细胞生物学双语教案3(线粒体结构与功能).doc.pdf

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1、细胞生物学双语教案 3（线粒体结构与功能）Mitochondrial SubstructureMitochondria contain two membranes,separated by a space.Both are thetypical unit membrane(railroad track)in structure.Inside the spaceenclosed by the inner membrane is the matrix.This appears moderately denseand one may find strands of DNA,ribosomes,or

2、small granules in the matrix.The mitochondria are able to code for part of their proteins with thesemolecular tools.The above cartoon shows the diagram of the mitochondrialmembranes and the enclosed compartments.How are mitochondria organized to be powerhouses?The food we eat is oxidized to produce

3、high-energy electrons that are convertedto stored energy.This energy is stored in high energy phosphate bonds in amolecule called adenosine triphosphate,or ATP.ATP is converted fromadenosine diphosphate by adding the phosphate group with the high-energybond.Various reactions in the cell can either u

4、se energy(whereby the ATP isconverted back to ADP,releasing the high energy bond)or produce it(wherebythe ATP is produced from ADP).Steps from glycolysis to the electron transport chain.Why are mitochondriaimportant?Lets break down each of the steps so you can see how food turns into ATP1energy pack

5、ets and water.The food we eat must first be converted to basicchemicals that the cell can use.Some of the best energy supplying foodscontain sugars or carbohydrates.bread,for example.Using this as anexample,the sugars are broken down by enzymes that split them into thesimplest form of sugar which is

6、 called glucose.Then,glucose enters the cellby special molecules in the membrane called“glucose transporters”.Once inside the cell,glucose is broken down to make ATP in two pathways.The first pathway requires no oxygen and is called anaerobic metabolism.This pathway is called glycolysis and it occur

7、s in the cytoplasm outside themitochondria.During glycolysis,glucose is broken down into pyruvate.Otherfoods like fats can also be broken down for use as fuel(see following cartoon).Each reaction is designed to produce some hydrogen ions(electrons)that canbe used to make energy packets(ATP).However,

8、only 4 ATP molecules canbe made by one molecule of glucose run through this pathway.That is whymitochondria and oxygen are so important.We need to continue thebreakdown process with the Krebs cycle inside the mitochondriain order toget enough ATP to run all the cell functions.The events that occur i

9、nside and outside mitochondria are diagrammed in theabove cartoon.Pyruvate is carried into the mitochondria and there it isconverted into Acetyl Co-A which enters the Krebs cycle.This first reactionproduces carbon dioxide because it involves the removal of one carbon fromthe pyruvate.2How does the K

10、rebs cycle work?How does the Krebs cycle work?The whole idea behind respiration in the mitochondria is to use the Krebs cycle(also called the citric acid cycle)to get as many electrons out of the food we eatas possible.These electrons(in the form of hydrogen ions)are then used todrive pumps that pro

11、duce ATP.The energy carried by ATP is then used forall kinds of cellular functions like movement,transport,entry and exit ofproducts,division,etc.The following explanation is very simple and focuseson only the pathway from pyruvate through the cycle.However,it illustratesthe process and its function

12、s.To run the Krebs cycle,you need several important molecules in addition to allthe enzymes.Consult your text for details about the enzymes themselves.This presentation will focus on the electron donors,carriers and acceptors.First,you need pyruvate,which is made by glycolysis from glucose.Next,youn

13、eed some carrier molecules for the electrons.There are two types of these:one is called nicotinamide adenine dinucleotide(NAD+)and the other is calledflavin adenine dinucleotide(FAD+).The third molecule,of course,isoxygen.Pyruvate is a 3 carbon molecule.After it enters the mitochondria,it is brokend

14、own to a 2 carbon molecule by a special enzyme(see text for more detailsabout the biochemistry of each step).This releases carbon dioxide.The 2carbon molecule is called Acetyl CoA and it enters the Krebs cycle by joiningto a 4 carbon molecule called oxaloacetate.Once the two molecules are joined,the

15、y make a 6 carbon molecule called citric acid(2 carbons+4 carbons=6carbons).That is where the Citric acid cycle got its name.from that firstreaction that makes citric acid.Citric acid is then broken down and modifiedin a stepwise fashion(see text for details)and,as this happens,hydrogen ionsand carb

16、on molecules are released.The carbon molecules are used to makemore carbon dioxide and the hydrogen ions are picked up by NAD and FAD(see below).Eventually,the process produces the 4 carbon oxaloacetate again.The reason the process is called a cycle,is because it ends up always where itstarted.with

17、oxaloacetate available to combine with more acetyl coA.What is“oxidative phosphorylation”?What is“oxidative phosphorylation”?First,some basic definitions.When you take hydrogen ions or electrons awayfrom a molecule,you“oxidize”that molecule.When you give hydrogen ions3or electrons to a molecule,you“

18、reduce”that molecule.When you givephosphate molecules to a molecule,you“phosphorylate”that m olecule.So,oxidative phosphorylation(very simply)means the process that couples theremoval of hydrogen ions from one molecule and giving phosphate moleculesto another molecule.How does this apply to mitochon

19、dria?As the Krebs cycle runs,hydr ogen ions(or electrons)are donated to the twocarrier molecules in 4 of the steps.They are picked up by either NAD or FADand these carrier molecules become NADH and FADH(because they now arecarrying a hydrogen ion).The following cartoon shows what happens next.These

20、electrons are carried chemicallyto the respiratory or electron transportchain found in the mitochondrial cristae(see cartoons above and below thisparagraph).The NADH and FADH essentially serve as a ferry in the lateralplane of the membrane diffusing from one complex to the next.At each site is ahydr

21、ogen(or proton)pump which transfers hydrogen from one side of themembrane to the other.This creates a gradient across the inner membranewith a higher concentration of Hydrogen ions in the intercristae space(this isthe space between the inner and outer membranes).The following cartoon shows the indiv

22、idual complexes in the electron transportchain.The electrons are carried from complex to complex by ubiquinone andcycochrome C.4The third pump in the series catalyzes the transfer of the electrons to oxygento make water.This chemiosmotic pumping creates an electrochemical protongradient across the m

23、embrane which is used to drive the energy producingmachine.the ATP synthase.This molecule is found in small elementaryparticles that project from the cristae.The cartoon below shows an elementaryparticle.Also see its projection from the inner membrane in the previous figureshowing the overview of th

24、e cristae.As stated above,this process requires oxygen which is why it is called aerobicmetabolism.The ATP synthase uses the energy of thehydrogen ion(also5called proton)gradient to form ATP from ADP and Phosphate.It also produceswater from the hydrogen and the oxygen.Thus,each compartment in themit

25、ochondrion is specialized for one phase of these reactions.This is how oxidation is coupled to phosphorylation:To review:NAD and FAD remove the electrons that are donated during someof the steps of the Krebs or Citric acid cycle.Then,they carry the electrons tothe electron transport pumps and donate

26、 them to the pumps.So,NAD andFAD are“oxidized”because they lose the hydrogen ions to the pumps.Thepumps then transport the hydrogens ions to the space between the twomembranes where they accumulate in a high enough concentration to fuel theATP pumps.With sufficient fuel,they“phosphorylate”the ADP.Th

27、at is how“oxidation”is coupled to“phosphorylation”.The hydrogens that get pumped back into the matrix by the ATP pump thencombine with the oxygen to make water.And that is very important because,without oxygen,they will accumulate and the concentration gradient needed torun the ATP pumps will not al

28、low the pumps to work.So,why do we need mitochondria?So,why do we need mitochondria?The whole idea behind this process is to get as much ATP out of glucose(orother food products)as possible.If we have no oxygen,we get only 4molecules of ATP energy packets for each glucose molecule(in glycolysis).How

29、ever,if we have oxygen,then we get to run the Krebs cycle to producemany more hydrogen ions that can run those ATP pumps.From the Krebscycle we get 24-28 ATP molecules out of one molecule of glucose converted topyruvate(plus the 4 molecules we got out of glycolysis).So,you can see howmuch more energ

30、y we can get out of a molecule of glucose if our mitochondriaare working and if we have oxygen.Importance of the cristaeYou can now appreciate the importance of the cristae.not only do they containand organize the electron transport chain and the ATP pumps,they also serve toseparate the matrix from

31、the space that will contain the hydrogen ions,allowingthe gradient needed to drive the pump.When the discussion focuses on howmitochondria move proteins into the matrix,you will see another reason whythis hydrogen ion(proton)gradient is so important!6As shown in the above cartoons,the molecules in t

32、he electron transport chainare found as a cluster organized in the cristae.These membrane shelves may bemore numerous in mitochondria that are more active in the production of ATP.Thus,they may increase the density of these membranes as the need arises.Theflight muscle of a hummingbird has many cris

33、tae in each mitochondrion,because the need is so great.Structure and function of the inner membrane and elementary particles.Mitochondria can be separated and the inner and outer membrane can bedissociated.This will result in a fraction containing only the inner membraneand the matrix.These have bee

34、n called mitoplasts.They are functional andhave helped us learn more about the compartmentation of mitochondria.Onecan open mitoplasts and view the inside membrane surface after negativelystaining the membranes.This deposits stain around any surface projections.With this method,one can see the eleme

35、ntary particles projecting from theinner surface of the cristae.These are the ATP synthase molecules(orelementary particles)discussed in the previous section.The cartoons in the previous section showed cytochrome C lying just outsidethe inner membrane.It is a loosely attached peripheral protein lyin

36、g in thespace contained by the cristae.In fact,if the outer membrane is removed,often the cytochrome C is lost and must be replaced to promote function of themitoplast.7How do cytochemists know that cytochrome C is on the inner membrane?Wecan do cytochemical tests for this cytochrome and the results

37、 are shown in thisfigure.Note that the enzyme reaction product is confined to the cristae and infact delineates the cristae.Unfortunately,as is the case with most enzymecytochemistry,the reaction product spreads and it looks like it fills theinter-membrane space.This reflects the orientation of cytochrome C.It is foundin space inbetween cristae membranes which suggests it is next to the outerleaflet of the cristae membrane,rather than the inner leaflet(opposite to that ofthe elementary particles,or ATP synthetase).8