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Cellular Metabolism 1. Free Energy a. Free energy is energy available for doing work i. Cells release free energy in exergonic reactions ii. Endergonic reactions require energy to occur 2. The Role of Enzymes a. Enzymes are catalysts that encourage exergonic reactions to occur i. Activation energy - energy required to start a reaction 1. Enzyme reduces activation energy of a reaction ii. Cofactors - may be metallic ions iii. Coenzymes - small organic molecules - many are what we call vitamins b. Enzyme action i. Associates with substrate in a very specific way. ii. Enzyme-substrate complex 1. Stresses chemical bond in substrate - less energy required to complete reaction iii. Enzymes are highly specific to a particular substrate 1. 3. succinic dehydrogenase only works on succinic acid Chemical energy transfer by ATP a. ATP - adenosine triphosphate i. Two very high energy bonds ii. Energy is released by hydrolysis of bonds - used in coupled reactions to cause an endergonic reaction to occur. 4. 1. ATP is the energy coupling agent 2. ATP doesn't last long Overview of respiration a. ATP is the common denominator of energy in cells i. How do you get the energy from food to ATP? ii. Redox reactions 1 Cellular Metabolism 1. Transfer of electrons from one compound to another 2. Electron donor = e- + electron acceptor + energy 3. Fire - wood is electron donor, oxygen is electron acceptor - lots of energy released in flames 4. b. Can have different final electron acceptors Two kinds of energy metabolism i. Aerobes - final electron acceptor is oxygen ii. Anaerobes - final electron acceptor is some other molecule c. Cellular respiration - the oxidation of fuel molecules with molecular oxygen as the final electron acceptor i. Overview - Figure 4-10 ii. Glycolysis - happens in cytoplasm 1. 2. Glucose is split into two 3-carbon sugars a. Start with Fructose 1,6 diphosphate b. Series of other intermediates c. ATP formed d. NAD- is reduced to NADH Step-by-step the glucose molecule is pulled apart and the energy in its bonds is turned into energy in the bonds of ATP 3. End point is pyruvic acid iii. Krebs Cycle - named for Hans Krebs who discovered it 1. Pyruvate is oxidized to acetyl coenzyme A a. Actually the coenzyme A carries the acetyl group b. Drops it into the Krebs Cycle c. Cycles until acetyl group is completely oxidized to carbon dioxide d. Produce more ATP's and NADH's and FADH2 2 Cellular Metabolism iv. Electron transport chain - Where do the reduced compounds go 1. NADH and FADH2 2. Down the electron transport chain - remember, these compounds carry electrons 3. Oxydative phosphorylation a. Complex chain of enzymes and transfers inside of mitochondria i. Electrons are passed from compound to compound in the mitochondrion ii. Build up of hydrogen ion in mitochondrion 1. Produces ATP - method is unclear 2. Electron is eventually attached to oxygen a. b. Forms water Lots of ATP produced v. Comparison of ATP and NAD production in Glycolysis and Krebs cycle plus oxidative phosphorylation d. 1. Total Glycolysis + Krebs Cycle + OP 2. Glucose + 2 ATP + 36 ADP +36P+6 O2 Æ 6 CO2 + 2 ADP + 36 ATP + 6 H2O 3. Glycolysis alone 4. Glucose + 2ADP +2P +2NAD+ Æ 2 pyruvics + 2NADH + 2 ATP Anaerobic Glycolysis 3 Cellular Metabolism i. What if there is no oxygen to be a final electron acceptor? 1. What if you are an anaerobe? 2. You can only do glycolysis 3. You are at a severe energy disadvantage 4. You can use an organic compound as a final energy acceptor a. Use the extra electrons to reduce pyruvic to lactic acid i. Done in animals b. Release 1 carbon as carbon dioxide and reduce what is left to ethanol i. Done in yeasts - alcoholic fermentation 5. Anaerobic glycolysis is 1/18th as efficient as complete oxidation of glucose a. If you live in an environment with no oxygen, this is what you are stuck with - anaerobic bacteria b. 6. Some extreme parasites What about normal animals? a. They are forced into situations where oxygen is limiting i. Diving - seals and whales ii. Humans under heavy exertion b. Do anaerobic glycolysis as produce lactic acid i. Incur an oxygen debt - lactic acid must be oxidized eventually ii. Feel the burn - that is the lactic acid building up in your muscles as you exercise iii. Gasping for breath - you are repaying the oxygen debt that you incurred 4