Q.55 In the history of photosynthetic research, the empirical reaction of photosynthesis
was first proposed for green plants (equation 1), followed by another reaction for
purple sulfur bacteria (equation 2), leading to a generalized equation for
photosynthesis (equation 3)

CO₂ + H₂O —light—> (CH₂O) + O₂ (equation 1)

CO₂ + 2H₂S —light—> (CH₂O) + H₂O + 2S (equation 2)

CO₂ + 2H₂A —light—> (CH₂O) + H₂O + 2A (equation 3)

where H2A in equation 3 is a generalized electron donor.
Which one of the following statements is DISPROVEN by equation 3?
(A) The source of oxygen produced in photosynthesis in green plants is CO2
(B) The source of oxygen produced in photosynthesis in green plants is H2O
(C) Light is essential in every form of photosynthesis
(D) Glucose is the end product in all forms of photosynthesis

The correct answer is option (A): “The source of oxygen produced in photosynthesis in green plants is CO₂”, and this statement is disproven by the generalized equation 3.​

Generalized equation of photosynthesis

Cornelis van Niel proposed a generalized equation for photosynthesis:

CO₂ + 2H₂A —^light—>(CH₂O) + H₂O + 2A

Here H₂A is any external electron (hydrogen) donor, and A is its oxidized form (for example, H₂O/O₂ in plants or H₂S/S in purple sulfur bacteria).

In green plants, the donor is water, so the specific equation becomes                      CO₂ + 2H₂O– ^light –>(CH₂O) + H₂O + O₂
, showing that oxygen arises from water, not carbon dioxide.​

Why option (A) is disproven

• Historically, it was once believed that oxygen released in photosynthesis came from CO₂, but isotopic labelling experiments with H₂¹⁸O proved that the evolved O₂ originates from water.​

• In equation 3, the oxygen‐containing product “2A” clearly derives from the generalized hydrogen donor H₂A; when H₂A is water (H₂O), A becomes O₂, so the source of oxygen is water rather than CO₂, directly contradicting statement (A).​

Thus equation 3 specifically disproves the claim that CO₂ is the source of oxygen in green‐plant photosynthesis, making (A) the correct choice.​

Why option (B) is not disproven

• When H₂A is water, equation 3 describes oxygenic photosynthesis of green plants:  CO₂ + 2H₂O → (CH₂O) + H₂O + O₂  ; in this case molecular oxygen comes from water.​

• Modern experimental evidence (Hill reaction, ¹⁸O labelling) supports this view, so statement (B) is actually correct and is supported rather than disproved by equation 3.​

Why option (C) is not disproven

• Equation 3 itself includes light as a reactant, emphasizing that the generalized photosynthetic process is light‑driven, whether in plants or photosynthetic bacteria.​

• The question uses only light‑dependent empirical equations (1–3), so nothing in these equations contradicts the statement that light is essential in every form of photosynthesis; equation 3 therefore does not disprove option (C), even though some organisms may use light very efficiently at low intensities.​

Why option (D) is not disproven

• Equation 3 produces a generalized carbohydrate unit (CH2O)(CH2O), not specifically glucose, but this is a conventional shorthand for “carbohydrate,” and the equations in the question do not compare different final carbon products across all phototrophs.​

• While many photosynthetic organisms channel fixed carbon into various sugars and polymers, equation 3 does not explicitly rule out glucose formation; therefore, it does not directly disprove statement (D), and the option that is clearly falsified remains (A).​​

Introduction

The source of oxygen in photosynthesis in green plants is a classic concept that often appears in competitive exams like GATE 2025 Life Sciences. The generalized photosynthesis equation
CO₂ + 2H₂A → (CH₂O) + H₂O + 2A clearly demonstrates that oxygen evolves from the external hydrogen donor, which is water in the case of green plants.​

This particular question asks which statement is disproven by equation 3, and a careful analysis of every option, in light of van Niel’s hypothesis and later experimental verification, identifies option (A) as the only statement directly contradicted.​

Detailed option‑wise explanation

Option (A): CO₂ as oxygen source

• Statement: “The source of oxygen produced in photosynthesis in green plants is CO₂.”

• Scientific position: Van Niel first generalized photosynthesis, suggesting that oxygen or other oxidized products originate from the external hydrogen donor H₂A.​

• In green plants, H₂A is water, and isotopic labelling has confirmed that the oxygen evolved during photosynthesis comes from water, not carbon dioxide, unequivocally refuting this older hypothesis.​

Because equation 3 encodes this principle, stating that 2A (oxygen, in plants) arises from H₂A (water), option (A) is explicitly disproven.​

Option (B): H₂O as oxygen source

• Statement: “The source of oxygen produced in photosynthesis in green plants is H₂O.”

• When H₂A is specified as H₂O in the generalized equation, the reaction becomes CO₂ + 2H₂O– ^light –>(CH₂O) + H₂O + O₂ , showing that molecular oxygen is derived from water.​

• Experiments using H₂¹⁸O demonstrated that the evolved oxygen gas contains the isotope from water, confirming water as the true oxygen source in oxygenic photosynthesis.​

Thus equation 3 actually supports option (B) rather than disproving it.​

Option (C): Light essential in all photosynthesis

• Statement: “Light is essential in every form of photosynthesis.”

• Equations 1–3 all include light as a necessary reactant, covering both oxygenic photosynthesis in plants and anoxygenic photosynthesis in bacteria.​

• The generalized equation was formulated specifically for light‑driven conversion of inorganic carbon into organic matter; within this conceptual framework, the role of light is fundamental and not challenged.​

Therefore, nothing in equation 3 contradicts the necessity of light; option (C) is not the statement being disproven in this question.​

Option (D): Glucose as universal end product

• Statement: “Glucose is the end product in all forms of photosynthesis.”

• The symbol (CH2O)(CH2O) in equation 3 is a generic representation of carbohydrate and does not explicitly limit the product to glucose, although glucose is a common storage and transport form in many plants.​

• Different photosynthetic organisms may channel carbon into diverse products (e.g., storage polysaccharides, organic acids), but the generalized equation does not enumerate these pathways and thus does not directly refute the possibility of glucose formation.​

Because the question focuses on what equation 3 disproves, and equation 3 mainly addresses electron donors and oxygen evolution, option (D) is not clearly falsified, unlike option (A).​

By understanding the generalized equation of photosynthesis and the historical experiments that established water as the source of oxygen, aspirants can reliably answer conceptual questions like this GATE 2025 item on the source of oxygen in photosynthesis in green plants.​