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					Photosynthesis
      Photosynthesis
Process  by which plants use
  light energy to make food.
 A reduction process that
    makes complex organic
    molecules from simple
          molecules.
         Autotrophs
Organisms   that can
 manufacture their own food
 from inorganic molecules
Self-feeding organisms
       PSN General Equation
6 CO2 + 6 H2O + light   C6H12O6 + 6 O2

Requires:
Chlorophyll
   PSN: a redox process
  Hydrogens  are added to
          Carbons.
 Water is a source for the
        Hydrogens.
Complex covalent bonds are
           made.
                PSN
Has  two chemical reactions:
1. Light Dependent Reactions
2. Light Independent Reactions
    Names are from “light” as a
   requirement, not where or when
             they occur.
           Light
A form of electromagnetic
         radiation.
  Visible light has the
necessary energy for use in
           PSN.
Too Hot            Too Cold
          Just Right
       Action Spectrum
Not  all colors are useable to
   the same degree for PSN.
Red and Blue light - absorbed
       and used in PSN.
  Green light - reflected or
           transmitted.
Photosynthesis Pigments

1. Chlorophylls
2. Accessory Pigments
         Chlorophylls
    Has   CHON and Mg.
  Several types possible.
Molecule has a lipophilic tail
 that allows it to dissolve into
          membranes.
 Contains Mg in a reaction
            center.
     Accessory Pigments
Absorb  light energy and
 transfer it to chlorophyll.
Ex: Carotene (orange).
     Xanthophyll (yellow)
      Fall Leaf Colors
 Chlorophyll  breaks down.
  N and Mg salvaged and
 moved into the stem for next
             year.
Accessory pigments remain
behind, giving the various fall
         leaf colors.
  Chloroplast Structure
 Double  outer membrane.
Inner membrane folded and
     stacked into grana.
    Stroma - liquid that
   surrounds the thylakoid
        membranes.
thylakoid
   sacs

      stroma
            Photosystems
Collection  of pigments that serve as a
                 light trap.
Made of chlorophyll and the accessory
                 pigments.
Two photosystems are known:
 Photosystem I ,which uses P700 ( a
 type of chlorophyll a energized by the
 frequency 700 nm), and Photosystem II
 which uses P680 ( a type of chlorophyll
 a energized by the frequency 680 nm)
Noncyclic Photophsphorylation

   Uses    Photosystem I and
           Photosystem II.
   Splits water, releasing H+,
         a pair of e-, and O2.
  Produces ATP and NADPH.
    (e- carrier similar to NADH)
Light Dependent Reactions

    Photosystem II: Electrons
        trapped by P680 are
      energized by light and
    accepted by the “primary”
         electron acceptor
Light Dependent Reactions

              As electrons move
              down the Electron
               Transport Chain
              they lose energy…
              this energy is used
               to produce ATP.
               Some transport
                 proteins, like
                ferredoxin and
             cytochrome, include
               nonprotien parts
               containing iron.
Light Dependent Reactions


                 Electrons are
                again energized
                  by light and
                 accepted by a
                   primary
                   acceptor
Light Dependent Reactions




                  Electrons lose energy as
                    they move down the
                   ETC … the energy is
                        used to make
                     NADPH…. The 2
                  electrons lost to Photo-
                   system II are replaced
                   when water is split….
                        This is called
                        “Photolysis”.
Light Dependent Reaction

 Same  thing as Noncyclic
    Photophsphorylation.
  Location - grana of the
        chloroplast.
Function - to split water and
  produce ATP and NADPH.
       Light Dependent Reaction
 Requirements             Products
 Light
                           O2 (released into the environment)
 Water
                           ATP
 ADP+ Pi
                           NADPH
 NADP+


H2O + ADP + Pi + NADP+ + light     ATP + NADPH + O2 + H+
   Chemiosmosis Model
 The  chloroplast produces
  ATP in the same manner as
      the mitochondria in
          Respiration.
Light energy is used to pump
    H+ across a membrane.
 When the H+ diffuses back,
       ATP is generated.
  Chemiosmosis Model
 H+ are pumped into the
      thylakoid space.
ATP and NADPH are made
 when the H+ diffuse into the
           stroma.
       Calvin-Benson Cycle….
 Light Independent Reactions
How  plants actually
 makes food
 (carbohydrates).
Don't require light …
 energy supplied by
 ATP made during
 Light Dependent
 Reactions
Also known as the
 Calvin cycle or C3 Ps.
      Calvin-Benson Cycle….
  Light Independent Reactions

 Function - to use ATP and NADPH to
          build food from CO2
Location - stroma of the chloroplast.
     Rubisco (RuBP)
  RibuloseBisPhosphate
       Carboxylase.
Enzyme that adds CO2 to an
    acceptor molecule.
Most important enzyme on
          earth.
   Calvin-Benson Cycle…
Light Independent Reactions

                              CO2 (1C) combines
                              with RuBP (5C) to
                               form 2 PGA (3C)




  Carboxylation: 6CO2 + 6 RuBP produce 12 PGA
 The energy from
  ATP and the H+
 from the NADPH
are used to convert
   12 PGA to 12
   PGAL (G3P)
6 ATP are used to convert 10 PGAL
to 6 RuBP… the remaining 2 PGAL
are used to make Glucose.
                             C3 PSN
   Requirements                  Products
    6 CO2                       C6H12O6
    18 ATP                      18 ADP + 18 Pi
    12 NADPH                    12 NADP+



6CO2 + 18 ATP + 12 NADPH+ + H+   18 ADP + 18 Pi12 NADP+ + 1 Glucose
Cyclic Photophosphorylation
Uses  Photosystem I only.
Requires light
Electrons from Photosystem I are
 used to produce ATP…. the electrons
 then return to Photosystem 1…. They
 are not incorporated into NADPH as
 in Non-Cyclic Photophosphorylation
e-
           Photorespiration
Rubisco  is the most abundant protein
 on earth.
Occurs when Rubisco accepts O2
 instead of CO2 as the substrate.
Generates no ATP.
Decreases PSN output by as much as
 50%.
Products of Photorespiration are
 broken down by peroxisomes…
 found near the chloroplasts.
    Photorespiration
May reflect a time when O2
 was less plentiful and CO2
   was more common.
  Alternate PSN Methods
1. C4 PSN
2. CAM PSN
         C4 PSN
 C4  plants high rate of
photosynthesis reduces to
  overall loss of water….
Therefore they are found in
 dry, hot climates…. This
 gives them an advantage
      over C3 plants.
             C4 PSN
Uses   a different enzyme, PEP, to
         initially capture CO2
Still uses C3 PSN to make sugar,
  but only does so in the bundle
 sheath cells…. The bundle cells
 are near the water source… this
  makes the C4 plants extremely
                efficient.
      PEP Carboxylase
Enzyme  used for CO2 capture
          in C4 PSN.
Can use CO2 down to 0 ppm.
Prevents photorespiration.
            C4 PSN
Found   in 19 plant families.
Characteristic of hot regions
 with intense sunlight.
Examples - sugarcane,
 Bermuda grass, crab grass
   C3 Ps        vs       C4 Ps
Photorespiration     No Photorespiration
Shade to full sun    Full sun only
High water use       Moderate water use
Cool temperatures    Warm temperatures
Slow to moderate     Very fast growth
  growth rates         rates
Cool season crops    Warm season crops
           CAM PSN
Crassulacean  Acid Metabolism
  Found in plants from arid
 conditions where water stress
         is a problem.
Examples - cacti, succulents,
   pineapples, many orchids.
          CAM PSN
Open stomata at night to take
           in CO2.
 The CO2 is stored as a C4
            acid.
 During the day, the acid is
 broken down and CO2 is fixed
         into sugar.
           CAM plants
Tissues  decrease in pH over
 night, rise in pH during day.
Avoid H2O stress by keeping
  stomata closed during the
              day.
Generally have slow growth.
           Factors That Affect
            Photosynthesis
1.   Light - quantity and quality.
2.   Temperature - too hot or too cold.
3.   CO2 - often limits C3 plants.
4.   Minerals - especially NPK and Mg.
Importance of Photosynthesis

1. Food - either directly or
 indirectly comes from plants.
2. Oxygen in the air.
3. CO2 balance.
4. Plant products.
5. Life on Earth.

				
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posted:11/11/2011
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