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CHAPTER 10 PHOTOSYNTHESIS

• 1. Plants and other autotrophs are the producers
  of the biosphere
• Chloroplasts are the site of photosynthesis in
  plants
• Photosynthesis takes place in two steps
• a) Light Cycle b) Calvin cycle

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Different feeding methods

• Autotrophs
• 1) produce their organic molecules from CO2
  other inorganic raw materials from the
  environment.
• 2) are the producers of the biosphere.
• - provide organic compounds for all
  non- autotrophic organisms.
• Photo-autotrophs
• Chemo-autotrophs

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• Photoautotrophs use light as the energy source.
• e.g. plants, algae, some other protists, and some
  prokaryotes.
• Chemoautotrophs use energy from inorganic
  substances, such as sulfur and ammonia.
• e.g. bacteria only .

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• Heterotrophs live on organic compounds produced
  by other autotrophs and organisms.
• These organisms are the consumers of the
  biosphere.
• E.g. animals.
• Decomposers heterotrophs that feed on dead
  organisms and on organic litter, like feces and
  fallen leaves.
• E.g. fungi

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chloroplasts and their relation to photosynthesis

• The green color of a leaf comes from the pigment
  chlorophyll, found in chloroplasts.
• Chloroplasts are organelles found in leaves/green
  parts of plants
• What is the purpose of chlorophyll?
• Absorbs light energy emitted from sun during
  photosynthesis.
• Photosynthesis converts it into chemical energy
  (ATP and NADPH)

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LEAF

• The stomata is an opening (pore) in the leaf.
  Allows exchange of gases.
• Plants can open/close stomata.

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• Chloroplast structure
• Two membranes surrounding the fluid portion, the
  stroma.
• Thyakoids are membranous sacs
• have internal aqueous space.
• Grana Thylakoids stacked into columns.

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Chlorophyll

• Chlorophyll is a pigment embedded in the
  thylakoid membrane.
• Its function is to capture light energy

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What is light?

• Light, is a form of electromagnetic energy,
• The entire range of electromagnetic radiation is
  the electromagnetic spectrum.
• Light travels in the form of waves.
• Different wavelengths have different energies.

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Wavelengths
Energy of light travels in waves. The shorter
the wavelength, the higher the energy The
longer the wavelength, the lower the energy.
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• visible light 380 to 750 nm,.
• visible range include colors violet, indigo,
  blue, green, yellow, orange and red

Fig. 10.5
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• Chlorophyll absorbs all colors except green.
• Green is reflected to our eye we see green.
• Chlorophyll absorbs E ? excites electrons to
  higher energy state.
• It passes the energy along in an electron
  transport chain (sound familiar?)
• The electron carriers are NADPH

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The Pathways of Photosynthesis

• The light reaction convert solar energy to the
  chemical energy of ATP and NADPH
• The Calvin cycle uses chemical energy ATP and
  NADPH in the synthesis of sugar

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Photosynthesis is a two step process, each with
multiple stages light and dark reaction

• Light Rxn
• 1) Occurs on thylakoid membrane.
• 2) convert solar energy to chemical energy
  (NADPH,ATP) with use of water.

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Pigments in Chloroplasts

• Chlorophyll a participates directly in the light
  reactions
• accessory pigments such as chlorophyll b ,
  carotenoids absorb light
• transfer energy to chlorophyll a.

Plant pigments
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What happens to light energy?

• Photons are absorbed in clusters of pigment
  molecules on thylakoid membrane.
• Clusters of pigments are called photosystems. (
  2 photosystems)
• Electrons in chlorophyll are excited to a higher
  energy state.
• This energy is transferred to a central
  chlorophyll called the reaction center.

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• PHOTOSYSTEM Light Harvesting unit
• Each photosystem is made of
• 1) Antenna pigments such as chlorophyll b,
  carotenoids
• 2) reaction center chlorophyll a
• 3) Primary electron acceptor

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• 1) Antenna pigments attract photons diffuse
  amt photons hitting reaction center.
• (to protect it from too much radiation)
• 2) Reaction center chlorophyll receives excited
  electron
• 3) Primary electron acceptor accepts e- from
  reaction center

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Where does electrons energy go?

• The energy captured is used to make NADPH and ATP
  
• with electron transport chain built into the
  thylakoid membrane
• Two paths they can take
• Cyclic flow makes ATP
• Non-Cyclic makes NADPH

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• Photolysis
• Enzymatic split of water
• We end up with hydrogen ions and O
• The hydrogen ions electrons are used to fill the
  hole that was left in chlorophyll that gave up
  its electrons (when excited)
• Oxygen instantly combines and O2 given off

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• Splitting of water generates electrons and
  protons in the thylakoid space
• Generating more protons in thylakoid space than
  in stroma (unequal distribution)
• So, ATP made via chemiosmosis.

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• Recap Light Reaction
• 1) Chlorophyll on the thylakoid membrane of
  chloroplasts are arranged in photosystems.
• 2) When chlorophyll absorbs visible light, it
  shoots electron into higher orbital.
• 3) This energy is used to make ATP, and NADPH via
  chemiosmosis and electron transport chain,
  respectively.
• 4) Water is split and oxygen is given off as
  waste, hydrogens electron used to fill hole in
  chlorophyll, as well as used for H gradient to
  make ATP.

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How do plants make sugar?

• The Calvin cycle produces sugar
• It uses CO2 from the atmosphere
• ATP and NADPH synthesized during light reaction
  supply energy and electrons respectively

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Dark reaction or Calvin cycle

• Fixes Carbon from atmosphere
• Takes place in the stroma
• uses energy from the light reaction (ATP and
  NADPH) to reduce carbon to sugar.

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The Calvin cycle has three phases

• carbon fixation phase
• Reduction Phase
• Regeneration of Ribulose biphosphate

1911-1997
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• Carbon fixation phase
• CO2 molecule is attached to a 5-C sugar,
  ribulose bisphosphate (RuBP).
• catalyzed by enzyme RuBP carboxylase or rubisco.

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• Reduction
• 3-phosphoglycerate accepts phosphate group from
  ATP to form 1,3 bisphosphoglycerate.
• 1,3. bisphosphate glycerate receives a pair of
  electrons from NADPH and reduces it to G3P.

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• Regeneration of RuBP
• In this way five G3P molecules are rearranged to
  regenerate 3 RuBP molecules.
• 3 ATP (one per RuBP) to complete the cycle

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• Overall rxn for Calvin cycle
• C5 CO2 ATP NADPH ---gt C6H12O6
• However, immediate product is a three-carbon
  sugar, glyceraldehyde-3-phosphate (G3P).
• To make one glucose molecules would require six
  cycles and the fixation of six CO2 molecules.
• For fixing one CO2, it requires 3 ATP and 2NADPH
  molecules
• To generate one glucose molecule
• 18 ATP molecules are used
• 12 NADPH derived ATP molecules
• endergonic rxn

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Summery of the Calvin cycle

• 1) Occurs in stroma
• 2) Carbon fixation.
• 3)This new piece of carbon backbone is reduced
  with electrons provided by NADPH.
• 4) ATP from light rxn powers Calvin cycle.
• 5) Produces G3P (3-C sugar)
• -then goes on to produce glucose

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• On a global scale,
• photosynthesis is the most important process to
  the welfare of life on Earth.
• Each year photosynthesis synthesizes 160 billion
  metric tons of carbohydrate per year.
• (That is 60 trillion copies of your bio. Text
  book, or 17 stacks of books from the Earth to the
  sun)
• We need our plants!