Question

In half leaf experiment, a part of the leaf is enclosed in a test tube containing KOH soaked cotton, while the other half is exposed to air and then setup is placed in light for some time. It was later found that part of the leaf, which was exposed to air tested positive for starch. Does this indicate that?
(a) Light is essential for photosynthesis.
(b) Oxygen is liberated in photosynthesis.
(c) Water is essential for photosynthesis because, in KOH soaked leaf, starch synthesis does not occur as water reacts with KOH and it becomes unavailable for photosynthesis.
(d) Carbon dioxide is essential for photosynthesis because in KOH soaked leaf, starch synthesis does not occur as $CO_2$ is absorbed by KOH, so $CO_2$ is not available for photosynthesis.

Answer 1

Hint: The above mentioned experiment is also known as Moll's half- leaf experiment which is being conducted to show that a particular gas is very much essential for the conduction of photosynthesis. This gas also helps in restricting the burning process.

Correct step by step answer:
In half leaf experiment, It was later found that part of the leaf, which was exposed to air tested positive for starch. This indicates that carbon dioxide is essential for photosynthesis because in KOH soaked leaf, starch synthesis does not occur as $CO_2$ is absorbed by KOH, so $CO_2$ is not available for photosynthesis.

Additional Information:
- During the photosynthesis process, the light energy transfers electrons from water (H2O) to carbon dioxide ($CO_2$), to produce carbohydrates.
- During this transfer, the $CO_2$ is reduced as it receives electrons, and the water is oxidized as it loses electrons.
- As the final product of the reaction, the oxygen is produced along with carbohydrates. In the above- mentioned experiment, when a part of the leaf is enclosed in a test tube containing KOH soaked cotton, this will cause absorption of $CO_2$ by KOH (Potassium hydroxide), as a result, the leaf sealed inside the glass container is now in the air without carbon dioxide.
- After some time, when the leaves were tested for starch, the leaf which was in the sealed container will not turn blue- black like the other leaves which were not there within the sealed glass container. This proves that without carbon dioxide, the process of photosynthesis cannot take place.
- The process of photosynthesis is used by plants, algae, and certain bacteria to absorb energy from sunlight and convert it into chemical energy.
- With the help of the Half- Leaf experiment, scientists were studying the natural process photosynthesis to get some help in the development of clean fuels and sources of renewable energy.
So the correct answer is, '(d) Carbon dioxide is essential for photosynthesis because, in KOH leaf, starch synthesis does not occur as $CO_2$ is absorbed by KOH, so $CO_2$ is not available for photosynthesis'.

Note:
- The Potassium Hydroxide (KOH) absorbs carbon dioxide which is necessary for photosynthesis.
- The leaf which was kept in KOH solution does not change to blue black due to the absence of carbon dioxide
- During the photosynthetic reaction the $CO_2$ is reduced as it receives electrons, and the water is oxidized as it loses electrons.