Photosynthesis: Capturing Energy

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Photosynthesis Capturing Energy

• Chapter 8

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Autotrophs

• Carbon fixation is the process of building
  complex carbon compounds from simple carbon
  compounds.
• Organisms that make their own food are
  autotrophs they use CO2 as a carbon source, and
  combine it with water to make carbohydrates
• Photoautotrophs Use radiant energy to make their
  own food (Plants, Algae, and Cyanobacteria)
• Photoautotrophs provide nearly all the energy
  used by living systems on Earth

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Photoautotrophs and Chemoautotrophs

• Photoautotrophs Organisms that use radiant
  energy in order to carry out carbon fixation
• Photosynthetic plants, algae and bacteria
• Use light energy to make ATP and carbohydrate
• Chemoautotrophs Organisms that use chemical
  energy only to cause carbon fixation and to build
  structure
• Certain bacteria

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Heterotrophs

• Heterotrophs Organisms that gain energy by
  eating other organisms
• Animals
• Nonphotosynthetic plants,
• Nonphotosynthetic unicellular organisms(such as
  protists)
• Bacteria
• Fungi

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The Sun
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The Visible Spectrum of Light

• Photosynthesis usesonly small visible portion of
  the electromagneticspectrum
• Wavelengths of visiblelight most important
  forphotosynthesis.
• The symbol forwavelength is ?

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How Electrons Capture Energy

• Electrons can absorb radiant energy.
• Radiant energy comes in parcels called photons
• When electrons absorb energy, they hop to a
  higher shell.
• When electrons release energy, drop back to the
  lower shell.
• The energy released is a kind of light energy
  called fluorescence.

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Absorption of Light Energy

• Light energy is absorbed by electrons
• The energy causes electrons to jump shells the
  more energy absorbed, the further away electrons
  move from the nucleus
• The energy may be shed as fluorescence
• Or transferred in the form of an electron to
  another molecule

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Photosynthesis
6CO2 12H2O light energy ? C6H12O6 6O2
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Leaf Structure

• Leaves have a layered organization
• The mesophyll tissue (middle layers of cells) is
  the main site of photosynthesis
• Sap flows through the veins

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The Chloroplast

• The site of light harvesting or energy capture
• The site of the start of carbohydrate synthesis

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Chlorophylls are Leaf Pigments

• Chlorophylls collect light energy (absorbs it) in
  a resonant porphyrin group that hangs out like a
  kite on the surface of the thylakoid
• Chlorophyll a initiates the light-dependent
  reactions
• Chlorophyll b is an accessory pigment
• Carotenoids are yellow and orange pigments that
  capture light energy and pass electrons to
  chlorophyll

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The Structure of Chlorophyll

• Note the double bonds and the Mg2 ion
• The positive charges on the Mg2 ion attract
  electrons
• The electrons bounce around the porphyrin ring

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Engelmanns Experiment 1883

• Engelmann sought to determine the wavelengths of
  light most important for photosynthesis
• He illuminated a strand of Spyrogyra (a green
  alga) with the spectrum of light from a prism
  while observing through a microscope
• Aerobic bacteria were attracted to the regions of
  high oxygen production i.e. regions of
  photosynthesis

Aerobic bacteria
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Photosynthesis in the Chloroplast

• The light-dependent reactions (the harvesting of
  light) occur on thylakoid membranes
• The carbon fixation reactions (formation of
  carbohydrate) occur in the stroma

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Photosynthesis
6CO2 12H2O light energy ? C6H12O6 6O2

• Carbon dioxide is reduced to sugar
• Water is converted to oxygen

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Three Energy Carriers of Photosynthesis

• NADH
• NADPH
• Much like NADH except that it bears a phosphate
• Phosphate is attached to the sugar group
• ATP

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Two Major Steps in Photosynthesis

• Light-Dependant Reactions
• Cyclic photophosphorylation
• Non-cyclic photophosphorylation

• Light-Independent reactions
• Calvin cycle (C3)

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The Light Dependant Reactions

• Water molecules are split apart, producing
  electrons and hydrogen ions, and O2 gas is
  released.
• Electrons from the split water are passed along
  an electron transport chain
• Energy storing ATP molecules are produced
• Hydrogen from the split water is transferred from
• NADP ? NADPH and used in the light independent
  reactions

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Capturing Light Energy

• Photons are absorbed by chlorophyll
• Energy (as an electron) falls from one
  chlorophyll to the next.

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Two Different ways to Photosynthesize in the
Light-Dependant Reactions

• 1. Cyclic photophosphorylation
• e- run in a cycle
• makes ATP
• No carbohydrate made
• Uses only P700
• 2. Noncyclic photophosphorylation
• e- derived from splitting of water
• Releases O2
• Makes lots of ATP
• Makes carbohydrate
• Uses P700 and P680

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Cyclic Photophosphorylation

• Photosystem I
• P700 reaction center
• Energized e- hops to an e- acceptor protein
• e- falls from protein complex to protein
  complex, losing energy as it cascades down

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Cyclic Photophosphorylation Limitations

• Does not produce any carbohydrate
• Found in certain primitive plants
• Found also in bacteria
• Also seen in plants that have sufficient
  carbohydrate but need ATP
• But still makes some ATP

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Noncyclic Photophosphorylation

• Plants that need a lot of energy must make a
  large quantities of carbohydrate.
• They use noncyclic photophosphorylation
• Both PII (P680) and PI (P700) are used.
• The 680/700 designations indicate the peak
  wavelengths absorbed by each chlorophyll reaction
  center

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Steps of Non-cyclic Photophosporylation

• Photosystem II (P680) uses light energy to split
  a water molecule rips it apart into electrons,
  protons and oxygen
• e- cascades from protein complex to protein
  complex losing energy.
• The e- gets re-energized by photosystem I (P700)
  and falls down another cascade of protein
  complexes.
• The energy from the e- cascades is used to pump
  H from the split water molecule into the
  intermembrane space of the thylakoid.
• The H from the water ultimately is used to make
  ATP and NADPH

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Noncyclic Photophosphorylation
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The Electron Transport Chain
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The Chloroplast ATP Synthase

• The H forms a proton gradient.
• The H moves from H? L
• H are transported through the ATP synthase
• Makes ATP by combining ADP and phosphate
• The H that have been pumped into the stroma are
  used to make NADPH

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The Light Independent Reactions

• So-called because they do not directly need light
  (radiant energy)
• They occur in the stroma of the chloroplast
• They fix carbon to make carbohydrate
• Named the Calvin Cycle

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The Light Independent Reactions

• CO2 is combined with RuBP to yield a 6C sugar
• Enzyme RuBP carboxylase
• 6C sugar is broken into TWO 3C sugars
• NADPH and ATP supply the energy for the
  conversion
• Most of the 3C sugar gets recycled into the
  Calvin cycle
• The remainder gets converted into sucrose

Takes place in the stroma of the chloroplast
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The Calvin Cycle

• CO2 uptake
• Catalyzed by the enzyme RUBISCO ribulose
  bisphosphate carboxylase
• ATP and NADPH from the light-dependant reactions
  are used as energy sources to rearrange the 3-C
  sugars

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• Most of the 3-C sugars are sent to the cytoplasm
  to make glucose
• The remaining 3-C sugars are recycled back into
  the Calvin cycle to regenerate RUBP

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The Calvin Cycle is very expensive in terms of
Energy

• CO2 is the carbon source
• For each glucose to be formed, 18 ATP and 12
  NADPH are used!

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