MitochondriaStructureandFunction-TheoreticalDetails

Inanattempttobeconciseandunderstandable,introductorylevelcoursesandtextbooksfrequentlypresentconceptsthataretechnicallycorrect,butleadtomisconceptionsonthepartofthestudentbecausetheyomittoomuch.IndiscussionsaboutmitochondriastudentsfrequentlycomeawaywithasuperficialunderstandingofthetruenatureofKrebs"cycle,electrontransport,respiratorycontrol,andoxidativephosphorylation.

Thesepagesweredesignedtosupplementlaboratoryworkwithmitochondriabyprovidingbackgroundinasmuchdetailasthestudentmightwish.Studentsatanylevelarelikelytoencountertermswithwhichtheyareunfamiliar.Aglossaryoftermsisincludedinthisprojectforyourconvenience.Abriefoverviewofmitochondriastructureandfunctionispresentedjusttogetyoustarted.Youcanthenwindyourwaythroughthemainpathofdetailedinformation.

Structure

Mitochondriaarebacteria-sizedorganelles,foundinthecytoplasmofvirtuallyalleukaryoticcells.Theyareespeciallyabundantincellsandpartsofcellsthatareassociatedwithactiveprocesses.Forexample,inflagellatedprotozoaorinmammaliansperm,mitochondriaareconcentratedaroundthebaseoftheflagellumorflagella.Incardiacmuscle,mitochondriasurroundthecontractileelements.Hummingbirdflightmuscleisoneoftherichestsourcesofmitochondriaknown.Thus,fromtheirdistributionaloneonesUSPectsthattheyareinvolvedinenergyproduction.

Weknownowthatmulticellularorganismsprobablycouldnotexistwithoutmitochondria.Mitochondriamakeefficientuseofnutrientmolecules,requiringoxygenintheprocess.Theyare,infact,whyweneedoxygenatall.

Thedouble-membranedmitochondrioncanbelooselydescribedasalargewrinkledbagpackedinsideofasmaller,unwrinkledbag.Thetwomembranescreatedistinctcompartmentswithintheorganelle,andarethemselvesverydifferentinstructureandinfunction.

Theoutermembraneisarelativelysimplephospholipidbilayer,containingproteinstructurescalledporinswhichrenderitpermeabletomoleculesofabout10kilodaltonsorless(thesizeofthesmallestproteins).Ions,nutrientmolecules,ATP,ADP,etc.canpassthroughtheoutermembranewithease.

Theinnermembraneisfreelypermeableonlytooxygen,carbondioxide,andwater.Itsstructureishighlycomplex,includingallofthecomplexesoftheelectrontransportsystem,theATPsynthetasecomplex,andtransportproteins.Thewrinkles,orfolds,areorganizedintolamillae(layers),calledthecristae(singlular:crista).Thecristaegreatlyincreasethetotalsurfaceareaoftheinnermembrane.Thelargersurfaceareamakesroomformanymoreoftheabove-namedstructuresthaniftheinnermembranewereshapedliketheoutermembrane.

Themembranescreatetwocompartments.Theintermembranespace,asimplied,istheregionbetweentheinnerandoutermembranes.Ithasanimportantroleintheprimaryfunctionofmitochondria,whichisoxidativephosphorylation.

ThematrixcontainstheenzymesthatareresponsIBLeforthecitricacidcyclereactions.Thematrixalsocontainsdissolvedoxygen,water,carbondioxide,therecyclableintermediatesthatserveasenergyshuttles,andmuchmore.Becauseofthefoldsofthecristae,nopartofthematrixisfarfromtheinnermembrane.Thereforematrixcomponentscanquicklyreachinnermembranecomplexesandtransportproteins.

Electronmicrographshaverevealedthethreedimensionalstructureofmitochondria.However,sincemicrographsarethemselvestwodimensional,theirinterpretationcanbemisleADIng.

Textsfrequentlyshowapictureofa"typical"mitochondrionasabacteria-sizedellipsoid(perhaps0.5by1micrometer).However,theyvarywidelyinshapeandsize.Electronmicrographsseldomshowsuchvariation,becausetheyaretwo-dimensionalimages.

Isolatedmitochondria,suchasfromhomogenizedmuscletissue,showaroundedappearanceinelectronmicrographs,implyingthatmitochondriaaresphericalorganelles.

Mitochondriainsitucanbefreeinthecytoplasmorpackedinamongmorerigidstructures,suchasamongthemyofibrilsofcardiacmuscletissue.Incellssuchasmuscle,itisclearthatmitochondriaarenotspherical,andoftenarenotevenellipsoid.Insometissues,themitochondriaarealmostfilamentous,acharacteristicthattwodimensionalmicrographsmayfailtoreveal.

Aplanarsectioncutsthroughoneorseveralpartsoftheorganelle,makingitappearthatthereismorethanone.Theimageweseeofacircularorellipsoidalorganellemaydisguisethetruenatureofthemitochondrion.