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Types of Photosynthesis Certain plants have developed ways by u7Q47l

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									Photosynthetic Process
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  THE BASICS OF PHOTOSYNTHESIS
• Almost all plants are photosynthetic
  autotrophs (self producing), as are some
  bacteria and prtozoas
 – Autotrophs generate their own organic matter
   through photosynthesis
 – Sunlight energy is transformed to energy
   stored in the form of chemical bonds




        (a) Mosses, ferns, and
        flowering plants         (c) Euglena   (d) Cyanobacteria
Light Energy Harvested by Plants &
  Other Photosynthetic Autotrophs




     6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
WHY ARE PLANTS GREEN?
                         Plant Cells
                         have Green
                         Chloroplasts


      The thylakoid
      membrane of the
      chloroplast is
      impregnated with
      photosynthetic
      pigments (i.e.,
      chlorophylls,
      carotenoids).
  THE COLOR OF LIGHT SEEN IS THE
       COLOR NOT ABSORBED

• Chloroplasts
  absorb light                           Reflected
                               Light       light
  energy and
  convert it to
  chemical energy


                    Absorbed
                    light


                           Transmitted   Chloroplast
                           light
   Photosynthesis occurs in chloroplasts
• In most plants, photosynthesis occurs
  primarily in the leaves, in the chloroplasts
• A chloroplast contains:
  – stroma, a fluid
  – grana, stacks of thylakoids
• The thylakoids contain chlorophyll
  – Chlorophyll is the green pigment that captures
    light for photosynthesis
• The location and structure of chloroplasts
                                               Chloroplast
                          LEAF CROSS SECTION                 MESOPHYLL CELL
              LEAF


                                                 Mesophyll




                           CHLOROPLAST         Intermembrane space

                                                                        Outer
                                                                        membrane



                               Granum                                   Inner
                                                                        membrane
      Grana      Stroma                                           Thylakoid
                                        Stroma      Thylakoid     compartment
         Thylakoid
                     Thylakoid Membrane




                        Thylakoid Space
Granum
        Chloroplast Pigments
• Chloroplasts contain several pigments
– Chlorophyll a
– Chlorophyll b
(Chlorophyll a (alpha) absorbs well at a wavelength
  of about 450 nm but its primary absorption is at
  675nm in the long red wavelengths.
  Chlorophyll b (beat) absorbs most effectively at
  blue 470 but also with shorter peaks at 430 and
  640nm)
– Carotenoids
– Xanthophyll
                Fall Colors


• During the fall, the green chlorophyll pigments
  are greatly reduced revealing the other
  pigments.

• Carotenoids are pigments that are either red or
  yellow.
      Chlorophyll Molecules

• Located in the thylakoid membranes.
• Chlorophyll have Mg+ in the center.
• Chlorophyll pigments harvest energy (photons)
  by absorbing certain wavelengths (blue-420
  nm and red-660 nm are most important).
• Plants are green because the green
  wavelength is reflected, not absorbed.
Chlorophyll a & b
                     •Chl a has a methyl
                     group

                     •Chl b has a carbonyl
                     group

                     Porphyrin ring
                      delocalized e-




               Phytol tail
     Absorption of Chlorophyll




Absorption




             violet   blue   green   yellow   orange   red
                                wavelength
Different pigments absorb light
           differently
AN OVERVIEW OF PHOTOSYNTHESIS


• Photosynthesis is the
  process by which
  autotrophic organisms
  use light energy to
  make sugar and oxygen
  gas from carbon dioxide
  and water
       Carbon    Water                    Glucose   Oxygen
       dioxide                                       gas
                         PHOTOSYNTHESIS
 AN OVERVIEW OF PHOTOSYNTHESIS
 • The light reactions
                                 Light
   convert solar                                     Chloroplast
   energy to chemical
   energy                                            NADP
    – Produce ATP & NADPH                             ADP
                                                      +P
                                                                   Calvin
• The Calvin cycle makes                 Light
                                         reactions
                                                                   cycle

  sugar from carbon
  dioxide
  – ATP generated by the light
    reactions provides the energy
    for sugar synthesis
  – The NADPH produced by the
    light reactions provides the
    electrons for the reduction of
    carbon dioxide to glucose
    Steps of Photosynthesis
• Light hits reaction centers of chlorophyll,
  found in chloroplasts
• Chlorophyll vibrates and causes water
  to break apart.
• Oxygen is released into air
• Hydrogen remains in chloroplast
  attached to NADPH
• “THE LIGHT REACTION”
    Steps of Photosynthesis

• The DARK Reactions= Calvin Cycle
• CO2 from atmosphere is joined to H
  from water molecules (NADPH) to form
  glucose
• Glucose can be converted into other
  molecules with different flavors!
           Redox Reaction
• The transfer of one or more electrons from
  one reactant to another.

• Two types:
  1. Oxidation
  2. Reduction
        Oxidation Reaction
• The loss of electrons from a substance.
• Or the gain of oxygen.


                      Oxidation



 6CO2 + 6H2O            C6H12O6 + 6O2
                         glucose
        Reduction Reaction
• The gain of electrons to a substance.
• Or the loss of oxygen.



           Reduction



  6CO2 + 6H2O  C6H12O6 + 6O2
                          glucose
• Two types of
  photosystems
  cooperate in the
  light reactions
                                       ATP
                                       mill




                     Water-splitting          NADPH-producing
                     photosystem                photosystem
 1. Light Reaction (Electron Flow)
• Occurs in the Thylakoid membranes

• During the light reaction, there are two
  possible routes for electron flow.

  A. Cyclic Electron Flow
  B. Noncyclic Electron Flow
      A. Cyclic Electron Flow

•   Occurs in the thylakoid membrane.
•   Uses Photosystem II only
•   P700 reaction center- chlorophyll a
•   Uses Electron Transport Chain (ETC)
•   Generates ATP only

       ADP +    P        ATP
Cyclic Photophosphorylation (addition of
  phosphate to ADP to make ATP.)
• Process for ATP generation associated with some
  Photosynthetic Bacteria
• Reaction Center => 700 nm
Plants produce O2 gas by splitting H2O
• The O2 liberated by photosynthesis is made
  from the oxygen in water (H+ and e-)
 In the light reactions, electron transport
     chains generate ATP, NADPH, & O2

• Two connected photosystems collect
  photons of light and transfer the energy to
  chlorophyll electrons
• The excited electrons are passed from the
  primary electron acceptor to electron
  transport chains
  – Their energy ends up in ATP and NADPH
     Chemiosmosis powers ATP
     synthesis in the light reactions
• The electron transport chains are arranged
  with the photosystems in the thylakoid
  membranes and pump H+ through that
  membrane
  – The flow of H+ back through the membrane is
    harnessed by ATP synthase to make ATP
  – In the stroma, the H+ ions combine with NADP+
    to form NADPH
                  Chemiosmosis
            SUN
                          H+ H+ (Proton Pumping)

Thylakoid                   E
                  PS II           T                  PS I
                                           C

                                                         high H+
                                               H+   H+
                                                         concentration
                          H+ H+   H+ H+

                                                                     Thylakoid
                                  H+       ATP Synthase              Space




                  ADP + P                                     low H+
                                               ATP
                                      H+                      concentration
  B. Noncyclic Electron Flow
• Occurs in the thylakoid membrane

• Uses PS II and PS I

• P680 rxn center (PSII) - chlorophyll a

• P700 rxn center (PS I) - chlorophyll a

• Uses Electron Transport Chain (ETC)

• Generates O2, ATP and NADPH
Noncyclic Photophosphorylation
• Photosystem II regains electrons by splitting
  water, leaving O2 gas as a by-product
                                                   Primary
                                              electron acceptor

           Primary
      electron acceptor




                                                                  Photons




                           Energy for
                          synthesis of

                                               PHOTOSYSTEM I


       PHOTOSYSTEM II       by chemiosmosis
  B. Noncyclic Electron Flow
• ADP +                   ATP
              P
  (Reduced
• NADP + H
  )      +                NADPH
  (Reduced)

• Oxygen comes from the splitting of
  H2O, not CO2
              H 2O         1/2 O2 + 2H+
              (Oxidized)
How the Light Reactions Generate ATP and NADPH
                                                     Primary           NADP
                                                     electron
                                                     acceptor
                                        Energy
                    Primary             to make                 3
                    electron
                    acceptor        2


                                                                    Light




        Light


                                                     Primary
                                                     electron
                                                     acceptor


                    Reaction-
              1      center                       NADPH-producing
                   chlorophyll                      photosystem



                  Water-splitting
                  photosystem
 2 H + 1/2
Summary—Light Dependent
       Reactions

a. Overall input
      light energy, H2O.
b. Overall output
       ATP, NADPH, O2.
       Light Independent Reactions
              aka Calvin Cycle
Carbon from CO2 is
 converted to glucose

(ATP and NADPH
  drive the reduction
 of CO2 to C6H12O6.)
     Light Independent Reactions
            aka Calvin Cycle
CO2 is added to the 5-C sugar RuBP by the
  enzyme rubisco.
This unstable 6-C compound splits to two
  molecules of PGA or 3-phosphoglyceric acid.

PGA is converted to Glyceraldehyde 3-phosphate
 (G3P), two of which bond to form glucose.

G3P is the 3-C sugar formed by three turns of the
 cycle.
Summary—Light Independent
       Reactions

a. Overall input
     CO2, ATP, NADPH.
b. Overall output
     glucose.
  Review: Photosynthesis uses light
    energy to make food molecules

• A summary of
                                        Chloroplast
  the chemicalLight

  processes of
  photosynthesis       Photosystem II
                          Electron
                         transport                    CALVIN
                           chains                     CYCLE      Stroma
                       Photosystem I




                                                                     Cellular
                                                                     respiration
                                                                     Cellulose
                                                                     Starch
                                                                     Other
                      LIGHT REACTIONS             CALVIN CYCLE       organic
                                                                     compounds
Photorespiration (Competing Reactions)
• Occurs under the following conditions:
   – Intense Light (high O2 concentrations, hot, dry,
     bright days)
   – High heat (Stomatas close)
• Rubisco grabs CO2, “fixing” it into a carbohydrate in
  the light independent reactions.
• O2 can also react with rubisco, inhibiting its active site
   – not good for glucose output
   – wastes time and energy (occupies Rubisco)
• So Fixation of O2 instead of CO2.
• Produces no sugar molecules or no ATP.
• Photorespiration is estimated to reduce
  photosynthetic efficiency by 25%
       Types of Photosynthesis
                  C3

                                   C4

                                   CAM




Rubisco: the world’s busiest enzyme!
     Types of Photosynthesis
• Certain plants have developed ways to limit
  the amount of photorespiration
  – C3 Pathway
  – C4 Pathway*
  – CAM (Crassulacean Acid Metabolism)
    Pathway*
  * Both convert CO2 into a 4 carbon intermediate
     C4 Photosynthesis
        C3 Photosynthesis : C3 plants
• Called C3 because the CO2 is first incorporated
  into a 3-carbon compound.
• Stomata are open during the day.
• RUBISCO, the enzyme involved in
  photosynthesis, is also the enzyme involved in
  the uptake of CO2.
• Photosynthesis takes place throughout the leaf.
• Adaptive Value: more efficient than C4 and
  CAM plants under cool and moist conditions and
  under normal light because requires less
  machinery (fewer enzymes and no specialized
  anatomy)..
• Most plants are C3.
          C4 Photosynthesis : C4 plants

• Called C4 because the CO2 is first incorporated into a
  4-carbon compound.
• Stomata are open during the day.
• Uses PEP Carboxylase for the enzyme involved in the
  uptake of CO2. This enzyme allows CO2 to be taken
  into the plant very quickly, and then it "delivers" the
  CO2 directly to RUBISCO for photsynthesis.
• Photosynthesis takes place in inner cells (requires
  special anatomy called Kranz Anatomy)
        C4 Photosynthesis : C4 plants

• Adaptive Value:
   – Photosynthesizes faster than C3 plants under high
     light intensity and high temperatures because the
     CO2 is delivered directly to RUBISCO, not allowing it
     to grab oxygen and undergo photorespiration.
   – Has better Water Use Efficiency because PEP
     Carboxylase brings in CO2 faster and so does not
     need to keep stomata open as much (less water lost
     by transpiration) for the same amount of CO2 gain for
     photosynthesis.
• C4 plants include several thousand species in at least 19
  plant families. Example: fourwing saltbush pictured here,
  corn, and many of our summer annual plants.
  CAM Photosynthesis : CAM plants. CAM
  stands for Crassulacean Acid Metabolism


• Called CAM after the plant family in which it was first
  found (Crassulaceae) and because the CO2 is stored in
  the form of an acid before use in photosynthesis.
• Stomata open at night (when evaporation rates are
  usually lower) and are usually closed during the day.
  The CO2 is converted to an acid and stored during the
  night. During the day, the acid is broken down and
  the CO2 is released to RUBISCO for photosynthesis
         CAM Photosynthesis : CAM plants.
• Adaptive Value:
   – Better Water Use Efficiency than C3 plants under arid conditions
     due to opening stomata at night when transpiration rates are lower
     (no sunlight, lower temperatures, lower wind speeds, etc.).
   – May CAM-idle. When conditions are extremely arid, CAM plants
     can just leave their stomata closed night and day. Oxygen given
     off in photosynthesis is used for respiration and CO2 given off in
     respiration is used for photosynthesis. This is a little like a
     perpetual energy machine, but there are costs associated with
     running the machinery for respiration and photosynthesis so the
     plant cannot CAM-idle forever. But CAM-idling does allow the
     plant to survive dry spells, and it allows the plant to recover very
     quickly when water is available again (unlike plants that drop their
     leaves and twigs and go dormant during dry spells).
• CAM plants include many succulents such as cactuses and agaves
  and also some orchids and bromeliads
               Leaf Anatomy
• In C3 plants (those that do C3
  photosynthesis), all processes occur in the
  mesophyll cells.

      Mesophyll cells




       Bundle sheath
       cells
             C4 Pathway

• In C4 plants
  photosynthesis occurs
  in both the mesophyll
  and the bundle sheath
  cells.
             C4 Pathway
• CO2 is fixed into a 4-
  carbon intermediate
• Has an extra enzyme–
  PEP Carboxylase
  (Phosphoenolpyruvat
  e carboxylase) that
  initially traps CO2
  instead of Rubisco–
  makes a 4 carbon
  intermediate
              C4 Pathway
• The 4 carbon intermediate
  is “smuggled” into the
  bundle sheath cell
• The bundle sheath cell is
  not very permeable to CO2
• CO2 is released from the
  4C malate  goes through
  the Calvin Cycle
                              C3 Pathway
   How does the C4 Pathway
     limit photorespiration?
• Bundle sheath cells are far from the
  surface– less O2 access
• PEP Carboxylase doesn’t have an
  affinity for O2  allows plant to collect a
  lot of CO2 and concentrate it in the
  bundle sheath cells (where Rubisco is)
           CAM Pathway
• Fix CO2 at night and
  store as a 4 carbon
  molecule
• Keep stomates
  closed during day to
  prevent water loss
• Same general
  process as C4
  Pathway
 How does the CAM Pathway
   limit photorespiration?
• Collects CO2 at night so that it can be
  more concentrated during the day
• Plant can still do the calvin cycle during
  the day without losing water
                         CAM Plants
  Night (Stomates Open)              Day (Stomates Closed)




                           Vacuole

         C-C-C-C           C-C-C-C            C-C-C-C
CO2       Malate                               Malate
                            Malate

                                                        CO2
                                                              C3

C-C-C
                   ATP                 C-C-C                   glucose
 PEP
                                     Pyruvic acid
           Summary of C4
           Photosynthesis
• C4 Pathway
  – Separates by
    space (different
    locations)
• CAM Pathway
  – Separates
    reactions by
    time (night
    versus day)
   Bio cell uses photosynthesis to generate
                   electricity
• Bio cell inserted into a cactus and a graph showing the
  intensity of the electric current generated as a function
  of light that fell on the plant (in black, glucose, and red,
  O2).Scientists at the research institute CNRS, France,
  changed the chemical energy generated by
  photosynthesis of a plant into electrical energy.
• The research demonstrates a new route for artificial
  photosynthesis , a promising area of research that aims
  to develop a strategy for conversion of sunlight into
  electricity even more efficient and more environmentally
  friendly than solar cells .
      Artificial photosynthesis

• Artificial photosynthesis is a research field
  that attempts to replicate the natural
  process of photosynthesis,
  converting sunlight, water, andcarbon
  dioxide into carbohydrates and oxygen.
  Sometimes, splitting
  water into hydrogen and oxygen by using
  sunlight energy is also referred to
  as artificial photosynthesis.
         Photoelectrochemical cell
• Research is being done into finding catalysts that can convert
  water, carbon dioxide, and sunlight to carbohydrates. For the
  first type of catalysts, nature usually uses the oxygen evolving
  complex. Having studied this complex, researchers have made
  catalysts such as blue dimer to mimic its function, but these
  catalysts were very inefficient. Another catalyst was engineered
  by Paul Kögerler, which uses four ruthenium atoms.
• The carbohydrate-converting catalysts used in nature are
  the hydrogenases. Catalysts invented by engineers to mimic
  the hydrogenasesinclude a catalyst by Cédric Tard,[3] the
  rhodium atom catalyst from MIT,[4] and the cobalt catalyst from
  MIT. Dr. Nocera of MIT is receiving funding from the Air Force
  Office of Scientific Research to help conduct the necessary
  experiments to push forward in catalyst research.
        Dye-sensitized solar cell
Possibly the most exciting technological development in
nanotechnology is a photovoltaic cell that uses photosynthesis to
generate electricity. The first solar photovoltaic chip was made
using ground-up spinach tissue by scientist Shuguang Zhang at
MIT. He was building on work by a group of researchers who had
earlier figured out how to harness energy from a plant. That
group was able to extract electrical current using a plant’s
photosynthesis for a period of three weeks. Zhang’s chip
converted approximately 12% of the light energy absorbed to
electrical current. This compares to the 24% efficiency of silicon
power cells. In the future, it is hoped that by adding layers of
chips, efficiency will be increased to 20%. size of this
photosynthetic solar chip is Ten to twenty nanometers or, small
enough to fit about a hundred of them in the width of a human
hair. Would result in lightweight computers and other electronic
devices, not to mention more environmentally friendly.
Electricity Generation by
Photosynthetic Biomass
                   Advantages
• Dye-sensitized cells can be made at one-fifth of the price
  of silicon cells.
• The solar energy can be immediately converted and
  stored, unlike in PV cells, for example, which need to
  convert the energy and then store it into a battery (both
  operations implying energy losses). Furthermore,
  hydrogen as well as carbon-based storage options are
  quite environmentally friendly.
• Renewable, carbon-neutral source of energy, which can
  be used for transportation or homes. Also the
  CO2 emissions that have been distributed from fossil
  fuels will begin to diminish because of the photosynthetic
  properties of the reactions.
            Disadvantages
• Artificial photosynthesis cells (currently)
  last no longer than a few years[ (unlike PV
  and passive solar panels, for example,
  which last twenty years or longer).
• The cost for alteration right now is not
  advantageous enough to compete
  with fossil fuels and natural gas as a viable
  source of mainstream energy.

								
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