Jan. 18, 2011 B4730/5730 Plant Physiological Ecology Subcellular Processes II Chloroplast and Mitochondrion Biology • Chloroplasts and mitochondria arose from endosymbiotic evolution – Both affected by bacterial antibiotics – Both reproduce by binary fission, neither can reproduce by themselves – Both have own genome and chimeric proteins – Both have ATPase to generate ATP from chemiosmosis • Differences between chloroplasts and mitochondria – Membrane structure – Direction of pH gradient during chemiosmosis Respiration • Four steps respiration – – – – 1) Glycolysis 2) Pyruvate Transport 3) Krebs cycle 4) Electron transport chain and oxidative phosphorylation • Glycolysis in cytoplasm – Results in two pyruvate molecules • Pyruvate Transport removes CO2 • Krebs cycle occurs in the mitochondrial matrix – Remaining 2 C molecule is broken into carbon dioxide • Glycolysis and the Krebs cycle both pass electrons to NAD+ and form NADH • Electron transport chain receives the electrons from NADH and passes them to O2 – Powers pH gradient and ATPase Respiration Accounting • Two paths of energy production during respiration – Most energy flows from glucose -> NADH -> electron transport chain -> proton-motive force -> ATP – Some energy comes directly from substrate-level phosphorylation • Each NADH can yield ~3 ATP and FADH2 ~2 ATP – Some NADH shuttles or transport proteins use 1 ATP resulting in a net gain of 2 • ~38 ATP generated under optimum conditions – 34 ATP from oxidative phosphorylation – 4 ATP from substrate-level phosphorylation – Some energy from proton-motive force may be used for other work • Respiration pathways used for other purposes Fig. 7.10 Brooker Biology 2007 Fermentation • Aerobic conditions occur when oxygen is present and anaerobic conditions occur when oxygen is absent – Glycolysis occurs under aerobic or anaerobic conditions because NAD+ is the oxidizing agent – Glycolysis produces two ATP in aerobic or anaerobic conditions • Fermentation occurs under anaerobic conditions and is limited by the supply of NAD+ • Fermentation requires two phases – 1) glycolysis – 2) regeneration of NAD+ by transferring electrons to pyruvate from NADH Light Properties and Photosynthesis • Light behaves both as a wave and a particle – Photons have no mass but contain distinct amount of energy – Amount of energy in photon inversely proportional to its wavelength – Sun radiates entire electromagnetic spectrum but visible light passes through the earth’s atmosphere easiest • When light meets matter three possible outcomes – 1) Reflectance or light bounces off the matter – 2) Transmittance or light passes through the matter – 3) Absorption or light is absorbed by the matter Photosynthesis I • Photosynthesis is composed of two stages – light reactions in the thylakoids – Calvin cycle in the stroma • The light reactions convert solar energy to chemical energy – Light absorbed by chlorophyll drives a transfer of electrons from water to NADP+ to form NADPH – Water is split and oxygen is given off as a waste product – ATP is formed through photophosphorylation – No sugar is produced Fig. 8.2 Brooker Biol. 2007 Photosynthesis II • Calvin cycle or dark reactions incorporate CO2 from air into organic compounds – Carbon fixation incorporation of CO2 into organic compounds – Fixed carbon is reduced to carbohydrates by NADPH – Additional energy provided by ATP – ATP and NADPH provided by the light reactions – Dark reactions do not need light but usually occur during the daytime 3 RuBP (15C) Carbon Fixation 5 G3P (15C) 6 PGA (18C) Reduction 3 ATP Rubisco 3 CO2 (3C) RuBP Regeneration Calvin Cycle 6 ATP 6 NADPH 6 G3P (18C) Output 1 G3P (3C) Pinus radiata (closed) and Populus deltoides (open); Ow et al. GCB 09 Photosynthesis, O2 and H2O • Plants face two major problems – 1) whenever stomata open to allow CO2 to diffuse to the locations of carbon fixation, H2O invariably leaves – 2) Rubisco fixes both CO2 and O2 • Transpiration loss of H2O from plants – Stomatal physiology tries to maximize photosynthesis while minimizing transpiration – Stomatal closure decreases CO2 concentrations and increases O2 concentrations promoting O2 fixation • Photorespiration fixation of O2 by Rubisco – Photorespiration requires light – Photorespiration produces no ATP – Photorespiration uses organic material from the Calvin cycle Alternative Pathways of Photosynthesis • Three major photosynthetic pathways based on which molecule first incorporates CO2 – 1) C3 plants fix CO2 into 3-PGA (3 carbon) – 2) C4 plants initially fix CO2 into a 4 carbon molecule before passing it to the Calvin cycle – 3) CAM plants initially fix CO2 into organic acids • C4 and CAM photosynthetic pathways minimize transpiration and photorespiration at the cost of additional energy for carbon fixation – Temporal or spatial separation – Light reactions same for all pathways