Photosynthesis Ch. 7 Ms. Haut

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PhotosynthesisCh. 7Ms. Haut
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Basics of Photosynthesis

• All cells need energy to carry out their
  activities
• All energy ultimately comes from the sun
• Photosynthesisprocess in which some of the solar
  energy is captured by plants (producers) and
  transformed into glucose molecules used by other
  organisms (consumers).
• 6CO2 6H2O C6H12O6 6O2

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Basics of Photosynthesis

• Glucose is the main source of energy for all
  life. The energy is stored in the chemical
  bonds.
• Cellular Respirationprocess in which a cell
  breaks down the glucose so that energy can be
  released. This energy will enable a cell to
  carry out its activities.
• C6H12O6 6O2
  6CO2 6H2O energy

enzymes
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Basics of Photosynthesis

• Autotrophorganisms that synthesize organic
  molecules from inorganic materials (a.k.a.
  producers)
• Photoautotrophsuse light as an energy source
  (plants, algae, some prokaryotes)
• Heterotrophorganisms that acquire organic
  molecules from compounds produced by other
  organisms (a.k.a. consumers)

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Leaf Anatomy
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Photosynthesis redox process

• Oxidation-reduction reaction
• Oxidation-loss of electrons from one substance
• Reduction-addition of electrons to another
  substance

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A Photosynthesis Road Map

• Photosynthesis is composed of two processes
• The light reactions convert solar energy to
  chemical energy.
• The Calvin cycle makes sugar from carbon dioxide.

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Figure 7.4
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The Nature of Sunlight

• Sunlight is a type of energy called radiation
• Or electromagnetic energy.
• The full range of radiation is called the
  electro-magnetic spectrum.
• Light may be reflected, transmitted, or absorbed
  when it contacts matter

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Chloroplasts Natures Solar Panels

• Chloroplasts absorb select wavelengths of light
  that drive photosynthesis.
• Thylakoids trap sunlight

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Photosynthetic Pigments

• Pigments-substances that absorb light (light
  receptors)
• Wavelengths that are absorbed disappear
• Wavelengths that are transmitted and reflected as
  the color you see

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Plant Pigments

• Chlorophyll a absorbs blue-violet and red
  light, thus appears green
• Accessory pigments
• Absorb light of varying wavelengths and transfer
  the energy to chlorophyll a
• Chlorophyll b-yellow-green pigment
• Carotenoids-yellow and orange pigments

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Photosynthesis 2 stages

• Light reactionsconvert light energy to chemical
  bond energy in ATP and NADPH
• Occurs in thylakoid membranes in chloroplasts

• Calvin Cyclecarbon fixation reactions assimilate
  CO2 and then reduce it to a carbohydrate
• Occurs in the stroma of the chloroplast
• Do not require light directly, but requires
  products of the light reactions

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Light reactions produce ATP and NADPH that are
used by the Calvin cycle O2 released
Calvin Cycle produces ADP and NADP that are
used by the light reactions glucose produced
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How Photosystems Harvest Light Energy

• Photosystem assemblies of several hundred
  chlorophyll a, chlorophyll b, and carotenoid
  molecules in the thylakoid membrane
• form light gathering antennae that absorb photons
  and pass energy from molecule to molecule
• Photosystem Ispecialized chlorophyll a molecule,
  P700
• Photosystem IIspecialized chlorophyll a
  molecule, P680

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Light Reactions

• Light drives the light reactions to synthesize
  
• NADPH and ATP
• Includes cooperation of both photosystems, in
• which e- pass continuously from water to
• NADP

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• When photosystem II absorbs light an e- is
  excited in the reaction center chlorophyll (P680)
  and gets captured by the primary e- acceptor.
• This leaves a hole in the P680

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• To fill the hole left in P680, an enzyme extracts
  e- from water and supplies them to the reaction
  center
• A water molecule is split into 2 H ions and an
  oxygen atom, which immediately combines with
  another oxygen to form O2

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• Each photoexcited e- passes from primary e-
  acceptor to photosystem I via an electron
  transport chain.
• e- are transferred to e- carriers in the chain

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• As e- cascade down the e- transport chain, energy
  is released and harnessed by the thylakoid
  membrane to produce ATP
• This ATP is used to make glucose during Calvin
  cycle

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• When e- reach the bottom of e- transport chain,
  it fills the hole in the reaction center P700 of
  photosystem I.
• Pre-existing hole was left by former e- that was
  excited

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• When photosystem I absorbs light an e- is excited
  in the reaction center chlorophyll (P700) and
  gets captured by the primary e- acceptor.
• e- are transferred by e- carrier to NADP
  (reduction reaction) forming NADPH
• NADPH provides reducing power for making glucose
  in Calvin cycle

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Chemiosmosis

• Energy released from ETC is used to pump H ions
  (from the split water) from the stroma across the
  thylakoid membrane to the interior of the
  thylakoid.
• Creates concentration gradient across thylakoid
  membrane
• Process provides energy for chemisomostic
  production of ATP

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Light reactions produce ATP and NADPH that are
used by the Calvin cycle O2 released
Calvin Cycle produces ADP and NADP that are
used by the light reactions glucose produced
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The Calvin Cycle Making Sugar from Carbon Dioxide

• Carbon enters the cycle in the form of CO2 and
  leaves in the form of sugar (glucose)
• The cycle spends ATP as an energy source and
  consumes NADPH as a reducing agent for adding
  high energy e- to make sugar
• For the net synthesis of this sugar, the cycle
  must take place 2 times

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The Calvin Cycle Carbon Fixation

• 3 CO2 molecules bind to 3 molecules of ribulose
  bisphosphate (RuBP) using enzyme, RuBP
  carboxylase (rubisco)
• Produces 6 molecules of
  3-phosphoglycerate (3-PGA)

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The Calvin Cycle Reduction

• 6 ATP molecules transfer phosphate group to each
  3-PGA to make 6 molecules of 1,3-diphosphoglycerat
  e
• 6 molecules of NADPH reduce each 1,3-bisphosph.
  to make 6 molecules of glyceraldehyde 3-phosphate
  (G3P)

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The Calvin Cycle Regeneration of RuBP

1. One of the G3P exits the cycle to be used by the
   plant the other 5 molecules are used to
   regenerate the CO2 acceptor (RuBP) 3 molecules
   of ATP are used to convert 5 molecules of G3P
   into RuBP3

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The Calvin Cycle Regeneration of RuBP

• 3 more CO2 molecules enter the cycle, following
  the same chemical pathway to release another G3P
  from the cycle.
• 2 G3P molecules can be used to make glucose

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Calvin Cycle
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Special Adaptations that Save Water

• C3 Plantsplants that only use Calvin Cycle to
  fix carbon
• During dry conditions C3 plants conserve water by
  closing stomata
• Plants then fix O2 to RuBP rather than CO2, since
  CO2 cant enter the plant (photorespiration)
• This yields no sugar molecules or ATP

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Special Adaptations that Save Water

• C4 Plantsplants that incorporate CO2 before the
  Calvin cycle

• Different plant anatomy
• Bundle-sheath cellsthylakoids not stacked
• Calvin cycle confined to chloroplasts of
  bundle-sheath cells
• Mesophyll cells loosely arranged

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C4 Plants

• In the mesophyll, CO2 is added to
  phosphenolpyruvate (PEP) to form oxaloacetate
  (4-carbon compound)
• PEP carboxylase-high affinity to CO2 and no
  affinity for O2, thus no photorespiration
  possible

• Oxaloacetate converted to malate (4-carbon
  compound)

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C4 Plants

• Mesophyll export malate through plasmodesmata to
  bundle-sheath cells
• Malate releases CO2 , which is then fixed by
  rubisco in the Calvin cycle

• Process minimizes photorespiration and enhances
  sugar production by maintaining a CO2
  concentration sufficient for rubisco to accept
  CO2 rather than oxygen

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Crassulacean acid metabolism (CAM)

• CAM plantssucculent plants that open their
  stomata primarily at night and close them during
  the day (opposite most plants)

• At night, CO2 is taken in by open stomata and
  incorporated into a variety of organic acids.
• Organic acids stored in vacuoles of mesophyll
  cells until morning, when stomata close

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• During daytime, light reactions supply ATP and
  NADPH for the Calvin cycle.
• At this time, CO2 is released from the organic
  acids made the previous night and is incorporated
  into sugar in the chloroplast

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The CAM and C4 pathways

• Are similar in that CO2 is first incorporated
  into organic intermediates before it enters the
  Calvin cycle
• Differ in that the initial steps of carbon
  fixation in C4 plants are structurally separate
  from the Calvin cycle in CAM plants, the two
  steps occur at separate times
• Regardless of whether the plant uses C3, C4, or
  CAM pathway, all plants use the Calvin Cycle to
  produce sugar from CO2

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How Photosynthesis Moderates Global Warming

• Photosynthesis has an enormous impact on the
  atmosphere.
• It swaps O2 for CO2.

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How Photosynthesis Moderates Global Warming

• Greenhouses used to grow plant indoors
• Trap sunlight that warms the air inside.
• A similar process, the greenhouse effect,
• Warms the atmosphere.
• Is caused by atmospheric CO2.

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Global Warming

• Greenhouse gases (CO2, CH4, CFCs) are the most
  likely cause of global warming, a slow but steady
  rise in the Earths surface temperature.
• Destruction of forests may be increasing this
  effect.
• Combustion of fossil fuels

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Global Warming Consequences

• Polar ice caps melting
• Rise in sea level and flooding of current
  coastline
• New York, Miami, Los Angeles underwater
• Change in types of plantsmore adapted to warmer
  temps. and less water

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References

• Unless otherwise noted, pictures are from
  Essential Biology with Physiology, 2nd edition.
  Campbell, Reece, and Simon. (2007).