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Chapter 7 PHOTOSYNTHESIS. Life Depends on Photosynthesis ] What if there is a nuclear winter? F What is it F What would cause it? F What are the repercussions?

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Text of Chapter 7 PHOTOSYNTHESIS. Life Depends on Photosynthesis ] What if there is a nuclear winter? F What...


  • Life Depends on PhotosynthesisWhat if there is a nuclear winter?What is itWhat would cause it?What are the repercussions?

  • What is a photosynthate?Glucose and other carbohydratesRuns photorespirationMakes amino acids, starch, cellulose, rubber, quinine, spices, etc

  • Who came first: Autotrophs or Heterotrophs?Heterotrophs CREATED a heavy CO2 atmosphere.Almost became extinct when resources disappeared.Environmental pressure allowed heterotrophs to begin to photosynthesizeAutotrophes kick out TONS of O2

  • Who came first: Autotrophs or Heterotrophs?, contHow did the rise of autotrophes change the earth?Decrease CO2 in atmosphere so climate got coldCreated polar ice capsLowered ocean levelsO2 levels increased to above todays levelsJurassic much higher


  • A. LightVisible light makes up only a small portion of the electromagnetic spectrum. Sunlight consists of:4% Ultraviolet (UV) radiation44% Visible light52% Infrared (IR) radiation

  • Characteristics of Visible Light:is a spectrum of colors ranging from violet to red (ROY G BIV)consists of packets of energy called photons photons travel in waves, having a measurable wavelength () = distance a photon travels during a complete vibration [measured in nanometers (nm)]

  • A photons energy is inversely related to its wavelength......the shorter the , the greater the energy it possesses.Which of the following photons possess the greatest amount of energy?Green photons = 530nmRed photons = 660nmBlue photons = 450nm

  • What happens to light when it strikes an object?reflected (bounces off)Only absorbed wavelengths of light function in photosynthesis. Visible light provides just enough energy to excite or energize moleculestransmitted (passes through)absorbed

  • B. Photosynthetic PigmentsMolecules that capture photon energy by absorbing certain wavelengths of light.1. Primary pigmentsBacteriochlorophyll - green pigment found in certain bacteria.Chlorophylls a & b - bluish green pigments found in plants, green algae & cyanobacteria.

  • Chlorophyll a is the dominant pigment in plant cells.Central Mg atom with 4 N atoms area where energy transfer occurs

    Tail anchors molecule to chloroplast

  • 2. Accessory PigmentsCarotenoids - red, orange, yellow pigments found in plants, algae, bacteria & archaea.Xanthophylls red and yellow pigments found in plants, algae & bacteria.Fucoxanthin brown pigment found in brown algae, diatoms, & dinoflagellatesPhycoerythrin - red pigment found in red algae.Phycocyanin - blue pigment found in red algae & cyanobacteria.Bacteriorhodopsin purple pigment found in halophilic archaeaEach pigment absorbs a particular range of wavelengths.

  • C. Chloroplasts (hyperlink chloroplasts)Sites of photosynthesis in plants & algae.Concentrated in mesophyll cells of most plants.

  • Chloroplast structure:Stroma - gelatinous matrix; contains ribosomes, DNA & various enzymes.Thylakoid - flattened membranous sac; embedded with photosynthetic pigments.

  • D. PhotosynthesisOccurs in two stages:Light reactions (pink hyperlink)- harvest photon energy to synthesize ATP & NADPH. Carbon reactions (Calvin cycle) - use energy from light reactions to reduce CO2 to carbohydrate. 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

  • Overview of Photosynthesis

  • 1. Light Reactionsrequire lightoccur in thylakoids of chloroplastsinvolve photosystems I & II (light harvesting systems).Photosystems contain antenna complex that captures photon energy & passes it to a reaction center.

  • Light Reactions of Photosynthesis


    Uses light and waterSynthesizes ATP and NADPHO2 released here (comes from water)Begins in thylakoid membrane4 clusters of proteins involved2 clusters make 2 photosystemsother 2 clusters form electron transport chain (ETC)notice different light lengths

  • Light Reaction, contPhotosystem II happens firstSUNAntenna ComplexChlorophyll a in reaction center P680e-1st carrier molecule in ETCEnergy (electron) goes to Photosystem IATP, produced from ADP, comes out of thylakoid and into stroma for Calvin cyclePowers CalvinCollects photons sends electronsReceives electrons sends electronsRedox reactionsOxygen is released here when H2O is split

  • Light reaction, contPhotosystem I is nextSun1st carrier molecule in new ETCAntenna complexElectrons change (reduce) NaDP+ to NaDPHChlorophyll a in reaction center P700e-Powers Calvin

  • ATP Production by Chemiosmotic Phosphorylation

  • 2. Carbon Reactions (Calvin cycle; C3 cycle) do NOT require light (occur in both darkness & light as long as ATP & NADPH are available)occur in stroma of chloroplastsrequire ATP & NADPH (from light reactions), and CO2

  • Calvin Cycle

  • CALVIN CYCLE (aka carbon cycle, aka C3 cycle)

    Found in plants that only use calvin: cereals, peanuts, tobacco, spinach, most trees and lawn grassesC3 named for 3 carbon compound: phosphoglyceric acid (PGA)- 1st stable compound in pathwayCalvin discovered this systemGoal of calvin cycle: Fix C from CO2 into organic compounds (glucose)Occurs in both light and dark (as long as ATP and NADPH is availableCO2 enters through stoma (plural: stomata)Powered by ATP/NADPH from light reactionDiagram on pg 117 is goodRubisco most abundant and important protein in the world85% of plants use this system

  • Calvin cycle, contRuBp (ribulose BiphosphateCO2 from the atmosphereUnstable 6 Carbon moleculePGA3-CPGA3-CCombines with(assisted by rubisco- an enzyme)(uses ATP and NADPH as energy from LR) BecomesMore ATP & NADPH used herePGALC3H6O3Converts to glucose then to other carbohydrateCan be rearranged to form RuBP (starts cycle over)(Phosphoglyceraldehyde: 1st carbohydrate product of Calvin)(C3H6O3)

  • Plants that use only the Calvin cycle to fix carbon are called C3 plants.Ex. cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees & lawn grasses.

  • Photosynthetic EfficiencyOnly .037% of atmosphere is CO2Yet 200 billion tons of carbon (from CO2 used to make glucoseIf each photosystem gets all the CO2 possible, efficency is only 30%- reality is much lower!Cloudy days are only .1% efficientCultivated plants only 3% efficientGreatest natural efficiency evening primrose 8%Sugarcane 7%

  • E. PhotorespirationProcess that counters photosynthesis.Occurs when stomata close under hot, dry conditions:O2 levels in plant increaseCO2 levels in plant decreaseUnder these conditions, rubisco fixes O2 (rather than CO2).Thus, PGAL is NOT produced.

  • PhotorespirationOccurs when conditions are low CO2 or high O2Rubisco uses O2 instead of CO2 as substrateResults in loss of Carbon for calvin (carbon) reactionHigh temperatures complicate things: fixing and releasing CO2 occurs at the same rate. No net C gain, no glucose producedStomata too open (trying to get more CO2) means dehydration

  • C4 PhotosynthesisAdaptations that allow certain plants to conserve water and reduce photorespiration at higher temperatures.Found in sugarcane, corn, millet, sorghum, all flowering plants in hot, open environments (about .4% of all plants)Use 4-carbon compound to concentrate carbon within special cellsCO2 is fixed in mesophyll cells firstThis keeps CO2 concentration 20-120 times greater than in C3 cycleNot efficient in normal climates: used 2 ATP for every carbon that is moved from the mesophyll cellsThen it is fixed as normal via Calvin cycleThese plants require about half as much water due to recycling of compounds during the pathway

  • C4 Respiration

    C4 plants reduce photorespiration by physically separating the light reactions and Calvin cycle.C4 plants even fix CO2 when stomata begin to close preserves water plants require the water of C3 CO2 malic acid migrates to bundle sheath cells and enters C3 glucose (instead of PGA)

  • Leaf anatomy of a C4 plantC4 Photosynthesis:Light reactions occur in chloroplasts of mesophyll cells.Calvin cycle occurs in chloroplasts of bundle sheath cells.

  • 2. CAM PhotosynthesisCAM plants reduce photorespiration by acquiring CO2 at night. Night:mesophyll cells fix CO2 as malic acidmalic acid is stored in vacuoles.Day:malic acid releases CO2 which enters Calvin cycle.Malic acid

  • CAM (crassulcean acid metabolism)RespirationIncludes cacti, pineapple, Spanish moss, orchids, some firns and wax plants (10% of all plant species)Take in CO2 at nightFix it in calvin cycle the next dayStomata stay closed during the day to preserve waterPlants acidic at night more alkaline in dayMalic acid formed in large vacuoles in same cells that contain chloroplastsMalic acid enters chloroplast during the day where C3 begins

    *Photosynthesis is the process by which plants, algae & cyanobacteria capture the energy in sunlight and convert it into chemical energy.Many consider photosynthesis to be the most important chemical process on earth, because it was not until photosynthesis began about 2 billion years ago that oxygen began to build up in the earths atmosphere. All oxygen in the air we breathe has cycled through photosynthetic organisms. ******Of these 3 types of radiation, we are primarily concerned with visible light because it provides the right amount of energy to power photosynthesis. [UV radiation is too powerful, while infrared radiation is not powerful enough]

    *1 nanometer = a billionth of a meterThe wavelengths of visible light range between 390 (violet end) and 760 nanometers (red end).*Blue photons are the most energetic because they have the shortest wavelengths.*Reflected or transmitted wavelengths determine the color of the object. Leaves appear to be green because the pigments they possess reflect green wavelengths of light.Objects that reflect all wavelengths of light (absorb none) are white, while objects that reflect none (absorb all) are black. ****Chlorophylls absorb red (600-700nm) & blue (400-500nm) wavelengths of light best. Since these are the primary photosynthetic pigments, photosynthesis occurs maximally under red & blue lights (reason why grow-lamps have a purple hue).Accessory pigments function to capture wavelengths of light that chlorophylls cannot. They then pass that energy to the chlorphylls. Carotineoids absorb blue wavelengths (460-550nm) of light best.Phycoerythrin absorbs green & yellow wavelengths of light best. Since phycoerythrins are not found in plants, green wavelengths of light contribute little to photosynthesis in plants.Phycocyanin absorbs yellow wavelengths of light best. *Are usually 40-200 chloroplasts / cell.Note stoma (opening) in cross section of leaf. they function in gas exchange, allowing CO2 to enter & O2 to exit leaf. However, stoma will close during hot dry conditions to conserve water. As we shall see later, this will impair photosynthesis.*Double membrane surrounds stroma.Granum = stack of thylakoids.*Photosynthesis will be described as it occurs in most eukaryotic cells.**Antenna complex contains about 300 chlorophyll molecules & 50 accessory pigments.Reaction center contains a pair of reactive chlorophyll a molecules.Reaction center of photosystem I contains a pair of P700 chlorophyll a molecules (P stands for pigment; they absorb light energy mostly at 700nm).Reaction center of photosystem II contains a pair of P680 chlorophyll a molecules (they absorb light energy mostly at 680nm). *Both photosystems and electron transport chains are embedded in thylokoid membranes.Light reactions begin with photosystem II.1. Light strikes PSII, exciting 2 electrons, which are passed to an electron acceptor. [Electrons lost from PSII must be replaced - replacement electrons are obtained by splitting water (O2 is released as a byproduct of the light reactions)].2. Electrons (from PSII) flow down ETC, providing energy for production of ATP by chemiosmotic phosphorylation.3. Light strikes PSI, exciting 2 electrons, which are passed to an acceptor molecule. Electrons reaching bottom of ETC are passed to PSI as replacement electrons.4. Excited electrons flow down a 2nd ETC, providing energy for production of NADPH. Note: electrons released when water was split eventually end up in NADPH!!!!****ATP is produced as electrons from PSII flow down the electron transport chain toward PSI.1. As electrons flow down the chain, energy is released. Energy is used to pump H+ (protons) from the stroma into the thylakoid space, creating a proton gradient. 2. Protons within the thylakoid space flow back into the stroma through channels called ATP synthases.3. As protons flow through the ATP synthase, ADP is phosphorylated, forming ATP.

    The coupling of ATP formation to energy release from a proton gradient is called chemiosmotic phosphorylation.

    *Called Calvin cycle in honor of the American biochemist Melvin Calvin.Called C3 cycle because CO2 is fixed as a 3C compound (PGA).NADPH is often the limiting factor of carbon reactions, because cells have only 1 mechanism for its production (light reactions of photosynthesis). NADPH cannot be made at night, so when it runs out, carbon reactions cease.Not likely that ATP will be a limiting factor of carbon reactions because cells have other mechanisms for making ATP.

    *1. Carbon fixation:The enzyme rubisco fixes CO2 [attaches CO2 to the 5-carbon sugar, ribulose biphosphate (RuBP)]. The resulting 6C compound is unstable & immediately splits to form two 3C molecules (PGA). Rubisco is one of the most important & abundant enzymes in the world.2. PGAL synthesis:The energy in ATP & NADPH is used to convert PGA PGAL (phosphoglyceraldehyde).PGAL is the direct carbohydrate product of the carbon reactions.3. PGAL molecules are siphoned off & combined to form glucose, sucrose, starch & other organic molecules.4. Regeneration of RuBP:Some of the PGAL is rearranged to regenerate RuBP. [essential step in perpetuating the cycle]***85% of all plant species are C3 plants.*Stoma close on hot dry days (to conserve water). Thus, CO2 is steadily being depleted, while O2 is steadily increasing inside the plant.Photorespiration severely hampers photosynthesis in C3 plants.

    **Called C4 photosynthesis because carbon dioxide is fixed as a 4C compound [malic acid] before it enters the Calvin cycle.C4 plants include sugarcane, corn, millet & sorghum.About 0.4% of plant species are C4 plants.**C4 plants have an additional biochemical pathway that allows them to fix CO2 even when levels within the plant fall very low.CO2 is fixed initially in mesophyll cells using the enzyme PEP carboxylase. PEP carboxylase has a high affinity for CO2. PEP carboxylase converts CO2 into a 4C compound, malic acid.Mesophyll cells actively pump malic acid into bundle sheath cells. CO2 is released & enters Calvin cycle (fixed by rubisco). This adaptation keeps CO2 levels high in bundle sheath cells, so rubisco functions optimally (photorespiration does not occur).Note: C4 plants dominate in hot, dry environments because they have a distinct advantage over C3 plants (able to inhibit water loss & reduce photorespiration).However, C4 plants are not as abundant in other habitats because they are at an energetic disadvantage. They must use use energy to pump malic acid into bundle sheath cells. *CAM plants include cacti, pineapple, Spanish moss, orchids, some ferns & the wax plant.About 10% of plant species are CAM plants.