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Lect., No. : Dr. Rihab Edan Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways All organisms produce ATP by releasing energy stored in glucose and other sugars. 1- Plants make ATP during photosynthesis. 2- All other organisms, including plants, must produce ATP by breaking down molecules such as glucose. Aerobic respiration : the process by which a cell uses O2 to "burn" molecules and release energy. C6H12O6 + 6O2 6CO2 + 6H2O Note: this reaction is the opposite of photosynthesis This reaction takes place over the course of three major reaction pathways 1- Glycolysis 2- The Krebs Cycle 3- Electron Transport Phosphorylation (chemiosmosis) Glycolysis (glyco = sugar; lysis = breaking) Goal: break glucose down to form two pyruvates Who: all life on earth performs glyclolysis Where: the cytoplasm Glycolysis produces 4 ATP's and 2 NADH, but uses 2 ATP's in the process for a net of 2 ATP and 2 NADH. NOTE: This process does not require O2 and does not yield much energy. The First Stage of Glycolysis 1- Glucose (6C) is broken down into 2 PGAL's (3C) 2- This requires two ATP's 1 Lect., No. : Dr. Rihab Edan The Second Stage of Glycolysis 2 PGAL's (3C) are converted to 2 pyruvates This creates 4 ATP's and 2 NADH's The net ATP production of Glycolysis is 2 ATP's Krebs Cycle (citric acid cycle, TCA cycle) Goal: take pyruvate and put it into the Krebs cycle, producing NADH and FADH2 Where: the mitochondria There are two steps:The Conversion of Pyruvate to Acetyl CoA The Krebs Cycle proper In the Krebs cycle, all of Carbons, Hydrogens, and Oxygeng in pyruvate end up as CO2 and H2O. The Krebs cycle plus the Conversion of Pyruvate produces 2 ATP's, 8 NADH's, and 2FADH2's per glucose molecule. The Conversion of Pyruvate to Acetyl CoA for Entry Into the Krebs Cycle 2 NADH's are generated. 2 CO2 are released. 2 Lect., No. : Dr. Rihab Edan The Krebs Cycle Krebs Cycle Animation 6 NADH's are generated 2 FADH2 is generated 2 ATP are generated 4 CO2's are released Therefore, for each glucose molecule that enters into the Krebs cycle (including the preparatory conversion to Acetyl CoA), the net production of products are: 8 NADH 2 FADH2 2 ATP 6 CO2 Remember, glycolysis produced 2 ATP and 2 NADH, so there is a net production of 4 ATP and 10 NADH up to now. 3 Lect., No. : Dr. Rihab Edan Electron Transport Phosphorylation (Chemiosmosis) Goal: to break down NADH and FADH2, pumping H+ into the outer compartment of the mitochondria. Where: the mitochondria In this reaction, the ETS creates a gradient which is used to produce ATP, quite like in the chloroplast. Electron Transport Phosphorylation typically produces 32 ATP's ATP is generated as H+ moves down its concentration gradient through a special enzyme called ATP synthase. Net Energy Production from Aerobic Respiration Glycolysis: 2 ATP Krebs Cycle: 2 ATP Electron Transport Phosphorylation: 32 ATP Each NADH produced in Glycolysis is worth 2 ATP (2 x 2 = 4) - the NADH is worth 3 ATP, but it costs an ATP to transport the NADH into the mitochondria, so there is a net gain of 2 ATP for each NADH produced in gylcolysis. Each NADH produced in the conversion of pyruvate to acetyl COA and Krebs Cycle is worth 3 ATP (8 x 3 = 24). Each FADH2 is worth 2 ATP (2 x 2 = 4). 4 + 24 + 4 = 32 Net Energy Production: 36 ATP. 4 Lect., No. : Dr. Rihab Edan Anaerobic Respiration Goal: to reduce pyruvate, thus generating NAD+ Where: the cytoplasm Why: in the absence of oxygen, it is the only way to generate NAD+ and ADP. Alcohol Fermentation: Occurs in yeasts and in many bacteria. The product of fermentation, alcohol, is toxic to the organism. Lactic Acid Fermentation Occurs in humans and other mammals. The product of Lactic Acid fermentation, lactic acid, is toxic to mammals. This is the "burn" felt when undergoing strenuous activity. 5 Lect., No. : Dr. Rihab Edan The only goal of fermentation reactions is to convert NADH to NAD+ (to use in glycolysis). No energy is gained. Note differences - fermentation - 2 ATP's produced, aerobic respiration - 36 ATP's produced. Thus, the evolution of an oxygen-rich atmosphere, which facilitated the evolution of aerobic respiration, was crucial in the diversification of life. Photosynthesis: 6 CO2 + 6 H2O C6H12O6 + 6 O2 Respiration: C6H12O6 + 6 O2 6 CO2 + 6 H2O Notice that these reactions are opposites - this is important since the earth is a closed system. All life has a set amount of natural materials to work with, so it is important that they all be cycled through effectively and evenly. Energy Yields: Glucose: 686 kcal/mol ATP: 7.5 kcal/mol 7.5 x 36 = 270 kcal/mol for all ATP's produced 270 / 686 = 39% energy recovered from aerobic respiration Related Catabolic Processes Beta Oxidation Fats consist of a glycerol backbone with two or three fatty acids connected to it The body absorbs fats and then breaks off the fatty acids from the glycerol Glycerol is converted to glyceraldehyde phosphate, an intermediate of glycolysis The fatty acids are broken down into two-carbon units which are then converted to acetyl CoA. o An eight-carbon fatty acid can produce 4 acetyl CoA's o Each acetyl CoA is worth 12 ATP's (3 NADP, 1 FADH2, 1 ATP) o Therefore, this short fatty acid is worth 48 ATP's, a fat with three chains of this length would be worth 144 ATP's! o This is why fats are such a good source of energy, and are hard to lose if you want to lose weight. A comparison between Plants and Animals Animal cells and Plant cells contain mitochondria! o However, animal cells contain many more mitochondria than plant cells. Animal cells get most of their ATP from mitochondria. Plant cells get most of their ATP from the chloroplast o The ATP generated from the mitochondria is only used when the plant cannot generate ATP directly from the light-dependent reactions Ref. : Basic Notes and pictures obtained from the following link: http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect12. 6