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Chapter 6


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York University
BIOL 1000
Julie Clark

Tanya Sivamanoharan Wed, Nov, 8/11 Biology- chap# 6 textbook notes CELLULAR RESPIRATION 6.1- The Chemical Basis of Cellular Respiration -most energy enters biosphere by photosynthesis -photosynthesis traps light energy and use it to convert carbon dioxide and water into organic molecules (sugars) -oxidizing energy rich molecules, cellular respiration extracts potential energy and convert it to ATP (chemical energy) -the complete oxidation of food results in formation of carbon dioxide, which is released into atmosphere 6.1a- Food as Fuel -glucose and gasoline molecules are good fuel molecules b/c of an abundance of hydrogen in form of carbon-hydrogen (C-H) bonds -in any atom, electron further from nucleus contains more energy than closer electrons, therefore electrons gain energy when moving away from nucleus and lose energy as they approach nucleus -electrons in C-H bond contain high energy and can be removed easily -molecules with oxygen contain less potential energy b/c of high electronegativity -more electronegative atoms, the tighter the atoms are held to nucleus -fat molecule is almost entirely C-H bonds, so contain more calories per weight compared to proteins and carbohydrates 6.1b- The Principle of Redox -potential energy within fuel molecule released by oxidation -oxidation/oxidizedloss of electrons by a molecule -oxidation of molecule is linked to a reduction reaction where another molecule gains the electrons -reduction/reducedgain of electrons by molecule -oxidation and reduction are coupled processes that only happen together/at once -oxygen is essential for oxidation reactions -high affinity and electronegativity of oxygen make it an ideal terminal electron acceptor of cellular respiration 6.1c- Cellular Respiration is Controlled Combustion -oxidation of glucose results in transfer of electrons to oxygen, yielding carbon dioxide, water, and the release of energy -carbon dioxide is the common product for complete oxidation of all organic molecules - fully oxidized carbon molecule (carbon dioxide) contains no usable energy -in cellular respiration, oxidation of food molecules occurs in presence of enxymes called hydrogenases -hydrogenases facilitate transfer of electrons from food to a molecule that acts as energy carrier/shuttle -most common energy carrier is nicotinamide adenine dinucleotide (NAD) -during respiration, dehydrogenases remove 2 hydrogen atoms from substrate molecule and transfer 2 electrons and 1 proton to NAD+, resulting in its reduction to NADH -the other proton is simply released -efficiency of energy transfer between food molecules and NAD+ is high, with little heat/energy loss -the potential energy carried in NADH is used to synthesize ATP 6.2- Cellular Respiration: An Overview -primary goal of cellular respiration to transform potential energy in food molecules into form that can be used for metabolic processes, adenosine triphosphate (ATP) -proteins and lipids can be oxidized by cellular respiration but oxidation of glucose utilizes entire respiratory pathway 6.2a- The Three Parts of Cellular Respiration -cellular respiration is divided into 3 parts (pg# 119 fig:6.7) 1. glycolysisenzymes break down a molecule of glucose into 2 pyruvate molecules. Some ATP and NADH are also synthesized. 2. citric acid cycle/Krebs cycleacetyl coenzyme A (acetyl-coA) from oxidation of pyruvate enters a metabolic cycle, where it is oxidized completely into carbon dioxide. Some ATP and NADH are also synthesized. 3. electron transport/chemiosmosis The NADH synthesized by glycolysis and Krebs cycle is oxidized, with the liberated electrons being passed into ETC, where they are transferred to oxygen, producing water. Free energy released during ETC is used to create a proton gradient across the membrane, and the gradient tis what synthesizes excess ATP. -all 2 stages are import. to extract max. amount of energy from a molecule of glucose --not all organisms contain all 3 stages 6.2b- The Mitochondrian -in prokaryotes, glycolysis and Krebs cycle occur in cytosol of cell, and ETC occurs on internal membranes derived from plasma membrane -in eukaryotes, the Krebs cycle and ETC occur in in specialized organ mitochondrion -mitochondrion membrane-bound organelle often referred to as powerhouse of cell b/c it contains the process of Krebs cycle and ETC which generate most ATP of cells -mitochondrion composed of 2 membranes the outer membrane and the inner membrane, which together make up the intermembrane space -although prokaryote do not have mitochondrion, they possess complete reactions for cellular respiration -glycolysispyruvate oxidationKrebs cycle ETC 6.3- Glycolysis -glycolysis is 1 step of reactions that extracts energy from sugar molecules -glycolysis consists of 10 sequential enzyme-catalyzed reactions that leads to the oxidation of 6-carbon glucose to 2 3-carbon pyruvate molecules -potential energy released during glycolysis leads to synthesis of NADH and ATP 6.3a- Glycolysis is an Ancient Pathway st -glycolysis is 1 of the 1 metabolic pathways studied and best understood -it is considered the most ancient of all metabolic pathways support by 3 facts: 1.glycolysis is universal, found in prokaryotes, eukaryotes, and plants 2.unlike other stages of cellular resp., glycolysis does not require oxygen, which only became abundant on earth only 2.5 billion years ago, 1.5 years after life developed 3.glycolysis happens in cytosol of all cells and therefore does not require ETCs or subcellular components in order to operate 6.3b- The Reactions of Glycolysis -the net energy input and output of glycolysis: glucose 2 pyruvate+ 2 H2O 2 ADP+ 2Pi 2 ATP (LOOK AT PAGE# 121 FIG: 6.10) 2 NAD+ 2 NADH+ 2H+ st 1.glycolysis has an energy input and energy output phase. The 1 5 steps require energy input and the final 5 steps do energy output. Glycolysis consumes 2 ATP molecules to phosphorylate glucoseglucose-6- phosphate and fructose-6-phosphatefructose-1,6-biphosphate. Consumption of 2 ATP produces 4 ATP and 2 NADH molecules. 2.no carbon is lost! the 6 carbon in glucose is split into 2 3-carbon pyruvate molecules. However, since glucose is oxidized, the potential energy in 2 pyruvate molecules is lower than of glucose. 3. ATP is made by substrate-level phosphorylation. During glycolysis, ATP is produced by substrate-level phosphorylation. Subsrate-level phosphorylation is also used during Krebs cycle to make ATP. 6.4- Pyruvate Oxidation and Krebs Cycle -the 2 molecules of 3-carbon pyruvate, product of glycolysis is contain almost 75% of energy of glucose -extracting the remaining energy as ATP and in electron carriers (NADH) from the 2 pyruvate molecules is done in pyruvate oxidation and Krebs cycle 6.4a- Bridging Glycolysis and the Krebs Cycle -reactions of Krebs cycle take place in mitochondrial matrix (powerhouse of cell) -pyruvate, the product of glycolysis can simply diffuse into the outer membrane of the mitochondria through the large pores, but to cross the inner membrane a pyruvate-specific membrane carrier is needed -once pyruvate is in matrix, it is converted into a molecule called acetyl- coA by a process called pyruvate oxidation -conversion of pyruvate to acetyl-coA starts with 1.decarboxylation reaction whereby the carboxyl (COO-) of pyruvate is lost as carbon dioxide -the carboxyl group (carbon dioxide) contains very little energy (no C-H bonds) -after decarboxylation, comes 2.oxidation of the remaining 2 carbon molecules, producing acetate -3.dehydrogenation reaction transfers 2 electrons and a proton to NAD+ forming NADH -lastly 4.the acetyl group reacts with coenzyme A, forming acetyl-coA -an acetyl-coA still contains 3 C-H bonds which contain energy, that is then extracted by the Krebs cycle! (LOOK AT PG#122 FIG: 6.12) 6.4b- The Krebs Cycle -Krebs cycle contains 8 enzyme-catalyzed reactions, 7 are soluble enzymes located in mitochondrial matrix and 1 enzyme is bound to inner mitochondrial membrane -the reactions of Krebs cycle results in the oxidation of acetyl groups to carbon dioxide accompanied by the synthesis of ATP, NADH, and another nucleotide based molecule called FAD (reduced form FADH2) -the net reactants of products of one cycle of the Krebs cycle: 1 acetyl-coA + 3 NAD+ + 1 FAD+ + 1 ADP+ 1Pi + 2 H2O 2 CO2 + 3 NADH + 1 FADH2 + 1 ATP + 3H+ +1coA -b/c 1 molecules of glucose is converted into 2 molecules of pyruvate the reactants and products of the Krebs cycle is doubled to account for 2 pyruvate thus 2 acetyl-coA molecules (LOOK AT PG# 124 FIG: 6.14) 6.5- Electron Transport and Chemiosmosis -after the Krebs cycle, all the carbon from glucose is completely oxidized and released as carbon dioxide -besides energy as ATP, the potential energy from glucose now exists in the form of NADH and FADH2 -the ETC and chemiosmosis extract the potential energy in these molecules and synthesize more ATP 6.5a- The Respiratory electron Transport Chain -the respiratory ETC is found on inner mitochondrial membrane in eukaryotes -the chain facilitates the transfer of electrons from NADH2 and FADH2 to oxygen -the chain consists of 4 protein complexes: -complex 1: NADH dehydrogenase -complex 2:succinate dehydrogenase -complex 3: cytochrome complex -complex 4: cytochrome oxidase -complex 2 is a single peripheral proteins and the others are made o multiple proteins put together -electron flow from one complex to another is done by 2 electron mobile shuttles: -shuttle 1: ubiquinonehydrophobic molec. found in core of membrane shuttles electrons from complex 1 and 2 to complex 3 -shuttle 2: cytochrome C located on intermembrane space side of the membrane and transfers electrons from complex 3 to complex 4 6.5b-
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