystic fibrosis
4. lysosome
e. familial hypercholesterolemia
5. cell membrane
f. Lesch-Nyhan syndrome
6. cell surface receptors
7. How does Duchenne Muscular Dystrophy (DMD) demonstrate that the cytoskeleton,
cell membrane, and extracellular matrix interact?
8. Describe a disorder caused by:
a. faulty cell adhesion
b. a deficient lysosomal enzyme
c. an abnormal or missing peroxisomal protein
d. a cytoskeletal abnormality
ANSWERS TO ADDITIONAL QUESTIONS
1. The two proteins function together to maintain muscle cell integrity, so when either is
abnormal, the result can be muscle weakness.
2. An individual without telomerase would never develop specialized tissue, and would
not survive very far into prenatal development.
3. Peroxisomes
4. Bone and cartilage cells in people with achondroplasia do not receive sufficient
signals to divide and, therefore, do not produce enough cells to build bones to
normal size.
5. Lesch-Nyhan disease
6. a. 4
b. 3
c. 1
d. 5
e. 6
f. 2
7. Dystrophin touches parts of the cytoskeleton and ECM and is part of the cell
membrane.
8. a. Wounds cannot heal
b. Tay Sachs disease, in which lipid accumulates in nerve cells.
c. Adrenoleukodystrophy, in which very-long chain fatty acids accumulate in the
brain and spinal cord.
d. Spherocytosis, in which red blood cells balloon out