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Lecture-9-Cellular-R_38381 - Cellular respiration converts...

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Unformatted text preview: Cellular respiration: converts the potential energy stored in fuel molecules to a usable form (i.e., ATP’s) that can do cellular work. Photosynthesis (chloroplasts) Stored chemical energy (carbohydrates, fats, (organic molecules) proteins, nucleic acids) Aerobic (+ O2) Anaerobic (– O2) Broken down by the catabolic pathways of... Fermentation • Incomplete breakdown • Products: small organic molecules • Net energy trapped: 2 ATP Cellular Respiration • Complete breakdown • Products: H20, CO2 • Net energy trapped: 38 ATP Glucose most often used as a fuel source C6H12O6 + 6 O2 6 H2O + 6 CO2 + Energy ΔG = – 686 kcal/mol energy released by cells: • Catabolic pathways yield energy through redox reactions; such reactions relocate electrons & release energy stored in organic molecules use energy to make ATP’s – Redox reactions involve the transfer of electrons (e–) from one reactant to another: Oxidation: substance loses electrons Oxidizing agent (Y) is Reduction: substance gains electrons the electron acceptor Reducing agent is the electron donor • Redox reactions can also result in changing the degree of electron sharing in a covalent bond • Electron transfers in redox reactions require energy to pull an electron from an atom. – The energy required correlates to the electronegativity of an atom – Electrons lose potential energy when transferred from less, to more, electronegative atoms release of chemical energy (‐∆G; can do work) e– Transfer to another molecule • Redox reactions comprise cellular respiration: [Glucose] • Hydrogen‐rich organic molecules are “fuel” sources for metabolism: hydrogen bonds are sources of electrons with high potential energy; carbohydrates, fats, proteins. – Oxidize these to release chemical energy Harvest of Energy from Fuel Molecules Occurs in Steps • Energy from glucose can be released all at once via combustion: C6H12O6 O2 CO2 H2O Releases 686 kcal/mol of energy • Such a process cannot result in the efficient transfer of energy to do work rather catabolize glucose in series of enzyme catalyzed reactions, some of which remove electrons (as H atoms) transferred ultimately to oxygen (at the end). • Energy harnessed in electron transfers • Requires electron carriers to execute: NAD+ (coenzyme) • NAD+ (nicotinamide adenine dinucleotide); oxidized form – works with dehydrogenase enzymes: 2H [2e‐, 2 protons] Glucose [Reduced form] Portion of NAD molecule where chemistry occurs • Reduced electron carriers (NADH’s) formed during glucose catabolism go to the electron transport chain. • NADH’s give up their electrons to proteins of the electron transport chain, where the e–’s are transferred in steps & energy is released, forming ATP’s: • Oxygen serves as the terminal (last) electron acceptor in this chain; is reduced to H2O. • Electron transport chain proteins located in the inner mitochondrial membrane (cristae) Cellular respiration Stages of Cellular Respiration Glycolysis (outside mitochondrion) 1 Glucose 2ATP (6 carbons) Energy Investing Rxns: Requires energy to split glucose C6H12O6 2 ADP 2 x 3 carbon molecules 2 NAD+ 2 NADH 2 ATP 2 x C3H5O3P Substrate level phosphorylation 2 ATP Energy Harvesting Form ATP’s and NADH 2 ADP 2 ADP Glycolysis can occur in the absence of oxygen (anaerobic) 2 Pyruvates (3 carbon molecule) Summary [2 x C3H4O2] • Substrate level phosphorylation during cellular respiration yields ATP by a substrate acting as the donor of the phosphate to ADP. • Yields much less ATP than oxidative phosphorylation (electron transport chain) • In the presence of oxygen, pyruvate formed from glycolysis enters the mitochon‐ drion. Pyruvate is oxidized to acetyl‐CoA & loses CO2. • Attachment of Coenzyme A (CoA) 2 Acetyl‐CoA’s 2 Pyruvates makes molecule Forms 2 NADH more reactive. Citric acid cycle (in mitochondria) End Result: Oxidation of glucose is completed Energy capture → (NADH, FADH2, ATP) 1 turn of cycle = 1 acetyl‐CoA enters cycle; make 1 ATP, 3 NADH, 1 FADH2 per turn Per 1 glucose: 2 ATP, 6 NADH, 2 FADH2 FADH is an electron carrier similar to NAD+ Acetyl‐CoA CO2 also formed Oxidative Phosphorylation Yields the Bulk of ATP in Cellular Respiration • Glycolysis and the citric acid cycle directly yield a total of 4 ATPs per glucose molecule via substrate level phosphorylation. – Most of the energy extracted from glucose is contained in the reduced electron carriers, NADH (10) and FADH2 (2) • NADH & FADH2 transfer chain give up electrons to the electron – NADH, FADH2 are oxidized here – Electron transfers yield energy that are coupled to ATP formation, a type termed oxidative phosphorylation • Electron transfer chain molecules located in the inner mitochondrial membrane (cristae) • Electron transport chain components: multi‐protein complexes I, II, III, and IV; III & IV are cytochromes. non‐protein: Q (ubiquinone) • Electron carriers are alternately reduced & oxidized; energy released along the way. • NADH & FADH2 are oxidized at different entry points. • Electronegativity of components arranged from low to high (oxygen is highest) “downhill”; energy releasing; ‐∆G 2H+ + ½ O2 H2O Chemiosmosis • ATP synthase is multi‐protein complex found in multiple copies within the inner mitochondrial membrane. • ATP synthase is a proton (H+) pump which uses the energy of an existing proton gradient to power ATP synthesis. • This process is termed chemiosmosis • ATP synthase is a molecular motor that binds H+ ions causing it to spin. Turning activates catalytic sites producing ATP from ADP + P. • The H+ gradient that drives ATP synthase is the function of the electron transfer chain. [H+ gradient = pH gradient] H+ H+ H+ H+ H+ Electron Transport Chain Intermembrane space ATP Synthesis High [H+] Inner mitochon. membrane Matrix Lower [H+] • Complexes I, III, and IV of the electron transport chain couple energy release from electron transfer to the pumping of protons into the intermembrane space. A proton gradient is generated. • Protons can travel down their gradient through ATP synthase. Final Tally of ATPs • • • • Total = 36 – 38 ATPs 2.5‐3.3 ATP per NADH x 10; 1.5‐2.0 ATP per FADH2 x 2 Oxidative phosphorylation accounts for 90% of ATP formed Efficiency of respiration: 40% Location of Cell Respiration Processes in Mitochondria Glycolysis electron transport chain & ATP synthases citric acid cycle reactions • Fermentation enable cells to produce ATP in the absence of oxygen; they are incomplete oxidations. • Fermentation relies on glycolysis, plus reactions that regenerate NAD+: – NAD+ NADH transfer e–’s to pyruvate or similar molecule – ATP’s formed by glycolysis (substrate level phosphorylation) • Compared to cellular respiration, fermentation produces 19X less ATP (2 vs. 38); lacks oxidative phosphorylation. Alcohol fermentation Lactic acid fermentation Metabolism of Other Fuel Molecules • A variety of molecules beyond glucose are used as fuel sources: other sugars, proteins, lipids, etc. • These fuel molecules funnel into different points of the cellular respiration reactions. • Alternately, intermediates of cellular respiration are used for anabolic pathways (biosynthesis) Summary: Cell Respiration • Glucose + 6 O2 6 H2O + 6 CO2 + Energy – Redox reactions (What is oxidized/reduced above?) • Three stages: (glycolysis, citric acid cycle, electron transport chain) – Cytoplasmic & Mitochondrial locations of reactions – Release of free energy as reactions proceed are captured to produce ATP’s, NADH, & FADH2 • Chemiosmosis oxidation of NADH, FADH2 by electron transport chain create proton gradient – Protons diffuse thru ATP synthase (ADP + P ATP); oxidative phosphorylation (contrast with substrate level phosphorylation) • ATP totals of process; efficiency of glucose oxidation. • Fermentation: anaerobic, less efficient; are incomplete oxidations that rely on regenerating NAD+. • Proteins, lipids feed into cellular respiration reactions ...
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