131. Which of the following types of plastids is formed first when Etioplast types are exposed to light?
1. Gerontoplast
2. Amyloplast
3. Pregranal plastid
4. Mature Chloroplast

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Question:

Which of the following types of plastids is formed first when Etioplast types are exposed to light?

1. Gerontoplast

2. Amyloplast

3. Pregranal plastid

4. Mature Chloroplast

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Correct Answer:

3. Pregranal plastid

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Detailed Explanation:

Plastids are specialized organelles found in plant cells that perform essential functions such as photosynthesis, storage, and pigment synthesis. Among the different types of plastids, etioplasts play a critical role in plant development, particularly under low light conditions.

When etioplasts are exposed to light, a series of transformations occurs, leading to the formation of other plastid types. Here is how this transformation process unfolds:

Etioplasts and Light Exposure:

• Etioplasts are plastids found in plants that grow in the dark or under very low light. They contain protochlorophyllide, a precursor to chlorophyll, and lack fully developed thylakoid membranes. Under light, these etioplasts undergo a conversion process to become fully functional chloroplasts, which are essential for photosynthesis.

Formation of Pregranal Plastids:

• The first plastid type formed when etioplasts are exposed to light is the pregranal plastid. This type represents an intermediate stage in the transformation process. During this stage, the protochlorophyllide is converted into chlorophyll, and the thylakoid membranes start to form but have not yet developed into the organized stacks of granules that are characteristic of mature chloroplasts.

• The pregranal plastids gradually develop into mature chloroplasts as the process continues, allowing the plant to begin photosynthesis efficiently.

Other Types of Plastids:

• Gerontoplasts are plastids found in aging plant tissues, typically involved in senescence and degradation processes. They are not formed directly from etioplasts but are instead associated with aging tissues.

• Amyloplasts are starch-storing plastids, which are typically found in tissues like roots and tubers. They are not involved in the light-induced transformation of etioplasts.

• Mature chloroplasts are the end product of the light-induced transformation of etioplasts. They are fully developed and contain the thylakoid structures necessary for photosynthesis.

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Conclusion:

When etioplasts are exposed to light, the first plastids to form are pregranal plastids, which serve as intermediates in the development of mature chloroplasts. This transformation is essential for the plant to begin photosynthesis, marking the critical step in the shift from dark-grown to light-exposed growth.