Photosynthesis

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Photosynthesis

• Chapter 10

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Photosynthesis

• Organisms that derive their food from other
  organisms are called heterotrophs.
• Organisms that make all their own food from ions
  and simple molecules are called autotrophs.
• Photosynthesis is the process of converting the
  energy in sunlight into chemical energy
• Most important chemical process on earth

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Photosynthetic Organisms

• Both prokaryotic and eukaryotic
• Single-celled and multi-cellular
• Many different pigments
• Land or aquatic

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Overview of Photosynthesis

• Summary equation
• Photosynthesis is a redox process
• Water is oxidized, carbon dioxide is reduced

6 CO2 12 H2O Light energy ? C6H12O6 6 O2
6 H2 O
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Overview of Photosynthesis

• Two processes
• The light reactions
• Occur in the grana
• Split water, release oxygen, produce ATP, and
  form NADPH
• 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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Photosynthesis Overview
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The Capture of Light Energy
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Chloroplasts

• Photosynthesis occurs in the chloroplasts
• All green parts of the plant contain
  chloroplasts, but leaves are 1º sites of
  photosynthesis
• Chloroplast contains fluid-filled sacs called
  thylakoids
• The space inside a thylakoid is its lumen
• The fluid between the thylakoids and the inner
  membrane is the stroma.

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Chloroplasts
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Chloroplasts

• Chlorophyll is the primary pigment of
  photosynthesis
• Located in the chloroplasts in thylakoid
  membranes
• Gas exchange through stomata
• CO2 in and O2 out

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How Does Chlorophyll Capture Light Energy?

• The electromagnetic spectrum is the range of
  wavelengths of electromagnetic radiation
• Shorter wavelengths have more energy than longer
  wavelengths (e.g., red, infrared).
• Packets of light are called photons and each
  photon or wavelength has a specific amount of
  energy
• Pigments can absorb this energy

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Photosynthetic Pigments

• Certain pigments absorb wavelengths of light
• Many types of pigments
• Chlorophyll, carotenoids, xanthophylls,
  anthocynanins
• Color of pigment is color of light that is
  reflected
• Pigments that absorb all light are black
• Pigments that absorb no light are white
• Green pigments reflect green light

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Absorption of Light

• A photon of energy is absorbed by the pigment
• Energy causes an outer electron to jump to a
  higher energy shell
• Excited electron has a higher potential energy
• Energy is passed on to ATP

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Light Reactions
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Light Reactions

• Occur in the thylakoid membranes of the
  chloroplasts grana
• Light energy is used to make ATP
• The ATP is used to drive the transfer of an
  electron from water to NADP, to make NADPH
• Temporarily stores energy as NADPH and water is
  split giving off O2
• Absorb solar energy and convert it to chemical
  energy stored in ATP and NADPH

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Photosystems

• Chlorophyll molecules are organized into
  photosystems
• Composed of a reaction center surrounded by a
  number of light-harvesting complexes
• The 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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Non-cyclic Electron Flow

• Photon energy is absorbed by PSII
• Electron in P680 is excited to primary electron
  acceptor
• Water molecule is split and 2 electrons are taken
  
• Electrons are passed down electron transport
  chain
• Electron fall releases energy to make ATP

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Non-cyclic Electron Flow

• Photon energy is absorbed by PSI and electrons
  are excited again
• Primary electron acceptor captures electrons
• Electrons are passed down a second electron
  acceptor
• Enzyme NADP reductase transfers electrons to
  NADP making NADPH

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Cyclic Electron Flow

• Uses PSI but not PSII
• Only generates ATP
• Calvin cycle uses up more ATP than NADPH
• Happens when low of ATP

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Chemiosmosis in the Thylakoid
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The Calvin Cycle
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Calvin Cycle

• Occurs in the stroma
• Reactions are light-independent
• Require the ATP and NADPH produced by the
  light-dependent reactions.
• Similar to citric acid cycle because it
  regenerates after molecules enter and leave the
  cell
• Different because
• Anabolic- builds sugar and consumes energy

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Calvin Cycle

• The Calvin cycle has three phases
• Carbon fixation- incorporates CO2 one at a time
• Come in attached to RuBP
• Catalyzed by rubisco
• Reduction- 1,3-bisphosphoglycerate is reduced to
  glyceraldehyde-3-phosphate
• One G3P leaves the cycle, the rest are recycled
  to regenerate RuBP
• Regeneration of the CO2 acceptor- the G3P
  molecules are rearranged to form RuBP

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Alternate Forms of Carbon Fixation
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Photorespiration

• Most plants are C3 plants, which take CO2
  directly from the air and use it in the Calvin
  cycle
• In these types of plants, stomata on the leaf
  surface close when the weather is hot
• This causes a drop in CO2 and an increase in O2
  in the leaf (reduces loss of water)
• Photorespiration may then occur and produce CO2
  for the Calvin cycle

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C4 Plants

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

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C4 Plants
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CAM Plants

• Succulent plants, jade, cacti, pineapples
• Open their stomata during the night
• Fix carbon in a variety of organic acids
• Stored in the mesophyll cells
• Incorporate carbon into 4-carbon molecules
• Called crassulacean acid metabolism (CAM)

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Storage of Sugar
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Carbohydrate Storage

• G3P molecules used to make glucose and fructose,
  which can combine to form sucrose
• Glucose can be temporarily stored in the
  chloroplast as starch
• Intermediates in the production of glucose from
  G3P can be used to fuel ATP production via
  glycolysis

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