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Chapter 7 Vocabulary: Heterotrophs Autotrophs Aerobic cellular respiration Anaerobic cellular respiration Dehydrogenations Electron transport chain Substrate level phosphorylation Oxidative phosphorylation ATP synthase Fermentation Outline 7.1 Overview of Respiration Autotrophs Heterotrophs Cellular respiration – oxidation of food to produce energy Dehydrogenation Redox reactions – electrons take their energy with them (energy determined by orbital position) Aerobic Anaerobic Fermentation Electron carriers (NADH and FADH2) Uses a step wise pathway to harness max amount of energy (electron transport chain) ATP is energy Made by Substrate level phosphorylation (glycolysis and Kreb’s) Oxidative phosphorylation (ETC) 7.2 Glycolysis: Splitting Glucose Occurs in the cytoplasm Breaks down glucose into 2 pyruvate molecules 3 things happen through glycolysis: Priming (endergonic stage) Cleavage (6 carbon glucose split into 2 3 carbon molecules – ultimately pyruvate) Oxidation (which results in ATP formation during exergonic stage) First stage is endergonic (requires energy): Glucose is phosphorylated by ATP (glucose 6 phosphate) *This is the phosphorylation that traps glucose inside the cell!!! Rearranged into fructose 6 phosphate (this is a reversible isomerization) Glucose is phosphorylated by ATP (fructose 1,6 bisphosphate) (It costs 2 ATP to start glycolysis) Phosphorylation provides energy necessary to make molecule reactive – push over threshold. Second stage is exergonic: This second phosphorylation causes the double phosphorylated molecule to split into 2 molecules of glyceraldehyde 3 phosphate (actually it splits into one molecule of glyceraldehyde 3 phosphate and one molecule of dihydroxy acetone, but the dihydroxy acetone is converted to glyceraldehyde 3 phosphate.) Glyceraldehyde 3 phosphate is converted to an unstable intermediate and gives one of its phosphate groups to ADP ATP Glyceraldehyde 3 phosphate also liberates electrons and hydrogen which are picked up by the electron carrier NAD+ NADH Several reactions producing intermediates H2O is released from pathway PEP (phospho-enol pyruvate) is converted to pyruvate when it looses a phosphate group to ADP ATP This is substrate level phosphorylation – direct transfer of a phosphate from a substrate of a reaction to another molecule (ADP in this case) NET PRODUCTS: 2 molecules of pyruvate 2 ATP 2 NADH 2 H2O End products will move on Don’t bit off more than you or your students can chew as you go through the chemistry and the nomenclature here. At the end of the day we are looking for an overall understanding of this pathway. Students will NOT need to know the 9 intermediates involved, but sometimes looking at the chemistry can help them understand what’s happening. NAD+ must be present for glycolysis to continue. Can be regenerated by: Aerobic respiration (oxygen will ultimately accept the electrons from NADH returning it to NAD+) Fermentation (organic molecules can accept electrons from NADH returning it to NAD+) Anaerobic respiration (an inorganic molecule other than oxygen will act as TEA regenerating NAD+) I don’t know why this alternative is not mentioned in the book and I think it confuses the issue between anaerobic respiration and fermentation for it not to be mentioned. 7.3 The Oxidation of Pyruvate to Produce Acetyl-CoA Occurs in the mitochondrion There is a transport protein to bring pyruvate into the inner mitochondrial membrane. An enzyme removes one C from pyruvate which is released as CO2. Acetyl Co-A is formed for the Krebs’ cycle. Also formed is 1 NADH each (so 2 per glucose) NET PRODUCTS: 2 CO2 2 NADH I am a fan of tracking carbons. If pyruvate has 3 and acetyl Co-A has 2 this is how we are able to create CO2. 7.4 Kreb’s Cycle Occurs in the mitochondrion Cyclic pathway Acetyl Co-A transfers two carbons to oxaloacetate forming citrate (Kreb’s is also called the Citric Acid Cycle due to citrate being the first stable intermediate) An isomerization produces isocitrate An oxidation reaction produces NADH and a decarboxylation produces CO2. The remaining intermediate is alpha-ketoglutarate. In a similar oxidation / decarboxylation reaction succinyl-CoA is formed and NADH and CO2 are released. Substrate level phosphorylation produces ATP and results in succinate being left in the cycle. You can mention the role of GTP here if you want to, but I tend to gloss over it. A weaker oxidation (not associated with a decarboxylation) produces FADH2 and fumarate. The addition of water creates malate. The oxidation of malate to regenerate oxaloacetate also produces NADH. As the cycle turns it produces CO2, ATP, NADH, FADH2 and regenerates oxaloacetate NET PRODUCTS (from one pyruvate from two pyruvate): 3 NADH 6 NADH 1 FADH2 2FADH2 1ATP 2 ATP 2 CO2 4 CO2 Students do not need to know intermediates here either; but I always mention them and show a picture of chemical structures and names. Debatably, the most important thing generated here are the electron carriers because of the amount of energy they will be able to produce in the ETC. 7.5 Electron Transport Chain and Chemiosmosis Electron carriers delivered to the inner mitochondrial membrane from glycolysis and Kreb’s NADH and FADH2 donate electrons which flow along the electron transport chain in the inner mitochondrial membrane. H are pumped out and allowed to come back in through ATP synthase producing ATP. Spent electrons are picked up by the terminal electron acceptor (oxygen for aerobic respiration) Oxygen also picks up hydrogen and water is produced O2 plus the 10 NADH and 2 FADH2 from Glycolysis, Oxidation of pyruvate and Krebs 34 ATP and 4 H2O This is oxidative phosphorylation!!! They need to have an appreciation for this part of the process. 7.6 Energy Yield of Aerobic Respiration TOTAL NET YIELD FROM AEROBIC RESPIRATION: Glycolysis 2ATP Krebs 2 ATP ETC 34 ATP 38 ATP Energy yield can fluctuate. NADH from glycolysis can’t enter mitochondria; it must pass its electrons to transport proteins. This costs ATP. NADH passes electrons at a point on the chain that produces 3 ATP. FADH2 makes only 2 ATP. ***The textbook provides a modified energy yield of 32 ATP due to alternate calculations of ATP generated from electron carriers. It assumes NADH produces 2.5 ATP and FADH2 produces 1.5. While I have recently found some discussion on this I am NOT inclined to teach it this way. In my opinion it is a case of researchers trying to make the actual equal the theoretical.*** They have a hard enough time keeping up with all the letters and numbers why add to the confusion! 7.7 Regulation of Aerobic Respiration Cells control the rate of cellular respiration through a system of feedback inhibition Excess ATP shuts down the pathway and stops ATP production As ATP is used by the cell it triggers the production of more. 7.8 Oxidation Without Oxygen Anaerobic cellular respiration uses the same pathway already described, but it does not use oxygen as its terminal electron acceptor Sulfur, nitrate, carbon dioxide or even inorganic metals serve as TEAs Fermentation Starts with glycolysis Glycolysis yields 2 ATP, 2 pyruvate, 2 NADH An organic substance formed from glucose is the TEA Fermentation regenerates NAD+ Alcoholic fermentation: Pyruvate is converted to acetaldehyde Acetaldehyde accepts electrons from NADH and becomes ethanol (alcohol). CO2 is produced Prominent in baking and brewing industries Baking – CO2 makes bread light and airy, ethanol evaporates away Brew – yeast feed on sugar in grapes and produce ethanol. Can occur in fungi, plants and bacteria, yeast Lactate fermentation: Pyruvate accepts electrons from NADH This regenerates NAD+ Electron transfer converts pyruvate to lactate Used to produce yogurt, cheese, sauerkraut, etc. Muscles use when there is an O2 debt Lactate can be converted back to pyruvate when O2 is available again Occurs in animals, some fungi and some bacteria Our muscles have slow-twitch muscle fibers for prolonged activity using aerobic respiration. These cells are darkened due to the large numbers of myoglobin (bind and store oxygen) Our muscles also have fast-twitch muscle fibers for short bursts of immediate intense use. These cells are paler and perform anaerobic fermentation when needed. Muscle composition is the difference between great sprinters and great marathon runners. 7.9 Catabolism of Proteins and Fats Nucleic Acids: nitrogenous bases converted to nitrogenous waste, carbons enter glycolysis or Kreb’s Proteins: amino group (nitrogen group) is removed as waste and carbon backbone goes into glycolysis and/or Kreb’s Fats: glycerol goes into glycolysis and carbon backbone goes into Kreb’s 7.10 Evolution of Metabolism Carbon degradation Glycolysis Anoxygenic photosynthesis Oxygen forming photosynthesis Nitrogen fixation Aerobic respiration I never have time to cover this section This chapter is a pathway chapter. Students need to understand all the steps in the pathway with particular interest on starting and ending molecules and what is produced (electron carriers, ATP, CO2, etc). Be careful not to loose the forest for the trees. It is more important that they know Kreb’s produces electron carriers for the ETC than it is that oxaloacetate is the molecule that accepts acetyl-CoA. If you mention differences in ATP yield you should only do so as examples of diversity. It is more important that they know textbook yields than specializations. Students don’t need to know the molecular mechanisms for fermentation, but should know what it is accomplishing. You don’t need to cover evolution of metabolism. I have also included a document on nutrition. I like to spend a day talking about alternate energy (7.9 expanded) and how it relates to what they eat. I also like for them to bring in food labels or to log what they eat and what’s in it a few days prior to this discussion. It makes it more meaningful when they are looking at their list and Monster has 100 calories and 27g of sugar per 8oz and there are 24oz in the can they drank. That’s 300 calories and almost 90g of sugar! If they only need 2200 calories a day that’s 13% of their calories in a single drink! 1 sausage egg McMuffin (sorry McDonald’s) has 450 calories and 27g of fat (that is 47% of the fat recommended for an entire day!) There are tons of sites they can look up calorie content, but they cannot forget that serving size doesn’t always mean what’s in the entire container. Like Coke is labeled for 8oz knowing there are 12oz in a can!