6.1 Biologists use microscopesand the tools of biochemistry tostudy cells

6.2 Eukaryotic cells have internalmembranes that compartmentalizetheir functions

6.3 The eukaryotic cell's genetic instructionsare housed in the nucleus and carried outby the ribosomes

The endomembrane system regulatesprotein traffic and performs metabolicfunctions in the cell

6.5 Mitochondria and chloroplastschange energy from one formto another

6.6 The cytoskeleton is a network of fibersthat organizes structures and activities inthe cell

6.7 Extracellular components and connectionsbetween cells help coordinate cellularactivities

The cell is the fundamental unit of life. The complexi­ties in the processes of life are reflected in the complex­ities of the cell. It is easy to become overwhelmed bythe number of new vocabulary terms for this array ofcell structures. The diagram on the following page pro­vides an organizational framework for the wealth ofdetail found in "a tour of the cell."

33

-

manufacture,transport, export,digestion, storage

energytransformations

support, commLmication, cOimection

for

structural support and movement

RNA synthesis

protein production

for

for

c. What does an SEM show best?

a. Define cytology.

d. What advantages does light microscopy have over bothTEM and SEM?

INTERACTIVE QUESTION 6.1

b. What do cell biologists use a TEM to study?

about 2 nm (nanometers = 10-3 fLm),about a hundredtimes greater than that of the light microscope.

In a scanning electron microscope (SEM), an elec­tron beam scans the surface of a specimen that is usu­ally coated with a thin gold film. The beam exciteselectrons from the specimen, which are detected andtranslated into an image on a video screen. This imageappears three-dimensional.

In a transmission electron microscope (TEM), abeam of electrons is passed through a thin section of aspecimen stained with atoms of heavy metals. Electro­magnets, acting as lenses, then focus the image onto ascreen or film.

New techniques such as super-resolution microscopyare providing high-resolution images. Modem cell biol­ogy integrates cytology with biochemistry to understandrelationships between cellular structure and function.

mitochondria,chloroplasts

provides

nuclear envelope,rough and smooth ER,

Golgi apparatus, vesicles,lysosomes, vacuoles,plasma membrane

such --l nucleus ~foras

includes

cell wall, plasmodesma (plants);ECM, desmosomes, gap and

tight jW1Ctions(animals)

organelles

include

endomembranesystem

----1 THE CELL '--

~ has performsI STRUCTURES 1 that serve a1/1'i0.... vanetyof

include ""-J I ribosomes I for

34 Unit 2: The Cell

The cell is the basic structural and functional unit of all

organisms. In the hierarchy of biological organization,the capacity for life emerges from the structural order ofthe cell. All cells are related through common descent,but evolution has shaped diverse adaptations.

6.1 Biologists use microscopes and thetools of biochemistry to study cells

Microscopy Cells were first described by RobertHooke in 1665. The glass lenses of a light microscope(LM) refract (bend) the visible light passing through aspecimen such that the projected image is magnified.Magnification is the ratio of the size of an image to thereal size of the object. Resolution is a measure of theclarity of an image and is determined by the mini­mum distance two points must be separated to beseen as separate. The resolving power of the lightmicroscope is limited, so that details finer than 0.2 fLm(micrometers = 10-3 mm) cannot be distinguished.Staining specimens and using techniques such as flu­orescence, phase-contrast, and confocal microscopyimprove visibility by increasing contrast betweenstructures.

Many cellular structures, including most membrane­enclosed organelles, cannot be resolved by the lightmicroscope. Their structures were first revealed whenthe electron microscope (EM) was developed in the1950s. The electron microscope focuses a beam of elec­trons through a specimen or onto its surface. The shortwavelength of the electron beam allows a resolution of

Cell Fractionation Cell fractionation is a techniquethat separates organelles and other subcellular struc­tures of a cell so that they can be identified and theirfunctions studied. Cells are broken apart and thehomogenate is separated into component fractions bycentrifugation at increasing speeds.

6.2 Eukaryotic cells have internalmembranes that compartmentalize theirfunctions

Comparing Prokaryotic and Eukaryotic Cells Allcells are bounded by a plasma membrane, which enclosesa semifluid medium called cytosol. All cells containchromosomes and ribosomes.

Only members of the domains Bacteria and Archaeahave prokaryotic cells, which are cells with no nucleusor membrane-enclosed organelles. The DNA ofprokaryotic cells is concentrated in a region called thenucleoid. Eukaryotic cells have a true nucleus enclosedin a double membrane and numerous organellessuspended in cytosol. Cytoplasm refers to the regionbetween the nucleus and the plasma membrane, andalso to the interior of a prokaryotic cell.

Most bacterial cells range from 1 to 10 fLmin diame­ter, whereas eu~aryotic cells are 10 times larger, rang­ing from 10 to 100 fLm. The small size of cells isinfluenced by geometry. Area is proportional to thesquare of linear dimension, while volume is propor­tional to its cube. The plasma membrane surroundingevery cell must provide sufficient surface area forexchange of oxygen, nutrients, and wastes relative tothe volume of the cell.

A Panoramic View of the Eukaryotic Cell Membranescompartmentalize the eukaryotic cell, providing localenvironments for specific metabolic functions and par­ticipating in metabolism through membrane-boundenzymes. All types of eukaryotic cells share manycommon structures, although there are also importantdifferences between them.

t.-} ....-""~c •. ,~, .. _.

INTERACTIVE QUESTION 6.2

Chapter 6: A Tour of the Cell 35

6.3 The eukaryotic cell's geneticinstructions are housed in the nucleusand carried out by the ribosomes

The Nucleus: Information Central The nucleus issurrounded by the nuclear envelope, a double mem­brane perforated by pores. A protein pore complex liningeach pore regulates the movement of materialsbetween the nucleus and the cytoplasm. The innermembrane is lined by the nuclear lamina, a layer ofprotein filaments that helps to maintain the shape ofthe nucleus. A nuclear matrix of fibers extends through­out the nucleus.

Most of the cell's DNA is located in the nucleus,where it is organized into units called chromosomes,which are made up of chromatin, a complex of DNAand proteins. Each eukaryotic species has a character­istic chromosomal number. Individual chromosomes

are visible only when condensed in a dividing cell.The nucleolus, a dense structure visible in the non­

dividing nucleus, synthesizes ribosomal RNA (rRNA)and combines it with protein to assemble ribosomalsubunits, which then pass through nuclear pores to thecytoplasm.

Ribosomes: Protein Factories Ribosomes are com­

posed of protein and ribosomal RNA. Most of the pro­teins produced on free ribosomes are used within thecytosol. Bound ribosomes, attached to the endoplasmicreticulum or nuclear envelope, usually make proteinsthat will be included within membranes, packaged intoorganelles, or exported from the cell.

.fNTERACTIVEQUEsnON 6.3

How does the nucleus control protein synthesis in thecytoplasm?

6.4 The endomembrane system regulatesprotein traffic and performs metabolicfunctions in the cell

c. Proportionally how much more volume would it have?

a. Describe the molecular structure of the plasmamembrane.

b. If a eukaryotic cell has a diameter that is 10times thatof a bacterial cell, proportionally how much more sur­face area would the eukaryotic cell have?

••• II

The endomembrane system of a cell consists of thenuclear envelope, endoplasmic reticulum, Golgi appa­ratus, lysosomes, vesicles, vacuoles, and the plasmamembrane. Although not identical, all these mem­branes are related either through direct contact or bythe transfer of membrane segments by membrane­bound sacs called vesicles.

The Endoplasmic Reticulum: Biosynthetic FactoryThe endoplasmic reticulum (ER) is a membranoussystem that is continuous with the nuclear envelope

:J--~ _

36 Unit 2 The Cell

and encloses a network of interconnected tubules orcomDartments called cisternae. Ribosomes are at-

.1

tached to the cytoplasmic surface of rough ER; smoothER lacks ribosomes.

Smooth ER serves diverse functions in different

cells: Its enzymes are involved in phospholipid andsteroid (including sex hormone) synthesis, carbohy­drate metabolism, and detoxification of drugs and poi­sons. Barbiturates, alcohol, and other drugs increase aliver cell's production of smooth ER, thus leading to anincreased tolerance (and thus reduced effectiveness)for these and other drugs. Smooth ER also functions instorage and release of calcium ions during musclecontraction.

Proteins intended for secretion are manufactured bymembrane-bOlmd ribosomes and then threaded into

the lumen of the rough ER.Many are covalently bondedto small carbohydrates to form glycoproteins. Secre­tory proteins are transported from the rough ER inmembrane-bound transport vesicles.

Rough ER also manufactures membranes. Enzymesbuilt into the membrane assemble phospholipids, andmembrane proteins formed by bound ribosomes areinserted into the ER membrane. Transport vesiclestransfer ER membrane to other parts of the endomem­brane sys tern. -.

The Golgi Apparatus: Shipping and Receiving CenterThe Golgi apparatus consists of a stack of flattenedsacs. Vesicles that bud from the ER join to the cis face ofa Golgi stack, adding to it their contents and membrane.

'" NTERf\(::JI){I Q.U ESTION 6./1.

According to the cisternal maturation model, Golgiproducts are processed and tagged as the cisternaethemselves progress from the cis to the trans face. Cly­coproteins often have their attached carbohydratesmodified. The Colgi apparatus of plant cells manufac­tures some polysaccharides, such as pectins. Colgiproducts are sorted into vesicles, which pinch off fromthe trans face of the Colgi apparatus. These vesicles mayhave surface molecules that help direct them to theplasma membrane or to other organelles.

Lysosomes: Digestive Compartments In animal cells,lysosomes are membrane-enclosed sacs containinghydrolytic enzymes that digest macromolecules. Lyso­somes provide an acidic pH for these enzymes.

In some protists, lysosomes fuse with food vacuoles todigest material ingested by phagocytosis. Macrophages,a type of white blood cell, use lysosomes to destroyingested bacteria. Lysosomes also recycle a cell's ownmacromolecules by fusing with vesicles enclosing dam­aged organelles or small bits of cytosol, a process knownas autophagy.

Vacuoles: Diverse l\1aintenance Compartments Vac­uoles are large vesicles. Food vacuoles are formed as aresult of phagocytosis. Contractile vacuoles pumpexcess water out of freshwater protists. Vacuoles inplant cells may store organic compounds and inorganicions for the cell, or contain dangerous metabolic by­products and poisonous or unpalatable compounds,which may protect the plant from predators. A large

Name the components of the endomembrane system shown in this diagram, and list the functions of each of these membranes.

a.

b.

c.

d.

e.

f.

g.

a.

, J

Chapter 6: A Tour of the Cell 37

~ ~ , ". ~ • -):- .--, ..,t ~

INllRACTlVE"QUESTION 6.5 ~:" - -~ '.< "._~ _-..., ~ ;;0.., _ ~

Sketch a mitochondrion and a chloroplast and label theirmembranes and compartments.

IIIlIill

6.6 The cytoskeleton is a network of fibersthat organizes structures and activities inthe cell

Roles of the Cytoskeleton: Support and Motility Thecytoskeleton is a network of protein fibers that givemechanical support and function in cell motility (ofboth internal structures and the cell as a whole). Thecytoskeleton interacts with special proteins calledmotor proteins to produce cellular movements.

Components of the Cytoskeleton The three maintypes of fibers involved in the cytoskeleton aremicrotubules, microfilaments, and intermediatefilaments.

Why are peroxisomes not considered part of the endomem­

brane system?

Peroxisomes: Oxidation Peroxisomes are oxidative

organelles filled with enzymes that function in a vari­ety of metabolic pathways, such as breaking downfatty acids for energy or detoxifying alcohol and otherpoisons. An enzyme that converts hydrogen peroxide(HzOz), a toxic by-product of these pathways, to wateris also packaged into peroxisomes. GLyoxysomes,foundin plant seeds, contain enzymes that convert fatty acidsto sugars for emerging seedlings.

central vacuole is found in mature plant cells andencloses a solution called cell sap. A plant cell increasesin size with a minimal addition of new cytoplasm as itsvacuole absorbs water and expands.

The Endomembrane System: A Review As mem­branes move from the ER to the Golgi apparatus andthen to other organelles, their compositions, functions,and contents are modified.

6.5 Mitochondria and chloroplasts changeenergy from one form to another

Cellular respiration, the metabolic processing of fuelsto produce ATP, occurs within the mitochondria ofeukaryotic cells. Photosynthesis occurs in thechloroplasts of plants and photosynthetic protists,which produce sugars from carbon dioxide and waterby absorbing solar energy.

The Evolutionary Origins of Mitochondria and Chloro­plasts According to the endosymbiont theory, bothmitochondria and chloroplasts originated as prokaryoticcellsengulfed by an ancestral eukaryotic cell. The doublemembranes surrounding mitochondria and chloroplastsappear to have been part of the prokaryotic endosyrnbiont.These organelles grow and reproduce independentlywithin the cell. They contain a small amount of DNA andribosomes (both similar to prokaryotic structures), andthey synthesize some of their proteins.

Mitochondria: Chemical Energy Conversion Twomembranes, each a phospholipid bilayer with uniqueembedded proteins, enclose a mitochondrion. A narrowintermembrane space exists between the smooth outermembrane and the convoluted inner membrane. The

folds of the inner membrane, called cristae, create a largesurface area and enclose the mitochondrial matrix.

Many respiratory enzymes, mitochondrial DNA, andribosomes are housed in this matrix. Other respiratoryenzymes and proteins are built into the inner membrane.

Chloroplasts: Capture of Light Energy Two mem­branes separated by a thin intermembrane spaceenclose a chloroplast. Inside the inner membrane is amembranous system of connected flattened sacs calledthylakoids, inside of which is the thylakoid space. Pho­tosynthetic enzymes are embedded in the thylakoids,which may be stacked together to form structures calledgrana. Chloroplast DNA, ribosomes, and many en­zymes are found in the stroma, the fluid surroundingthe thylakoids.

Plastids are plant organelles that also include amy­lop lasts, which store starch, and chromoplasts, whichcontain pigments.

I

II, : 38 Unit 2: The CellII

All eukaryotic cells have microtubules, which arehollow rods constructed of columns of globular pro­teins called tubulins. Microtubules change lengththrough the addition or subtraction of tubulin dimers.In addition to providing the supporting framework ofthe cell, micro tubules separate chromosomes duringcell division and serve as tracks along which organellesmove with the aid of motor molecules.

In animal cells, micro tubules grow out from aregion near the nucleus called a centrosome. A pair ofcentrioles, each composed of nine sets of triplet micro­tubules arranged in a ring, is associated with the cen­trosome and replicates before cell division. Fungi andplant cells lack centrosomes with centrioles andevidently have some other microtubule-organizingcenter.

Cilia and flagella are locomotor extensions of someeukaryotic cells. Cilia are numerous and short; flagellaoccur one or two to a cell and are longer. Many protistsuse cilia or flagella "to move through aqueous media.Cilia or flagella attached to stationary cells of a tissuemove fluid past the cell. A signal-receiving cilium, suchas the primary cilium found on vertebrate animal cells,transmits environmental signals to a cell's interior.

Both cilia ~d flagella are composed of two singlemicro tubules surrounded by a ring of nine doublets ofmicrotubules (a nearly universal "9 + 2" arrangement),all of which are enclosed in an extension of the plasmamembrane. A basal body, with a "9 + 0" pattern ofmicrotubule triplets, anchors a cilium or flagellum inthe cell. ATP drives the sliding of the microtubule dou­blets past each other as the two "feet" of large motorproteins called dyneins alternately attach to adjacentdoublets, pull down, release, and reattach. In conjunc­tion with anchoring cross-linking proteins and radialspokes, this action causes the bending of the flagellumor cilium.

Microfilaments, probably present in all eukaryoticcells, are solid rods consisting of a twisted double chainof molecules of the globular protein actin. Thus, they arealso called actin filaments. Cortical microfilaments form anetwork just inside the plasma membrane that creates asemisolid or gel consistency in this cytoplasmic layercalled the cortex. Microfilaments form the supportivecore of small cytoplasmic extensions called microvilli.

In muscle cells, thousands of actin filaments inter­

digitate with thicker filaments made of the motor pro­tein myosin. The sliding of actin and myosin filamentspast each other causes the contraction of muscles.

Actin and myosin also interact in localized contrac­tions such as cleavage furrows in animal cell divisionand amoeboid movements. Actin subunits reversiblyassemble into micro filament networks, driving theconversion of cytoplasm from sol to gel during the for­mation of cellular extensions called pseudopodia.Actin filaments interacting with myosin may propelcytoplasm forward into pseudopodia. Cytoplasmicstreaming in plant cells appears to involve bothactin-myosin interactions and sol-gel conversions.

Intermediate filaments are intermediate in size be­tween microtubules and micro filaments and are more

diverse in their composition. Intermediate filaments ap­pear to be important in maintaining cell shape. The nu­cleus is securely held in a web of intermediate filaments,and the nuclear lamina lining the inside of the nuclearenvelope is composed of intermediate filaments.

6.7 Extracellular components andconnections between cells help coordinatecellular activities

Cell Walls of Plants Plant cell walls are composed ofmicro fibrils of cellulose embedded in a matrix of poly­saccharides and protein.

Fill in the following table to organize what you have learned about the components of the cytoskeleton. You may wish torefer to the textbook for additional details.

Cytoskeleton Component Structure and MonomersFunctions

Microtubules

a.b.

Microfilaments (actin filaments)

c.d.

Intermediate filaments

e.f.

centro- = the center; -soma = a body (centrosome: astructure in the cytoplasm of animal cells thatfunctions as a microtubule-organizing center)

chloro- = green (chloroplast: the site of photosynthesisin plants and photosynthetic protists)

cili- = hair (cilium: a short cellular appendage contain­ing microtubules)

cyto- = cell (cytosol: the semifluid portion of thecytoplasm)

-ell = small (organelle: a membrane-enclosed structurewith a specialized function, suspended in thecytosol of eukaryotic cells)

endo- = inner (endomembrane system: the system of mem­branes inside and surrounding a eukaryotic cellthat includes the plasma membrane, nuclear enve­lope, endoplasmic reticulum, Golgi apparatus,lysosomes, vesicles, and vacuoles)

eu- = true (eukaryotic cell:' a cell with a membrane­enclosed nucleus and organelles)

extra- = outside (extracellular matrix: the meshwork ofglycoproteins, polysaccharides, and proteogly­cans surrounding animal cells)

flagell- = whip (flagellum: a long cellular appendagespecialized for locomotion)

Chapter 6: A Tour of the Cell 39

b. Listsome of the functions of the extracellular matrix ofanimal cells.

'. _ -'~l,;::,',--_'f:-- -~':FE"-':---T---- __'_m __,.

I NTERACTIVE'qU ESTION

The Cell: A Living Unit Greater Than the Sum of ItsParts The compartmentalization and the many spe­cialized organelles typical of cells exemplify the princi­ple that structure correlates with function. The intricatefunctioning of a living cell emerges from the complexinteractions of its multiple parts.

a. Return to your sketch of plant cells in Interactive Ques­tion 6.8 and draw in a plasmodesma.

Desmosomes (also called anchoring junctions) are rein­forced by intermediate filaments and rivet cells intostrong sheets. Gap junctions (also called communicatingjunctions) are cytoplasmic connections that allow forthe exchange of ions and small molecules between cellsthrough protein-lined pores.

The primary cell wall secreted by a young plant cell isrelatively thin and flexible. Microtubules in the cell cortexappear to guide the path of cellulose synthase, determin­ing the pattern of cellulose fibril deposition and thus thedirection of cell expansion. Adjacent cells are glued to­

gether by the middle lamella, a thin layer of polysaccha­rides (called pectins). When they stop growing, somecells secrete a thicker and stronger secondary cell wallbetween the plasma membrane and the primary cell wall.

Sketch two adjacent plant cells, and show the location ofthe primaryand secondary cellwallsand the middle lamella.

The Extracellular Matrix (ECM) of Animal CellsAnimal cells secrete an extracellular matrix (ECM)composed primarily of glycoproteins and othercarbohydrate-containing molecules. Collagen formsstrong fibers that are embedded in a network of proteo­glycan complexes. These large complexes form whenmultiple proteoglycans, each consisting of a small coreprotein with many carbohydrate chains, attach to a longpolysaccharide. Cells may be attached to the ECM byfibronectins and other glycoproteins that bind tointegrins, proteins that span the plasma membrane andbind, via other proteins, to microfilarnents of thecytoskeleton. Thus, information about changes outsidethe cell can be communicated through a mechanical sig­naling pathway involving fibronectins, integrins, and themicrofilarnents of the cytoskeleton. Signals from the ECMappear to influence the activity of genes in the nucleus.

Cell Junctions Plasmodesmata are channels in plantcell walls through which the plasma membranes ofbordering cells connect, thus linking most cells of aplant into a living continuum. Water, small solutes, andeven some proteins and RNA molecules can movethrough these channels.

There are three main types of cell junctions betweenanimal cells. At tight junctions, proteins hold adja­cent cell membranes tightly together, creating animpermeable seal across a layer of epithelial cells.

~---------------------

40 Unit 2: The Cell

glyco- = sweet (glycoprotein: a protein covalently bondedto one or more carbohydrates)

lamin- = sheet/layer (nuclear lamina: a netlike array ofprotein filaments lining the inner surface of thenuclear envelope)

lyso- = loosen (lysosome: a membrane-enclosed sac ofhydrolytic enzymes that digest macromoleculesin animal cells)

micro- = sm~ll; -tubul = a little pipe (microtubule: a hol­low rod of tubulin proteins that makes up part ofthe cytoskeleton in all eukaryotic cells)

nucle- = nucleus; -oid = like (nucleoid: a non­membrane-bound region in prokaryotic cellswhere the DNA is concentrated)

Structure Your Knowledge

phago- = to eat; -kytos = vessel (phagocytosis: a type ofendocytosis in which large particulate substancesare taken up by a cell)

plasm- = molded; -desma = a band or bond (plasmodesma:

an open channel in a plant cell wall, connectingthe cytoplasm of adjacent cells)

pro- = before; karyo- = nucleus (prokaryotic cell: a celllacking a membrane-enclosed nucleus andorganelles)

pseudo- = false; -pod = foot (pseudopodium: a cellularextension of amoeboid cells used in moving andfeeding)

thylaco- = sac or pouch (thylakoid: a flattened membra­nous sac inside a chloroplast)

vacu- = empty (vacuole: a relatively large vesicle with aspecialized function)

1. In the following table, write the organelles or structures that are associated with each of the listed functions inanimal cells.

-.Functions

Associated Organelles and Structures

Cell division

a.

Information storage and transferal

b.

Energy conversions

c.

Manufacture of proteins, membranes, and other products

d.

Lipid synthesis, drug detoxification

e.

Digestion, recycling

f.

Oxidation, conversion of H202 to water

g.

Structural integrity

h.

Movement

i.

Exchange with the environment

j.

Cell-to-cell connections

k.

2. Write the functions of each of the following plant cell structures.

Plant Cell Structures Functions

Cell wall

a.

Central vacuole

b.

Chloroplast

c.

Amyloplast

d.

Plasmodesmata

e.

3. Label the indicated structures in the following diagram of an animal cell.

5. _

L _

q.---

p.---

{ o.1._;I

I

J~_-------------

Chapter 6: A Tour of the Cell 41

Ij

1,1' 42 Unit 2: The Cell

4. Create a diagram or flowchart in the following space to trace the development of a secretory product (such as adigestive enzyme) from the DNA code to its export from the cell,

••

MULTIPLE CHOICE: Choose the one best answer.

1. Resolution of a microscope refers to

a. the distance between two separate points.

b. the sharpness or clarity of an image.

c. the amount of magnification of an image.

d. the depth of focus on a specimen's surface.

e. the wavelength of light.

2. The detailed structure of a chloroplast can be seenwith the best resolution by usinga. transmission electron microscopy.

b. scanning electron microscopy.

c. phase-contrast light microscopy.

d. deconvolution microscopy.

e. confocal fluorescence microscopy.

3. In an animal cell fractionation procedure, one ofthe last pellets from the series of centrifugationswould most likely containa. ribosomes. d. lysosomes.

b. peroxisomes. e. nuclei.c. mitochondria.

4. Which of the following is / are not found in aprokaryotic cell?a. ribosomes d. a and c

b. plasma membrane e. a, b, and cc. mitochondria

5. The cells of an ant and an elephant are, on average,the same size; an elephant just has more cells. Whatis the main advantage of small cell size?a. Small cells are easier to organize into tissues and organs.

b. A small cell has a larger plasma membrane surfacearea than does a large cell, facilitating the exchange ofsufficient materials with its environment.

c. The cytoskeleton of a large cell would have to be solarge that cells would be too heavy.

d. Small cells require less oxygen and nutrients than dolarge cells.

e. A small cell has a larger surface area relative to cyto­plasmic volume, facilitating sufficient exchange ofmaterials.

6. Proteins that function within the cytosol are gener­ally synthesizeda. by ribosomes bound to rough ER.

b. by free ribosomes.

c. by the nucleolus.

d. within the Golgi apparatus.

e. by mitochondria and chloroplasts.

7. The pores in the nuclear envelope provide for themovement of

a. proteins into the nucleus.b. ribosomal subunits out of the nucleus.

c. mRNA out of the nucleus.

d. signal molecules into the nucleus.e. all of the above.

8. A fluorescent green tag is attached to a protein be­ing synthesized and inserted into an area of roughER membrane. Which of the following is a possibleroute for that protein to follow?

a. rough ER ---7 Golgi ---7 lysosome ---7 nuclear membrane---7 plasma membrane

b. rough ER ---7 transport vesicle ---7 Golgi ---7 vesicle ---7

plasma membrane ---7 food vacuole

c. rough ER ---7 nuclear envelope ---7 Golgi ---7 smooth ER---7 lysosome

d. rough ER ---7 transport vesicle ---7 Golgi ---7 smooth ER---7 plasma membrane

e. rough ER ---7 transport vesicle ---7 Golgi ---7 vesicle ---7

extracellular matrix

9. A growing plant cell elongates primarily bya. increasing the number of vacuoles.

b. synthesizing more cytoplasm.c. taking up water into its central vacuole.

d. synthesizing cellulose fibrils in an orientation per­pendicular to the axis of cell elongation.

e. producing a secondary cell wall.

10. Which of the following is not a similarity amongnuclei, chloroplasts, and mitochondria?a. They contain DNA.

b. They are bounded by two (or more) phospholipid bi­layer membranes.

c. They can divide to reproduce themselves.

d. They are derived from the endoplasmic reticulumsystem.

e. Their membranes are associated with specific proteins.

11. The contractile elements of muscle cells area. intermediate filaments.b. centrioles.

c. microtubules.

d. microfilaments (actin filaments).e. fibronectins.

12. Microtubules are components of all of the followingexcepta. centrioles.

b. the spindle apparatus for separating chromosomes incell division.

c. tracks along which organelles can move using motorproteins.

d. flagella and cilia.

e. the cleavage furrow that pinches apart cells in animalcell division.

13. Which of the following characteristics are shared bycristae of mitochondria, thylakoids, and microvilli?a. have a large surface area of membrane that enhances

their particular function

b. are bounded by a double membrane

c. have a shape reinforced by microfilaments

d. are not derived from the endoplasmic reticulum systeme. arose by endosymbiosis of an aerobic bacterium, a

photosynthetic bacterium, and a motile bacterium,respectively

14. The innermost portion of the cell wall of a plant cellspecialized for support is thea. primary cell wall.

b. secondary cell wall.c. middle lamella.

d. plasma membrane.

e. cytoskeleton.

Chapter 6: A Tour of the Cell 43

15. Plasmodesmata in plant cells are similar in function toa. desmosomes.

b. tight junctions.

c. gap junctions.d. the extracellular matrix.

e. integrins.

16. Which of the following structures is / are involved

in receiving external information and relaying itinto a cell?

a. a primary ciliumb. fibronectins

c. integrins

d. micro filaments of the cytoskeletone. all of the above

17. Which of the following is incorrectly paired with itsfunction?

a. peroxisome-contains enzymes that break downH202

b. nucleolus-produces ribosomal RNA, assembles ribo­some subunits

c. Golgi apparatus-processes, tags, and ships cellularproducts

d. lysosome-food sac formed by phagocytosise. ECM (extracellular matrix)-supports and anchors

cells, communicates information with inside ofcell

Answer questions 18-22 using the following five choices.

a. muscle cells in the thigh muscle of a runner

b. pancreatic cells that manufactures digestiveenzymes

c. macrophages that engulf bacteria

d. epithelial cells lining digestive tract

e. ovarian cells that produce estrogen (a steroidhormone)

18. In which cells would you expect to find the mosttight junctions?

19. In which cells would you expect to find the mostlysosomes?

20. In which cells would you expect to find the mostsmooth endoplasmic reticulum?

21. In which cells would you expect to find the mostbound ribosomes?

22. In which cells would you expect to find the mostmitochondria?