Photosynthesis

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Photosynthesis

• 6 H2O 6 CO2 ? C6H12O6 6 O2
• Light
• Enzymes
• Chlorophyll

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Review.

• Autotrophs (producers)
• Photoautotrophs
• Chemoautotrophs
• Heterotrophs (consumers)

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Sunlight as an energy source.

• Sunlight is a form of radiant energy (travels in
  waves)
• Electromagnetic Spectrum
• Range of different types of radiant energy
• One portion is the Visible Light Spectrum
• 380-750 nm
• Contains colored light of different wavelengths
• Light is absorbed by special pigments in
  chloroplast

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Electromagnetic Spectrum
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Pigments

• Molecules responsible for gathering suns energy
• Chlorophyll
• Green pigments
• Two types a b
• Absorb mostly blue and red light, reflect green
• Carotenoids (ie carotene)
• Red and orange pigments
• Absorb light in other regions of spectrum
• Xanthophylls and Anthocyanins
• Red, purple, brown

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• Light is a form of energy, so any compound
  (pigment) that absorbs light also absorbs the
  energy of the light
• When chlorophyll absorbs light, the energy is
  transferred directly to electrons in chlorophyll,
  which raises the energy level of the electrons

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Plants are specialized for photosynthesis..

• Photosynthesis primarily takes place in leaves
• Leaf cells contain chloroplasts
• Chloroplasts contain chlorophyll chromoplasts
  (contain carotenoids)

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Cross Section of a Leaf

• Palisade layer
• Spongy layer
• Upper and lower epidermis
• How does the structure of the leaf match the
  function of the leaf?

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Structure of a Chloroplast
How is a chloroplast like a to-go order from
IHOP?
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Photosynthesis Overview

• Photosynthesis takes place in 3 steps
• 1. Sunlight energy is captured
• 2. Light Dependent Reactions
• Light energy is converted to chemical energy (ATP
  and NADPH)
• 3. Light Independent Reactions (Calvin Cycle)
• ATP and NADPH power the synthesis of organic
  molecules, using carbon from carbon dioxide

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Elodea Demo
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Chloroplast structure
Click to view animation.
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Concept Map
Section 8-3
Photosynthesis
includes
takes place in
uses
use
take place in
to produce
to produce
of
Go to Section
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How do the high energy electrons get transported
to different parts of the cell?

• Special carrier molecules transport electrons
  (and the energy they contain) from chlorophyll to
  other parts of the cell/chloroplast
• Example NADP
• Accepts and holds 2 high energy electrons along
  with a H ion
• NADP ? NADPH
• NADPH contains sunlight energy trapped in
  chemical form (as part of electrons)

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Now we have two high energy molecules that will
be produced and used in photosynthesis..

• ATP (converted from ADP, contains high energy
  bond between second and third phosphate groups)
• NADPH (converted from NADP, contains high energy
  electrons and a H ion)

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Light Dependent Reactions

• Photosystems
• Clusters of pigments (chlorophyll carotenoids)
  embedded within thylakoid membranes of
  chloroplasts
• 2 Types
• Photosystem I absorb light with a wavelength of
  700 nm
• Photosystem II absorb light with a wavelength of
  680 nm

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Reaction Center

• Molecules of chlorophyll a within a photosystem
  where electrons are excited (boosted to higher
  energy levels) after being hit with the right
  amount of energy from a photon strike (sunlight)

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Photosystem II

• First to act during light reactions
• Responsible for production of ATP
• 1. Photons of light strike a chloroplast and are
  absorbed by P.S. II molecules
• 2. Electrons from reaction center are excited
• 3. Excited electrons leave chlorophyll molecule
  and jump to next membrane protein

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• 4. Water molecule is split, replacing the
  electrons lost by Photosystem II
• H atoms (protons) used later
• O atoms immediately join to form O2 gas
  (byproduct)
• 5. Excited electron passed through electron
  transport chain (ETC) a series of membrane-bound
  protein and pigment molecules in thylakoid
  membrane
• 6. Energy from electrons fuels ATP production
• One ETC molecule is a proton pump uses energy of
  electrons to pump protons into thylakoid space
  

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• -protons build up inside thylakoid until the
  pressure drives them through a specific protein
  channel ATP synthase
• -Chemiosmosis making ATP by forcing protons
  through a membrane channel
• -ATP is released into stroma of chloroplast to
  be used in Calvin Cycle to make sugars

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Photosystem I

• 1. Electrons in Photosystem I are excited
• 2. Excited electrons leave reaction center and
  are accepted by a membrane protein
• 3. Electrons from Photosystem II travel through
  ETC to Photosystem I.
• Electrons are low in energy now, replace the
  electrons that have been lost from P.S. I from
  sunlight

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• 4. Excited electrons from P.S. I travel along
  another ETC to a special protein in membrane that
  adds hydrogens carrying high energy electrons to
  NADP
• 5. NADP ? NADPH and is released into the stroma
  of chloroplast to be used in production of
  organic molecules
• NADPH still contains energy from photon strike

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Photosystems work together..

• NADPH not produced from the action of Photosystem
  I alone
• Electrons continually passed from Photosystem II
  to Photosystem I, which then passes electrons to
  NADPH
• Photosystem II electrons are replaced by
  electrons extracted from the splitting of water

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Light Dependent Reactions
Hydrogen Ion Movement
Chloroplast
Photosystem II
ATP synthase
Inner Thylakoid Space
Thylakoid Membrane
Stroma
Electron Transport Chain
Photosystem I
ATP Formation
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Noncyclic pathway of ATP and NADPH formation
Click to view animation.
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Cyclic pathway
Click to view animation.
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Light Independent ReactionsCalvin-Benson Cycle

• Chemical energy of ATP and NADPH is used in
  carbon fixation
• Carbon Fixation Carbon atoms from CO2
  incorporated into organic molecules in a series
  of enzyme-catalyzed reactions
• Products required from Light Dependent reactions!

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Calvin-Benson Cycle

• Uses enzymes found in chloroplasts stroma
• Enzyme forms a cycle because they regenerate the
  starting materials for further reactions
• 6 CO2 molecules required to make one six-carbon
  sugar

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Process

• 1. Carbon atom from CO2 is added to a
  five-carbon molecule RuBP (starting material) by
  rubisco enzyme
• 2. Six-carbon molecule is unstable, so it
  immediately splits into two three-carbon
  molecules (PGA)
• 3. Energy from ATP (phosphate group) and NADPH
  (e- and H atoms) is added to three-carbon
  compound.

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• 4. Energized three-carbon molecules (PGAL) may
  combine to make glucose (2 PGAL) or may be used
  to make other organic compounds.
• 5. Other three-carbon molecules used to
  regenerate the starting material the five-carbon
  compound using energy from ATP
• 6. Cycle begins again

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Calvin-Benson Cycle
CO2 Enters the Cycle
Energy Input
ChloropIast
5-Carbon Molecules Regenerated
6-Carbon Sugar Produced
Sugars and other compounds
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Light-independent reactions
Click to view animation.