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Photosynthesis & Respiration Photosynthesis Pathway http://www.ualr.edu/~botany/photosynthesis.gif Leaves & Leaf Structure • Raw Materials of Photosynthesis Enter Cells of Leaf • Visible Light only Small Portion of Electromagnetic Spectrum • Plants Use Light Energy mostly in Visible Light Range for PS • Red & Blue Wavelengths most Important for PS – Water & Carbon Dioxide • Products of Photosynthesis Leave Leaf – Captured by Chloroplasts & Used to Initiate PS Reactions – Sugar & Oxygen http://techalive.mtu.edu/meec/module19/Page4.htm Photosynthetic Reactions • • • • • • Nature of Light Photos (light) Synthesis (to put together) Light Energy to Chemical Energy Life on Earth Depends on this Process Supplies Our Oxygen Overall Equation http://www.coloradocollege.edu/dept/ev/courses/EV112/ImagesII/image003.gif • Overall Equation Deceptively Simple • Complex Set of Physical & Chemical Reactions must Occur in Coordinated Manner for Synthesis of Carbohydrates 1 Chlorophyll Accessory Pigments • Chlorophyll Is a Complex Molecule • All Photosynthetic Organisms Have Chlorophyll a • Accessory Pigments Absorb Energy that Chlorophyll a Does not Absorb • Pigment Is any Substance that Absorbs Light • Color of the Pigment Comes from Wavelengths of Light Reflected • Chlorophyll – Green Pigment Common to all Photosynthetic Cells – Absorbs all Wavelengths of Visible Light Except Green – Chlorophyll b – Xanthophylls – Carotenoids (Beta-Carotene) http://www.nyu.edu/pages/mathmol/library/photo/ • If a Pigment Absorbs Light Energy, 1 of 3 Things Will Occur • Energy Is Dissipated as Heat • The Energy may Be Emitted Immediately as a Longer Wavelength • Energy may Trigger a Chemical Reaction, as in PS Chloroplasts • Organelles in a Plant Cell • Location of Photosynthesis – Chlorophyll Triggers a Chemical Reaction when It Is Associated with Proteins Embedded in a Membrane (as in a Chloroplast) http://www.ualr.edu/~botany/chlorophyll.jpg • Chloroplast Has 3 Membrane Systems http://www.lclark.edu/~seavey/Bio100_03/Lecture%20notes/lecture_Feb_11.h tml Photosynthesis Stages – Form 3 Compartments • 2-Stage Process 1. Outer Membrane 2. Inner Membrane 3. Intertwined & Stacked Network of more Membranes – Light Reactions • Require Light to Occur • Involves Actual Harnessing of Light Energy • Occur in\on Grana – Dark Reactions – Thylakoids: Wafer-Like Structures – Granum/Grana: Stack of Thylakoids – Stroma: Areas between Grana • Photosynthesis Takes Place in the Structures http://www.wellesley.edu/Biology/Courses/Plant/chloro.html • Do not Need Light to Occur • Involve Creation of Carbohydrates • Uses Products of Light Reaction to Form C-C Bonds of Carbohydrates • Occur in Stroma http://www.daviddarling.info/images/chloroplast.jpg 2 2. Photophosphorylation Light Reactions 1. Electron Transfer – Light Strikes Magnesium (Mg) Atom in Center of Chlorophyll Molecule – Light’s Energy Excites Mg Electron & It Leaves Orbit from Mg Atom – Electron Can Be Converted to Useful Chemical Energy – Excited Electron (plus Additional Light Energy) Provides Energy – Phosphate Group Can Be Added to Compound Called Adenosine Diphosphate (ADP) – Yields Adenosine Triphosphate (ATP) – ATP Is Important Stored Energy Molecule http://bioweb.wku.edu/courses/Biol120/images/Photophosphryl.htm http://www.sirinet.net/~jgjohnso/lightreactionproject.html ATP • ATP = Adenosine---(PO4-)---(PO4-)---(PO42-) • String of 3 Phosphate Groups Is Held Together by Covalent Bonds • When Bond that Attaches 1 of the Phosphate Groups onto ATP Is Broken, It Becomes ADP • Adenosine---(PO4-)---(PO42) + (PO42) + Energy 3. Photolysis (Hill Reaction) – 2 Water Molecules Are Split into H & O – H Is Attached to a Molecule Called Nicotinamide Adenine Dinucleotide Phosphate (NADP) – It Becomes NADPH2 – O Is Given off as O Gas – 2 H20 + NADP + light NADPH2 + O2 http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookatp.html • ATP & NADPH2 Are Common Energy-Carrying Molecules in all Plant & Animal Cells • ATP Gives up the Phosphate Group when It Is Involved in a Chemical Reaction • Same Thing Happens when NADPH2 Gives off the Hydrogen Atoms as Part of a Reaction • ATP & NADPH2 Are Renewable or Recyclable Energy Sources http://plantphys.info/plant_physiology/lightrxn.shtml Dark Reactions • ‘Calvin Cycle’ • ‘Carbon Reactions Pathway’ • Do not Require Light Energy to Occur – Do Require Energy Captured by Light Reactions http://www.ualr.edu/~botany/calvincycle.gif 3 • Occur at same Time as Light Reactions • Cease Soon if Light Energy Is not Available to Make Light Reaction Products – Exception: some Xerophytes • 2 Main Steps – Carbon Dioxide Fixation – Sugar Formation • Occur in the Stroma of the Chloroplasts • C4 Plants – CO2 Fixation for Many Plants of Dry or Tropical Origins – Plants Use a Different Enzyme Called PEP Carboxylase for CO2 Fixation • Much Higher Affinity for CO 2 than Rubisco • At Low CO2 Pressures, Rubisco Doesn’t Distinguish Well between O2 & CO2 so Stomata usually Have to Be Wide Open for PS to Occur Calvin Cycle/Dark Reactions 1. Carbon Dioxide Fixation • CO2 Incorporated into a 3-Carbon or 4Carbon Chain • C3 Plants – Most Plants Use Enzyme Called RuBP Carboxylase (RuBisCo) to Carry out CO2 Fixation – Occurs in Mesophyll Cells – Creates a 3-Carbon Product Ready for Sugar Formation – Called C3 Plants because 1st Stable Carbon Chain Made from CO2 Has 3 Carbons • PEP Carboxylase’s Transport of CO2 to Bundle Sheath Allows C4 Plants to Build Up Higher Concentration of CO2 than Normally Found in the Mesophyll Cells of C3 or C4 Plants • Where Does 1st Part of Calvin Cycle Occur? – Bundle Sheath Cells of C4 Plants – Mesophyll Cells of C3 Plants – Creates a 4-Carbon Product – 4-Carbon Chain Transported into Bundle Sheath Cells where CO2 Is Released & then Immediately Fixed by Rubisco as Part of C3 Cycle – Same Fixation with Rubisco as in C3 Plants but in Bundle Sheath Cells, not Mesophyll Cells http://www2.mcdaniel.edu/Biology/botf99/photodark/c4.htm PEP Carboxylase vs. Rubisco • PEP Carboxylase Works Well at Warm Temps but Not Optimum at Cool Temps • Why C4 Grasses Called ‘Warm Season’ Grasses & Why They Don’t Compete Well with C3 Grasses at Cooler Temps • C4 Grasses Have an Edge in Dry Warm Sites or Open Sunny Sites C3 & C4 Plants Differ in Water Use • C4 Plants Can Produce 3 Times as Much Dry Matter/Unit of Water as C3 Plants – Can Keep Leaf Stomata Closed during Mid-Day & Extract every Last CO2 Molecule in the Leaf 4 • Stomata Closed during the Day CAM Plants – CO2 Is Released from the 4-Carbon Product – Normal Light & Dark Reactions occur without Stomata Opening – Allows the Plants to Conserve Water during the Day • Crassulacean Acid Metabolism • Another Type of C4 PS Carried out only by Xerophytes • Stomata Open at Night • When Water Is Adequate, these Plants usually Carry out C3 PS – Plants Fix CO2 into a 4Carbon Product – 4-Carbon Product Stored Overnight in Vacuole http://www.ualr.edu/~botany/c4andcam.jpg Calvin Cycle/Dark Reactions 2. Sugar Formation Photosynthesis Logistics • • Carbon Chain Formed in Step 1 Is Converted to Glucose 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O • Carbon Dioxide Source – CO2 Enters Leaves through Stomata by Diffusion – Dissolves in Water to Become HCO3- – C6H12O6 • Water Source – From Roots Upward • Oxygen Output – Diffuse out through Stomata – Used in Respiration or other Reactions – Plants Are Net Oxygen Producer http://www.ualr.edu/~botany/starch_sucrose.jpg http://www.westga.edu/~geosci/wgmc/plants_pics.htm • 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O Factors Affecting Photosynthesis • Water Output – Fate of Water Produced by PS • Used in other Reactions or Plant Transport • Water Vapor out of Stomata • Sugar/Glucose Output • Converted to Sucrose – Main Plant Sugar, Water Soluble • Converted to Starch or other Storage Carbs – Water-Soluble – Common Form of Stored Carbs • Converted to Cellulose or other Structural Carbohydrate – Not Water Soluble • 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O • Availability of CO2 – – – – – CO2 Supply Diminishes if Stomata Close Air Movement Replaces CO2 Taken up by Plants Normal [CO2] Is ~400 ppm (0.04%) Increase Can Increase Photosynthetic Rates Occurring due to Environmental Changes • Availability of Water – Water not a Limiting Factor for PS – Water Stress Can Causes Stomata to Close plantbiology.stanford.edu/ imagecollection.html 5 • • 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O • Temperature • Light Quality (Color) – Increasing Temp will Increase Rate of PS, within Normal Ranges – Below Normal Ranges, PS Slows or Stops – Chlorophyll Absorbs Red & Blue Wavelengths – Called Photosynthetically Active Radiation (PAR) • Cytoplasm (Liquid inside Cells) Slows Moving – Above Normal Ranges • Light Duration (Photoperiod) • Light Intensity (Brightness) • Proteins may Change Shape • Membranes may Become too Leaky – Light Saturation Point • PS Reaches Its Maximum Point • Increasing Light Intensity no Longer Increases PS Rate • Leaf Chlorophyll Content – About 20 to 100 Chloroplasts/Leaf Mesophyll Cell http://www.firstrays.com/plants_and_light.htm • 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O • • C3 Plants Have Optimum PS from about 5575°F – Can Carry out PS from 32-95°F – C3 Grasses Called Cool-Season • C4 Plants Optimum PS 75-95°F – Can Carry out PS from 55-105°F – PEP Enzyme Deactivates below 55°F – C4 Grasses Called Warm-Season 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O • Carbohydrate Translocation – Sugars not Moved out of Mesophyll Cells can Inhibit PS – As more Sugars Are Needed, It can Increase the Rate of PS • ‘Source-Sink Relationship’ • Leaf Age – Young, Mature Leaves Have Greatest Rate & Output of PS – Young, Immature Leaves Have High Rate of PS but Use more of what They Produce for Their Own Growth – Mature Leaves have Slower PS Rates – Defoliation of Young or Young + Mature Leaves of a Plant Drains the Plant http://www.emc.maricopa.edu/faculty/farabee/BIOBK/pressflo.jpg Respiration • Free Energy Is Released & Incorporated into Form that Can Be Readily Used for Maintenance & Development • Net Reaction Appears Reverse of PS • C6H12O6 + 6O2 + 40 ADP + 40 Phosphates → 6 CO2 + 6 H2O + 40 ATP • Turns Carbs into Usable Chemical Energy (ATP) for many Other Plant Reactions including PS • All Living Plant & Animal Cells Carry Out Respiration • When Does Respiration Occur? – – – – Same Time as PS During Night Developing & Ripening Fruit Dormant Seeds http://www.biol.lu.se/cellorgbiol/dehydrogenase/pop_sv.html 6 Mitochondria Aerobic Respiration • • • • • Occurs in Mitochondria • Membrane-Enclosed Organelles Distributed through Cytosol of Most Eukaryotic Cells • Convert Potential Energy of Food Molecules into ATP Requires Oxygen Main Type of Respiration in most Situations Breakdown of Glucose back to CO2 & Water Not All Energy in Glucose Is Converted to ATP Formation – Only about 40% Efficient – Extra Energy Is Given off as Heat http://www.kathleensworld.com/mitochondria.jpg http://www.science.siu.edu/plant-biology/PLB117/JPEGs%20CD/0077.JPG 2. Krebs Cycle 3 Main Steps 1. Glycolysis – Breakdown of Glucose to a 3-Carbon Compound Called Pyruvate – Occurs in Cytosol – Some ATP & NADH Are also Formed – – – – ‘Tricarboxylic Acid Cycle (TCA Cycle)’ ‘Citric Acid Cycle’ Occurs in Mitochondrial Matrix Cyclic Series of Rxs Break Down Pyruvate to CO2 & Carbon Skeletons – Skeletons Used in other Metabolic Pathways – Step where CO2 Is Given off by Plant – 10 NADH Are Generated • Storage Energy Molecules – NADH Is Formed from NAD – Similar Type of Energy-Storing Rx as NADP + H2 → NADPH2 • NAD + H → NADH http://www.med.unibs.it/~marchesi/glycpth2.gif 3. Electron Transport Chain – – ‘Oxidative Phosphorylation’ Series of Proteins in Mitochondria Helps Transfer Electrons (e-) from NADH to Oxygen • Releases a Lot of Energy – – Occurs on Mitochondrial Inner Membrane (Proteins Bound to Membrane) Released Energy Is Used to Drive Reaction ADP + P → ATP http://www.sp.uconn.edu/~bi107vc/images/mol/krebs_cycle.gif Anaerobic Respiration • ‘Fermentation’ • Low-Oxygen Environments – Wet or Compacted Soils – After Strong Exertion • ATP Still Produced from Glucose but Not as Efficiently as with Aerobic Respiration • Many ATP Are Made – – Oxygen Is Required for this Step Water Is Produced http://www.uccs.edu/~rmelamed/MicroFall2002/Chapter%205/ch05.htm http://www.jracademy.com/~vinjama/2003pics/fermentation%5B1%5D.jpg 7 • C6H12O6 + O2 → 2 CH2O5 + 2 H2O + 2 ATP • Same Rx Used to Produce Alcohol from Corn or to Make Wine or other Consumed Alcohol Photorespiration Aerobic: C6H12O6 + 6O2 + 40 ADP + 40 Phosphates → 6 CO2 + 6 H2O + 40 ATP • Only 2 ATP Are Formed Instead of 40 from Aerobic Respiration – Plant Soon Runs out of Energy – Can Suffer from Toxic Levels of Ethanol http://www.marietta.edu/~spilatrs/biol103/photolab/photresp.html • Respiration Driven by Light Energy • Scientists Realized that Some Plants Have Faster Respiration Rate in Light than in Dark • Occurs in Chloroplasts & Other Structures in a Photosynthetic Cell • Rubisco can React with Oxygen to Start Slightly Different Series of Rxs – Result in Loss or No Net Gain of Dry Matter for Plant – Less ATP Is Produced Factors Influencing Photorespiration • O2 : CO2 Ratio – C4 Plants Have Little Photorespiration • Light Intensity – Increasing Light Intensity Will Increase Energy for Photorespiration Process & for PS – C3 Plants Light-Saturate at Lower Light Intensities than C4 Plants • Temperature – Aerobic Respiration & Photorespiration Increase with Temp http://www.botany.hawaii.edu/faculty/webb/BOT311/BOT311-00/PSyn/Image81.gif • Net Photosynthesis Rate – C4 Plants Generally Have Net of 2 to 3 Times that of C3 Plants – C4 Plants often Called Efficient Plants & C3 Plants Called Non-Efficient Plants – A Few C3 Plants Have Low Respiration & Similar Rates as C4 Plants – Cooler Temps Are the only Time when C3 Plants Have Higher Net Rates than C4 Plants Factors Affecting Respiration • Kind of Cell or Tissue – Young & Developing Cells (Meristematic Areas) usually Have Higher Respiration Rates – Developing & Ripening Fruit & Seeds, too • Temperature Inside Plant Cell – Can Have Net Dry Matter Loss at High Temps where Respiration Exceeds PS http://www.botany.hawaii.edu/faculty/webb/BOT311/BOT311-00/PSyn/Image81.gif 8 • Oxygen – Low O2 Can Reduce Aerobic Respiration – Low O2 Can Reduce Photorespiration • Light – Can Enhance Rate of Photorespiration – No Direct Effect on other Forms of Respiration • [Glucose] – Need Adequate # to Carry out Respiration – Reductions can Occur Light Compensation Point • [CO2] – Higher CO2 Levels Reduce Rate of Respiration – Seldom Occurs except if O2 Levels Are Limited • [ATP] – Higher [ATP] Reduces Rate of Respiration – Usually Occurs when Other Metabolic Processes Have Slowed or Stopped • Plant Injury – Injury Increases Respiration – Plant’s Growth Rate Increases in Attempt to Recover – Synthesizes Compounds to Fight Pests – Some Herbicides Kill Plants by Disrupting or Affecting Respiration Endosymbiotic Theory • Level of Light Intensity where Rate of Respiration (CO2 Produced) Equals Rate of PS (CO2 Consumed) • Greater Light Intensity = Net Dry Matter (Carbohydrate Accumulation) • Lower Light Intensity = Net Dry Matter Loss • Generally Reached for Plants Grown Outdoors • Maybe Not for Full Sun Plants Grown in Shade or Houseplants Grown Indoors in Inadequate Light http://www.daviddarling.info/encyclopedia 9