Photosynthesis: Light Dependent and Light Independent Reactions

IV) Light Dependent Reactions

• Purpose: Harvest energy for the next step by splitting water, releasing electrons, O2, and H+.
• Importance: Releases O2 to the atmosphere, making it hospitable for aerobic life.
• Location: Occurs in thylakoid membrane compartments (grana).

Types of Electron Flow

1. Non-cyclic Electron Flow

   • Involves Photosynthesis I and II (starts with II).
   • Process:
     • Sunlight hits P680 (Photosynthesis II) chlorophyll → Excites electrons to a higher state.
     • Photolysis: Water splitting
       • [ H2O + Sunlight → 2 H+ + 1/2 O2 + 2 electrons ]
     • Excited electrons move down the electron transport chain (similar to respiration).
     • Energy conversion: ADP → ATP (used in next steps).
     • Electrons replace those in P700 (Photosynthesis I), receiving more photons.
     • Electrons pass through another transport system → NADP+ becomes NADPH (carries electrons to the next step).

2. Cyclic Electron Flow

   • Involves only Photosynthesis I.
   • Produces ATP but no NADPH.
   • Provides extra ATP needed for the light-independent reactions.

ATP Formation

• Via chemiosmosis (similar to mitochondria during respiration).
  • H+ from water and transport steps build up in compartments, creating a gradient.
  • H+ flows down the gradient through ATP Synthase, catalyzing ADP to ATP conversion.
  • ATP and NADPH are released in the stroma for the next step.

V) Light Independent Reactions (Calvin Cycle)

• Location: Occurs in the stroma.
• Process:
  • CO2 diffuses into stroma, carbon fixation occurs, producing G3P (3-carbon sugar).
  • G3P can be made into sugars and other compounds (e.g., starch).
• Steps:
  • 3 CO2 + 3 RuBP (5-carbon sugar) → 18 carbon compound (catalyzed by rubisco).
  • 6 ATP and 6 NADPH provide energy and electrons to form 6 G3P.

Overview of Calvin Cycle

• Reactants:
  • CO2 from atmosphere.
  • Regenerated compounds from the cycle.
  • ATP and NADPH from light-dependent reactions.
• Products:
  • G3P (3-carbon sugar, precursor to glucose and other carbohydrates).
  • Compounds to be used in the cycle again.
  • Compounds regenerated by light-dependent reactions.

VI) Alternate Photosynthesis in Arid Climates

Stomata Function

• Purpose: Allows gas exchange, closes in heat to prevent water loss, reducing CO2 intake.
• Problem: Low CO2 concentration leads rubisco to react with O2, which is unproductive.

C4 and CAM Plants

• C4 Plants (e.g., grasses, sugar cane, corn):

  • CO2 is fixed into a 4-carbon organic acid in mesophyll cells.
  • Acid transported to bundle sheath cells for the Calvin Cycle.

• CAM Plants (e.g., succulents, cacti, pineapples):

  • Stomata open at night, close during the day.
  • Convert CO2 to organic acids at night, releasing CO2 for the Calvin Cycle during the day.