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 CO 2 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 O 2 –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 FADH 2 ~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 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 CO 2 from air into organic compounds –Carbon fixation incorporation of CO 2 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 CO 2 (3C) 6 PGA (18C) 1 G3P (3C) 5 G3P (15C) 3 RuBP (15C) 6 G3P (18C) Carbon Fixation Reduction 6 ATP 6 NADPH 3 ATP RuBP Regeneration Output Calvin Cycle Rubisco

Pinus radiata (closed) and Populus deltoides (open); Ow et al. GCB 09

Photosynthesis, O 2 and H 2 O Plants face two major problems –1) whenever stomata open to allow CO 2 to diffuse to the locations of carbon fixation, H 2 O invariably leaves –2) Rubisco fixes both CO 2 and O 2 Transpiration loss of H 2 O from plants –Stomatal physiology tries to maximize photosynthesis while minimizing transpiration –Stomatal closure decreases CO 2 concentrations and increases O 2 concentrations promoting O 2 fixation Photorespiration fixation of O 2 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 CO 2 –1) C 3 plants fix CO 2 into 3-PGA (3 carbon) –2) C 4 plants initially fix CO 2 into a 4 carbon molecule before passing it to the Calvin cycle –3) CAM plants initially fix CO 2 into organic acids C 4 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