Cellular Energetics PDF

!"#$"#$ Chapter 12 Cellular Energetics Actin filaments Yellow tube-like net work of m...

!"#$"#$ Chapter 12 Cellular Energetics Actin filaments Yellow tube-like net work of mitochondria in a human bone cancer (osteosarcoma) cell. Mitochondria Nucleus Ch 12 Opener: Lodish et al. Mol Cell Biol 8th Ed 1 Aerobic and anaerobic respiration ! When O2 is available and used as the final recipient of electrons transported via the electron transport chain, the respiratory process that converts nutrient energy into ATP is called aerobic respiration. ! If some molecule other than O2, for example the weaker oxidant sulfate (SO42–) or nitrate (NO3–), is the final recipient of the electrons in the electron transport chain, the process is call anaerobic respiration. 2 !  !"#$"#$ The complete aerobic oxidation of one molecule of glucose yields 30-32 molecules of ATP. The overall reaction is: C6H12O6 + 6 O2 + 30 Pi + 30 ADP + 30 H+ (or 32 ADP) Glucose 6 CO2 + 30 ATP + 36 H2O (or 32 ATP) Glucose oxidation in eukaryotes takes place in four stages: Stage 1: Glycolysis ! 2 pyruvate + 2 ATP + 2NADH Stage 2: Citric Acid Cycle ! NADH + FADH2 Stage 3: Electron transport chain ! H+ gradient 3-5 ATP Stage 4: ATP synthesis ! 28-30 additional ATP are produced (made at stage 4) Total ATP = 30-32 3 Glycolysis Substrate-level phosphorylation: ATP is formed through enzyme catalyzed joining of ADP and Pi ;it does not require proton-motive force. The overall reaction for the first stage of glucose metabolism: C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi 2 C3H4O3 + 2 NADH + 2 ATP + 2 H+ 4 #  !"#$"#$ Glycolysis: the preparatory phase !"#$%&' 1 1 ,&'#-(.+$& ;6< !"#$%&'()*+,%&+,-.' !"#$%"#"&'#$& 2 2 ($#)&*+$& /0#$.%&'()*+,%&+,-.' 3 !"#$%"#/ 0*123#-(.+$&/ ;6< 3 4 /0#$.%&'(12)*+,%&+,-.' 4 567#6+$& 4 8*(#$&99 67,340%83-$'.%9' +,%&+,-.' %"#$%"+3&9 5 ! ! ($#)&*+$& !"3$'0-"4',34'(5*+,%&+,-.' 5 : Lehninger 4th Ed. Fig. 14.2a Glycolysis: the payoff phase !"#$%&'"(%)#(%*+,-)./-)'0%*123 6 2;A 6'()**+,'7*"(7* 89%"#$%",-*5 2B9:C 6 7*"(7+#&*0,$* ! >?+,@A/-)./-).4"#$%&'0%*123 7 !"#$%"#&'()*+,-* 2 9:; 7./0,$* 2 +,-)./-).4"#$%&'0%*123 8 !"#$%"#&'()*+,-* 8 12-,$* 2,-)./-).4"#$%&'0%*123 9 30#',$* 22 V of energy are used to carry out this oxidation-reduction reaction ! 1 mole of photons (680 nm wavelength; red light) is equivalent to a 1.8 V change in redox potential ! It is theoretically possible for 1 photon of red light to boost an electron to an energy level needed to reduce NADP+ under standard conditions (1.14 V) ! However, the process is accomplished in the cell through the combined action of 2 different light-absorbing reactions "# Electron flow and O2 evolution in chloroplast PSII PSII reaction center contains two molecules of chlorophyll a, two accessory chlorophylls, two pheophytins, two quinones (QA and QB) and one non-heme Fe atom. Pheophytin is a chlorophyll 167 molecule lacking a central Mg2+ ion. These small molecules are bound to two two proteins in e- (P680) ! Chl ! Pheo ! Q A ! QB ! PQ PSII, called D1 and D2. P680 is the strongest biological oxidant known. It can oxidize water to generate O2 and H+ ions. Purple bacteria use H2S and H2 as electron donors to reduce chlorophyll a+ in a linear electron flow. 2 H2O ! 4 H+ lumen + O2 + 4 e- !" Fig. 6.11a Cell and Molecular Biology 7th Ed by Karp #$  1/25/25 Electron flow and O2 evolution in chloroplast PSII Water is a very stable molecule made up of tightly held H and O atoms; splitting of water is most thermodynamically challenging reaction known to occur in living organisms. Splitting water in lab requires use of strong electric current or temperature approaching 2000°C. A plant does it on snowy mountainside using only energy of visible light. P680+ is most powerful oxidizing agent yet discovered in a living system. This redox potential is sufficiently strong to pull tightly held (low-energy) electrons from H2O (redox potential of +0.82 V), thus splitting the molecule (photolysis) Formation of 1 O2 molecule following reaction: 2 H2O à 4 H+ lumen + O2 + 4 e-; This reaction requires simultaneous loss of 4 electrons from 2 H2O and use of 4 photons. Four photons can split 2 H2O producing 1 O2. 51 How does PSII accomplish H2O splitting? One PSII reaction center can generate only one positive charge (P680+) or oxidizing equivalent at a time. Closely associated with PSII D1 protein at its luminal surface is a cluster of 5 metal ions – 4 Mn2+ ions and 1 Ca2+ ion. The Mn/Ca cluster is stabilized and protected by a number of peripheral proteins that form oxygen-evolving complex. The Mn/Ca cluster accumulates 4 successive "+" charges by passing electrons one-at-a- time to the nearby P680+. Transfer of electrons from the Mn/Ca cluster to P680+ is accomplished by passing electron through tyrosine (Tyr167) residue of D1 protein, neutralizing P680. After each electron is transferred to P680+ regenerating P680, the pigment is re-oxidized back to P680+ following absorption of another photon by the photosystem. The stepwise accumulation of 4 "+" charges by Mn/Ca cluster is driven by the successive absorption of 4 photons of light by the PSII photosystem. Once 4 "+" charges accumulate, PSII oxygen-evolving complex catalyzes the removal of 4 electrons from 2 closely bound H2O molecules; this then leads to O2 release. 52 26  !"#$"#$ !"#$%&'(#))#*+ , In 1940, Biophysicist Robert Emerson discovered that the rate of plant photosynthesis generated by light of wavelength of 700 nm can be greatly enhanced by adding light of shorter wavelength (higher energy). , He found that a combination of light, say 600 and 700 nm, supports a greater photosynthesis than the sum of the rates for the two separate wavelengths. , This is called Emerson effect. , Photosynthesis in plants involved the interaction of two separate photosystems, PSI and PSII. , PSI is driven by light of wavelength 700 nm or less. , PSII is driven by light of shorter wavelength (