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Metabolism Part 2: Krebs cycle
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  1. 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

  2. What was the first stage of metabolism? Anabolism Sorry – anabolism is when your cell builds large molecules like proteins. Glycolysis RIGHT! ‘Glyco’ = sugar ‘lysis’ = breaking Catabolism Glucogenesis Sorry – catabolism refers to breaking down cell molecules to get energy from them ‘Gluco’ = sugar ‘genesis’ = making Does your cell MAKE sugar in the first step of metabolism?

  3. Where did glycolysis take place? Mitochondria Mitochondria can’t cope with sugar molecules. They need smaller molecules. Ribosomes Ribosomes are involved in creating protein – not in breaking anything down Nucleus Cytoplasm Sorry – the nucleus is where your DNA is stored. Your cell doesn’t do much breaking down of things in there – it might damage the DNA! Right! As soon as glucose crosses the cell membrane and enters the cytoplasm, the glycolysis enzymes grab it.

  4. What does glycolysis break glucose into? 2 Pyruvic acids Right! Glycolysis breaks a 6-carbon glucose into two 3-carbon pyruvic acids. Carbon dioxide and water Glycolysis doesn’t break glucose down that far. 2 pairs of electrons Sugar Glycolysis takes off 2 pairs of electrons, but there is a lot more than electrons to a glucose molecule. Glucose already is a sugar.

  5. Here’s a summary of glycolysis Two molecules of pyruvic acid (3 carbons each) One molecule of glucose (6 carbons) Energy released!! Net gain of 2 ATPs AND – 4 electrons are removed Handed to 2 molecules of NAD

  6. 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.

  7. The glycolysis tutorial used a cooking metaphor. • Remember this guy? • Now he’s handing those pyruvic acids on to another set of enzymes. Energy released!! Clip art from Microsoft

  8. 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

  9. 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

  10. 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.

  11. Removing a carbon also involves removing 2 electrons One acetyl molecule (2 carbons ) One molecule of pyruvic acid (3 carbons) One carbon is lost as CO2 AND – 2 electrons are removed Handed to a molecule of NAD

  12. What’s NAD? An electron carrier That’s right! NAD and FAD are used to hold electrons, so the electrons can’t damage the cell. A product of Cell respiration NAD isn’t made during cell respiration. It’s needed for cell respiration. A cell organelle A sugar molecule NAD isn’t a sugar. It’s actually more like the compounds that make up DNA. Organelles are really large compared to the molecules we’re dealing with.

  13. 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

  14. Pyruvic acid Acetyl molecule Glucose • 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. Two CO2 molecules

  15. Pyruvic acid Acetyl molecule Glucose ATP ATP ATP Two CO2 molecules from each acetyl

  16. Pyruvic acid Acetyl molecule Glucose ATP • For every glucose you eat, you get: • 2 ATPs from glycolysis • 2 acetyl molecules  2 more ATPs from the Krebs cycle. ATP Two CO2 molecules from each acetyl ATP ATP

  17. Acetyl molecule • AND electrons are taken off in the Krebs cycle: • For every acetyl molecule • 3 pairs of high-energy electrons  handed to NAD • 1 pair of lower-energy electrons  handed to FAD Two CO2 molecules from each acetyl

  18. When a pair of electrons is handed to NAD, what happens? NAD is oxidized NAD is reduced Right! Remember GER: Gain of Electrons is Reduction. Remember LEO: Loss of Electrons is Oxidation. Did the NAD lose electrons? CO2 is produced Remember GER: Gain of Electrons is Reduction. Did the acetyl molecule gain or lose electrons? Acetyl is reduced CO2 was formed, but not by handing electrons to NAD.

  19. Let’s review metabolism so far.Which pathway breaks glucose into pyruvic acids? Glycolysis Glucogenesis ‘gluco’ = glucose ‘genesis’ = creation So this would be making glucose, not breaking it! Right! Glycolysis does the initial breakdown of sugar. Coenzyme A The Krebs cycle can’t use glucose. It will only metabolize 2-carbon compounds. Krebs cycle Sorry, Coenzyme A is the messenger that carries the broken-down glucose into the Krebs cycle. Redo Questions

  20. How many carbons does a glucose molecule have? 6 2 sorry, that’s too small! Right! Glucose is C6H12O6. 8 Sorry, that’s too small! 4 Sorry, that’s too big! Redo Questions

  21. How many carbons does a pyruvic acid molecule have? 4 2 sorry, that’s too small! That’s too big – if you broke glucose in half, would you get two 4-carbon compounds? 6 That’s right! When you split a 6-carbon glucose in half, you get two 3-carbon pyruvic acids. 3 The glucose only had 6 carbons! So if you broke it, would you still end up with 6-carbon compounds? Redo Questions

  22. How many net ATPs did glycolysis make? 4 2 That’s right! Glycolysis did make 4 ATPs, but it used 2. So what was the net GAIN in ATPs? 6 Sorry – recheck the pathway. 3 We wish! Glycolysis is just not efficient enough to make this many ATPs. Redo Questions

  23. How can the pyruvic acids get to the Krebs cycle for further breakdown? Electron carriers take them into the mitochondrion Coenzyme A takes them into the nucleus Sorry – the nucleus doesn’t break down pyruvic acids, so there is no point taking them in there. The problem with this answer is that electron carriers carry electrons, not pyruvic acids. FAD takes them into the cytoplasm Right! Coenzyme A takes them into the mitochondrion There are two problems with this. First, FAD is an electron carrier and can’t carry pyruvic acids. Second, the pyruvic acids are already in the cytoplasm! Redo Questions

  24. What happens to pyruvic acids on the way into the mitochondria? Glycolysis enzymes break them into CO2 Coenzyme A breaks them down into CO2 This isn’t quite right. Coenzyme A can’t completely break down pyruvic acids. If glycolysis could do this, the pyruvic acids wouldn’t need to go into the mitochondria. They are turned into Lactic acids Right! The Krebs cycle likes 2-carbon compounds, so Coenzyme A breaks a carbon off each pyruvic acid. Coenzyme A breaks one carbon off them This can happen, but it only happens if there is no Oxygen and the cell cannot run the Krebs cycle. Redo Questions

  25. When a carbon is taken off the pyruvic acids, what are they called? Lactic acid Coenzyme A Sorry – Coenzyme A is the molecule that carries them. Sorry – lactic acid is a 3-carbon compound made when electrons are handed to pyruvic acid. Acetyl Glycogen would be a molecule that made glucose. These little molecules are made FROM glucose. Glycogen Right! The two-carbon group is an acetyl group. When Coenzyme A is carrying one, we call it acetyl CoA. Redo Questions

  26. What do the Krebs cycle enzymes do to the acetyl molecule? Break it down into two molecules of CO2 Reduce it, forming lactic acid Sorry – this is what happens to pyruvic acid if it CAN’T enter the Krebs cycle That’s right! The Krebs cycle breaks the acetyl molecule down and the CO2 is exhaled. Pass it back to the cytoplasm Sorry- the whole process is aimed at breaking glucose down, not forming it. Put them together to make glucose That would not make much sense, after Coenzyme A did all that work to move it from the cytoplasm into the mitochondria. Redo Questions

  27. How many ATPs will the Krebs cycle make when it breaks down one acetyl molecule? 3 1 Right! Sorry- the Krebs cycle is not this efficient. 4 Sorry- the Krebs cycle is not this efficient. 2 Sorry- the Krebs cycle is not this efficient. Redo Questions

  28. Pyruvic acid Acetyl molecule Glucose ATP • For every glucose you eat, you get: • 2 ATPs from glycolysis • 2 acetyl molecules  2 more ATPs from the Krebs cycle. ATP Two CO2 molecules from each acetyl ATP ATP

  29. 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?

  30. 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.

  31. Every time a molecule was broken, electrons were removed. • Electrons were removed during glycolysis • And by Coenzyme A • And by the Krebs cycle

  32. Pyruvic acid Acetyl molecule Glucose 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 Two CO2 molecules from each acetyl

  33. So all in all: Glucose Pyruvic acid Pyruvic acid 12 pairs of electrons were removed as the glucose was broken down. Acetyl Acetyl CO2 CO2

  34. Where did all those electrons go? They were exhaled Into the nucleus That would be really bad. The electrons would damage the DNA and mutate the cell. Sorry- there’s not an easy way to exhale electrons. Onto electron carriers This would cause damage to the blood vessels. Into the blood Right! Remember NAD and FAD, the electron carriers?

  35. So all in all: Glucose Pyruvic acid Pyruvic acid 10 pairs of electrons were given to NAD 2 pairs were given to FAD Acetyl Acetyl CO2 CO2

  36. By the end of the Krebs cycle, you have: 10 NADs filled with electrons … and two FADs filled with electrons.

  37. How would you describe this? The NADs and FADs have been reduced The glucose has been reduced The glucose has been broken down – but it has lost electrons. Remember LEO: Loss of Electrons is Oxidation. That’s right! Gain of Electrons is Reduction – the NADs and FADs have gained electrons. The NADs and FADs have been oxidized The pyruvic acid has been broken down – but it has lost electrons. Remember LEO: Loss of Electrons is Oxidation. The pyruvic acid has been reduced Remember LEO: Loss of Electrons is Oxidation. The NADs and FADs have GAINED electrons, so they haven’t been oxidized.

  38. 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

  39. After the Krebs cycle has been running a while… The inner space of the mitochondrion is full of reduced NADs and FADs.

  40. And buzzing around all those electrons … Are swarms of H+ ions. What a mess!

  41. 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. ‘

  42. 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

  43. 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.

  44. 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.

  45. 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.

  46. 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…

  47. Here’s our mitochondrion , full of reduced electron carriers and H+ ions. Click here to zoom in

  48. 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.

  49. 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.

  50. 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.