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Chapler6: A TourofllieCell
36
37
TheCell
I funPAMBNTAL UNITOFLIFE j
'perforais
that serve a
STRUCTURES
ribosomes
organeilcs
protein production
•for
membrane-bound
-such—I nucleus |
—includes—
cell surfaces
tight junctions(animals)
Cliapter Review
The cell is the basic sbfuchiial and functional unit of all
livingoiganisms.In the hieraichyof biological oiganization, the capacity for life emerges from the structural
order of the cell. All cells are related through common
descent,but evolutionhas shaped diverseadaptations.
6.1 To study cells, biologists use
microscopes and the tools
of biochemistry
Microscopy The glass lenses of light microscopes
(LMs)refract (bend) the visiblelight passing through a
specimensuch that the projected image is magnified.
Magnificatiou is the ratio of this projected image to the
realsize of the object.Resolution is a measureof how clear
an imageis,and isdeterminedby the minimiundistance
tv^o points must be separated to be distinguished. The
resolutionof the lightmicroscope is limitedby the short
est wavelengthof visiblelight,so that detailsfinerthan
0.2 pm (micrometers = 10"' mm) cannot be resolved.
Staining of specimens and using techniques such as
fluorescence, phase-contrast, and confocal microscopy
improve visibilityby increasingcontrast between struc
tures that are largeenough to be resolved.
Mostsubcellularstructures,or oiganelles, cannot be
revived by the lightmicroscope. Cellswerediscovered
by Robert Hookc in 1665, but their ultrastructure was
laigely unknown until the developmentof the electron
microscope (EM) in the 195Qs. The electron microscope
proteins.
6.3 The eukaryotic cell's genetic
instructions are housed in the nucleus
spun slowly, and dense structures settle to form a
and carried out by the ribosomes
pellet. The supernatant is centrifuged at increasing
speeds, each time isolatingsmaller and smaller cellular
components in the pellet. Ultracentrifuges can spin at
speeds up to 130,000 rpm.
TheNucleus:GeneticLibraryof the Cell The nucleus
is surrounded by the nuclear envelope, a double
membrane perforated by poresthat regulate the move
6.2
Eukaryotic cells have internal
membranes that compartmentalize their
ment of large macromolecules between the nucleus
and thecytoplasm. Theinnermembrane is linedby the
nuclearlamina,a layerof proteinfilaments that helps
closes a semifluid medium called cytosol. All cells con
structtual support and movement
tain chromosomes and ribosomes. Only members of
the domains Bacteria and Archaea have prokaryotic
called chromatin. Each euk^otic species has a char
cells, which are cells with no nucleus or membrane-
acteristic chromosomal number. Individual chromo
•for—[^pport, communication, connection
enclosed otganelles. The DNA of prokaryotic cells is
concentrated in a region called the nucleoid. Eukary
somes are visible only when coiled and condensed in a
dividing cell.
functions
manufacture,
— for—
Comparing Prokaryotic and Eukaryotic Cells All
cells are bounded by a plasma membrane, which en
transport, export,
digestion, storage
cell wail, plasmodesma (plants);
ECM, desmosomes, ^p and
boundenzymes. Membranes are composed ofa bilayer
of phospholipid molecules associated with diverse
to maintain the shape of the nucleus. There is evidence
of a framework of fibers, called a nuclear matrix, ex
tending through the nucleus.
Most of the cell's DNA is located in the nucleus,
where it is organized into units called chromosomes,
nudear envelope,
rough and smooth ER,
Golgiapparatus, vesicles,
iysosomes, vacuoles,
pbsma membrane
provides'
and junctions —include
energy
transformations
for
chloroplasts
system
' replication,
RNA synthesis
for
mitochondria,
endomembrane
FUNCTIONS
variety of '
Isolating Otganelles by Cell Fractionation Cell fractionation is a technique that separates major organellesof a cellso that they can be identified and their
functions can be studied. Cellsare homogenized and
the resulting cellularsoup is separated into component
fractions by centrifugation. TTie homogenate is first
fbctises a beam of electrons through the specimen. The
short wavelength of the electronbeam allows a resolu
tionof about2 nm (nanometers = 10"' pm),a hundred
times greater than that of the light microscope.
In a scanning election microscope (SEM),an electron
beam scans the surface of a specimen usually coated
with a thin gold film,excitingelectronsfrom the speci
men, which are detected and translated into an image on
a videoscrcen. Thisimageappears three-dimensional.
In a transmission electron microscope (TEM), a
beam of electronsis passed through a thin sectionof a
specimen stained with atoms of heavy metals, and
electromagnets, acting as lenses, focus the image onto
whichare made up ofa complex of DNAand proteins
otic cells have a true nucleus enclosed in a nuclear en
The nucleolus, a dense structure visible in the non-
velopeand numerousorganellessuspended in cytosol.
Cytoplasm refers to the entire region t>etween the nu
dividing nucleus, synthesizes ribosomal RNA and
cleus and the plastxtamembrane, and also to the interior
of a prokaryotic cell.
Mostbacterialcellsrange from 1 to 10 jim in diame
cytoplasm.
combines it with protein to assemble ribosomal sub-
units that pass through nuclear pores to the
ter,whereaseukaryoticcellsare ten times larger, rang
ing from 10 to 100 jim.
•
The small size of cells is dictated by geometry and
the requirements of metabolism.Area is proportional
to the square of linear dimension, while volume is
INTERACTIVE QUESTION
6.3
How does the nucleus control protein synthesis in the
cytoplasm?
proportional to its cube. The plasma membrane sur
rounding every cell must provide sufficient surface
area for exchangeof oxygen, nutrients, and wastes rel
ative to the volume of the cell.
a screen or film.
Modem cell biology integrates cytology with bio
chemistry to understand relationships between cellular
• INTERACTIVE QUESTION
structure and function.
a. If a eukaryotic cell has a diameter that is 10 times that
6.2
of a bacterial cell, proportionally how much more sur
face area would the eukarotic cell have?
• INTERACTIVE QUESTION
6.1
a. Define cytology.
b. Proportionally how much more volume would it have?
Ribosomes: Protein Factories in the Cell
Ribosomes
are particlescomposed of protein and ribosomalRNA.
Most of the proteins produced on free ribosomes are
used within thecytosol. Bound ribo^mes, attached to
the endoplasmic reticulum or nuclearenvelope, usu
ally make proteins that will be included within mem
branes,packaged into organelles, orexportedfromthe
cell.
b. What do cell biologisU use a TEMto study?
c
6.4 The endomembrane system regulates
protein traffic and performs metabolic
What does an SEM show best?
A Panoramic View of the Eukaryotic Cell Mem
d. What advantages does light microscopy have over TEM
and SEM?
branes compartmentalize the eukaryoticcell, provid
ing local environments for specific metabolic Unctions
and participating in metabolism through membrane-
functions in the cell
The endomembrane system of a cell consists of the nu
clearenvelope, endoplasmic reticulum, Golgi apparatus.
40
Unit tim: TheCell
Chapters: A Tourof theCeU
Ihylakoids, inside of which is the thylakoid space
.Thylakoids may bestacked together to form struc
tures called grana.
H INTERACTIVE QUESTION
6.5
Sketch a mitochondrion and a chloroplast and labef their
membranes and compartments.
.41
ComponenU of tite Cytoskeleton The three main
ypes offibers involved inthe cytoskeleton are micro*n eukaryotic
™"=™®'amcnts,
and microtubules,
intennediate filaments.
All
cells have
which are
hollow
constructed ofcolumns ofglobular pro
teins called tubulins. In addition to providing the sup
the extensionand retractionof pseudopodia. Actinfil
amentsinteracting withmyosinmaypropelcytoplasm
forward intopseudopodia. Cytoplasmic streamingin
plantcellsappears to involve bothactin-myosin inter
tobeimportantinmaintaining cellshape.Thenucleus is
securely held in a web of intermediate filaments, and
actions and sol-gel conversions.
lopeiscomposed of intermediate filaments.
tra^ along which oiganelles move with the aid of
between microtubules and microfilaments and are more
porting framework ofthe cell, microtubules serve as
diverse in their composition. Intermediate fibers appear
the nudear lamina lining the inside of the nuclear enve
Intermediate filaments are intermediate in size
motor molecules.
In many cells, microtubules radiate cwt from a re
gion near thenucleus called a centrosome. Inanimal
ceils, a pairofcentrioles, each composed ofnine sets
01 triplet microtubules arranged ina ring, isassoci
ated with the centrosome and replicates before cell
• INTERACTIVE QUESTION
6.7
Fill inthefollowing table to organize what you have learned about thecomponents ofthecytoskeleton. You may wish to
refer to the textbook for additional details.
division.
aiia and flagella are locomotor extensions of some
eukaryotic cells. Cilia are numerous and short; flagella
Mcur one ortwo to acell and are longer. Many protists
^e cilia or flagella to move through aqueous media.
Ciha orflagella attached to stationary cells ofa tissue
Peroxisomes: Oxidation Peroxisomes are mem-
brane-encloscd compartments filled with enzymes
that function in avariety ofmetabolic pathways, such
as breakmg down fatty adds for eneigy or detoxifying
alcohol and other poisons. An enzyme that converts
hydrogen peroxide (H2O2), a toxic by-product ofthese
pathways, towater isalso packaged into peroxisomes.
a INTERACTIVE QUESTION
6.6
Why are peroxisomes not considered part of the endomembranesystem?
6.6 The cytoskeleton is a network of fibers
that organizesstructuresand activities
move fluid past the cell.
Both dlia and flagella are composed of two single
microtubules surrounded by a ring ofnine doublets of
nucrotubules (a nearly universal "9 + 2" arraneement), all ofwhich are endosed inanextension ofthe
plasma membrane. Abasal body, shucturally identical
drives the sliding of the microtubule doubleU past
each other as arms, composed of the motor protein
dynein, alternately attadi to adjacent doublets, pull
to a centnole, anchors the tubules in thecell ATP
both mtemal shuctures and the ceU asa whole), and
interior. The cytoskeleton interacts with special proteins
called motor proteins toproduce cellular movements.
c.
d.
e.
f.
Intermediate
filaments
6.7 Extracellular components
and connectionsbetween cellshelp
Actin and myosin also interact in localized contrac
tion such ascleavage furrows inanimal cell division
and amoeboid movements. AcUn subunits revereibly
assemble into microfilaments and then networks, driv
ing the conversion of cytoplasm from sol to gel during
The Extracellular Matrix (ECM)of Animal Cells An
imal cells secrete an extracellular matrix (ECM) com
posed primarily ofglycoproteins. Collagen, the most
coordinate cellular activities
CellWalls ofPlants Plantcellwallsarecomposed of
microfibrils ofcellulose embedded ina matrix ofpoly-
• INTERACTIVE QUESTION
each other causes the contraction ofmuscles.
transmit mechanical signals from thecell's surface toits
Microfilaments
(actin filaments)
mywm. Thesliding ofacHn and myosin filaments past
Roles ofthe Cytoskeleton;Support, Motility, and Reg
give mechanical support, function incell motility (of
b.
Theprimary cellwallsecreted bya young plantcell
is relatively thinand flexible. Adjacent cells are glued
together by the middlelamella, a thin layer of polysaccharides (called pectins). When they stopgrowing,
some cellssecrete a thicker and strongersecondary
cell wall between the plasma membrane and the pri
digitate with thicker filaments made of the protein
Functions
Structure and Monomers
a.
down, release, and reattach. In conjunction with an
choring cross-linking proteins and radial spokes, this
action ca^ the bending of the flagellum orcUium
Microfilaments, probably present in all eukaryotic
cells, are solid rods consisting ofa twisted double
Cham ofmolecules ofthe globular protein actin. Also
called actm filaments, microfilaments function in
support, forming a network just inside the plasma
membrane and the core of small cytoplasmic exten
sions called microvilli.
In muscle celk, thousands of actin filaments inter-
in the cell
ulation The cytoskeleton is a network of fibers that
Cytoskeleton
Microtubules
saccharides and protein.
mary cell wall.
6.8
Sketch two adjacent plant cells, and show the location of
the primaryand secondarycellwallsand the middlelamella.
abundant glycoproteinof the ECM, formsstrong fibers
that are embedded in a network of proteoglycancom
plexes. Proteoglycans consist ofa small core protein
with many attachedcarbohydratechains.Cellsmaybe
attached to the ECMby fibronectins and other glyco
proteins that bind tointegriits, receptor proteins that
span the plasma membrane and bind to microfila
ments of the cytoskeleton. Thus, information about
changes inside andoutside thecell canbeexchanged
through a mechanical signaUng pathway involving fi
bronectins, integrins, and the microfilaments of the cy
toskeleton. Signals from the ECMappear to influence
the activity of genesin the nucleus.
Intercellular functions
Plasmodesmata are channels
in plant cell walls through which the plasma mem
branes of bordering cells connect, thus linking most
cells of a plant into a livingcontinuum.Water, small
solutes, and even some proteins and RNAmolecules
can move through these channels.
There are three main types of intercellular junctions
between animal cells.At tight junctions, proteins hold
adjacent cell membranes tightly together, creating an
impermeable seal across a layer of epithelial cells.
Desmosomes, reinforced by intermediate filaments,
44
Cliuptert: AToiiroflheCcll
Uiiil Tiw; The Ccll
45
6. Which of the following is incorrfcf/i/paired with its
"' 3. Label the indicated structures in lliis diagram of an animal ccll.
function?
MULTIPLK CHOICE: Choose the oiw best iiiisivcr.
M
1. Which of the following is/are not found in a
priikiiryotic cell?
a. ribiisomcs
b. plasma membrai\e
c. mitochondria
d. a and c
c. a, l>, and c
2. Resolution of a microscope is
a. the distance between two separate points.
b. the sharpness or clarity of an image.
c. the degree of magnification of an image.
d. the depth of focus on a specimen's surface.
e. the wavelength of light.
3. Which of the following is not a similarity among
the nucleus, chloroplasts, and mitochondria?
a. They contain DNA.
b. They are boimded by two phospholipid bilayer
membranes.
c. They can divide to reproduce themselves.
d. They are derived from the endoplasmic reticulum system.
c. Their membranes are associated with specific
proteins.
4. The pores in the nuclear envelope provide for the
a. peroxisome—contains enzymes that break
down H2O2
b. nucleohis—produces ribosomal RNA, assem
bles ribosome subunits
c. Golgi apparatus—processes, tags, and ships cel
lular products
d. lysosome—foiKl sac formed by phagcKytosis
e. ECM (extracellular matrix)—supports and an
chors cells, commimicates information with in
side ol cell
7. The cytoskcleton is composed of which type of
molecule?
a. protein
b. cellulose
d. phosphohpid
e. calcium phosphate
c. chitin
8. A growing plant ccll elongates primarily by
a. increasing the number of vacuoles.
b. synthesizing more cytoplasm.
c. taking up water into its central vacuole.
d. synthesizing more cellulose.
e. producing a secondary cell wail.
9. The innermost portion of the cell wall of a plant
cell specialized for support is the
a. primary cell wall.
d. plasma membrane.
b. secondary cell wall.
e. plasmodesmata.
c. middle lamella.
movement of
a. proteins into the nucleus.
b. ribosomal subunits out of the nucleus.
4. Create a diagram iir flow chart in the space
to tracc the development of a sccretory product
(such as a digestive enzyme) from tlx." DNA codi'
to its export from the cell.
10. Contractile elements of musclc cells arc
a. intermediate filaments.
c. mRNA out of the nucleus.
b. centrioles.
d. signal molecules into the nucleus.
c. microtubides.
c. all of the above.
d. actin (ilaments (microfilaments).
5. The ultrastructure of a chloroplast could be seen
with the best resolution using
a. transmission electron microscopy.
b. scanning electron microscopy.
c. phase-contrast light microscopy.
d. cell fractionation.
e. fluorescence microscopy.
e. fibronectins.
11. Micnitubules are components of all of the follow
ing except
a. centrioles.
b. the spindle apparatus for separating chnimosomes in cell division.
c. tracks along which organelles can move using
motor molecules.
d. tlagella and cilia.
e. the pinching apart of the cytoplasm in animal
46
U»ii(Ttiw; TheCell
brane system?
nuclear
12. o7 the following, which is probably the mostcom
mon route for membrane flow in the endomemmembrane -♦ plasmamembrane
a. rough ER -> Golgi -» lysosomes
vesicles-»plasma membrane
ER->Golgi-»
b.rough ER -*transport vesicles -> Golgi -»
envelope-»rough
smooth ER -> lysosomes
c. nuclear
vesicles
smooth ER -*
Golgi
d. rough ER -+vesicles -»Golgi
plasma membrane
e. smooth ER-»vesicles
peroxisomes
synthesized
13, Proteins tobeused within the cytosol arcgenerally
a. byribosomes bound torough ER.
b. by free ribosomes.
c. by the nudeolus.
e. bymitochondria and chloroplasts.
d. within the Golgi apparatus.
14, Plasmodesmata in plant cells are similar in func
d. theextracellular matrix.
tion to
a. desmosomes.
e. integrins.
c. gap junctions.
b. tight junctions.
pellet formed would most likely contain
e. nuclei.
d- lysosomes.
15, Inan animal cell fractionation procedure, thefirst
c. mitochondria.
a. the extracellular matrix.
b. ribosomes.
Use the cells described as follows loanswer questions 16-20.
a. muscle cell in the thigh muscle of a long
distance runner
b. pancreatic cell that manufactures digestive
enzymes
bacteria
c. macrophage (white blood cell) that engulfs
d. epithelial celllining digestive tract
hormone)
e. ovarian cell that produces estrogen (a steroid
tight junctions?
16. In which cell would you expect to find the most
lysosomes?
17. In whichcell would you expect lo find the most
18. In which cell would you expect to find the most
smoothendoplasmicreticulum?
19. In which cellwould you expect to find the most
bound ribosomes?
20. In whichcell would you expectto find the most
mitochondria?
FILL INTHE BLANKS with the appropriate cellular or-
ganelle orstructure.
ucts to various locations
1. transports membranes andprod
2. infoldings oftheiimer mitochonenzymes
drial membrane with attached
cans, and fibronectins
3. consists of collagen, proteoglymentswithenzymes that transfer
4. specialized' metabolic compart
hydrogen to oxygen, producing
chloroplasts
5. stacks of flattened sacs inside
6. anchoring structure forcilia and
flagella
7, semifluid medium between nu
cleusand plasmamembrane
8. system of fibers that maintains
cell shape, anchors organelles
9. connection betweenanimal celk
that createsimpermeablelayer
vacuoleof plant cells
10. membrane surrounding central
I
vStruclure Your Knowledge
with each of these functions.
h.
Chapter 6: ATour oftheCell
Associated Organellesand Structures
43
1. The table below lists the general functions performed by an animal cell. List the cellular structures associated
a.
Functions
Cell division
b.
f.
e.
d.
c.
Information storage
Energy conversions
and transferal
Manufacture of membranes
Lipid synthesis, drug
and pnxlucts
Digestion,recycling
8-
detoxitotion
Conversion of H2O2
Structural integrity
i.
to water
Movement
ik.
Exchange with
Cell to cell connections
environment
a.
Functions
2. This table lists struchit«s that are found inplant cells. Fill inthe hmctions of these structures.
CeUwall
b.
Plant Cell Structures
Central vacuole
d.
c.
e.
Chloroplast
Plasmodesmata
Amyloplast
—