Question

Which one of the following equation suggest that ${ O }_{ 2 }$ released during photosynthesis comes from water
a) $6C{ O }_{ 2 }^{ 18 }+12{ H }_{ 2 }O\longrightarrow 6{ O }_{ 2 }^{ 18 }+{ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+6{ H }_{ 2 }{ O }^{ 18 }$
b) $6C{ O }_{ 2 }+12{ H }_{ 2 }{ O }^{ 18 }\longrightarrow 6{ O }_{ 2 }+{ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+6{ H }_{ 2 }{ O }^{ 18 }$
c) $6C{ O }_{ 2 }^{ 18 }+12{ H }_{ 2 }{ O }\longrightarrow 6{ O }_{ 2 }^{ 18 }+{ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+6{ H }_{ 2 }{ O }$
d) $6C{ O }_{ 2 }+12{ H }_{ 2 }{ O }^{ 18 }\longrightarrow 6{ O }_{ 2 }^{ 18 }+{ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+6{ H }_{ 2 }{ O }$

Answer 1

Hint: Photosynthesis is an anabolic process by which chloroplast of green plants and other phototroph’s synthesize carbohydrate (glucose) and evolve molecular oxygen as a by- product, using $C{ O }_{ 2 }$, ${ H }_{ 2 }{ O }$, and sunlight.

Complete Step by Step Answer:
- Photosynthesis is chemically a redox reaction in which water is oxidized and carbon dioxide is reduced.
- Till 1930 it was believed that photosynthesis is just the reverse of respiration
$6C{ O }_{ 2 }+6{ H }_{ 2 }{ O }\xrightarrow [ Light ]{ Chlorophyll } { C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+6{ O }_{ 2 }$
- An experiment conducted by Ruben and Kamen in 1940 proved that oxygen evolved during photosynthesis comes from water, not from $C{ O }_{ 2 }$ for the experiment they used heavy radioisotopes of oxygen in the water and $C{ O }_{ 2 }$_.
- In experiment 1 when photosynthesis occurs in the presence of ${ H }_{ 2 }{ O }$ and $C{ O }_{ 2 }^{ 18 }$ the heavy isotopes were found in the evolved ${ O }_{ 2 }$
$C{ O }_{ 2 }+2{ H }_{ 2 }{ O }^{ 18 }\xrightarrow [ Light ]{ Chlorophyll } { C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+{ H }_{ 2 }{ O }+{ O }_{ 2 }^{ 18 }$
- In experiment 2, when photosynthesis occurs in the presence of normal ${ H }_{ 2 }{ O }$ and $C{ O }_{ 2 }^{ 18 }$, the isotopic oxygen is not evolved
$C{ O }_{ 2 }^{ 18 }+2{ H }_{ 2 }{ O }\xrightarrow[ Light ]{ Chlorophyll } { C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }^{ 18 }+{ H }_{ 2 }{ O }^{ 18 }+{ O }_{ 2 }$
- The experiment for the first time proved that the oxygen evolved during photosynthesis comes from water.
- Then the conventional equation was modified as a molecule of ${ H }_{ 2 }{ O }$ is not sufficient for the evolution of 6 molecules of $C{ O }_{ 2 }$, now the correct balanced equation of photosynthesis is
$6C{ O }_{ 2 }+12{ H }_{ 2 }{ O }^{ 18 }\longrightarrow 6{ O }_{ 2 }^{ 18 }+{ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+6{ H }_{ 2 }{ O }$.
So,the correct answer is (d), ”$6C{ O }_{ 2 }+12{ H }_{ 2 }{ O }^{ 18 }\longrightarrow 6{ O }_{ 2 }^{ 18 }+{ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 }+6{ H }_{ 2 }{ O }$”.

Additional information:
- The leaves are made up of very small cells. Inside this cell, tiny structures are present called chloroplast. Each chloroplast contain a green chemical chlorophyll which gives leaves their green color
- Chlorophyll absorbs solar energy. It is this energy that is used to split water molecules into hydrogen and oxygen then oxygen is released from the leaves into the atmosphere. Hydrogen and $C{ O }_{ 2 }$ are used to form glucose or food for plants.

Note:
- Three things in the plant's environment affect the rate of photosynthesis:
(1) Light Intensity: this is one of the most important factors. As light intensity increases, the rate of photosynthesis initially increases.
(2) $C{ O }_{ 2 }$ Levels: Increasing the level of $C{ O }_{ 2 }$ stimulates photosynthesis.
(3) Temperature: Raising the temperature accelerates the chemical reactions involved in photosynthesis. The rate of photosynthesis increases as temperature increases. The rate of photosynthesis generally peaks at a certain temperature, and photosynthesis begins to decrease when the temperature is further increased.