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					                                         Energy Capture
                                         Photosynthesis
                                         Energy Capture
                                         Photosynthesis
   How do cells acquire E
   Has to come from somewhere
   Easy answer: burn glucose
   Where does glucose come from
   C, H, O: present everywhere in natural world
   Assembled through photosynthesis
                                              Photosynthesis
   Green organisms photosynthesize
   Location: organelles called chloroplasts
   2 separate sets of reactions
   2 different sites within the chloroplast
      Membrane
      Stroma—fluid filled space
                                       Photosynthesis and Energy
   Plants and other ―green‖ organisms use the energy from sunlight to grow
      Many organisms eat the plants
           Obtain energy through cellular respiration
      Many organisms are dependent upon the O2 produced
           Also used in cellular respiration
   Virtually all life is dependent upon sunlight
      Dependent upon photosynthesis
                                       Photosynthesis and Energy
   Plants are photosynthetic organisms
   Some non-plants are also photosynthetic
      Algae
      Some bacteria
                                              Photosynthesis
                                            Terrestrial plants

                                         Photosynthesis
                                  Algae: macro and microscopic
                                         Photosynthesis:
                                     Photosynthetic Bacteria
                                     Energy Capture, Release
   Coupled processes
   Look at basic reactions
   Change direction of arrow
   Go from to the other
                                    Photosynthesis and Energy
 The chemical formula for photosynthesis is the reverse of the formula for cellular respiration
                                    Photosynthesis

    CO2 + H2O + (light) energy  sugar + O2

                                        Cellular respiration
 Sugar + O2  CO2 + H2O + energy (ATP)
                                    Photosynthesis and Energy
 Photosynthesis
     Electrons take a trip up an energy hill
     Energy is stored in food
 Cellular respiration
      Electrons take a trip down an energy hill
      Energy is extracted from food to form ATP
                                    Photosynthesis and Energy
 Photosynthesis and cellular respiration display symmetry
      Photosynthesis uses light energy to make food
      Cellular respiration ―burns‖ this food to release and capture energy as ATP
                                   Components of Photosynthesis
 Photosynthesis begins with the absorption of sunlight by leaves
      Only a portion of the light falling on them is absorbed

                                   Components of Photosynthesis
 Sunlight is composed of a spectrum of energetic rays
       Measured by ―wavelengths‖
       Short ultraviolet rays
       Visible light rays
       Longer infrared rays
                                      Components of Photosynthesis
   Visible light includes all the colors of the rainbow
      Red – violet
   Photosynthesis relies upon portions of the visible light spectrum
      Mainly blue and red wavelengths
                                         Light and Photosynthesis
   Need light
   Ocean photosynthesis: only top layer
   Light doesn’t penetrate much beyond 60 meters
   Surface layers productive
   Abyssal zone: chemosynthesis


                                   Components of Photosynthesis
 Plant leaves contain pigments
      These pigments absorb some wavelengths of light and reflect others
         Red and blue wavelengths are largely absorbed
         
         Green wavelengths are largely scattered
         
 Leaves appear green because most of the green light is scattered (reflected)
    When it hits your eye, you perceive ―green‖
                                 Components of Photosynthesis
 Most photosynthesis on planet done by unicellular organisms
 Floating in oceans, lakes
                                 Components of Photosynthesis
Leaf anatomy
 A plant’s leaves are the primary sites of photosynthesis
    Leaves contain epidermal layers on their top and bottom surfaces
    Mesophyll cells lay sandwiched between the epidermal layers
                                 Components of Photosynthesis
Leaf anatomy
 Epidermis
    ―Skin‖ of the leaves
    Photosynthetically inactive
    Protective function
        e.g., Minimizes water loss
    Contains pores on underside called stomata
        
         Allow exchange of CO2 and O2
                               Components of Photosynthesis
Leaf anatomy
 Stomata
    Allow CO2 and O2 exchange and H2O loss
    Present in large numbers
                               2
        1,000 – 100,000 per cm of a plant leaf
 Mesophyll cells
    Photosynthetically active
    Contains all component parts of plants cells in general, including chloroplasts

                                  Components of Photosynthesis
Leaf anatomy
 Chloroplasts are the site of photosynthesis in plants and algae
    Organelles of endosymbiotic origin
    One to several hundred per cell
    Outer membrane
         Eukaryotic origin
    Inner membranes
         Bacterial origin
                                  Components of Photosynthesis
Chloroplast anatomy
 Stroma
    Liquid material internal to the chloroplast’s inner membrane
 Thylakoids
    Network of membranes within the stroma
    Sometimes stacked like coins to form grana
 All the steps of photosynthesis take place in either the stroma or the thylakoids

                               Components of Photosynthesis
Chloroplast anatomy
 Pigment molecules are embedded within the thylakoids
    Chlorophyll a
        Primary pigment in photosynthesis
    Other accessory pigments
        Assist chlorophyll a
 These pigments absorb certain wavelengths of light

                                    Components of Photosynthesis
 Two essential stages in photosynthesis
     Light-dependent reactions
         Electrons are removed from water and boosted to a higher energy level
             Light is required
     Light-independent Reactions: Calvin cycle
         Electrons are added to CO2 to make sugar


                                    Components of Photosynthesis
 Photosystems
     Working units of the light reactions
           Photosystems I and II
     Organized complex of molecules within the thylakoids
           Contain a few hundred pigment molecules
     Collect solar energy and transform it into chemical energy
                                   Components of Photosynthesis
 Photosystems
     Antennae complex
        Includes most of the pigment molecules
        Absorb sunlight and transfer it to the reaction center
     Reaction center
        Pair of chlorophyll a molecules
        Receive solar energy
        Transform solar energy into chemical energy


                                   Components of Photosynthesis
 Light is absorbed by antenna pigments
    Energy is transferred to chlorophyll a molecules in the reaction center
 Electrons in reaction center are moved up an energy hill
     Achieve a higher energy state
     Transferred through a series of molecules
 Solar energy is converted into chemical energy
                                          Light Reactions
 Light energy collected by antenna molecules of photosystem II
     Energy transferred to reaction center
     Electrons are
           Boosted in energy level
           Transferred to primary electron acceptor

                                          Light Reactions
 Chlorophyll is now missing electrons
    Electrons are removed from H2O to replace chlorophyll a’s missing electrons
    ―Splitting of water‖
    O2 is produced as a byproduct
                                          Light Reactions
 High-energy electrons from photosystem II are passed through a series of electron transport
  molecules
    Similar to ETC in cellular respiration
    Electrons fall back down an energy hill
        Return to ground state
    ATP is synthesized
    Upon reaching ground state, these electrons are given to photosystem I
                                          Light Reactions
 Photosystem I
    Similar to photosystem II
    Slightly different reaction center
    Reaction center of photosystem I also receives solar energy
        Electrons are boosted to a higher energy state



                                          Light Reactions
 Chlorophyll is now missing electrons
     Electrons are removed from H2O to replace chlorophyll a’s missing electrons
     ―Splitting of water‖
     O2 is produced as a byproduct
                                          Light Reactions
 Photosystem I absorbs energy and promotes electrons to a higher energy state
 These electrons are added to a primary electron acceptor
     Transferred down a short energy hill
         
           Deposited on NADP+
               +
          NADP reduced to form NADPH
             Reduced coenzyme similar to NADH
             High energy molecule


                                             Light Reactions
 Photosystem I electrons elevated
                       +
     Added to NADP + to form NADPH
 Photosystem II electrons elevated
     Fall down energy hill, ATP produced
     Replace electrons gone from photosystem I
 Electrons taken from water
 H2O split to replace PS II electrons
    
       H20  2H+ + ½ O2 + 2 e-

    Products: NADPH, ATP, O2

                                           Light Reactions

    The early Earth’s atmosphere lacked O2

    21% of the Earth’s current atmosphere is O2
      This O2 is a product of photosynthesis
          When water is split to yield electrons, O2 is incidentally formed
             
                H20  2H+ + ½ O2 + 2 e-
      This waste product of photosynthesis is critically important to humans and many other
        species
                            Light-Independent Reactions: Calvin Cycle
   In the second stage of photosynthesis, the high energy molecules formed in the light
    reactions will be used
   Occurs in the stroma of chloroplasts
   Carbon dioxide is reduced into sugars
      CO2  sugar
   This process requires energy
      The energy is supplied by the products of the light reactions
          ATP and NADPH

                                  Light-Independent: Calvin Cycle
 Like the Krebs cycle, the Calvin cycle is circular
 Four phases
       Carbon fixation
       Energizing the sugar
       Production of sugar
       Regeneration of starting materials
                                        Calvin Cycle
Carbon fixation
The enzyme ―rubisco‖
 Facilitates carbon fixation
 ―The most important protein on Earth‖
 The most abundant protein on Earth
 Constitutes approximately 30% of the biomass in a typical plant leaf

                                              Calvin Cycle
Energizing the sugar
 ATP and NADPH are ―spent‖ to reduce and energize these three-carbon sugars
     6 NADPH and 6 ATP spent
     Products are 6 sugars called G3P
           High-energy sugars
                                            Calvin Cycle
Exit of the product
 One G3P molecule exits the Calvin cycle
     This is the product of the Calvin cycle
     This exiting G3P molecule is readily converted to glucose in reactions outside of the Calvin
      cycle
 The other 5 G3P molecules remain in the Calvin cycle

                                        Calvin Cycle
Regeneration of RuBP
 The 5 G3P molecules remaining in the Calvin cycle are used to regenerate RuBP
    5 G3P  3 RuBP
 This conversion requires energy
    3 ATP are spent

                                         Calvin Cycle
Recap
 3 CO2 + 3RuBP   6 PGA
 6 PGA 6 G3P
    6 ATP and 6 NADPH spent
    One G3P removed as product
 5 G3P  3 RuBP
    3 ATP are spent
 Overall accomplishment: 3 CO2  1 G3P
                                         Calvin Cycle
 The product of photosynthesis is a sugar
    G3P, not glucose
 G3P can be readily converted into many other molecules
    e.g., Two G3P  glucose

                                    Photosynthesis Summary
 The ultimate product of photosynthesis is the whole plant
     G3P is converted into sugars, starches, proteins, etc.
     G3P is ultimately converted into all of the organic molecules present within the plant
 Photosynthesis produces approximately 155 billion tons of material per year

				
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