Photosynthesis

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Photosynthesis
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Introduction to Photosynthesis

• Life is solar powered.
• photosynthesis captures light energy from the sun
  and converts into chemical energy that is stored
  in sugars and other organic molecules, ATP..
• Photosynthesis nourishes almost all of the living
  world directly or indirectly. Some Bacteria do
  not need it.
• Photosynthesis has 3 stages. 1. Energy is
  captured from the sun light. 2. Light energy is
  converted into chemical energy which is stored as
  ATP and NADPH. 3. The energy stored in ATP and
  NADPH powers the formation of organic compounds
  using CO2.

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Global View

• Autotrophs are the producers of the biosphere
  they get their energy from in organic sources
  like light.
• There are photoautotrophs and chemoautotrophs.
• Heterotrophs live on organic compounds produced
  by other organisms, autotrophs. Consumers.
• Almost all heterotrophs are entirely relianton
  photoautotrophs for food and for oxygen
• So the circle of life starts with photosynthesis.

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Where Does Photosynthesis Occur

• Occurs in the chloroplasts of plant or
  photosynthetic organism cells.
• Any green part of a plant contains chloroplasts
• But, leaves are the major site of photosynthesis
  for most plants.
• There are about 500,000 chloroplasts per square
  millimeter of leaf surface.
• color of a leaf comes from chlorophyll, the green
  pigment in the chloroplasts. It reflects green
  wavelengths of light.
• Chloroplasts are found mainly in mesophyll, the
  middle tissue of a leaf.

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Cont.

• O2 exits and CO2 enters the leaf via tiny pores,
  stomata, in the leaf.
• Veins carry water from the roots and carry off
  sugar from mesophyll cells to other plant areas

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Chloroplast Structure
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Explanation of How Light is Absorbed and How it
Works
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The Light Reactions (Step 1)

• The thylakoids convert light energy into the
  chemical energy of ATP and NADPH.
• How When a molecule absorbs a photon of light,
  one of that molecules electrons is elevated to
  an orbital with more potential energy so it is
  now Excited.
• Photons get absorbed by clusters of pigment
  molecules in the thylakoid membranes called
  photosystems.
• chlorophyll is organized along with proteins and
  smaller organic molecules into photosystems.
• photosystems act as a light-gathering antenna
  complex made of chlorophyll a, chlorophyll b,
  and carotenoid pigment molecules.

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Light Reaction Cont
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Light Reaction Cont

• When any antenna molecule absorbs a photon, the
  photon gets passed from molecule to molecule
  until it arrives at a specific chlorophyll a
  molecule, the reaction center.
• Here it finds a primary electron acceptor that
  removes the excited electron from the reaction
  center or chlorophyll a molecule. This starts the
  light reactions
• Each photosystem - - reaction-center chlorophyll
  and primary electron acceptor surrounded by an
  antenna complex - works in the chloroplast as a
  light-gathering unit.

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Light Reaction Cont..

• 2 Kinds of Photosystems Photosystem I has a
  reaction center chlorophyll, the P700 center,
  that has an absorption peak at 700nm. Photosystem
  II has a reaction center with a peak at 680nm.
• They work together to use light energy to make
  ATP and NADPH.
• There are 2 routes the excited electron can take
  the Non-Cyclic path and the Cyclic path the
  non-cyclic is more common so we will focus on it.
  

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Light Reaction Cont. Non-Cyclic Path

• Main route, produces both ATP and NADPH.
• Step 1. photosystem II absorbs light, the excited
  electron is captured by the primary electron
  acceptor, and departs from the reaction center
• Step 2. An enzyme splits water, converting into 2
  H ions and O2. The Hydrogen replaces the
  electrons that just left and the oxygen is
  released as a byproduct, lucky for us.
• Step 3. The excited electrons pass along an
  electron transport chain, ETC, before ending up
  at the photosystem I reaction center

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Light Reaction Cont. Non-Cyclic Path

• Step 4. As these electrons move along the ETC
  their energy is used to make ATP.
• Step 5. The electrons gather in the P700 center
  of photosystem I and are then captured by a 2nd
  primary electron acceptor which sends them down
  another ETC.
• Step 6. these electrons are passed from the
  transport chain to NADP, creating NADPH.
• NADPH will carry these high-energy electrons to
  the Calvin cycle. Also know as Dark Reactions
  since they do not need light to run.

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Overview of Light Reactions
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Dark Reactions (The Calvin Cycle)

• Is a cycle so it regenerates its own molecules.
• CO2 enters the cycle and leaves as sugar. We all
  knew plants take in Carbon dioxide and release
  oxygen, now we are beginning to understand WHY.
• This cycle uses the energy in the ATP and NADPH
  that was created in the Light reactions to
  produce sugar.
• sugar product of the Calvin cycle is not glucose,
  but a three-carbon sugar, glyceraldehyde-3-phospha
  te (G3P).

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The Calvin Cycle

• Each turn of the Calvin cycle fixes one carbon
• To make one G3P molecule, the cycle must run 3
  times, fixing three molecules of CO2.
• To produce one glucose molecule six cycles and
  the fixation of six CO2 is needed
• The Calvin cycle has 3 phases Phase 1 carbon
  fixation phase, each CO2 molecule is attached to
  a five-carbon sugar, ribulose bisphosphate
  (RuBP).
• catalyzed by RuBP carboxylase or rubisco
• six-carbon intermediate splits in half to form
  two molecules of 3-phosphoglycerate

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Phase 1
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Phase 2 Reduction Phase

• each 3-phosphoglycerate receives another
  phosphate group from ATP to form 1,3
  bisphosphoglycerate.
• pair of electrons from NADPH reduces,Gain, each
  1,3 bisphosphoglycerate to G3P.
• Electrons from NADPH change a carboxyl group to
  a carbonyl group.

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Phase 2 Reduction Phase
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Phase 3 Regeneration Phase

• regeneration of the CO2 acceptor (RuBP), these
  five G3P molecules are rearranged to form 3 RuBP
  molecules.
• cycle must spend 3 more molecules of ATP (one per
  RuBP) to complete the cycle and prepare for the
  next.

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There is more then one way to skin a cat C3,C4,
and CAM Plants

• C3 plants are what we just talked about. But,
  some plants go about getting energy by using
  different molecules then what we just discussed.
  They fix carbon differently due to their
  environmental conditions, usually hot and dry.
• Read summary 7 pg 129 also explained on pg.
  126-127

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References

• Jack Brown M.S. Biology
• Starr and Taggart The Unity and Diversity of
  Life 10th edition 2004 Thomson Brookes/Cole
• Campbell and Reece Biology 6th edition. 2002
  Benjamin Cummings.
• Microsoft Encarta Encyclopedia 2004
• Raven and Johnson Holt Biology 2004 Holt,
  Rinehart and Winston.