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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.
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CHAPTER OUTLINE
2.1 Introducing Cells
1. Cells comprising the body are termed somatic cells. They are diploid and contain two
copies of the genome. Gametes, sperm and egg cells, are haploid. Stem cells, which
are diploid, can reproduce themselves and differentiate for repair and replacement of
specialized cells.
2.1 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 rough endoplasmic reticulum (ER), smooth ER, and Golgi body 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. Secretions bud off in
vesicles and leave the cell.
4. Lysosomes contain enzymes that degrade cellular debris.
5. Peroxisomes house enzymes that detoxify certain substances, break down lipids,
and synthesize bile acids.
6. 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 a bilayer of phospholipids.
3. 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
The Cell Cycle
1. The cell cycle consists of interphase, when a cell is not dividing, and mitosis.
2. 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.
Replicated chromosomes have two sister chromatids attached at their centromeres.
3.
4.
5.
6.
7.
Non-dividing cells may become arrested during interphase and enter a quiescent
phase (G0).
In mitotic prophase, replicated chromosomes condense, a spindle forms, and the
nuclear membrane breaks down. In metaphase, chromosomes align down the center
of the cell (equator or metaphase plate). In anaphase, centromeres part, one
chromatid from each pair is pulled to opposite ends of the cell. In telophase, the cell
pinches in the middle (cytokinesis), and the two new cells separate.
The cell cycle is tightly controlled and regulated at several “checkpoints.”
A cellular clock that limits the number of divisions is based on shrinking telomeres.
Crowding, hormones, and growth factors are extracellular influences on mitosis.
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.
4. Stem cells persist in many adult tissues and have the potential to replace injured or
diseased tissue.
Stem Cells in Health Care
1. 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.
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>
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