Photosynthesis

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Photosynthesis

• Chapter 10
• A.P. Biology
• Liberty Senior High School
• Mr. Knowles

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Photosynthesis

• Conversion of unusable light energy into usable
  chemical energy.

C6H12O6
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The Players

• Prokaryotic Photosynthesizers
• Purple Sulfur Bacteria
• Cyanobacteria
• Eukaryotic Photosynthesizers
• Protists (Algae)
• Plants (single-celled and multicellular)

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Photosynthesis

• Two Step Process
• Light-Dependent Reactions (Light
  Reactions)-produce ATP and NADPH.
• Light-Independent Reactions (Calvin Cycle)-fix
  CO2 into sugars.

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• Light Reactions
• Occur in the grana.
• Split water, release oxygen, produce ATP, and
  form NADPH

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• The Calvin Cycle
• Occurs in the stroma
• Forms sugar from carbon dioxide, using ATP for
  energy and NADPH for reducing power

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• An overview of photosynthesis

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Light Dependent Reactions

• Pigments absorb the light energy.
• Different pigments are used in different
  organisms.
• Each with different absorption spectra.
• Why?

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• A photosystem
• Is composed of a reaction center surrounded by a
  number of light-harvesting complexes

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• Light-harvesting complexes
• Consist of pigment molecules bound to particular
  proteins.
• Funnel the energy of photons of light to the
  reaction center.

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Light-Dependent Reactions

• Photon captured by pigments arranged in a
  network with proteins embedded in a membrane-
  Photosystem (I and II).
• In bacteria, this membrane is the cell membrane.
• In protists (algae) and plants, this membrane is
  the thylakoid of the chloroplasts.

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Photosystems

• In plants, the reaction center molecule is a type
  of chlorophyll a called P700 for photosystem l.
• In the sulfur bacteria, the reaction center
  molecule is P870 for photosystem l.

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The First Photosynthesizers

• The purple sulfur bacteria more than 3 billion
  years ago.
• Used P870 in photosystem l.
• The excited electron is ejected from the pigment
  and travels in a circular path. Used to power a
  proton pump to make ATP- chemiosmosis.
• This circular movement is called - Cyclic
  Photophosphorylation.

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A Comparison of Chemiosmosis in Chloroplasts and
Mitochondria

• Chloroplasts and mitochondria
• Generate ATP by the same basic mechanism
  chemiosmosis
• But use different sources of energy to accomplish
  this

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• The spatial organization of chemiosmosis
• Differs in chloroplasts and mitochondria

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• In both organelles
• Redox reactions of electron transport chains
  generate a H gradient across a membrane
• ATP synthase
• Uses this proton-motive force to make ATP

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Cyclic Photophosphorylation.

• Produces ATP from light energy.
• Major limitation no biosynthesis (no
  carbohydrates made from CO2. Therefore, no long
  term storage of energy.
• These bacteria must find other sources of
  hydrogen to reduce CO2.
• Inefficient.

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How do you make a better photosynthesizer?

• The Evolutionary Process Continues!
• Make another Photosystem-The Advent of Noncyclic
  Photosysthesis

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Noncyclic Electron Flow

• Noncyclic electron flow
• Is the primary pathway of energy transformation
  in the light reactions

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• Produces NADPH, ATP, and oxygen

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• A mechanical analogy for the light reactions

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Enter the Competition! Cyanobacteria

• Another photosystem was created.
• Photosystem II uses a slightly different form of
  chlorophyll a called P680 (absorbs shorter
  wavelengths of light, more energy).
• Excited electron enters the electron transport
  chain and powers proton pumps.
• This leads to chemiosmosis and the synthesis of
  ATP.

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Photosystem II

• The excited electron is eventually passed to the
  P700 molecule in Photosystem I.
• Next, Photosystem I can absorb another photon and
  excites an electron.
• This electron provides the reducing power in the
  form of NADPH.

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Photosystems II and I

• Together, these are called Noncyclic
  Photophosphorylation.
• Why does II come before I?
• Why does noncyclic photo. Produce NADPH instead
  of NADH?
• How is the excited electron replaced in the P680
  of Photosystem II?

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Making Oxygen Gas!

• The P680 is a strong oxidizer and it removes an
  e- from the Z protein.
• The Z protein obtains an e- from H2O.
• Z enzyme
• H2O H OH-
• OH- reassembled into O2 and H2O.
• H remain in the thylakoid space.

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In summary

• The Light Reactions
• Photosystem II makes ATP from light energy.
• Photosystem I makes NADPH from light energy.
• Both occur in the thylakoid spaces of
  chloroplasts.

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Cyclic Electron Flow

• Under certain conditions
• -Photoexcited electrons take an alternative path.
• -Calvin cycle requires more ATP and NADPH.
• -This path makes up the difference.

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• In cyclic electron flow
• Only photosystem I is used
• Only ATP is produced

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• The light reactions and chemiosmosis the
  organization of the thylakoid membrane

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Worlds Largest Organism!
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The Dark Reactions

• Use the ATP and the NADPH made in the light
  reactions and build sugar molecules from the CO2
  in the atmosphere.
• Needs a source of energy (from ATP) and the H to
  reduce the CO2 (from NADPH).
• The Calvin Cycle, Fig. 10.12 and 10.13.

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• Concept 10.3 The Calvin cycle uses ATP and NADPH
  to convert CO2 to sugar
• The Calvin cycle
• Is similar to the citric acid cycle
• Occurs in the stroma

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• The Calvin Cycle has three phases
• Carbon fixation
• Reduction
• Regeneration of the CO2 acceptor

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• The Calvin Cycle

Phase 1 Carbon fixation
Phase 3Regeneration ofthe CO2 acceptor(RuBP)
Phase 2Reduction
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The Calvin Cycle

• Also called the C3 cycle because of a 3-carbon
  molecule (PGA).
• It is the reverse of glycolysis.
• Taking C atoms from atmospheric CO2 and making
  sugars-Carbon Fixation.
• Occurs in the stroma.

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The Calvin Cycle

• First step is dependent on RuBP carboxylase
  enzyme (RUBISCO) which adds C from CO2.
• When the temperature gt28 C or the concentration
  of CO2 falls, RUBISCO will also oxidize instead
  of adding Cs- Photorespiration- the opposite of
  the Calvin Cycle.

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RUBISCO Activity
gt28 C
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Photorespiration

• Loses 1/4 to 1/2 of all fixed C that enters the
  Calvin Cycle.
• The C3 plants would not efficiently
  photosynthesize in tropical climates.
• Enter the competition!

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• Concept 10.4 Alternative mechanisms of carbon
  fixation have evolved in hot, arid climates
• On hot, dry days, plants close their stomata
• Conserving water but limiting access to CO2
• Causing O2 to build up

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Photorespiration An Evolutionary Relic?

• In photorespiration
• O2 substitutes for CO2 in the active site of the
  enzyme rubisco.
• The photosynthetic rate is reduced.

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C3 Leaf Structure
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C4 Leaf Structure
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C4 Plants

• C4 plants minimize the cost of photorespiration
• By incorporating CO2 into four carbon compounds
  in mesophyll cells
• Are exported to bundle sheath cells, where they
  release CO2 used in the Calvin cycle

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• C4 leaf anatomy and the C4 pathway

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Comparsion of C3 and C4 Plants

• C3 Plants
• Are temperate plants.
• Perform light and dark reactions in the same
  cell-mesophyll cells.

• C4 Plants
• Are tropical plants. (Corn and sugar cane).
• Perform light reactions in mesophyll cells but
  the Calvin cycle in bundle sheath cells.

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The Cost to C4 Plants

• It costs 2 ATP to transport each CO2 into a
  bundle sheath cell.
• The energetic cost of C4 Photosynthesis is twice
  that of C3 Photosynthesis.
• Photosynthesis is advantegous in hot climates.
• C4 plants outcompete C3 plants in tropical
  climates.

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C3 Leaf Structure
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C4 Leaf Structure
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What about plants in extremely hot climates?

• Must conserve water!

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Transpiration

• Water loss from the leaf tissue through the
  stomata. (Analogy Sweating.)

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CAM Plants

• CAM plants
• Open their stomata at night, incorporating CO2
  into organic acids
• During the day, the stomata close
• And the CO2 is released from the organic acids
  for use in the Calvin cycle

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CAM Plants

• Succulent plants must conserve water loss from
  stomata. Cacti and pineapple use another
  strategy.
• Crassulacean Acid Metabolism (CAM) Plants.
• The stomata are closed during the day to prevent
  water loss and reduce photorespiration by
  preventing CO2 from leaving the leaf. Open the
  stomata at night.

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CAM Plants

• Perform C3 and C4 pathways in the same cell
  (mesophyll) but at different times.
• The C4 pathway is used at night when the stomata
  are open. Prevent CO2 losses
• The C3 pathway is used during the day when the
  stomata are closed and there is a need to reduce
  water loss. The CO2 for making sugars during the
  day come from organic molecules made during the
  previous night, none from atmosphere.

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• The CAM pathway is similar to the C4 pathway

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The Importance of Photosynthesis A Review
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