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Metabolism Part 2: Krebs cycle This tutorial will take you through the basics of the Krebs cycle for SC 120 Click this button to move forward What was the first stage of metabolism? Sorry – anabolism is when your cell builds large molecules like Anabolism proteins. Sorry – catabolism refers to breaking Catabolism down cell molecules to get energy from them Main Menu RIGHT! ‘Glyco’Glycolysis = sugar ‘lysis’ = breaking ‘Gluco’ = sugar ‘genesis’ = making Glucogenesis Does your cell MAKE sugar in the first step of metabolism? Where did glycolysis take place? Mitochondria can’t cope with sugar molecules. They need Mitochondria smaller molecules. Sorry – the nucleus is where your DNA is stored. Your cell doesn’t do much Nucleus breaking down of things in there – it might damage the DNA! Main Menu Ribosomes are involved in creating Ribosomes protein – not in breaking anything down Right! As soon as glucose crosses the cell membrane and enters the Cytoplasm cytoplasm, the glycolysis enzymes grab it. What does glycolysis break glucose into? Right! Glycolysis breaks a 6-carbon glucose into two 32 Pyruvic acids carbon pyruvic acids. Glycolysis doesn’t break glucose down Carbon dioxide that far. and water Glycolysis takes off 2 pairs of electrons, but pairs of there is a2lot more than electronselectrons to a glucose molecule. Glucose already is a sugar. Main Menu Sugar Here’s a summary of glycolysis One molecule of glucose Two molecules of pyruvic acid (6 carbons) (3 carbons each) Energy released!! Net gain of 2 ATPs AND – 4 electrons are removed Main Menu Handed to 2 molecules of NAD Glycolysis was the first step. • It broke the glucose into pyruvic acids. • Now, those pyruvic acids will be passed into the mitochondria to be broken down further. Main Menu The glycolysis tutorial used a cooking metaphor. Energy released!! • Remember this guy? • Now he’s handing those pyruvic acids on to another set of enzymes. Clip art from Microsoft Main Menu Inside the mitochondria… • A whole group of enzymes is waiting. • These make up the KREBS CYCLE. • They will pass the pyruvic acid molecules along the line, gradually breaking them down to release energy, which they can use to make ATP. Clip art from Microsoft Main Menu The first step in the process is to take the pyruvic acid into the mitochondria. • This is done by a molecule called Coenzyme A. Clip art from Microsoft Main Menu Coenzyme A • Coenzyme A doesn’t just carry pyruvic acid. • It also modifies it, by taking a carbon off the end of it. • The Krebs Cycle enzymes will only accept 2-carbon compounds. Main Menu Removing a carbon also involves removing 2 electrons One molecule of pyruvic acid One acetyl molecule (3 carbons) (2 carbons ) One carbon is lost as CO2 AND – 2 electrons are removed Main Menu Handed to a molecule of NAD What’s NAD? That’s right! NAD and FAD are used to hold An electron electrons, so the electronscarrier can’t damage the cell. Organelles are really large compared to the A cell we’re organelle molecules dealing with. Main Menu NAD isn’t made during cell A product of respiration. It’s Cellfor respiration needed cell respiration. NAD isn’t a sugar. It’s actually more like the A sugar molecule compounds that make up DNA. The 2-carbon molecule is passed to the Krebs cycle enzymes • They pass it along and break it into two CO2 molecules Clip art from Microsoft Main Menu Glucose Pyruvic acid Acetyl molecule What about ATP? • The glycolysis pathway made 2 ATPs when it broke glucose. •The Krebs cycle makes another ATP for every acetyl molecule it breaks down. Main Menu Two CO2 molecules Glucose ATP Pyruvic acid ATP Acetyl molecule ATP Two CO2 molecules from each acetyl Main Menu Glucose Pyruvic acid For every glucose you eat, you get: ATP • 2 ATPs from glycolysis ATP • 2 acetyl molecules 2 more ATP ATPs from the Krebs cycle. ATP Main Menu Acetyl molecule Two CO2 molecules from each acetyl Acetyl molecule AND electrons are taken off in the Krebs cycle: For every acetyl molecule • 3 pairs of high-energy electrons handed to NAD Two CO2 molecules from each acetyl Main Menu • 1 pair of lower-energy electrons handed to FAD When a pair of electrons is handed to NAD, what happens? Right! Remember GER: Gain of Electrons is NAD is reduced Reduction. Remember LEO: Loss of Electrons is Oxidation. NAD is oxidized Did the NAD lose electrons? Remember GER: Gain of Electrons is Reduction. Acetyl is reduced Did the acetyl molecule CO2 was formed, but not by handing electrons to CO2 is produced NAD. gain or lose electrons? Main Menu Let’s review metabolism so far. Which pathway breaks glucose into pyruvic acids? ‘gluco’ = glucose ‘genesis’ = creation Glucogenesis So this would be making glucose, not breaking it! The Krebs cycle can’t use glucose. It will only metabolize 2-carbon Krebs cycle compounds. Main Menu Right! Glycolysis does the initial breakdown of sugar. Glycolysis Sorry, Coenzyme A is the messenger that carries Coenzyme Aglucose the broken-down into the Krebs cycle. Redo Questions How many carbons does a glucose molecule have? sorry, that’s too small! 2 Sorry, that’s too small! 4 Main Menu Right! Glucose is C6H12O6. 6 Sorry, that’s too big! 8 Redo Questions How many carbons does a pyruvic acid molecule have? sorry, that’s too small! 2 That’s right! When you split a 6-carbon glucose in half, you get 3 two 3carbon pyruvic acids. Main Menu That’s too big – if you broke glucose in half, 4 two 4would you get carbon compounds? The glucose only had 6 carbons! So if you broke 6 still end up it, would you with 6-carbon compounds? Redo Questions How many net ATPs did glycolysis make? That’s right! 2 Sorry – recheck the pathway. 3 Main Menu Glycolysis did make 4 ATPs, but it used 2. So 4 net GAIN what was the in ATPs? We wish! Glycolysis is just not efficient enough to make this6 many ATPs. Redo Questions How can the pyruvic acids get to the Krebs cycle for further breakdown? Sorry – the nucleus Coenzyme A takes doesn’t break down themso there pyruvic acids, the nucleus is nointo point taking them in there. The problem with this Electron carriers answer is that electron take them the carriers carry into electrons, mitochondrion not pyruvic acids. Right! Coenzyme A takes them into the mitochondrion There are two problems with this. FAD First, takes FAD is them an electron carrier and can’t carry pyruvic into the acids. Second, the pyruvic acids arecytoplasm already in the cytoplasm! Main Menu Redo Questions What happens to pyruvic acids on the way into the mitochondria? This isn’t quite right. Coenzyme A breaks Coenzyme A can’t them down completely breakinto down CO2 pyruvic acids. If glycolysis could do this, the pyruvic acids Glycolysis enzymes wouldn’t needinto to go into break them CO 2 the mitochondria. Right! The Krebs cycle likes 2-carbon compounds, so Coenzyme A breaks a carbon Coenzyme A breaks off each one pyruvic carbonacid. off them This can happen, but it only happens if there They are turned intois no Oxygen the cell Lacticand acids cannot run the Krebs cycle. Main Menu Redo Questions When a carbon is taken off the pyruvic acids, what are they called? Sorry – Coenzyme A is the molecule that carries them. Coenzyme A Sorry – lactic acid is a 3carbon compound made Lactic acid when electrons are handed to pyruvic acid. Glycogen would be a molecule that made glucose.Glycogen These little molecules are made FROM glucose. Right! The two-carbon group is an acetyl Acetyl group. When Coenzyme A is carrying one, we call it acetyl CoA. Main Menu Redo Questions What do the Krebs cycle enzymes do to the acetyl molecule? Sorry – this is what happensReduce to pyruvic it, acid if it CAN’T the formingenter lactic acid Krebs cycle That’s right! The Krebs Break it down into cycle breaks the acetyl two molecules molecule down and the of CO2 CO2 is exhaled. Sorry- the whole process is aimed at Put them together breaking glucose down, to make glucose not forming it. That would not make much sense, after Coenzyme A did Pass all that workittoback moveto it from the cytoplasm into the the cytoplasm mitochondria. Main Menu Redo Questions How many ATPs will the Krebs cycle make when it breaks down one acetyl molecule? Right! 1 Sorry- the Krebs cycle is not this efficient. 2 Main Menu Sorry- the Krebs cycle is not this efficient. 3 Sorry- the Krebs cycle is not this efficient. 4 Redo Questions Glucose Pyruvic acid For every glucose you eat, you get: ATP • 2 ATPs from glycolysis ATP • 2 acetyl molecules 2 more ATP ATPs from the Krebs cycle. ATP Main Menu Acetyl molecule Two CO2 molecules from each acetyl Four ATPs? You went through all that for four measly ATPs? • That’s not enough ATPs to keep you alive. • When scientists studied the actual amount of ATP cells got from a glucose, they found the cells got way more than 4. • Cells get more like 36 ATPs from one glucose molecule. How are they doing it? Main Menu The review quiz left something out. • • • • It asked about glycolysis And about the Krebs cycle And about coenzyme A And about pyruvic acids and acetyl molecules • But it didn’t mention ELECTRONS. Main Menu Every time a molecule was broken, electrons were removed. • Electrons were removed during glycolysis • And by Coenzyme A • And by the Krebs cycle Main Menu Glucose Pyruvic acid Acetyl molecule Glycolysis removed 2 pairs of electrons Then Acetyl CoA removed another pair from each pyruvic acid And the Krebs cycle took 4 more pairs off each acetyl group Main Menu Two CO2 molecules from each acetyl So all in all: Glucose Pyruvic acid Acetyl CO2 Main Menu 12 pairs of electrons were removed as the glucose was broken down. Pyruvic acid Acetyl CO2 Where did all those electrons go? That would be really bad. The electrons Into the nucleus would damage the DNA and mutate the cell. Sorry- there’s not an easy way to exhale They were exhaled electrons. This would cause damage to the blood vessels. Into the blood Right! Remember NAD and FAD, Onto the electron carriers? electron carriers Main Menu So all in all: Glucose Pyruvic acid Acetyl CO2 Main Menu 10 pairs of electrons were given to NAD 2 pairs were given to FAD Pyruvic acid Acetyl CO2 By the end of the Krebs cycle, you have: 10 NADs filled with electrons … Main Menu and two FADs filled with electrons. How would you describe this? The glucose has been brokenThe down – but it has glucose lost has electrons. been reduced Remember LEO: Loss of Electrons is Oxidation. That’s right! Gain of Electrons is Reduction The NADs and FADs – thehave NADs andreduced FADs been have gained electrons. The pyruvic acid has been broken down – but it has lost electrons. LEO: The Remember pyruvic acid Losshas of Electrons is been reduced Oxidation. Remember LEO: Loss of Electrons is Oxidation. The Theand NADs and FADs NADs FADs have GAINED electrons, so they haven’t have been oxidized been oxidized. Main Menu The mitochondrion is like one bag inside another. This is the outer space of the mitochondrion This is the inner space of the mitochondrion, where the Krebs cycle enzymes are. This is the inner mitochondrial membrane This is the outer mitochondrial membrane Main Menu After the Krebs cycle has been running a while… The inner space of the mitochondrion is full of reduced NADs and FADs. Main Menu And buzzing around all those electrons … • Are swarms of H+ ions. What a mess! Main Menu When your electron carriers are full, metabolism must stop. • Then the cell won’t be able to make any more ATP, and it will die. ‘ Main Menu Something has to take out the trash. • This is a job for OXYGEN! • An Oxygen atom can pick up two electrons, and then two H+ will follow the electrons. O + 2 e- + 2 H+ H2O Clip art from Microsoft Main Menu O+2 e +2 + H H2O • Oxygen is your cell’s janitor. • It takes the dangerous electrons and H+ ions and carries them out as harmless water. • This is the reason you need Oxygen to live. Main Menu Some cells can live without Oxygen • They have to use some other molecule as a ‘janitor.’ • For instance, some cells use Sulfur, and they produce H2S when the sulfur carries the electrons and H+ ions out. • The next time you wade in a muddy pond, see if you can smell the H2S made by these cells. Main Menu Humans can only use Oxygen to carry out the electrons and H+ ions. • We are obligate aerobes. • ‘Obligate’ means obliged – we have no choice. • ‘Aerobes’ means Oxygen users. Main Menu All this has explained why we need Oxygen, • But not where those extra 32 ATPs are made. • They are made in the process of loading up the Oxygen with the electrons and H+ ions. • Let’s look back inside the mitochondrion… Main Menu Here’s our mitochondrion , full of reduced electron carriers and H+ ions. • Click here to zoom in Main Menu Inner space of mitochondrion – this is where the Reduced electron carriers and the H+ ions are Outer space of mitochondrion is separated from the inner space by a semi-permeable membrane. The reduced electron carriers and H+ ions cannot diffuse through that membrane, so they must stay in the inner space – unless a protein lets them cross. Main Menu Inner space of mitochondrion – this is where the Reduced electron carriers and the H+ ions are These are proteins in the membrane – the cytochromes. They can let electrons and H+ ions through. This set of proteins is called the ELECTRON TRANSPORT CHAIN. Main Menu The reduced NAD dumps its electrons into one of the cytochromes. Now it can go back and do its job, picking up new electrons. The H+ ions stay right here, though. They were never interested in the NAD – only in its electrons. So now, they want to hang around the cytochrome that has the electrons. Main Menu The cytochrome’s job is to let the electrons and H+ through the membrane. Click on the first cytochrome to see it do its thing. Main Menu See that again The thing is - the cytochrome lets electrons go both ways across the membrane, but H+ ions can only go one way. Click on the first cytochrome again to see what this means. Main Menu See that again Now two of the H+ ions are stuck in the outer region of the mitochondrion. But the two electrons have gone back inside – to be handed to the next cytochrome. Click on the second cytochrome to see it do its thing. Main Menu See that again By now, I’m sure you know what the third cytochrome will do! Click on the third cytochrome to see it do its thing Main Menu See it all again There’s only one protein in the membrane that will let the H+ ions back through – ATP synthase. It takes the energy from the H+ ions and uses it to make ATP. ATP ATP synthase Click on the ATP synthase to see it do its thing. Main Menu See that again ATP Now Oxygen picks up the electrons and the H+ ions and carries them out of the cell. ATP synthase Click on Oxygen to see him do his thing Main Menu See that again Since three pairs of H+ ions followed the electrons across the membrane, the ATP synthase will make 3 ATPs from letting them back in. ATP ATP ATP ATP synthase Click on the ATP synthase to see it do its thing. Main Menu See it all again The reduced NAD dumps its electrons into the first cytochrome, so they go back and forth three times and lure 3 pairs of H+ across the membrane, which make 3 ATPs coming back in. Reduced FAD has less energy, and can only hand its electrons to the second cytochrome. They only cross the membrane twice and lure 2 pairs of H+ across, so only 2 ATP are made when the H+ go back across the membrane. Main Menu From metabolizing one molecule of glucose, Your cell produced 10 reduced NADs and 2 reduced FADs. Main Menu How many ATPs will they make? 10 reduced NADs = 30 ATPs … Main Menu and two reduced FADs = 4 ATPs. So all in all: Glucose Pyruvic acid Acetyl Glycolysis made 2 ATP The KREBS cycle made 2 ATP 10 reduced NAD = 30 ATP Pyruvic acid Acetyl 2 reduced FAD = 4 ATP CO2 Main Menu The total is 38 ATP CO2 Where is the Krebs cycle located? Sorry – that’s where glycolysis is. In the cytoplasm No, it’s inside an organelle. On the You’re close, but not quite right. In the outer space of the mitochondrion Right! Main Menu cell membrane In the inner space of the mitochondrion Where is the electron transport chain located? Sorry – since it uses lots of reduced NAD and FAD the cytoplasm from In the Krebs cycle, it’s in the same place as the Krebs cycle. It’s on a membrane, but not the cell Onmembrane. the Sorry- the endoplasmic reticulum is for making In the endoplasmic proteins. reticulum Right! It is in the inner membrane of the In the mitochondria mitochondrion. Main Menu cell membrane Redo Questions What are the proteins in the electron chain called? Right! cytochromes Sorry- chromatids are single copies of a chromosome chromatids Main Menu Sorry- chromosomes are the packages of DNA in chromosomes your cell’s nucleus. Cytoplasm is the material filling the cell. cytoplasm Redo Questions What do those proteins do? The electrons were Move electrons already in the inner into the inner space space! of the mitochondrion Right! This is partly true, but they do more. Move electrons into the outer space of the mitochondrion The electrons were Move electrons to already thespace inner thein inner space!and then back Main Menu Move electrons to the outer space and then back to the inner space to the outer space Redo Questions What do H+ ions do? They were already in the Move inner space! into the inner space of the mitochondrion Not quite. The electrons Move to can move back in the outer space through the and then back cytochromes, the H+ to the innerbut space ions can’t. That’s right! They follow the electrons into the Move outer intospace. the outer space of the mitochondrion They were already in the Move to inner the space! inner space Main Menu and then back to the outer space Redo Questions How does the electron transport chain make ATP? That’s what glycolysis dies, not the electron By breaking transport chain.glucose That’s right! The energy fromH+ theions H+pass ionsback is used into the inner space to make ATP. The electron transport chain doesn’t have any enzymes for breaking By breaking fats things. Not quite -- it’s not electrons the Electronsthat passrun through enzyme. an ATP-producing enzyme Main Menu through the ATP synthase Redo Questions How many ATPs do the electrons from one reduced NAD make? It’s more than that! How many times do they go 1 across the inner mitochondrial membrane? That’s right! It’s more than that! How many times do they go across the inner 2 mitochondrial membrane? It’s less than that! How many times do they go 4 across the inner mitochondrial membrane? Main Menu 3 Redo Questions How many ATPs do the electrons from one reduced FAD make? It’s more than that! How many times do they go 1 across the inner mitochondrial membrane? It’s less than that! How many times do they go 3 across the inner mitochondrial membrane? That’s right! It’s less than that! How many times do they go 4 across the inner mitochondrial membrane? 2 Main Menu Redo Questions How many ATPs do you get from the electron transport chain when you metabolize one molecule of glucose? This is the number of reduced NAD that send 10 electrons through the electron transport chain That’s the total number of ATPs made in 38 the glycolysis plus Krebs cycle plus the electron transport chain. That’s the number of ATPs you make in glycolysis, or 2 in the Krebs cycle. That’s right! 30 from the 10 reduced NADs, and 4 34 from the 2 reduced FADs. Main Menu Redo Questions This only leaves one question. If you remember, I said cells made 36 ATP from a molecule of glucose. Glucose Pyruvic acid Acetyl Glycolysis made 2 ATP The KREBS cycle made 2 ATP 10 reduced NAD = 30 ATP Pyruvic acid Acetyl 2 reduced FAD = 4 ATP CO2 The total is 38 ATP So why did I say 36? Main Menu CO2 Glucose Pyruvic acid Acetyl CO2 Main Menu The answer is these pairs of electrons. They were removed from glucose and pyruvic acid OUTSIDE the mitochondria! Pyruvic acid Acetyl CO2 Glucose Pyruvic acid Acetyl CO2 Main Menu These electrons were handed to NAD, but the NAD was in the wrong place. It took 2 ATPs to move that reduced NAD into the mitochondria, so it could pass its electrons to the electron transport chain. Pyruvic acid Acetyl CO2 Glucose Pyruvic acid Acetyl Glycolysis made 2 ATP The KREBS cycle made 2 ATP 10 reduced NAD = 30 ATP Pyruvic acid Acetyl 2 reduced FAD = 4 ATP CO2 Minus 2 used to move the NAD TOTAL = 36 ATP per glucose Main Menu CO2 This is the end of the metabolism tutorial. • Here’s a question to work on at home: • Any kind of food can be broken down into 2-carbon chunks and used by the Krebs cycle. • How much ATP would you get if you ate a 16-carbon fatty acid and broke it into 2carbon chunks and fed it into the Krebs cycle? Main Menu