Photosynthesis

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Photosynthesis

• Ch.6

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(6-1) Photosynthesis

• Biochemical pathway that converts light E into
  chemical E (organic cmpds/ carbs)

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Obtaining E

• Autotrophs use E from sun to make food
• Plants, algae, some bacteria
• Heterotrophs get E from food they consume
• Humans, birds

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Overview

• 2 step process
• Light Rxn light E is converted to chemical E
  temporarily stored in ATP carrier molecule
  NADPH
• Calvin Cycle (Dark Rxn) organic cmpds formed
  using CO2 stored chemical E

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Capturing Light E

• Light rxns begin w/ absorption of light in
  chloroplasts
• Inside a chloroplast are thylakoids stacked to
  form grana surrounded by stroma

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Light Pigments

• White light strikes an object its colors are
  reflected or absorbed
• Pigments cmpds that absorb light
• Colors not absorbed will be reflected back seen
  

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Light E from the Sun
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Chloroplast Pigments

• Thlyakoid membranes have pigments that absorb
  light E
• Chlorophyll a impt. in light rxns
• Accessory pigments assist in capturing light E
• Chlorophyll b
• Carotenoids

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Why so many pigments?
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Light Rxns

• Light E excites pigment e-
• Photosystem 1 2
• e- donated to e- transport chain (ETC)
• Moves protons (H) into thylakoid
• Makes NADPH

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Importance of H2O

• e- replacement in PS2 needs to occur or
  photosynthesis stops
• e- provided by H2O

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Chemiosmosis

• Making ATP using a chemical conc. gradient of H
  ions (from H2O) across the thylakoid membrane via
  the protein ATP synthase

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(6-2) Calvin Cycle

• Plants use E stored in ATP NADPH during light
  rxns to produce organic cmpds (sugars)
• Not dependent on light!
• Carbon fixation C-atoms from CO2 bonded into
  sugars

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Carbon Fixation

1. CO2 diffuses into stroma, combines with RuBP,
   splits
2. 1 new molecule uses E (ATP NADPH) to leave the
   cycle to make carbs
3. 1 new molecule gets converted back to RuBP
   restarts cycle

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Stomata

• Small pores on the undersides of leaves that
  allow gas exchange
• When stomata are partially closed H2O loss can be
  dec., but CO2 levels also dec., which inhibits
  C-fixation

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CO2 in H2O O2 out
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Alternative Pathways

• C4 fix carbon into a 4-C cmpd
• Partially close stomata during hottest part of
  day
• Ex corn, sugar cane, crab grass
• CAM operates at night
• Close stomata during the day
• Ex cacti, pineapples

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Rate of Photosynthesis

• Affected by
• Light intensity
• CO2 level
• Temp.

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