Early Experiments in Photosynthesis

1. The Leaf & KOH Experiment 🌿

When part of a leaf is sealed in a test tube with KOH-soaked cotton (absorbs CO2), and the other half is exposed to air, only the exposed part makes starch after sunlight exposure. This proves CO2 is essential for photosynthesis!

2. Joseph Priestley’s Discovery (1770) 🕯️🐭🌱

  • Burning candles or breathing mice “damaged” air in a closed jar (candle went out, mouse died).
  • But with a mint plant inside, the candle kept burning and the mouse stayed alive!
  • Conclusion: Plants restore whatever animals and candles remove from the air.

3. Jan Ingenhousz’s Sunlight Test ☀️💦

  • Repeated Priestley’s experiment in sunlight vs. dark.
  • In sunlight: Aquatic plants released bubbles (oxygen!) from green parts.
  • In darkness: No bubbles formed.
  • Conclusion: Sunlight and green plant parts are essential for oxygen release.

4. Julius von Sachs (1854) 🥔🔬

  • Proved plants produce glucose (stored as starch) during growth.
  • Found chlorophyll (green substance) is inside special cell bodies called chloroplasts.

5. T.W. Engelmann’s Light Spectrum Experiment 🔆🧫

  • Split light with a prism → shone it on green algae (Cladophora) in bacteria-rich water.
  • Bacteria clustered most in blue and red light regions.
  • Conclusion: Photosynthesis is most efficient in blue/red light (matching chlorophyll absorption!).

6. Cornelius van Niel’s Bacterial Clue 🦠💡

  • Studied purple/green bacteria: Photosynthesis needs light to combine hydrogen (from compounds like H2S) with CO2.
  • In green plants, hydrogen comes from H2O → O2 is released.
  • Big insight: The O2 produced comes from water, not CO2!

The Photosynthesis Equation ✅

For green plants:

\[6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2\]

Fun fact: Radioisotope tests proved O2 comes from H2O! 💧➔💨

Where Photosynthesis Happens 🏞️

  • Mainly in green leaves (also stems, buds!).
  • Inside chloroplasts within mesophyll cells.
  • Chloroplast parts:
    • Grana (membranes): Trap light → make ATP/NADPH. (Light reactions) ☀️⚡
    • Stroma (fluid): Makes sugar → starch. (Dark reactions, but needs ATP/NADPH from light!) 🌙➔🍬

NEET Must-Know Concepts! 📚✨

  1. Priestley-Ingenhousz experiments: Plants need sunlight to restore air “damaged” by respiration/burning.
  2. Engelmann’s action spectrum: Blue/red light = most efficient for photosynthesis.
  3. Van Niel’s contribution: O2 released during photosynthesis comes from H2O, not CO2.
  4. Chloroplast structure: Light reactions (grana), dark reactions (stroma).
  5. Key equation: \(6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2\)