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Chapter 6

BIOL 1000 CHAPTER 6 textbook notes

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BIOL 1000
Nicole Nivillac

Chapter 6 – Cellular Respiration – Textbook NOTES 6.1 – The Chemical Basis of Cellular Respiration - The reactions of photosynthesis trap light energy and use it to convert CO2 and water into organic molecules such as sugars, which contain an abundance of free energy - Cellular respiration extracts the potential energy and converts it into ATP, a form of chemical energy that can be used by cell for energy-requiring reactions - The complete oxidation of food molecules results in formation of CO2 Food as Fuel - Common thing between glucose and gasoline: both are good fuel molecules that contain an abundance of hydrogen in the form of C-H bonds - Electrons associated with C-H bon are equidistant from both nuclei - Because of this, they contain high energy and can be easily removed - Molecules that have many oxygen have less potential energy because oxygen is highly electronegative and so a greater energy is required to remove these electrons - So ^ compared to proteins and carbs, fats contain more calories since it is mostly C-H bonds The Principle of Redox - Oxidation refers to the loss of electrons and the molecule afterward is said to be oxidized - Reduction reaction: another molecule gains the electrons or becomes reduced - Oxidation and reduction reactions are coupled processes (work together) - LEO GER: Loses electrons oxidation / Gains electrons reduction - Oxidation-reduction: redox reaction Cellular Respiration is Controlled Combustion - CO2 is the common product for the complete oxidation of all organic molecules - ^ Because it is fully oxidized carbon molecule CO2 contains no usable energy - In the cell, oxidation of glucose occurs via a series of enzyme-catalyzed reactions each with a small activation energy - In cellular respiration, the oxidation of food molecules occurs in the presence of a group of enzymes called dehydrogenases that facilitate the transfer of electrons from food to a molecule that acts as an energy carrier - The most common energy carrier is the coenzyme: Nicotinamide Adenine Dinucleotide (NAD+) - During respiration, the dehydrogenases removes 2 hydrogen atoms from a substrate molecule and transfers the 2 electrons but 1 proton to NAD+ resulting in its complete reduction to NADH: the other proton is released - The potential energy carried in NADH is used to synthesize ATP 6.2 – Cellular Respiration ^ Primary goal: to transform the potential energy found in food molecules into a form that can be used for metabolic processes: ATP (Adenosine Triphosphate) 3 Parts of Cellular Respiration 1) Glycolysis: Enzymes break down a molecule of glucose into 2 molecules of pyruvate. Some ATP and NADH is synthesized 2) Citric Acid Cycle: Acetyl coenzyme (acetyl-CoA) which is formed from the oxidation of pyruvate, enters a metabolic cycle where it is completely oxidized to CO2. ATP and NADH are synthesized 3) Electron Transport and Chemiosmosis: NADH produced is oxidized with the electrons being passed along an electron transport chain until they are transferred to O2, producing water. Energy released from this is used to establish a proton gradient to synthesize remaining ATP The Mitochondrion - Prokaryotes: glycolysis and the citric acid occur in cytosol and electron transport occurs on internal membranes from plasma membrane. Do not have mitochondria but processes can still occur - Eukaryotes: citric acid and electron transport occur in the mitochondrion - Mitochondrion: composed of the outer membrane and the inner membrane. Intermembrane space found between outer and inner and the matrix is the interior aqueous environment of organelle 6.3 – Glycolysis - This is the first set of reactions that extracts energy from sugar molecules - Lead to the oxidation of the 6-carbon sugar glucose producing 2 molecules of the 3 carbon compound (pyruvate) - Potential energy released in the oxidation leads to synthesis of NADH and ATP - This process is universal (found in both prokaryotes and eukaryotes - Does not require O2 - Occurs in cytosol requiring soluble enzymes so it does not require electron transport chain to operate Reactions of Glycolysis 1) Glucose receives a phosphate group from ATP producing glucose-6-phosphate (phosphorylation reaction) 2) Glucose—phosphate is rearranged into its isomer, fructose-6-phosphate (isomerization reaction) 3) Another phosphate group derived from ATP is attached to fructose-6-phosphate producing fructose-1,6-biphosphate (phosphorylation reaction) 4) Fructose-1,6-biphosphate is split into glyceraldehyde-3-phosphate (G3P) and dihydroxyanatone phosphate (DAP) (hydrolysis reaction) 5) The DAP produced is converted into G3P giving a total of 2 of these molecules per molecule of glucose (isomerization reaction) 6) 2 electrons and 2 protons are removed from G3P. Some energy released is trapped by addition of phosphate group (not derived from ATP). The electrons are accepted by NAD+ along with 1 proton. The other proton is released to the cytosol (redox reaction) 7) One of the 2 phosphate groups of 1,3-bisphosphoglycerate is transferred to ADP to produce ATP (substrate-level phosphorylation reaction) 8) 3-Phosphoglycerate is rearranged shifting phosphate group from the 3 carbon to produce 2- phosphoglycerate 9) Electrons are removed from one part of 2-phosphoglycerate and delivered to another part of the molecule. 10) The remaining phosphate group is removed from phosphoenolpyruvate and transferred to ADP. Reaction forms ATP and final product of glycolysis, pyruvate. (substrate-level phosphorylation reaction) - 2 molecules of ATP are consumed as glucose and fructose-6-phosphate become phosphorylated - ^ in total, 4 ATP and 2 NADH molecules are produced - No carbon is lost - Net of 2 ATP and 2 NADH - Potential energy in pyruvate is less than one molecule of glucose - During glycolysis, ATP is produced using a process called substrate-level phosphorylation - ATP synthesis requires an enzyme that transfers a phosphate group from a high-energy substrate molecule to adenosine diphosphate (ADP) producing ATP 6.4 – Pyruvate Oxidation and the Citric Acid Cycle - Pyruvate still contains 75% of energy from glucose Bridging Glycolysis and the Citric Acid Cycle - Citric acid located in mitochondrial matrix and the product of glycolysis, pyruvate, must pass through both the outer and inner mitochondrial membranes - Large pores in outer membrane allow pyruvate to simply diffuse through but inner membrane requires a pyruvate-specific membrane carrier - Once in the matrix, pyruvate is converted into a molecule called acetyl-CoA though process called pyruvate oxidation - The conversion of pyruvate to acetyl-CoA starts with a decaboxylation reaction whereby the carboxyl group of pyruvate is lost as CO2 - Dehydrogenation reaction leads to transfer of 2 electrons and 1 proton to NAD+ forming NADH - Acetyl group reacts with coenzyme A, forming the high-energy intermediate acetyl-CoA - Acetyl-CoA formed still contains 3 C-H bonds and can be further oxidized to release even more free energy - Extracting this energy is the purpose of the reactions that make up the citric acid cycle Citric Acid Cycle – Krebs Cycle 1) 2 carbon acetyl group carried by coenzyme A is transferred to oxaloacetate forming citrate 2) Citrate is rearranged into its isomer, isocitrate 3) Isocitrate is oxidized to a-ketoglutarate; one carbon is removed and released as CO and NAD+ i2 + reduced to NADH + H 4) A-Ketoglutarate is oxidized to sucinyl CoA; one carbon is removed and released as CO2 and NAD+ is reduced to NADH + H + 5) The release of CoA from succinyl CoA produces succinate: the energy released converts GDP to GTP which in turn converts ADP to ATP by substrate-level phosphorylation. Only ATP made directly in the citric acid cycle 6) Succinate is oxidized to fumarate, the 2 electrons and 2 protons removed from succinate are transferred to FAD, producing FADH 2 7) Fumarate is converted into malate by the addition of a molecule of water 8) Malate is oxidized to ocaloacetate, reducing NAD+ to NADH + H + The Citric Acid Cycle - Consists of 8 enzyme-catalyzed reactions: 7 soluble enzymes in mitochondrial matrix and 1 enzyme bound to matrix side of inner mitochondrial membrane - Combined, the reactions result in the oxidation of acetyl groups to CO2 accompanied by the synthesis of ATP, NADH and another nucleotide-based molecule, flavin adenine dinucleotide (FAD; reduced form is FADH2) - For each acetyl-CoA that enters the citric acid cycle, 3 NADH and FADH2 and a single molecule of ATP, generated by substrate-level phosphorylation are synthesized - In a complete turn of the cycle, 1 two-carbon acetyl unit is consumed and 2 molecules of CO2 are released, completing the conversion of all C atoms in glucose to CO2 - The CoA molecule that carried the acetyl group to the
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