Question

Assertion: If thylakoids isolated from chloroplasts are exposed to light in the presence of carbon dioxide and water, hexoses are not formed.
Reason: Carbon assimilation is a dark reaction and does not take place in the light.
A. Both assertion and reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct but reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Both assertion and reason are incorrect.

Answer 1

The chloroplasts are the cell organelles that are present in the cells of plants and many photosynthetic algae. The chloroplast absorbs the sunlight and conducts the process of photosynthesis for the generation of food from sunlight and carbon dioxide. It is responsible for the preparation of food in the form of starch. These are abundantly present in leaves where the maximum photosynthesis takes place.

Complete answer:
Chloroplast is a double membrane-bound structure which has a fluid-filled in it called the stroma. Stroma is a gel-like fluid containing proteins, ions, some enzymes, and other organic molecules in dissolved form, a circular DNA, ribosomes, and thylakoid system is observed floating in it. Thylakoids are flat sac-like structures that are arranged in stacks which are collectively called granum.
The main function of thylakoids is to perform a light reaction that takes place inside the thylakoids where the photosynthetic pigment ‘chlorophyll’ is present. The sunlight is absorbed by thylakoids and a light reaction is carried out. The light reaction produces two main products that are used for deriving energy, these are ATP and NADPH.
Further, these two products of the light reaction are used by the enzymes present in the stroma for the synthesis of carbohydrate molecules. Since the reaction taking place in stroma does not directly depend on the light; it is called a dark reaction.
In the question, it is given that isolated thylakoids are placed in light with carbon dioxide and water. In this case, only ATP and NADPH will be formed, as the synthesis of carbohydrate is a two-step reaction. One is the light reaction in which ATP and NADPH are formed and the other is the dark reaction which utilizes the product of light reaction as a substrate. Since in isolated thylakoids there is no stroma, consequently no enzymes are present which synthesize carbohydrates, so no hexoses are formed. Therefore, both the assertion and reason are correct and the reason is the correct explanation for the assertion.

Hence, the correct answer is Option A

Note: Chloroplasts are mostly lens-shaped and have about 3-10 micrometers of diameter and 1-3 micrometers thick. These can vary in shape depending on the type of organism. For example, green algae, Spirogyra has a ribbon-like spiral shaped chloroplasts, and Chlamydomonas has a cup-shaped chloroplast, but all are responsible for the same function that is production of food.