Lecture 5 presented

Lecture 5 presented - How do cells form chemical...

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Unformatted text preview: How do cells form chemical energy 1 High ­Energy Compound and Energy Storage • Energy released as a result of oxida=on ­reduc=on reac=ons must be conserved for cellular func=on – Usually conserved in high ­energy phosphate bonds • Typically, cells use compounds whose ΔG0’ is greater than  ­30kJ/mol as energy “currencies” in the cell. – The most important high energy compound in living cells is ATP 2 What Is the Role of ATP in Biochemical Energe=cs? ATP (adenosine triphosphate) captures and transfers free energy. ATP releases a large amount of energy when hydrolyzed. ATP can phosphorylate, or donate phosphate groups to other molecules. 3 What Is the Role of ATP in Biochemical Energe=cs? ATP is a nucleo=de. Hydrolysis of ATP yields free energy. ATP + H 2O ! ADP + Pi + free energy ΔG = –7.3 to –14 kcal/mol (exergonic) =  ­31.8 kJ/mol 4 Figure 8.5 ATP (Part 1) No=ce different types of bonds These two are “high energy” bonds 5 But it is not only ATP Cells use chemical energy to run endothermic reac=ons; Energy is in the form of high energy phospo ­ anhydride bonds: ATP being the most common ATP equivelents include: NTP (G, C, U) NDP (A, G, C, U) Acetyl~P Phosphoenol pyruvate 6 Examples of High energy bonds Not high energy 7 What Is the Role of ATP in Biochemical Energe=cs? The forma=on of ATP is endergonic: ADP + Pi + free energy ! ATP + H 2O Forma=on and hydrolysis of ATP couples exergonic and endergonic reac=ons. 8 Figure 8.6 Coupling of Reactions Exergonic and endergonic reactions are coupled. 9 Ques=on 1 Which is true for ADP a. consists of adenine, ribose, and three phosphate groups. b. is produced in an endergonic reac=on. c. can phosphorylate many different molecules. d. is poorer in energy than ATP. 10 10 How do cells generate usable energy? 11 11 Three ways cells generate usable energy Chemical reac=ons: Chemotrophy (burning) 1. Substrate level phosphoryla=on 2. Electron transport chains Light reac=ons: Phototrophy 3. Photosynthesis 12 12 Substrate ­level phosphoryla=on • Defini=on: – Produc=on of ATP by direct transfer of a high ­ energy phosphate molecule from a phosphorylated organic compound to ADP 13 Chemotrophs Fuels: Molecules whose stored energy can be released for use. A common fuel in organisms is glucose. C6 H12O6 + 6O2 ! 6CO2 + 6 H 2O + free energy 14 Chemotrophy Burning or metabolism of glucose: C6 H12O6 + 6O2 ! 6CO2 + 6 H 2O + free energy Glucose metabolism pathway traps the free energy in ATP: ADP + Pi + free energy ! ATP 15 Chemotrophy ΔG from complete combus=on of glucose = –686 kcal/mol Highly exergonic; drives endergonic forma=on of many ATP molecules. 16 How do cells generate usable energy? Energy transfer is simply the movement of electrons Think of this as electrical current, the movement of electrons from one compound to another, this movement releases energy that can be captured. This is the power of Redox reactions: 17 How do cells generate usable energy? Redox reac=ons: One substance transfers electrons to another substance Reduc1on: Gain of one or more electrons by an atom, ion, or molecule Oxida1on: Loss of one or more electrons • Also occurs if hydrogen atoms are gained or lost (H = H+ + e ­) 18 Oxidation and Reduction Are Coupled 19 REDOX • Organisms that obtain electrons come from an organic source, i.e. glucose, are called Organotrophs. • Organisms that obtain electrons from an inorganic source, i.e. Fe+2 are called Lithotrophs. 20 20 Figure 9.2 Oxida=on, Reduc=on, For example: the oxida=on of Methane and Energy 21 Electron Donor Energy Source Simple way to think about it: – The goal of a cell is to grow, reproduce, and survive. – What the cell can “eat” or use as an energy source (the electron ­donor) and “breath” (use as electron ­acceptor) is – dictated by the rela=ve degree of oxida=on/reduc=on of the chemicals. The greater the difference in the reduc=on poten=al redox couples, the more energy is released when they interact. 22 Transferring electrons • Primary electron donor: – first molecule to donate the electron • Terminal electron acceptor: – The final molecule to receive the electron • Carriers: – The transfer of electrons usually involves one or more intermediates between donor and acceptor • Net energy change: – is the difference in the reduc=on poten=al of the primary donor and the terminal acceptor 23 Classes of Carriers • Carriers: – The transfer of electrons usually involves one or more intermediates between donor and acceptor • Two general categories: – Freely diffusible carrier • Example: nico=namide ­adenine dinucleo=de (NAD+) and NAD ­phosphate (NADP+) – Those that are firmly anached to enzymes in the cytoplasmic membrane 24 Redox Carriers Coenzyme NAD+ is a key electron carrier in redox reac=ons. Two forms: NAD+ (oxidized) NADH + H+ (reduced) carries 2 e ­ and 2 H+ NADH+H+ has a very high electronega=vity  ­0.3 eV 25 NAD+ Is an Energy Carrier in Redox Reactions 26 NAD+ and NADP+ Reduc=on poten=al of NAD+/NADH (and NADP+/NADPH) is  ­0.32 e ­ volts – NADH (and NADPH) are good electron donors. NAD+ and NADP+ couples generally func=on in different capaci=es in the cell. – NAD+/NADH is involved in energy genera=ng (catabolic) reac=ons – NADP+/NADPH is involved primarily in biosynthe=c (anabolic) reac=ons 27 How do cells generate usable energy? Oxygen can accept electrons from NADH + H+: NADH + H + + 1 2 O2 ! NAD + H 2O + exergonic ΔG = –52.4 kcal/mol Oxidizing agent is molecular oxygen O2 28 NAD+ as a freely diffusible carrier • NAD+ and NADP + are electron plus H+ carriers, transpor=ng 2e ­ and 2H+ at a =me. • High on the electron tower – NADH (and NADPH) are good electron donors 29 The electron Tower The more electronegative a compound the more likely it is to give up electrons. The electron tower is composed of ½ redox reaction: Oxidized/Reduced The more electronegative (reduced), the higher the pair sits in the tower. CO2/glucose (-0.43) +24eGlucose readily gives up electrons to form CO2 Think of it this way, glucose easily burns forming CO2 30 30 Electron Tower The opposite is true for those at the other end. ½ O2/H2O (+0.82) 2eH2O is very stable; and O2 is very reactive Oxygen is very electropositive 31 31 Electron Tower Put these two half reactions together CO2/glucose (-0.43) +24e½ O2/H2O (+0.82) 2e- Which is actually: Glucose + O2 CO2 + H2O + ENERGY It is this energy that cells can capture!!! 32 32 How Do Cells Use Redox to generate ATP? Oxida1ve Phosphoryla1on. Remember, the word Oxida=ve, referrers to Oxida=on/Reduc=on reac=on genera=on of ATP. Molecular oxygen, (O2), is not essen=al!!!!!! We will start with O2 33 33 How Does Oxida=ve Phosphoryla=on Form ATP? Oxida1ve phosphoryla1on: ATP is synthesized by reoxida=on of electron carriers. Two stages: • Electron transport • Chemiosmosis 34 How Does Oxida=ve Phosphoryla=on Form ATP? Why does the electron transport chain have so many steps? Why not NADH + H + + 1 2 O2 ! NAD + H 2O + in one step? 35 How Does Oxida=ve Phosphoryla=on Form ATP? Too much free energy would be released all at once—it could not be harvested by the cell. In a series of reac=ons, each releases a small amount of energy that can be captured by an endergonic reac=on. 36 How Does Oxida=ve Phosphoryla=on Form ATP? Electron transport: Electrons from NADH +H+ and FADH2 pass through the respiratory chain of membrane ­associated carriers (Enzymes). Electron flow results in a proton concentra=on gradient across the membrane. 37 How Does Oxida=ve Phosphoryla=on Form ATP? The respiratory chain is located in the membrane. Energy is released as electrons are passed between carriers. Examples: protein complexes I, II, III, IV; Cytochrome c, ubiquinone (Q) 38 Figure 9.8 The Oxida=on of NADH and FADH2 in the Respiratory Chain 39 How Does Oxida=ve Phosphoryla=on Form ATP? During electron transport protons are also ac=vely transported. Protons accumulate in the intermembrane space and create a concentra=on gradient and charge difference̶poten=al energy! This proton ­mo1ve force drives protons back across the membrane. 40 How Does Oxida=ve Phosphoryla=on Form ATP? Chemiosmosis: Protons diffuse back into the mitochondria through ATP synthase, a channel protein. Diffusion is coupled to ATP synthesis. 41 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmo=c Mechanism 42 How Does Oxida=ve Phosphoryla=on Form ATP? ATP synthase: • F0 subunit—transmembrane • F1 subunit—projects into the cytosol and rotates to expose ac=ve sites for ATP synthesis. 43 The F0 F1 ATPase http://www.youtube.com/watch?v=aIMGS0cSoFI 44 44 Example of an Aerobic Electron Transport Chain 45  ­TCs: O Two different e Aerobic Respiration  ­ 2 versus NO3 Anaerobic respiration 46 Proton Motive Force A major function of the e-TC is to “energize” the membrane, forming a PMF or separation of charge across the membrane: H+ out The PMF can be used to do work, such as running the F0F1 ATPase. An energized membrane is essential for life! 47 47 Reverse Electron Flow What happens if PMF weakens? Cells can reestablish the PMF by running the F0F1 ATPase in reverse- pumping out protons. An energized membrane is essential for life! 48 48 A word about NADH and NADPH 49 Reducing Power Reducing Power is necessary for the synethsis of cellular components; both Catabolic and Anabolic pathways. Therefore both Energy (ATP) and reducing power (NADH/NADPH) is necessary for cell survival. Cells must balance the two! 50 PHOTOSYNTHESIS Harves=ng and u=lizing light energy and its conversion to forms the cell can use 51 How Does Photosynthesis Convert Light Energy into Chemical Energy? When a photon meets a molecule it can be: • Scanered—photon bounces off the molecule • Transmined—photon is passed through the molecule • Absorbed—molecule acquires the energy of the photon. The molecule goes from ground state to excited state 52 Figure 10.5 Exciting a Molecule (A) 53 Figure 10.5 Exciting a Molecule (B) 54 How Does Photosynthesis Convert Light Energy into Chemical Energy? Photons can have a wide range of wavelengths and energy levels. Molecules that absorb specific wavelengths in the visible range of the spectrum are called pigments. 55 Phototrophs Use light as an energy source – Photosynthesis can be carried out using bacteriochlorophyll Use H2S, H2O, H2, or organics as electron donor in photosynthesis • Oxygenic versus Anoxygenic photosynthesis Can obtain carbon from organic compounds or CO2 56 Photoautotrophs • Oxygenic photosynthesis – Carried out by green plants, algae, and some prokaryotes (example: cyanobacteria) – Oxygen is produced • Anoxygenic photosynthesis – Carried out by prokaryotes (example: purple phototrophic bacteria) – Oxygen is not produced 57 Photophosphoryla=on • Occurs in phototrophs that generate ATP by using light energy to create a charge separa=on • The electrons are extracted from water (H2O) or some other molecule (H2S for example) 58 How Does Photosynthesis Convert Light Energy into Chemical Energy? When a pigment molecule absorbs a photon the energy can be: • Released as heat and/or light • Transferred to another molecule • Used for a chemical reac=on 59 Figure 10.6 Absorp=on and Ac=on Spectra 60 How Does Photosynthesis Convert Light Energy into Chemical Energy? Pigments are arranged in antenna systems, or light ­ harvesBng complexes. A photosystem consists of mul=ple antenna systems and their pigments and surrounds a reac1on center. Pigments are packed together on membrane proteins. Excita=on energy passes from pigments that absorb short wavelengths to those that absorb longer wavelengths, and ends up in the reac=on center pigment. 61 62 Figure 10.8 Energy Transfer and Electron Transport 63 How Does Photosynthesis Convert Light Energy into Chemical Energy? The reac=on center converts light energy into chemical energy. The excited chlorophyll a molecule (Chl*) is a reducing agent (electron donor). A is an acceptor molecule (oxidizing agent). * Chl + A " Chl + A + ! 64 How Does Photosynthesis Convert Light Energy into Chemical Energy? a is the first in a chain of electron carriers on the membrane—electron transport, a series of redox reac=ons. The final electron acceptor is NADP+. NADP + e " NADPH + H + ! + 65 ...
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This note was uploaded on 10/11/2011 for the course BIS 2A taught by Professor Grossberg during the Summer '08 term at UC Davis.

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