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How Cells Harvest Chemical Energy Chapter 6 Overview • Photosynthesis • Aerobic respiration • Anaerobic respiration • Alternate sources of energy Components of a Reaction Reactants Intermediates Products A B C Endergonic vs. Exergonic Reactions Endergonic = Energy-requiring Exergonic = Energy-releasing Redox Reactions One molecule gives up electrons = oxidized One molecule gains electrons = reduced H+ atoms released simultaneously (are attracted to negative charge of electrons) Coenzymes pick up e-s & H+ from substrates & deliver to e- transfer chains Electron Transfer Chains Membrane-bound groups of enzymes/molecules Accept & give up e-s in sequence E-s enter chain at higher energy level than when they leave it (lose energy at each descending step of chain) e-s Substrate-Level Phosphorylation Formation of ATP by direct transfer of Pi group to ADP from intermediate NAD+ & FAD Coenzymes 1. Accept e-s & H+ from intermediates that form during glucose catabolism Become reduced = NADH & FADH2 2. NADH and FADH2 give up e-s & H+ to etransfer chains during final stages of aerobic respiration Become oxidized = NAD+ & FAD Autotrophs “Self-nourishing” Synthesize own food Obtain energy & organic compounds (e.g. C) from the physical environment Chemoautotrophs Have no enzymes to allow for complex metabolic reactions Obtain energy & C from simple inorganic & organic compounds e.g. CH4, H2S Photoautotrophs Contain light-sensitive molecules Can split H2O & use electrons Process releases lots of oxygen, which reacts rapidly with metals & creates toxic free radicals Early photoautotrophs existed when there was lots of Fe & metals everywhere Released O2 oxidized these metals & rusted them out O2 could then be released freely Over a few thousand years, O2 levels in sea & atmosphere increased Survival of the fittest: Most anaerobes died out because couldn’t neutralize toxic O2 radicals Chemoautotrophs with little or no O2 tolerance restricted to extreme & anoxic environments As O2 accumulated in the atmosphere, O atoms combined to form O3 = ozone layer (protects against lethal UV radiation from sun) Life was able to move out from the “darks” & live under open sky = diversification = evolution Photosynthesis The process by which photoautotrophs use light energy from the sun to make glucose, which can then be converted into ATP 12H2O + 6CO2 6O2 + C6H12O6 + 6H2O Respiration = breathing Cellular respiration = getting energy from food Organisms need usable energy in order to survive Obtained energy is converted into ATP chemical bond energy Can be used to do work e.g. metabolism Anaerobes Can’t tolerate O2 Make ATP via fermentation 1 glucose → 2 ATP Clostridium difficile e.g. first organisms, some prokaryotes & eukaryotes Aerobes Require O2 Make ATP via aerobic respiration (many also use anaerobic pathways) 1 glucose → 36+ ATP (vital for survival of large organisms) e.g. most eukaryotes, some prokaryotes Facultative Anaerobes Normally use aerobic pathways (i.e. use O2) Entamoeba histolytica Can switch to anaerobic pathways when O2 levels are low Mitochondria Membrane-bound organelles in most eukaryotic cells (# differs depending on cell type) Power source of cells – Production of ATP in presence of O2 – Convert NADH and FADH2 into ATP energy via oxidative phosphorylation Allow cell to produce lots of ATP simultaneously – Without mitochondria, complex animals wouldn’t exist Mitochondrion Structure Outer membrane Selectively permeable Inner membrane Highly impermeable Contains ATP synthase Has membrane potential Cristae ↑ surface area of inner membrane, which ↑ capacity to generate ATP Matrix Contains 100s of enzymes which oxidize pyruvate and fatty acids, and control the Krebs cycle Cellular Respiration The oxidation of food molecules (e.g. glucose) into CO2 & H2O Energy released is captured as ATP Used for all endergonic activities of cell Enzymes catalyze each step Intermediates formed at one step become substrates for enzyme at next step 2 phases of cellular respiration: Glycolysis: Glucose → 2 pyruvates Occurs in all cells Oxidation of pyruvate into CO2 and H2O: Energy-releasing pathways differ depending on cell & its needs 40% of energy from glucose is harvested Rest (60%) is lost as heat A working muscle uses 10 million ATP per second! Aerobic Respiration C6H1206 + 6O2 → 6CO2 + 6H2O Breakdown of glucose in presence of O2 3 stages of reactions: – Glycolysis – Krebs Cycle – Electron Transfer Phosphorylation Glycolysis – Glucose → 2 pyruvates – Occurs in cytosol Krebs Cycle – Pyruvate → CO2 + H2O + e-s – Occurs in mitochondria Electron Transfer Phosphorylation – Formation of lots of ATP Stage I: Glycolysis Glucose → 2 pyruvates “Universal energy-harvesting process of life” Initial energy-releasing mechanism for all cells Occurs in cytosol Coupled endergonic & exergonic reactions Endergonic Steps of Glycolysis Requires input of 2 ATP ATP #1 phosphorylates glucose Glucose → intermediate ATP #2 transfers Pi to intermediate Intermediate → PGAL + DHAP DHAP converts into PGAL = 2 PGAL enter next stage Exergonic Steps of Glycolysis Each PGAL gives 2 e-s + H+ to NAD+ 2 NAD+ → 2 NADH Intermediates each give Pi to ADP 2 ADP → 2 ATP (substrate-level phosphorylation) Pays back 2 ATP used in endergonic steps Intermediates each release H+ + OH 2 intermediates → 2 PEP Each PEP gives Pi to ADP 2 ADP → 2 ATP (substrate-level phosphorylation) 2 PEP → 2 pyruvate Sum Total of Glycolysis Glucose → 2 pyruvate + 2 NADH + 2 ATP From here, pyruvate can enter: – Aerobic pathway (Krebs cycle) – Anaerobic pathway (fermentation) (depends on cell & environmental conditions) Stage II: Krebs Cycle Pyruvate → CO2 + H2O (+ e-s) a.k.a. citric acid cycle Occurs in mitochondria Main function is to supply Stage III with e-s (in order to reduce NAD+ & FAD in stage III) Mitochondrial membrane proteins transport pyruvate into inner compartment Enzymes take 1 C from pyruvate C + O2 → CO2 Intermediates + coenzyme A → acetyl-CoA NAD+ is reduced into NADH Acetyl-CoA enters Krebs cycle Transfers 2 Cs to oxaloacetate → citrate Rearrangement of intermediates occurs 2 C released → 2CO2 3 NAD+ + H+ + e-s → 3 NADH ADP + Pi → ATP FAD + H+ + e-s → FADH2 Oxaloacetate regenerates so that cycle can run again In total, one turn of the cycle: 3 NADH + 1 FADH2 + 1 ATP Cycle repeats again for 2nd pyruvate molecule Remember 1 glucose → 2 pyruvates After both pyruvates are broken down: 6 NADH + 2 FADH2 + 2 ATP Sum Total of the Krebs Cycle With 2 NADH from acetyl-CoA formation: 2 pyruvate → 8 NADH + 2 FADH2 + 2 ATP + 6CO2 CO2 released into surroundings NADH & FADH2 deliver e-s and H+ to 3rd stage Stage III: Electron Transfer Phosphorylation H+ + e-s → H2O + ATP E-s delivered to electron transfer chains (ETCs) in inner mitochondrial membrane E- flow in ETCs drives phosphorylation of ADP → ATP (lots of it!) a.k.a. oxidative phosphorylation ATP formed by oxidation of NADH & FADH2 Responsible for high ATP yield NADH & FADH2 give e-s to ETCs Simultaneous release of H+ Energy released at each transfer of ETC At 3 transfers, released energy pumps H+ across mitochondrial membrane into outer compartment Concentration & electric gradients result across inner membrane = membrane potential H+ re-enters inner compartment by flowing down concentration gradient through ATP synthases Causes reversible change in shape of ATP synthases ADP + Pi → ATP (oxidative phosphorylation) At end of ETCs, O2 picks up e-s & H+→ H2O Sum Total of ET Phosphorylation H+ + e-s → H2O + 32 ATP In liver, heart, and kidney: – e-s from NADH delivered to different ETC entry point – H+ gradient makes 3 ATP (instead of 1) – Results in 34 ATP total Animation of ET Phosphorylation • http://vcell.ndsu.nodak.edu/animations/etc/ movie.htm • http://highered.mcgrawhill.com/sites/0072437316/student_view0/ chapter9/animations.html# In oxygen-starved cells, e-s have nowhere to go so get gridlocked No e- flow = no H+ gradients = no ATP forms Results in cell death because not enough ATP to sustain metabolic processes 3 different categories of poisons interfere with cellular respiration: • ETC Blockers • Inhibitors • Uncouplers ETC Blockers Block ETC at various steps of chain Starves cells of energy by prohibiting ATP synthesis Inhibitors Inhibit ATP synthase No passage of H+ through ATP synthase = no ATP generation Uncouplers Make mitochondrial membrane leaky to H+ Electron transport & O2 consumption continue but lack of H+ gradient = no ATP synthesis Sum Total of Aerobic Respiration Glycolysis Glucose → 2 pyruvate + 2 NADH + 2 ATP Krebs Cycle 2 pyruvate → 8 NADH + 2 FADH2 + 2 ATP + 6CO2 ET Phosphorylation H+ + e-s (from coenzymes) → 32 ATP + 6H2O Glucose + 6O2 → 6CO2 + 6H2O + 36 ATP (38 ATP in liver, heart, kidney) Cellular Respiration Video • http://video.google.com/videoplay?docid=1 463788471587082686&q=respiration+ninj a&total=2&start=0&num=10&so=0&type=s earch&plindex=0 Anaerobic Respiration Oxidation of molecules in absence of O2 Requires different electron acceptor at the end of it 2 stages: – Glycolysis – Various Energy-Releasing Pathways: • Alcoholic Fermentation • Lactate Fermentation • Anaerobic Electron Transfers Stage I: Glycolysis Glucose → 2 pyruvate + 2 NADH + 2 ATP Glucose is not broken down any further into CO2 or H2O All ATP comes from glycolysis (not enough energy to sustain large multicellular organisms) After Glycolysis Final stages in fermentation pathways do not generate more ATP Instead, they regenerate NAD+ so that it can act as an electron acceptor Pyruvate not moved into mitochondria (stays in cytosol & is converted into waste products that can be transported out of cells) Waste product depends on type of cell e.g. ethanol in yeast e.g. lactate in skeletal muscles, bacteria Alcoholic Fermentation 2 pyruvates → 2 acetylaldehydes + 2CO2 NADH gives up e-s & H+ to acetaldehyde → ethanol e.g. yeasts (Saccharomyces spp.) that ferment wine, bread, etc. Lactate Fermentation NADH gives e-s and H+ to pyruvate → lactate e.g. bacteria (Lactobacillus spp. and others) in cheese, yoghurt, etc. Certain organisms can couple aerobic & anaerobic respiration or switch from one mode to the other e.g. skeletal muscles associated with bones Variety of cell types within muscle fibres Slow-Twitch Muscle Fibres Lots of mitochondria & myoglobin Make ATP via aerobic respiration Used for steady, prolonged activity e.g. long-distance running, migration, etc. e.g. “dark meat” of birds Fast-Twitch Muscle Fibres Few mitochondria & no myoglobin Make ATP via lactate fermentation Used for short bursts of intense activity e.g. sprints, weight-training e.g. “white meat” of birds Anaerobic Energy Transfers Pathway of some archaeans & bacteria Inorganic compounds used as final eacceptors rather than O2 Aids in cycling of elements through biosphere Energy yield varies but is small Alternative Energy Sources • Glycogen Stores • Lipids • Proteins The Fate of Glucose When food is ingested, glucose is absorbed into the bloodstream Pancreas secretes insulin to make cells take up glucose faster Cells convert glucose to glucose-6-phosphate (intermediate of glycolysis) Can’t leave cell once phosphorylated Glycogen Stores When more glucose than necessary is taken in, is biosynthesized into glycogen (stored in liver and muscles) Only 1% of total energy stores Glycogen stores used up within 12 hours if regular meals aren’t eaten When blood glucose drops, pancreas secretes glucagon Liver cells convert stored glycogen → glucose & send back to blood Can then enter glycolysis pathway If excess carbs are eaten: Glucose → pyruvate → acetyl-CoA (aerobic respiration) Acetyl-CoA doesn’t enter Krebs cycle if excess glucose Enters lipid biosynthesis pathway instead ↑↑↑ carbs = fat Fat Stores Fat stored as triglycerides in adipose cells Triglyceride = glycerol + 3 fatty acid tails Between meals or during sustained exercise, fatty acids yield half of ATP needed by muscle, liver, kidney cells When blood glucose ↓, enzymes in adipose cells separate glycerol & fatty acids and release into blood Glycerol → PGAL Used in glycolysis Fatty acids → acetyl-CoA Used in Krebs cycle Protein Stores When proteins ingested, are broken down into amino acids Absorbed into bloodstream and taken up by cells to make more proteins, etc. If excess protein eaten, amino acids broken down Form acetyl-CoA, pyruvate, or intermediates of Krebs cycle “Reverse” pathways also exist Food molecules used for biosynthesis Requires ATP Summary Energy is required by all living organisms to sustain life Because energy flow is onedirectional, constant energy inputs are needed Interpreting Data This graph illustrates the free energy relative to oxygen of the electron transport chain. The solid blue circles are electron carrier molecules, and the light blue ovals represent protein complexes. From an energy standpoint, are these reactions endergonic or exergonic? a. Endergonic b. Exergonic c. Some are exergonic and others are endergonic. d. There is not enough information. Interpreting Data What would happen to the flow of electrons if oxygen were not present? a. The flow of electrons would continue but at a slower rate. b. The flow would cease and ATP production would stop. c. The presence of oxygen would have no effect.