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section 4 handout_answers

section 4 handout_answers - Bio1A Spring 2011 Discussion...

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Bio1A Spring 2011 Discussion Section Handout #4 Week of February 14, 2011 1. Label the reducing and oxidizing agents in the following reactions: a) C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O C6H12O6 becomes oxidized (reducing agent); O2 becomes reduced (oxidizing agent) The electron donor is called the reducing agent; the electron receptor is called the oxidizing agent Some redox reactions do not transfer electrons, but change the electron sharing in covalent bonds b) 6 H 2 O + 6 CO 2 → C 6 H 12 O 6 + 6 O 2 H2O becomes oxidized (reducing agent); CO2 becomes reduced (oxidizing agent) c) Pyruvate + NADH + H + → Lactate + NAD + Pyruvate becomes reduced (oxidizing agent); NADH becomes oxidized (reducing agent) 2 . Which enzymes in glycolysis are responsible for the following? a) Consumption of ATP Hexokinase (step 1), phosphofructokinase (step 3) b) Production of ATP Phosphoglycerokinase (step 7), pyruvate kinase (step 10) c) Reduction of NAD + to NADH + H + Triose phosphate dehydrogenase (step 6) 3. Describe how pyruvate enters the mitochondria, is converted to acetyl-CoA, and is incorporated into the citric acid cycle. Besides acetyl-CoA, what is produced during this process? Pyruvate enters the mitochondria via active transport. Pyruvate's carboxyl group (COO-) is removed and given off as a molecule of CO2. The 2-carbon fragment that remains is oxidized, forming a compound named acetate (the ionized form of acetic acid). An enzyme transfers the electrons from this oxidation to NAD+ to create NADH, a means of storing energy. Coenzyme A, a sulfur-containing compound derived from a B vitamin, is attached to the acetate by an unstable bond. Acetyl CoA is thus a molecule with high potential energy. It enters the citric acid cycle by adding its acetyl group to oxaloacetate, producing citrate. This process produces CO2 and NADH + H+. 4. Describe how isocitrate becomes alpha-ketoglutarate in step 3 of the citric acid cycle. Why is CO2 being released? Where is NADH getting its electrons. What is oxidized and reduced during this reaction? Isocitrate is oxidized to alpha-ketoglutarate, reducing NAD+ to NADH. Since the carboxyl group COO- is already fully oxidized, it has little chemical energy and is lost as CO2. NADH obtains its electrons from the oxidation of isocitrate. Isocitrate is oxidized, and NAD+ is reduced. 5. How are electrons transported in the electron transport chain? Electron carriers in the electron transport chain become increasingly more electronegative. Electron
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carriers alternate between reduced and oxidized states as they accept and donate electrons. Each component becomes reduced as it accepts electrons from the previous electron carrier, which has a lower affinity for electrons.
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