cellular organels.doc

8/21/2019 cellular organels.doc

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Life exhibits varying degrees of organization. Atoms are organized into

molecules, molecules into organelles, and organelles into cells, and so on.

According to the Cell Theory, all living things are composed of one or more

cells, and the functions of a multicellular organism are a consequence of the

types of cells it has. Cells fall into two broad groups pro!aryotes and

eu!aryotes. "ro!aryotic cells are smaller #as a general rule$ and lac! much of

the internal compartmentalization and complexity of eu!aryotic cells. %o

matter which type of cell we are considering, all cells have certain features in

common, such as a cell membrane, &%A and '%A, cytoplasm, and ribosomes.

(u!aryotic cells have a great variety of organelles and structures.

Cell Size and Shape | Back to Top

The shapes of cells are quite varied with some, such as neurons, being longer

than they are wide and others, such asparenchyma#a common type of plantcell$ and erythrocytes#red blood cells$ being equidimensional. )ome cells are

encased in a rigid wall, which constrains their shape, while others have a

flexible cell membrane #and no rigid cell wall$.

The size of cells is also related to their functions. (ggs #or to use the latin word,

ova$ are very large, often being the largest cells an organism produces. The

large size of many eggs is related to the process of development that occurs

after the egg is fertilized, when the contents of the egg #now termed a zygote$

are used in a rapid series of cellular divisions, each requiring tremendous

amounts of energy that is available in the zygote cells. Later in life the energymust be acquired, but at first a sort of inheritance*trust fund of energy is used.

Cells range in size from small bacteria to large, unfertilized eggs laid by birds

and dinosaurs. The realtive size ranges of biological things is shown in +igure

. -n science we use the metric system for measuring. ere are some

measurements and convesrions that will aid your understanding of biology.

meter / 00 cm / ,000 mm / ,000,000 1m / ,000,000,000 nm

centimenter #cm$ / *00 meter / 0 mm

millimeter #mm$ / *000 meter / *0 cm

micrometer #m$ / *,000,000 meter / *0,000 cm

nanometer #nm$ / *,000,000,000 meter / *0,000,000 cm
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+igure .)izes of viruses, cells, and organisms.-mages from "urves et al., Life

The )cience of 2iology, 3th (dition, by )inauer Associates #www.sinauer.com$

and 4 +reeman #www.whfreeman.com$, used with permission.
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The Cell Membrane | Back to Top

The cell membrane functions as a semi5permeable barrier, allowing a very fewmolecules across it while fencing the ma6ority of organically produced

chemicals inside the cell. (lectron microscopic examinations of cell

membranes have led to the development of the lipid bilayer model #also

referred to as the fluid5mosaicmodel$. The most common molecule in the

model is thephospholipid, which has a polar #hydrophilic$ head and two

nonpolar #hydrophobic$ tails. These phospholipids are aligned tail to tail so the

nonpolar areas form a hydrophobic region between the hydrophilic heads on

the inner and outer surfaces of the membrane. This layering is termed a bilayer

since an electron microscopic technique !nown as freeze5fracturing is able to

split the bilayer, shown in +igure 7.

+igure 7. Cell 8embranes from 9pposing %eurons #T(8 x3:;,


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Cholesterol is another important component of cell membranes embedded in

the hydrophobic areas of the inner #tail5tail$ region. 8ost bacterial cell

membranes do not contain cholesterol. Cholesterol aids in the flexibility of a

cell membrane.

"roteins, shown in +igure 7, are suspended in the inner layer, although the more

hydrophilic areas of these proteins >stic! out> into the cells interior as well as

outside the cell. These proteins function as gateways that will allow certain

molecules to cross into and out of the cell by moving through open areas of the

protein channel. These integral proteins are sometimes !nown as gateway

proteins. The outer surface of the membrane will tend to be rich in glycolipids,

which have their hydrophobic tails embedded in the hydrophobic region of the

membrane and their heads exposed outside the cell. These, along with

carbohydrates attached to the integral proteins, are thought to function in the

recognition of self, a sort of cellular identification system.

The contents #both chemical and organelles$ of the cell are termed protoplasm,

and are further subdivided into cytoplasm#all of the protoplasm except the

contents of the nucleus$ and nucleoplasm #all of the material, plasma and &%A

etc., within the nucleus$.
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The Cell Wall | Back to Top

%ot all living things have cell walls, most notably animals and many of the

more animal5li!e protistans. 2acteria have cell walls containing the chemical

peptidoglycan. "lant cells, shown in +igures : and 3, have a variety ofchemicals incorporated in their cell walls. Cellulose, a nondigestible #to

humans anyway$ polysaccharide is the most common chemical in the plant

primary cell wall. )ome plant cells also have ligninand other chemicals

embedded in their secondary walls.

The cell wall is located outside the plasma membrane. "lasmodesmataare

connections through which cells communicate chemically with each other

through their thic! walls. +ungi and many protists have cell walls although they

do not contain cellulose, rather a variety of chemicals #chitinfor fungi$.

Animal cells, shown in +igure ?, lac! a cell wall, and must instead rely on their

cell membrane to maintain the integrity of the cell. 8any protistans also lac!

cell walls, using variously modified cell membranes o act as a boundary to the

inside of the cell.

+igure :. )tructure of a typical plant cell.-mage from "urves et al., Life The

)cience of 2iology, 3th (dition, by )inauer Associates #www.sinauer.com$ and

4 +reeman #www.whfreeman.com$, used with permission.
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+igure 3. Lily "arenchyma Cell #cross5section$ #T(8 x


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+igure ?. Liver Cell #T(8 x@,300$.This image is copyright &ennis =un!el.

This image is copyright &ennis =un!el at www.&ennis=un!el.com, used with

permission.
http://www.denniskunkel.com/http://www.denniskunkel.com/


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The nucleus | Back to Top

The nucleus, shown in +igures ; and


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+igure


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The nuclear envelope, shown in +igure B, is a double5membrane structure.

%umerous pores occur in the envelope, allowing '%A and other chemicals to

pass, but the &%A not to pass.

+igure B.)tructure of the nuclear envelope and nuclear pores.-mage from"urves et al., Life The )cience of 2iology, 3th (dition, by )inauer Associates

#www.sinauer.com$ and 4 +reeman #www.whfreeman.com$, used with

permission.

+igure @.%ucleus with %uclear "ores #T(8 x


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Cytoplasm | Back to Top

The cytoplasm was defined earlier as the material between the plasma

membrane #cell membrane$ and the nuclear envelope. +ibrous proteins that

occur in the cytoplasm, referred to as the cytos!eletonmaintain the shape of thecell as well as anchoring organelles, moving the cell and controlling internal

movement of structures. (lements that comprose the cytos!eleton are shown in

+igure 0. 8icrotubulesfunction in cell division and serve as a >temporary

scaffolding> for other organelles. Actinfilaments are thin threads that function

in cell division and cell motility. -ntermediate filaments are between the size of

the microtubules and the actin filaments.

+igure 0. Actin and tubulin components of the cytos!eleton.-mage from

"urves et al., Life The )cience of 2iology, 3th (dition, by )inauer Associates#www.sinauer.com$ and 4 +reeman #www.whfreeman.com$, used with

permission.
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Vacuoles and vesicles | Back to Top

acuolesare single5membrane organelles that are essentially part of the outside

that is located within the cell. The single membrane is !nown in plant cells as a

tonoplast. 8any organisms will use vacuoles as storage areas. esicles are

much smaller than vacuoles and function in transporting materials both within

and to the outside of the cell.

Ribosomes | Back to Top

'ibosomesare the sites of protein synthesis. They are not membrane5bound

and thus occur in both pro!aryotes and eu!aryotes. (u!aryotic ribosomes are

slightly larger than pro!aryotic ones. )tructurally, the ribosome consists of a

small and larger subunit, as shown in +igure . . 2iochemically, the ribosome

consists of ribosomal '%A#r'%A$ and some ?0 structural proteins. 9ften

ribosomes cluster on the endoplasmic reticulum, in which case they resemble a

series of factories ad6oining a railroad line. +igure 7 illustrates the many

ribosomes attached to the endoplasmic reticulum. Clic! here for 'ibosomes#8ore than you ever wanted to !now about ribosomesD$

+igure . )tructure of the ribosome.-mage from "urves et al., Life The


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+igure 7.'ibosomes and "olyribosomes 5 liver cell #T(8 x


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(ndoplasmic reticulum, shown in +igure : and 3, is a mesh of interconnected

membranes that serve a function involving protein synthesis and transport.

'ough endoplasmic reticulum #'ough ('$ is so5named because of its rough

appearance due to the numerous ribosomes that occur along the ('. 'ough ('

connects to the nuclear envelope through which the messenger '%A #m'%A$

that is the blueprint for proteins travels to the ribosomes. )mooth ('E lac!s theribosomes characteristic of 'ough (' and is thought to be involved in transport

and a variety of other functions.

+igure :. The endoplasmic reticulum. 'ough endoplasmic reticulum is on the

left, smooth endoplasmic reticulum is on the right.-mage from "urves et al.,

Life The )cience of 2iology, 3th (dition, by )inauer Associates

#www.sinauer.com$ and 4 +reeman #www.whfreeman.com$, used with

permission.

+igure 3.'ough (ndoplasmic 'eticulumwith 'ibosomes#T(8 x;,?;0$.This image is copyright &ennis =un!el at www.&ennis=un!el.com, used with

permission.
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ol!i "pparatus and #ictyosomes | Back to Top

Folgi Complexes, shown in +igure ? and ;, are flattened stac!s of

membrane5bound sacs. -talian biologist Camillo Folgi discovered these

structures in the late B@0s, although their precise role in the cell was notdeciphered until the mid5@00s . Folgi function as a pac!aging plant,

modifying vesicles produced by the rough endoplasmic reticulum. %ew

membrane material is assembled in various cisternae #layers$ of the golgi.

+igure ?.)tructure of the Folgi apparatus and its functioning in vesicle5

mediated transport.-mages from "urves et al., Life The )cience of 2iology, 3th

(dition, by )inauer Associates #www.sinauer.com$ and 4 +reeman

#www.whfreeman.com$, used with permission.
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+igure ;. Folgi Apparatus in a plant parenchyma cell from Sauromatum

guttatum#T(8 x3?,


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image is copyright &ennis =un!el at www.&ennis=un!el.com, used with

permission.

$ysosomes | Back to Top

Lysosomes, shown in +igure


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Mitochondria | Back to Top

8itochondriacontain their own &%A #termed m&%A$ and are thought to

represent bacteria5li!e organisms incorporated into eu!aryotic cells over


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+igure @. 8uscle Cell 8itochondrion #T(8 x@0,@70$.This image is

copyright &ennis =un!el at www.&ennis=un!el.com, used with permission.

Mitochondria and endosymbiosis

&uring the @B0s, Lynn 8argulis proposed the theory of endosymbiosis to

explain the origin of mitochondria and chloroplasts from permanent resident

pro!aryotes. According to this idea, a larger pro!aryote #or perhaps early


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eu!aryote$ engulfed or surrounded a smaller pro!aryote some .? billion to protomitochondrion> and excess sugar provided by the

>protochloroplast>, while providing a stable environment and the raw materials

the endosymbionts required. This is so strong that now eu!aryotic cells cannot

survive without mitochondria #li!ewise photosynthetic eu!aryotes cannot

survive without chloroplasts$, and the endosymbionts can not survive outside

their hosts. %early all eu!aryotes have mitochondria. 8itochondrial division isremar!ably similar to the pro!aryotic methods that will be studied later in this

course. A summary of the theory is available by clic!ing here.

%lastids | Back to Top

"lastids are also membrane5bound organelles that only occur in plants and

photosynthetic eu!aryotes. Leucoplasts, also !nown as amyloplasts #and shown

in +igure 7$ store starch, as well as sometimes protein or oils.Chromoplasts

store pigments associated with the bright colors of flowers and*or fruits.

+igure 7. )tarch grains ina fresh5cut potato tuber. -mage from

http**images.botany.org*set5:*:500Bv.6pg .
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Chloroplasts, illustrated in +igures 77 and 7:, are the sites of photosynthesis in

eu!aryotes. They contain chlorophyll, the green pigment necessary forphotosynthesis to occur, and associated accessory pigments #carotenesand

xanthophylls$ inphotosystemsembedded in membranous sacs, thyla!oids

#collectively a stac! of thyla!oids are a granum Gplural / granaH$ floating in a

fluid termed the stroma. Chloroplasts contain many different types of accessory

pigments, depending on the taxonomic group of the organism being observed.

+igure 77. )tructure of the chloroplast.-mage from "urves et al., Life The


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+igure 7:. Chloroplast from red alga #Griffthsiaspp.$. x?,


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Chloroplasts and endosymbiosis

Li!e mitochondria, chloroplasts have their own &%A, termed cp&%A.

Chloroplasts of Freen Algae#"rotista$ and "lants #descendants of some of the

Freen Algae$ are thought to have originated by endosymbiosis of a pro!aryoticalga similar to livingProchloron#the sole genus present in the

"rochlorobacteria, shown in +igure 73$. Chloroplasts of 'ed Algae#"rotista$

are very similar biochemically to cyanobacteria#also !nown as blue5green

bacteria Galgae to chronologically enhanced fol!s li!e myself $H$.

(ndosymbiosis is also invo!ed for this similarity, perhaps indicating more than

one endosymbiotic event occurred.

+igure 73.Prochloron, a photosynthetic bacteria, reveals the presence of

numerous thyla!oids in the transmission electron micrograph on the left.Prochloronoccurs in long filaments, as shown by the light micrograph on the

right below. -mage from

http**www.cas.muohio.edu*Iwilson!g*bot@*mouseth*m@p:7.6pg.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossG.html#green%20algaehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossR.html#red%20algaehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#cyanobacteriahttp://www.cas.muohio.edu/~wilsonkg/bot191/mouseth/m19p32.jpghttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossG.html#green%20algaehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossR.html#red%20algaehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#cyanobacteriahttp://www.cas.muohio.edu/~wilsonkg/bot191/mouseth/m19p32.jpg


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Cell Movement | Back to Top

Cell movementE is both internal, referred to as cytoplasmic streaming, and

external, referred to as motility. -nternal movements of organelles are governed

by actin filaments and other components of the cytos!eleton. These filaments

ma!e an area in which organelles such as chloroplasts can move. -nternal

movement is !nown as cytoplasmic streaming. (xternal movement of cells is

determined by special organelles for locomotion.

The cytos!eleton is a networ! of connected filaments and tubules. -t extends

from the nucleus to the plasma membrane. (lectron microscopic studies

showed the presence of an organized cytoplasm. -mmunofluorescencemicroscopy identifies protein fibers as a ma6or part of this cellular feature. The

cytos!eleton components maintain cell shape and allow the cell and its

organelles to move.

Actin filaments, shown in +igure 7?, are long, thin fibers approximately seven

nm in diameter. These filaments occur in bundles or meshli!e networ!s. These

filaments are polar, meaning there are differences between the ends of the

strand. An actin filament consists of two chains of globular actin monomers

twisted to form a helix. Actin filaments play a structural role, forming a dense

complex web 6ust under the plasma membrane. Actin filaments in microvilli ofintestinal cells act to shorten the cell and thus to pull it out of the intestinal

lumen. Li!ewise, the filaments can extend the cell into intestine when food is to

be absorbed. -n plant cells, actin filaments form tracts along which chloroplasts

circulate.
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Actin filaments move by interacting with myosin, The myosin combines with

and splits AT", thus binding to actin and changing the configuration to pull the

actin filament forward. )imilar action accounts for pinching off cells during

cell division and for amoeboid movement.

+igure 7?. )!eletal muscle fiber with exposed intracellular actin myosin

filaments. The muscle fiber was cut perpendicular to its length to expose the

intracellular actin myosin filaments. )(8 J770. This image is copyright

&ennis =un!el at www.&ennis=un!el.com, used with permission.

-ntermediate filaments are between eight and eleven nm in diameter. They are

between actin filaments and microtubules in size. The intermediate fibers are

rope5li!e assemblies of fibrous polypeptides. )ome of them support the nuclear

envelope, while others support the plasma membrane, form cell5to5cell

6unctions.


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8icrotubules are small hollow cylinders #7? nm in diameter and from 700 nm5

7? 1m in length$. These microtubules are composed of a globular protein

tubulin. Assembly brings the two types of tubulin #alpha and beta$ together as

dimers, which arrange themselves in rows.

-n animal cells and most protists, a structure !nown as a centrosome occurs.The centrosome contains two centrioles lying at right angles to each other.

Centrioles are short cylinders with a @ K 0 pattern of microtubule triplets.

Centrioles serve as basal bodies for cilia and flagella. "lant and fungal cells

have a structure equivalent to a centrosome, although it does not contain

centrioles.

Cilia are short, usually numerous, hairli!e pro6ections that can move in an

undulating fashion #e.g., the protzoanParamecium, the cells lining the human

upper respiratory tract$. +lagella are longer, usually fewer in number,

pro6ections that move in whip5li!e fashion #e.g., sperm cells$. Cilia and flagellaare similar except for length, cilia being much shorter. They both have the

characteristic @ K 7 arrangement of microtubules shown in figures 7;.

+igure 7;. Cilia from an epithelial cell in cross section #T(8 x@@,?00$.%ote

the @ K 7 arrangement of cilia.This image is copyright &ennis =un!el at

www.&ennis=un!el.com, used with permission.


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Cilia and flagella move when the microtubules slide past one another. 2oth oif

these locomotion structures have a basal body at base with thesame

arrangement of microtubule triples as centrioles. Cilia and flagella grow by the

addition of tubulin dimers to their tips.

+lagella wor! as whips pulling #as in ChlamydomonasorHalosphaera$ or

pushing #dinoflagellates, a group of single5celled "rotista$ the organism

through the water. Cilia wor! li!e oars on a vi!ing longship #Parameciumhas


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%ot all cells use cilia or flagella for movement. )ome, such asAmoeba, Chaos

(Pelomyxa)and human leu!ocytes#white blood cells$, employpseudopodiato

move the cell. nli!e cilia and flagella, pseudopodia are not structures, but

rather are associated with actin near the moving edge of the cell. The formation

of a pseudopod is shown in +igure 7B.

+igure 7B.+ormation and functioning of a pseudopod by an amoeboid cell.-mage from "urves et al., Life The )cience of 2iology, 3th (dition, by )inauer

Associates #www.sinauer.com$ and 4 +reeman #www.whfreeman.com$, used

with permission.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#leukocyteshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html#pseudopodiahttp://www.sinauer.com/http://www.whfreeman.com/http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#leukocyteshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html#pseudopodiahttp://www.sinauer.com/http://www.whfreeman.com/


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cellular organels.doc

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