Photosynthesis - Light Dependent Reactions and the Calvin Cycle

Summary of scientific concepts / phenomena in the video

Photosynthesis (overview)

• Purpose: Use light energy to build carbohydrates (simple sugars, e.g., glucose).

• Net overall reaction (as stated): 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ (glucose) + 6 O₂

• Where reactants/products enter/leave the plant:

  • Water (H₂O): enters through roots
  • Carbon dioxide (CO₂): enters through stomata (tiny openings in leaves)
  • Oxygen (O₂): exits through stomata

• Key organelles and contrast with respiration:

  • Chloroplast: carries out photosynthesis
  • Mitochondria: carries out cellular respiration
  • The processes are described as opposites:
    • Photosynthesis: CO₂ + H₂O → glucose + O₂ (uses light)
    • Cellular respiration: glucose + O₂ → CO₂ + H₂O (releases energy)

---

Chloroplast structure and light-capturing pigments

• Chlorophyll: pigment that absorbs light
  • absorbs blue and red
  • reflects green → plants appear green
• Thylakoids: contain the light-dependent reactions
• Grana / granum: stacks of thylakoids
• Lumen: fluid inside the thylakoid
• Stroma: fluid inside the chloroplast but outside thylakoids

• Membranes: inner + outer membrane, with an intermembrane space (as described)

• Two stages of photosynthesis:

  • Light-dependent reactions: occur in thylakoid membranes
  • Light-independent reactions (Calvin cycle / “dark reactions”): occur in stroma

---

Light-dependent reactions (in thylakoids)

Inputs/outputs

• Reactants: water (H₂O), NADP⁺, ADP + phosphate
• Products: O₂, NADPH, ATP

---

Electron transport chain + photochemical events (stepwise outline)

1) Photosystem II (PSII; “680 nm”)

• Light excites electrons in chlorophyll
• Electrons move to plastoquinone (mobile electron carrier)
• Chlorophyll replaces lost electrons by oxidizing water
  • Water oxidation releases O₂
  • Produces electrons and hydrogen ions (H⁺)

2) Cytochrome b6f complex

• Electrons pass through the complex
• Complex pumps protons (H⁺) from stroma into the thylakoid lumen
• Builds an H⁺ concentration gradient (lumen high H⁺, stroma low H⁺)

3) Plastocyanin (copper-containing protein)

• Transfers electrons from cytochrome b6f to Photosystem I (PSI)

4) Photosystem I (PSI; “700 nm”)

• Another photon excites electrons again
• Electrons proceed to ferredoxin (iron-sulfur protein)
• Electrons go to NADP⁺ reductase

5) NADP⁺ reductase

• Uses electrons to reduce NADP⁺ → NADPH

6) ATP synthase via chemiosmosis

• H⁺ flows back through ATP synthase
• ATP synthase uses this to convert ADP + phosphate → ATP
• Mechanism described as chemiosmosis (proton-driven ATP production)

Overall theme: Light-driven electron flow generates O₂ (from water), NADPH (from NADP⁺), and ATP (via the proton gradient).

---

Calvin cycle / light-independent reactions (in stroma)

Purpose

• Convert CO₂ into carbohydrates, using sugar precursors and regeneration of the CO₂-accepting molecule.

Three major parts (as stated)

1. Carbon fixation
2. Reduction
3. Regeneration of RuBP (ribulose 1,5-bisphosphate)

Core reactions and enzymes (stepwise outline)

1. Carbon fixation

   • CO₂ reacts with RuBP, catalyzed by Rubisco
   • Produces 3-phosphoglycerate (3-PGA)
   • RuBP has 5 carbons; the process described includes intermediate 6-carbon breakdown into two 3-carbon molecules

2. Reduction (phosphorylation + electron-driven step)

   • Phosphorylation step: uses ATP to convert
     • 3-PGA → 1,3-bisphosphoglycerate
     • enzyme described as a kinase (video: “PGA kinase”)
   • Reduction step: uses NADPH to convert
     • 1,3-bisphosphoglycerate → G3P (glyceraldehyde 3-phosphate)
     • enzyme: G3P dehydrogenase
     • NADPH becomes NADP⁺

3. Regeneration

   • Out of multiple G3P molecules produced:
     • one is used to build sugars (e.g., glucose/fructose precursors)
     • the other G3P molecules are used to regenerate RuBP

---

Stoichiometry and cycle “net result” (as presented)

• Net Calvin cycle (per 3 CO₂):

  • 3 CO₂ → 1 G3P
  • Requires 9 ATP and 6 NADPH (as summarized)

• To make glucose (as described):

  • glucose requires 6 CO₂
  • results in 2 G3P, enabling glucose formation
  • thus double the requirements:
    • 18 ATP and 12 NADPH (as stated)

---

Researchers / sources featured

• No specific external researchers, textbooks, or named sources are cited in the provided subtitles.