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🧬 NEET Biology❓ Question

Priestley, Ingenhousz, Sachs, Engelmann, van Niel: kaun kya prove kiya? Mixing them up

Yash Tiwari11th Sample

2 Oct 2026 · 1 min read

Bhai photosynthesis ke early experiments wala part mere liye pure confusion hai. NCERT ke paragraph mein 5-6 names hain aur sab ne kuch alag prove kiya, mujhse sab mix ho jaata hai. Writing in English for the post so everyone can follow 😅

What I could understand:

  • Priestley: a candle burning in a closed space goes out, a mouse dies, but a mint plant in the same space made the air "good" again. So plants somehow restore air.
  • Ingenhousz: the plant must be in sunlight, and only the green parts do it.
  • Sachs: glucose is made in green parts and stored as starch.
  • Engelmann: used a prism and an alga with bacteria. Bacteria collected at some colours.
  • van Niel: something with bacteria and sulphur, and then "O2 comes from water".

What I can't tell is: which part of the story shows which conclusion? E.g. the Engelmann one: bacteria gathered at red and blue light, so what exactly was he proving? And why would a study on sulphur bacteria tell us that oxygen comes from water in green plants? That feels like a jump.

Is there some simple way to remember who did what? Test is on Friday and I need to actually remember this. Dhanyawaad in advance 🙏

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  • [+]Meera Patel11thSample· 17d ago

    Your summary of the first three is right. Engelmann: he split light with a prism onto a filamentous green alga (Cladophora) in a suspension of aerobic bacteria. The bacteria collected in the regions of blue and red light, where the alga was giving out the most oxygen. Conclusion: photosynthesis (oxygen release) is highest in blue and red wavelengths, which is basically the first action spectrum of photosynthesis. It matches the absorption peaks of chlorophyll a and b.

  • [+]Nisha Verma12thSample· 17d ago

    van Niel is the "jump" you mention. He studied purple and green sulphur bacteria, which photosynthesise using H₂S rather than H₂O as the hydrogen donor, and they release sulphur, not oxygen. His general equation: CO2+2H2A→light(CH2O)+2A+H2OCO_2 + 2H_2A \xrightarrow{light} (CH_2O) + 2A + H_2O where H2AH_2A is H2SH_2S in those bacteria, giving A=SA = S. By analogy, in green plants H2AH_2A is H2OH_2O, so the O₂ evolved must come from water and not from CO2CO_2. Later radioisotope experiments with ¹⁸O confirmed it.

    • [+]Yash Tiwari11thSample· 17d agoAuthor

      yaar this is the answer. H₂S → sulphur, so H₂O → oxygen. Wish teachers said it like this. Dhanyawaad Nisha!

  • [+]Aman YadavDropperSample· 17d ago

    Priestley is the one who found oxygen comes from CO2. Ingenhousz found it was water. Short version.

    • [+]Kavya NaikDropperSample· 17d ago

      Aman, that's not right. Priestley didn't find where oxygen comes from, he only showed that plants restore the air spoiled by a burning candle or a breathing animal. Ingenhousz showed it needs sunlight and green parts. The origin of oxygen (water) comes much later, from van Niel's idea and the isotope experiments. Please don't write it that way in the exam 🙏

      • [+]Aman YadavDropperSample· 17d ago

        Understood. Got it wrong. Corrected in notes.

  • [+]Ishita GoyalDropperSample· 17d ago

    Mnemonic for order: Priestley = Plants purify air. Ingenhousz = Illuminate (need light, green parts). Sachs = Starch. Engelmann = Examine colours. van Niel = via water. "P I S E V" can be read as "PISE-V"... not a word but I'm working on it. 😂

    Actually that's the whole chain from NCERT in one line.

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