Capturing Solar Energy: Photosynthesis

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Capturing Solar Energy Photosynthesis
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Photosynthesis

• Light energy captured and stored as chemical
  potential energy in the covalent bonds of
  carbohydrate molecules
• 6 CO2 6 H2O light ? C6H12O6 6 O2

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Photosynthesis

• Less Than 1 of the Sun's Energy Is Captured in
  Photosynthesis
• Sun energy drives reduction of carrier molecules
• Electrons in respiration loose energy going from
  sugar to oxygen
• Mitochondria use released energy to make ATP
• Electrons in photosynthesis must gain energy
  going from water to sugar
• Energy provided by the sun
• Occurs in 1 million billionths of a second

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Photosynthesis

• Light
• A. Light consists of units of energy called
  photons
• B. Photons possess differing amounts of energy
  
• C. Energy in visible light
• 1. Violet has short wavelength and high
  energy photons
• 2. Red has long wavelength and low energy
  photons
• D. Absorbed vs reflected
• E. Specific atoms can absorb only certain
  photons of light

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Pigments-molecules that absorb light

• A. Molecules that absorb light
• B. Types
• 1. Carotenoids-absorbs some green
• a. Absorb photons over a broad range,
  not highly efficient
• b. Include beta-carotene, vitamin A
  and retinal
• 2. Chlorophylls
• a. Absorb photons by excitation like
  the photoelectric effect
• 1. Complex ring structure called
  a porphyrin ring
• 2. Metal ion within a network of
  alternating single and double bonds(Fe)
• b. Absorb photons over a narrow
  range
• 1. Chlorophyll a absorbs in
  violet-blue range
• 2. Chlorophyll b absorbs in the
  red range
• 3. Wavelength not absorbed by
  chlorophylls reflected as green
• 4. Chlorophyll absorbs in a
  narrow range, but with great efficiency
• c. xanthrophyll

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Life depends on photosynthesis

• A. Foundation of energy for most ecosystems
• B. Source of oxygen
• C. Key component of the carbon cycle

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The mechanism of photosynthesis

• Chloroplasts are the sites of photosynthesis
• Have a membrane system within internal space
  (stroma)
• Arranged in disk-shaped sacks (thylakoids)
• The thylakoids contain light-harvesting
  photosynthetic pigments enzymes
• Internal membranes define space (lumen) that is
  separate from the rest of the stroma

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The mechanism of photosynthesis

• Photosynthesis occurs in two steps
• 1. Light-dependent reactions
• a. Provides the energy necessary to fix carbon
• b. Occurs in the thylakoid membranes
• c. Generates ATP

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LIMITATIONS

• 1. Geared only towards energy production(ATP)
• 2. Does not provide for biosynthesis(glucose
  synthesis)

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CYCLIC PHOTOSYNTHESIS

• PRIMATIVE FORM
• COMES IN TO PLAY ON ITS OWN IN FALL IN HIGHER
  PLANTS.

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Light-Dependent Reactions

• What happens during light reactions?
• During transport of electrons from PS II to PS I
• Some energy is harnessed to produce ATP
• Eventually, chlorophyll from PS II is oxidized
• Gets replacement electrons from water-photolysis

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Light-Dependent Reactions

• Energy of light has thus been captured in two
  forms
• The synthesis of NADPH from NADP
• Proton gradient across the thylakoid membrane
• Cannot be used directly to make food
• Must first be converted to ATP by chloroplast ATP
  synthase

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The mechanism of photosynthesis

• Energy carriers ATP and NADPH transport energy
  from the light-dependent reactions to the
  light-independent reactions

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The mechanism of photosynthesis

• 2. Light-independent reactions
• a. Uses energy of the light-dependent reaction
  to make sugar from CO2
• b. Occurs in the stroma

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Light-Independent Reactions

• Steps in Light-Independent Reactions
• CO2 joins with RuBP forming an unstable 6-C
  molecule
• Breaks into two 3-C PGA molecules
• This first step in Calvin-Benson/C3 cycle is
  catalyzed by enzyme
• Called ribulose biphosphate carboxylase (Rubisco)

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1 Carbon fixation combines CO2
with RuBP.
3 RuBP regeneration uses energy
and 10 G3P.
2 G3P synthesis uses energy.
2 G3P available for synthesis of organic
molecules.
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PHOTORESPIRATION

• Many land plants take up oxygen and release CO2
  in the light. There is a superficial resemblance
  to true respiration, but the process is much
  faster. However, it is normally masked by
  photosynthesis, which is even faster.
  Photorespiration differs from true respiration.
  Although plants do respire normally (with
  mitochondria, etc.) this is useful (produces ATP
  and NADH), and occurs mostly in the dark. In
  contrast, photorespiration is wasteful and occurs
  mostly in the light. Photorespiration appears to
  serve no useful purpose. Its main effect is to
  reduce the apparent rate of photosynthesis. Most
  of our important crops photorespire about half of
  their potential yield away!

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PHOTORESPIRATION

• A. O2 competes for CO2 with RuBP oxidizes it-high
  oxygen low carbon dioxide
• B. CO2 released without ATP or NADPH
• C. C3 lose 1/4 to 1/2 of carbon fixed-40
• D. C4 And CAM plants adapted to counter act
  this problem
• E. Rubisco takes oxygen makes phosphoglycerate
  and glycolate
• F. Goes to perioxisome-takes oxygen and makes a
  compound that goes to mitochondria to make
  carbon dioxide like respiration.

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mesophyll cell in C3 plant
(a)
C3 plants use the C3 pathway
Much photorespiration occurs under hot, dry
conditions.
In a C3 plant, most chloroplasts are in mesophyll
cells.
mesophyll cell in C4 plant
(b)
C4 plants use the C4 pathway
CO2 is captured with a highly specific enzyme.
bundle- sheath cells
Almost no photorespiration occurs in hot, dry
conditions.
In a C4 plant, both mesophyll and bundle-sheath
cells contain chloroplasts.
Much glucose synthesis occurs.
bundle-sheath cell in C4 plant
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ADAPTATIONS

• C4 plants-Hatch Slack plants
• a. Different leaf structure
• b. Bundle sheath surrounded by palisade
  mesophyll
• c. Grasses-Found in hot climates, lots of
  sun,above 300 C
• d. Uses about 2x ATP but stores CO2 at
  night or anytime stomates are open. Saves
  CO2 when plant can
• e. Cycle
• 1. CO2 is picked up by PEP in mesophyll-no
  rubisco
• 2. Converted to oxaloacetic acid then
  malic acid
• 3. Stored in this stable form
• 4. Malic converted to Pyruvic acid CO2
• 5. Pumped into bundle sheath thru
  plasmodesmata
• 6. Deeper than surface because there is
  less oxygen to cause
  photorespiration to occur

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C4 plants/Hatch Slack

• Utilize an alternate pathway to make sugars in
  dry environments
• Closing stomata to conserve water results in
  photorespiration in C3 plants

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CAM PLANTS

• a. Hot(desert) climates-high daytime temps,
  low soil moisture,intense light
• b. Stomates only open at night
• c. Central Vacuole stores malic acid
• d. Leaves vacuole and and releases CO2
• Diatoms
• 1. Have both C3 and C4 cycles
• 2. C3 in chloroplasts
• 3. C4 in cytosol
• 4. Uses because of low CO2 in the ocean