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Photosynthesis - PowerPoint PPT Presentation

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Photosynthesis. Chapter 8. Photosynthesis Overview. Energy for all life on Earth ultimately comes from photosynthesis 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants – chloroplasts.

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photosynthesis overview
Photosynthesis Overview
  • Energy for all life on Earth ultimately comes from photosynthesis

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

  • Oxygenic photosynthesis is carried out by
    • Cyanobacteria
    • 7 groups of algae
    • All land plants – chloroplasts
  • Thylakoid membrane – internal membrane
    • Contains chlorophyll and other photosynthetic pigments
    • Pigments clustered into photosystems
  • Grana – stacks of flattened sacs of thylakoid membrane
  • Stroma lamella – connect grana
  • Stroma – semiliquid surrounding thylakoid membranes
  • Light-dependent reactions
    • Require light
    • Capture energy from sunlight
    • Make ATP and reduce NADP+ to NADPH
  • Carbon fixation reactions or light-independent reactions
    • Does not require light
    • Use ATP and NADPH to synthesize organic molecules from CO2


discovery of photosynthesis
Discovery of Photosynthesis
  • Jan Baptista van Helmont (1580–1644)
    • Demonstrated that the substance of the plant was not produced only from the soil
  • Joseph Priestly (1733–1804)
    • Living vegetation adds something to the air
  • Jan Ingen-Housz (1730–1799)
    • Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)
F.F. Blackman (1866–1947)
    • Came to the startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly
    • Light versus dark reactions
    • Enzymes involved
C. B. van Niel (1897–1985)
    • Found purple sulfur bacteria do not release O2 but accumulate sulfur
    • Proposed general formula for photosynthesis
      • CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A
    • Later researchers found O2 produced comes from water
  • Robin Hill (1899–1991)
    • Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction
  • Molecules that absorb light energy in the visible range
  • Light is a form of energy
  • Photon – particle of light
    • Acts as a discrete bundle of energy
    • Energy content of a photon is inversely proportional to the wavelength of the light
  • Photoelectric effect – removal of an electron from a molecule by light
absorption spectrum
Absorption spectrum
  • When a photon strikes a molecule, its energy is either
    • Lost as heat
    • Absorbed by the electrons of the molecule
      • Boosts electrons into higher energy level
  • Absorption spectrum – range and efficiency of photons molecule is capable of absorbing
many pigments
Many pigments
  • Organisms have evolved a variety of different pigments
  • Only two general types are used in green plant photosynthesis
    • Chlorophylls
    • Carotenoids
  • In some organisms, other molecules also absorb light energy
  • Chlorophyll a
    • Main pigment in plants and cyanobacteria
    • Only pigment that can act directly to convert light energy to chemical energy
    • Absorbs violet-blue and red light
  • Chlorophyll b
    • Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb
structure of chlorophyll
Structure of chlorophyll
  • Porphyrin ring
    • Complex ring structure with alternating double and single bonds
    • Magnesium ion at the center of the ring
  • Photons excite electrons in the ring
  • Electrons are shuttled away from the ring
action spectrum
Action spectrum
  • Relative effectiveness of different wavelengths of light in promoting photosynthesis
  • Corresponds to the absorption spectrum for chlorophylls
other pigments
Other Pigments
  • Carotenoids
    • Carbon rings linked to chains with alternating single and double bonds
    • Can absorb photons with a wide range of energies
    • Also scavenge free radicals – antioxidant
      • Protective role
  • Phycobiloproteins
    • Important in low-light ocean areas
photosystem organization
Photosystem Organization
  • Antenna complex
    • Hundreds of accessory pigment molecules
    • Gather photons and feed the captured light energy to the reaction center
  • Reaction center
    • 1 or more chlorophyll a molecules
    • Passes excited electrons out of the photosystem
antenna complex
Antenna complex
  • Also called light-harvesting complex
  • Captures photons from sunlight and channels them to the reaction center chlorophylls
  • In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins
reaction center
Reaction center
  • Transmembrane protein–pigment complex
  • When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level
  • Light-energized electron can be transferred to the primary electron acceptor, reducing it
  • Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule
light dependent reactions
Light-Dependent Reactions
  • Primary photoevent
    • Photon of light is captured by a pigment molecule
  • Charge separation
    • Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule
  • Electron transport
    • Electrons move through carriers to reduce NADP+
  • Chemiosmosis
    • Produces ATP

Capture of light energy

cyclic photophosphorylation
Cyclic photophosphorylation
  • In sulfur bacteria, only one photosystem is used
  • Generates ATP via electron transport
  • Anoxygenic photosynthesis
  • Excited electron passed to electron transport chain
  • Generates a proton gradient for ATP synthesis
chloroplasts have two connected photosystems
Chloroplasts have two connected photosystems
  • Oxygenic photosynthesis
  • Photosystem I (P700)
    • Functions like sulfur bacteria
  • Photosystem II (P680)
    • Can generate an oxidation potential high enough to oxidize water
  • Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH
the connection
The connection
  • Photosystem I transfers electrons ultimately to NADP+, producing NADPH
  • Electrons lost from photosystem I are replaced by electrons from photosystem II
  • Photosystem II oxidizes water to replace the electrons transferred to photosystem I
  • 2 photosystems connected by cytochrome/ b6-f complex
noncyclic photophosphorylation
  • Plants use photosystems II and I in series to produce both ATP and NADPH
  • Path of electrons not a circle
  • Photosystems replenished with electrons obtained by splitting water
  • Z diagram
photosystem ii
Photosystem II
  • Resembles the reaction center of purple bacteria
  • Core of 10 transmembrane protein subunits with electron transfer components and two P680 chlorophyll molecules
  • Reaction center differs from purple bacteria in that it also contains four manganese atoms
    • Essential for the oxidation of water
  • b6-f complex
    • Proton pump embedded in thylakoid membrane
photosystem i
Photosystem I
  • Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules
  • Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron
  • Passes electrons to NADP+ to form NADPH
  • Electrochemical gradient can be used to synthesize ATP
  • Chloroplast has ATP synthase enzymes in the thylakoid membrane
    • Allows protons back into stroma
  • Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions
production of additional atp
Production of additional ATP
  • Noncyclic photophosphorylation generates
    • NADPH
    • ATP
  • Building organic molecules takes more energy than that alone
  • Cyclic photophosphorylation used to produce additional ATP
    • Short-circuit photosystem I to make a larger proton gradient to make more ATP
carbon fixation calvin cycle
Carbon Fixation – Calvin Cycle
  • To build carbohydrates cells use
  • Energy
    • ATP from light-dependent reactions
    • Cyclic and noncyclic photophosphorylation
    • Drives endergonic reaction
  • Reduction potential
    • NADPH from photosystem I
    • Source of protons and energetic electrons
calvin cycle
Calvin cycle
  • Named after Melvin Calvin (1911–1997)
  • Also called C3 photosynthesis
  • Key step is attachment of CO2 to RuBP to form PGA
  • Uses enzyme ribulosebisphosphatecarboxylase /oxygenaseor rubisco
3 phases
3 phases
  • Carbon fixation
    • RuBP + CO2 → PGA
  • Reduction
    • PGA is reduced to G3P
  • Regeneration of RuBP
    • PGA is used to regenerate RuBP
  • 3 turns incorporate enough carbon to produce a new G3P
  • 6 turns incorporate enough carbon for 1 glucose
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output of calvin cycle
Output of Calvin cycle
  • Glucose is not a direct product of the Calvin cycle
  • G3P is a 3 carbon sugar
    • Used to form sucrose
      • Major transport sugar in plants
      • Disaccharide made of fructose and glucose
    • Used to make starch
      • Insoluble glucose polymer
      • Stored for later use
energy cycle
Energy cycle
  • Photosynthesis uses the products of respiration as starting substrates
  • Respiration uses the products of photosynthesis as starting substrates
  • Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse
  • Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria
  • Rubisco has 2 enzymatic activities
    • Carboxylation
      • Addition of CO2 to RuBP
      • Favored under normal conditions
    • Photorespiration
      • Oxidation of RuBP by the addition of O2
      • Favored when stoma are closed in hot conditions
      • Creates low-CO2 and high-O2
  • CO2 and O2 compete for the active site on RuBP
types of photosynthesis
Types of photosynthesis
  • C3
    • Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)
  • C4 and CAM
    • Add CO2 to PEP to form 4 carbon molecule
    • Use PEP carboxylase
    • Greater affinity for CO2, no oxidase activity
    • C4 – spatial solution
    • CAM – temporal solution
c 4 plants
C4 plants
  • Corn, sugarcane, sorghum, and a number of other grasses
  • Initially fix carbon using PEP carboxylase in mesophyll cells
  • Produces oxaloacetate, converted to malate, transported to bundle-sheath cells
  • Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2
  • Carbon fixation then by rubisco and the Calvin cycle
the cost of doing business this way
The cost of doing business this way
  • C4 pathway, although it overcomes the problems of photorespiration, does have a cost
  • To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone
  • C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone
cam plants
CAM plants
  • Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups
  • Stomata open during the night and close during the day
    • Reverse of that in most plants
  • Fix CO2 using PEP carboxylase during the night and store in vacuole
cam plants in action
CAM plants in action
  • When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2
  • High levels of CO2 drive the Calvin cycle and minimize photorespiration
compare c 4 and cam
Compare C4 and CAM
  • Both use both C3 and C4 pathways
  • C4– two pathways occur in different cells
  • CAM – C4 pathway at night and the C3 pathway during the day