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What is the role of water in photosynthesis?
What is the role of water in photosynthesis?
Water acts as a hydrogen and electron donor in photosynthesis. It is broken down into hydrogen ions ($H^+$), electrons ($e^-$) and oxygen ($O_2$).
What is the balanced equation for the photolysis of water?
What is the balanced equation for the photolysis of water?
$2H_2O$ → $4H^+$ + $4e^-$ + $O_2$
What happens to the electrons released from water molecules during photolysis?
What happens to the electrons released from water molecules during photolysis?
The electrons released from water molecules compensate for the photoexcited electrons lost by the chlorophyll-a molecule in the reaction center of Photosystem II (P680).
What happens to the hydrogen ions ($H^+$) released from water during photolysis?
What happens to the hydrogen ions ($H^+$) released from water during photolysis?
What is the fate of oxygen produced during photolysis?
What is the fate of oxygen produced during photolysis?
Who first reported the role of water in photosynthesis?
Who first reported the role of water in photosynthesis?
Van Niel hypothesized that plants split water as a source of hydrogen, releasing oxygen as a byproduct.
Van Niel hypothesized that plants split water as a source of hydrogen, releasing oxygen as a byproduct.
Van Niel's observations were based on photosynthesis studies in bacteria that produce carbohydrates from $CO_2$ but do not release oxygen.
Van Niel's observations were based on photosynthesis studies in bacteria that produce carbohydrates from $CO_2$ but do not release oxygen.
How was Van Niel's hypothesis confirmed in 1940?
How was Van Niel's hypothesis confirmed in 1940?
In the first experiment, water containing $^{18}O$ was added to an alga suspension. The oxygen that evolved during photosynthesis was found to be radioactive.
In the first experiment, water containing $^{18}O$ was added to an alga suspension. The oxygen that evolved during photosynthesis was found to be radioactive.
In another experiment, carbon dioxide containing $^{18}O$ was used. The oxygen evolved did not contain this isotope.
In another experiment, carbon dioxide containing $^{18}O$ was used. The oxygen evolved did not contain this isotope.
These experiments proved that the oxygen released during photosynthesis originates from water, not $CO_2$.
These experiments proved that the oxygen released during photosynthesis originates from water, not $CO_2$.
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Study Notes
Role of Water in Photosynthesis
- Water (H₂O) and carbon dioxide (CO₂) are crucial raw materials for photosynthesis.
- Water acts as a hydrogen and electron donor in photosynthesis.
- During the light-dependent phase, water molecules are broken down (photolysis).
- The balanced equation for photolysis of water is: 2H₂O → 4H⁺ + 4e⁻ + O₂
- Released electrons from water replace those lost by chlorophyll in Photosystem II (P680).
- The 2H⁺ ions and 2e⁻ are temporarily used to produce NADPH.
- NADPH is used in the light-independent reactions to reduce CO₂.
- Oxygen produced during photolysis is released into the atmosphere.
Role of Water (Van Niel's Hypothesis)
- Van Niel (1931) proposed that water splits to provide hydrogen, releasing oxygen as a byproduct.
- This hypothesis was based on photosynthesis studies in bacteria which don't release oxygen.
- Experiments confirmed this using isotopes in 1940.
Experimental Confirmation
- Using 18O-labeled water in experiments with algae, researchers discovered the evolved oxygen was radioactive.
- Researchers used 18O-labeled carbon dioxide in experiments, discovering the evolved oxygen did not contain the isotope.
- This proved that oxygen released during photosynthesis comes from water, not carbon dioxide.
Summary Equation for Photosynthesis
- 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂
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