Ch. 7 Capturing Solar Energy: PHOTOSYNTHESIS // (Sid Science Kid...

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Transcript of Ch. 7 Capturing Solar Energy: PHOTOSYNTHESIS // (Sid Science Kid...

  • Ch. 7Capturing Solar Energy:PHOTOSYNTHESIS (Sid Science Kid Song)

  • CASE STUDY: Did the Dinosaurs Die from Lack of Sunlight? About 65 mya, the Cretaceous-Tertiary (K-T) extinction event brought the Cretaceous period to a violent end and life on Earth suffered a catastrophic blow. The fossil record indicates that a devastating mass extinction eliminated at least 50% of all forms of life known to exist at that time. The 160-million-year reign of the dinosaurs, including the massive Triceratops and its predator Tyrannosaurus rex, ended abruptly. It would be many millions of years before Earth become repopulated with a diversity of species even approaching that of the late Cretaceous. In 1980, Luis Alvarez, a Nobel Prize-winning physicist, his geologist son Walter Avarez, and nuclear chemists Helen Michel and Frank Asaro published a controversial hypothesis. They proposed than an invader from outer space-a massive asteroid-had brought the Cretaceous period to an abrupt and violent end. Their evidence consisted of a thin layer of clay deposited at the end of the Cretaceous period, found at sites throughout the world. Known as the K-T boundary layer, this clay deposit contains from 30 to 160 times the iridium level typical of Earths crust. Iridium is a silvery-white metal that is extremely rare in Earths crust, but abundant in certain types of asteroids.

  • How large must an iridium-rich asteroid have been to create the K-T boundary layer? The Alvarez team calculated this iridium-enriched space rock must have been at least 6 miles (10km) in diameter. In the Alvarez scenario, as the asteroid ploughed into Earth at nearly 45,000 mph, its impact released energy greater than 100 trillion tons of TNT, blasting out a plume of debris that likely extended halfway to the moon. The asteroids fragments, plummeting back into the atmosphere in a fiery shower, ignited widespread wildfires. A shroud of dust and soot blocked the suns rays, and cooling darkness enveloped Earth. Although the immediate destruction from fire, earthquakes, landslides, and tsunamis must have been almost unimaginable, these paled in contrast to the prolonged effects of the collision. How could an asteroid impact have eliminated half of all forms of life? Its most damaging long-term effect would have been disruptrion of the most important biochemical pathway on Earth: photosynthesis. What exactly occurs during photosynthesis? What makes this process so important that interrupting it ended the age of the dinosaurs?

  • Introduction to Photosynthesis7.1 What is Photosynthesis?

    7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy?

    7.3 The Calvin Cycle: How Is Chemical Energy Stored in Sugar Molecules?

  • 7.1 What is Photosynthesis energy required for all life forms is directly / indirectly derived from sunlightPhotosynthesis process by which light energy is captured and stored as chemical energy in bonds of organic molecules (i.e. sugar) organisms capable of trapping solar energyphotosynthetic protists, certain bacteria, land plants capture close to 100 billion tons of CARBON / yr photosynthetic organisms eventually feed all other forms of life.

  • Leaves and Chloroplasts Are Adaptations for PhotosynthesisLeaf structure: flat (increases surface area) & thin (allows sunlight to penetrate to reach light-trapping chloroplasts) Epidermis (upper and lower layers that consist of transparent cells) protect inner parts while light penetratesCuticle (transparent, waxy, waterproof covering of epidermis) reduces evaporation of water from leafStomata (stoma - mouth) adjustable pores in epidermis that allow CO2 to pass throughMesophyll (middle leaf) layers of cells where most chloroplasts are locatedVascular bundles (veins in leaf) supply water and minerals to leafs cells and transport sugar product to other parts of pantBundle sheath cells (surround vascular bundle) lack chloroplasts in most plants

  • Chloroplasts

  • Stomata Several Stomata Open Closed Showing Guard Cells

  • Chloroplast (photosynthetic organelle) found w/in mesophylldouble membrane

    Calvin cycle (Dark Reactions) capture C from CO2 to produce sugarLight Reactions - Sunlight chemical energy; H2O is split and rleases O2

  • CASE STUDY: Did the Dinosaurs Die from Lack of Sunlight? More than 2 billion years ago, some bacterial (prokaryotic) cells, through chance mutations, acquired the ability to harness the energy of sunlight. Exploiting this abundant energy source, early photosynthetic cells filled the seas. As their numbers increased, oxygen began to accumulate in the atmosphere, radically altering Earths environment. Later, plants evolved, made the transition to land, and eventually grew in luxuriant profusion. By Cretaceous period, plants had become sufficiently abundant to provide enough food to support plant-eating giants, such as th3e 8-ton Triceratops on which huge meat-eaters, such as the 40-foot-long Tyrannosaurus, may have preyed or scavenged. Then, as now, energy harvested through photosynthesis sustained nearly all forms of life.

  • Photosynthesis Consists of Light Rxns and the Calvin Cycle

    Involves dozens of rxns each catalyzed by separate enzymes2 different stages occur in 2 different locations but connected by energy-carrier moleculesLight Reactions (photo) chlorophyll w/in thylakoid membranes capture sunlight energy convert it to chemical energy stored in ATP + NADPH; water splits releases O2Calvin Cycle (Dark Reactions) (synthesis) enzymes in stroma use CO2 from air and chemical energy in ATP + NADPH to make 3-C sugar required to make glucose

  • 7.2 The Light Reactions: How Is Light Energy Converted to Chemical Energy

    light is electromagnetic radiation composed of packets of energy called photons - travels in a vacuum at186,000miles/sec other examples include microwaves, radio waves, X-rays, gamma rayseach type has a different wavelength and frequency


  • visible light wavelengths with energies able to alter biological pigments (chlorophyll) but not high enough to damage DNA; stimulates pigments in our eyes to see light that hits an object (leaf) can be absorbed (captured), transmitted (passed through) or reflected (bounced back).

    light reflected or transmitted seen as color of objectabsorbed light drives biological processes (photosynthesis) Chlorophyll a (chloroplast pigment) - absorbs violet, blue, and red light reflects GREEN Accessory pigmentschlorophyll b absorb add. wavelengths (blue and red-orange) missed by chlorophyll a reflects yellow-greenCarotenoids (i.e. beta carotene) absorb blue and green reflect yellow + orangebeta-carotene Vit A makes light capturing pigment in eyesWhy are leaves green if they contain Carotenoids?

  • The Light Reactions Occur in Association with the Thylakoid Membranes photosystem II and photosystem I cluster of chlorophyll and accessory pigments within thylakoid membranes

    electron transport chain (ETC) series of electron-carrier molecules adjacent to photosystems PS II ETC II PS I ETC I NADP

  • Photosystem II Uses Light Energy to Create a Hydrogen Ion Gradient and to Split WaterLight is absorbed by PS II and energy is passed to an e- in one of the chlorophyll a molecules w/in rxn ctrEnergized e- is ejected from chlorophyll a moleculeEnergized ejected e- is captured by a primary e- acceptor in rxn ctr High-energy e- is passed through ETC IIAs ETC II transfers e- along, some released energy is used to generate ATP; energy depleted e- enters rxn ctr of PS I replacing ejected e- thereLight strikes PS I, and energy is passed to the e- in rxn ctr chlorophyll moleculesEnergized e- is ejected from rxn ctr and captured by primary e- acceptorE- moves down ETC INADPH is formed when NADP+ in stroma picks up 2 energized e-, along with 1 H+

  • Figure 7-6 Energy transfer and the light reactions of photosynthesisH2OCO2ATPADPNADPHNADPlightreactionsCalvincyclesugarhighO2C6H12O6primaryelectronacceptorlightenergyeeenergy level of electronspigmentmoleculeselectrontransportchain IIeereaction centerchlorophyll a moleculesATPPhotosystem IIHO22H2OloweeeNADPHNADPHelectrontransportchain IPhotosystem I

  • - good leaf structure and light review; details about light rxns and Calvin cycle - different view of Light Rxns

  • The Hydrogen Ion Gradient Generates ATP by ChemiosmosisEnergy released as e- passes through ETC II is harnessed to pump H+ across thylakoid membrane and into thylakoid space

    2. High concentration gradient of H+ is generated

    During chemiosmosis, H+ flows down its concentration gradient through ATP synthase channels, which uses energy from gradient to generate ATP

    1 ATP made for every 3 H+ passed through the channel chemiosmosis

  • Case Study: Did the Dinosaurs Die from Lack of Sunlight?Scientists have calculated that the force of the asteroid impact blasted some debris so far into outer space that it took days to rain back down to Earth. Earths rotation while the debris was aloft meant that much of the material fell on regions far from the point of impact. The plummeting chunks of rocks would have made a flaming re-entry through Earths atmosphere, setting huge fires on nearly every continent. Oxygen levels were high in the Cretaceous atmosphere, intensifying the fires. A large portion of Earths vegetation was likely consumed by fire, and many of the plants that managed to survive the fires must have succumbed during the cold, dark global winter that began as the planet was encompassed by soot and dust. Plant-eating animals that survived the initial blast would have soon starved, especially enormous ones like the 12-ton Triceratops, which needed to consume hundreds of pounds of vegetation daily. Predators such as the Tyrannosaurus, which relied on plant-eaters for food, would have died soon afterward. During the Cretaceous, as now, interrupting the vital flow of solar energy captured by photosynthetic organisms would be catastrophic.

  • 7.3 The Calvin Cycle: How is Chemical Energy Stored in Sugar Molecules? good Calvin Cycle review our cells produce CO2 as they burn sugar for energy but only photosynthetic organisms (and chemosynthetic bacteria) can capture/fix carbon atoms w/in CO2.

    Calvin cycle

    discovered by Melvin Calvin, Andrew Benson, James Bassham in 1950s

    ATP + NADPH (product of light rxns) dissolve in stroma and power the synthesis of 3-C sugar glyceraldehyde-3-phosphate (G3P) from CO2cycle because it begins and ends with same 5-C sugar, ribulose biphosphate (RuBP)3 parts: 1) carbon fixation, 2) G3P synthesis, and 3) regeneration of RuBP

  • CALVIN CYCLE or C3 Pathway

    Carbon fixation rubisco enzyme 3 CO2 + 3 RuBP = 3 unstable 6-C molecules (split into) = 6 3-C PGA (phosphoglyceric acid) molecules

    2. G3P synthesis: 6 3-C PGA 6 3-C G3P molecules using energy donated by ATP and NADPH

    3. RuBP Regeneration: 5 G3P 3 RuBP using ATP (restarts cycle) 1 G3P exists the Calvin cycle

    GLUCOSE SYNTHESIS 2 G3P molecules that exit the Calvin cycle combine to form 1 6-C glucose molecule (outside the chloroplast)

  • Crash course on Photosynthesis

  • Light vs. Dark Rxnsthylakoids H2O, ADP, NADP+

    ATP, NADPH, and O2 energizes e- in PS I + II and jump from e- acceptors down ETCs2 energized e- 1 NADPHlost energy powers H+ ions through ATP synthase in chemiosmosishydrolysis supplies lost e- in PSII and O2stromaCO2, RuBP, ATP, NADPHC6H12O6 & RuBPRuBP + CO2 PGAPGA + ATP + NADPH 6 G3P 5 G3P 3 RuBP 1 G3P + 1 G3P glucose LocationReactants


  • sunlightwater uptakecarbon dioxide uptakeATPADP + PiNADPHNAD+glucosePoxygen releaseLIGHT INDEPENDENT REACTIONSLIGHT DEPENDENT REACTIONSnew waterLight and Dark Phase Compared

  • C4 Plants Capture Carbon and synthesize Sugar in Different Cells O2 can also bind to active site on rubisco enzyme (ex of comp. inhibition; usually in dry, hot conditions when stomata close)

    photorespiration prevents Calvin cycle from making sugar and wastes energy

    flowering plants evolved 2 different mechanisms to avoid photorespiration

    C4 pathway CAM (crassulacean acid metabolism)

    much more efficient at fixing carbon but require more energy (advantage only in warm, sunny, dry environments

    Kentucky bluegrass (C3) is taken over by spiky crabgrass (C4) in summer

  • C4 Pathway C4 plants contain chloroplastsin mesophyll and bundle sheath cells (unlike C3 plants)

    use enzyme PEP carboxylase to fix CO2 (highly selective for CO2unlike rubisco)

    PEP carboxylase causes CO2 to react with 3-C PEP to produce a 4-C oxaloacetate

    Oxaloacetate converts to malate which diffuses from mesophyll cells into bundle sheath cells (shuttles CO2)

    Malate breaks down 3-C pyruvate and releases CO2 creating high concentration in bundle sheath cells

    rubisco than fixes carbon with little comp. from O2 minimizing photorespiration

    Pyruvate transported back to mesophyll cells and ATP cells convert it back to PEP so cycle can continue

  • C4 pathway and CAM

  • CAM plants Capture Carbon and Synthesize Sugar at Different Times use C4 pathway but perform carbon fixation andsugar synthesis at different times (unlike C4 plants- use different structures)

    stomata of CAM plants open at night (cooler temps + high humidity) and CO2 is captured by mesophyll cells using C4 pathway

    malate (product of C4 pathway) shuttled into central vacuole (stored as malic acid) until daytime

    stomata close during day; malic acid re-enters cytoplasm as malate

    malate pyruvate PEP and CO2 releasedinto Calvin cycle to produce sugar

  • Stomata Several Stomata Open Closed Showing Guard Cells

  • Case Study: Did the Dinosaurs Die from Lack of Sunlight? Did an asteroid end the reign of dinosaurs? The Alvarez hypothesis wasinitially met with skepticism. If such a cataclysmic event had occurred, where was the crater? In 1991, scientists finally located it centered near the coastal town of Chicxulub on Mexicos Yucatan Peninsula. The crater, estimated at over 150mi in diameter and 10 mi deep, was filled with debris and sedimentary rock laid down during the 65.5 my since the impact. Ocean and dense vegetation hid most remaining traces from satellite images. Some paleontologists argue that the impact may have exacerbated more gradual changes in climate, to which the dinosaurs could not adapt. Such changes might have been caused by prolonged intense volcanic activity, such as occurred at a site in India at about the time of the K-T extinction. Volcanoes spew out soot and ash, and iridium is found in higher levels in lava from Earths molten mantle than in its crust. So furious volcanism could significantly reduce the amount of sunlight for plant growth, spew climate-changing gasses into the air, and also contribute to the iridium rich K-T boundary layer. In 2010, alternative hypotheses to the asteroid impact were dealt a blow when an expert group of 41 researchers published a review article in the prestigious journal Science. This publication analyzed the previous 20yrs of research by paleontologists, geochemists, geophysicists, climatologists, and sedimentation experts delaying with the K-T extinction event. The conclusion: Land and ocean ecosystems were destroyed extremely rapidly, and evidence overwhelmingly supports the asteroid impact hypothesis first proposed by the Alvarez group 30yrs earlier.

  • Photosynthesis Problems

    Click on link below and then select Begin Problem Set

    Try and answer the problem OR click on Tutorial for help and more information.