In ovules of Angiosperms, which one of the following is not a haploid cell?
Nucellus
Understanding the structure and ploidy levels of different cells within the ovule of Angiosperms is crucial for comprehending plant reproduction. The question asks us to identify which cell among the given options is not haploid (\(n\)). Let's examine each option:
Ploidy Levels in Angiosperm Ovule Cells
In a typical Angiosperm ovule, various tissues and cells are present, originating from different generations (sporophyte or gametophyte) and having different ploidy levels.
- Nucellus: The nucellus is the main mass of parenchymatous cells that forms the body of the ovule. It is derived from the maternal sporophyte tissue. The sporophyte generation in plants is diploid (\(2n\)). Therefore, the cells of the nucellus are typically diploid (\(2n\)).
- Synergid: Synergid cells are part of the embryo sac, which is the female gametophyte. The embryo sac develops from a functional megaspore through a process called megagametogenesis. The megaspore itself is haploid (\(n\)), resulting from meiosis of a diploid megaspore mother cell. All cells of the embryo sac, including synergids, the egg cell, and antipodal cells, are haploid (\(n\)). There are usually two synergids located near the egg cell at the micropylar end of the embryo sac.
- Egg: The egg cell is also a part of the female gametophyte (embryo sac). It is located at the micropylar end, often flanked by the synergids. Like the synergids, the egg cell develops from the haploid (\(n\)) functional megaspore and is therefore haploid (\(n\)).
- Antipodal: Antipodal cells are located at the chalazal end of the embryo sac, opposite the micropylar end. They are also components of the female gametophyte and are derived from the haploid (\(n\)) functional megaspore. Thus, antipodal cells are haploid (\(n\)).
Comparing Ploidy Levels
Let's summarize the ploidy of each cell type mentioned:
| Cell Type in Angiosperm Ovule | Typical Ploidy Level | Origin | Part of Sporophyte or Gametophyte? |
|---|---|---|---|
| Nucellus | Diploid (\(2n\)) | Maternal Sporophyte Tissue | Sporophyte |
| Synergid | Haploid (\(n\)) | Female Gametophyte (Embryo Sac) | Gametophyte |
| Egg | Haploid (\(n\)) | Female Gametophyte (Embryo Sac) | Gametophyte |
| Antipodal | Haploid (\(n\)) | Female Gametophyte (Embryo Sac) | Gametophyte |
The question asks which one is not a haploid cell. Based on our comparison, the Nucellus is diploid (\(2n\)), while the Synergid, Egg, and Antipodal cells are haploid (\(n\)). Therefore, the Nucellus is the cell that is not haploid.
Conclusion
Among the given options, the Nucellus is the only structure that is typically diploid, not haploid. The other cells listed (Synergid, Egg, and Antipodal) are integral parts of the haploid female gametophyte (embryo sac).
Revision Table: Angiosperm Ovule Cells
| Cell/Tissue | Ploidy |
|---|---|
| Nucellus | \(2n\) (Diploid) |
| Synergid | \(n\) (Haploid) |
| Egg | \(n\) (Haploid) |
| Antipodal | \(n\) (Haploid) |
| Megaspore Mother Cell | \(2n\) (Diploid) |
| Functional Megaspore | \(n\) (Haploid) |
| Central Cell Nuclei (before fusion) | \(n\) (Haploid) |
| Primary Endosperm Nucleus (after fusion) | \(3n\) (Triploid) |
| Zygote | \(2n\) (Diploid) |
Additional Information on Angiosperm Reproduction
The ploidy level of cells changes throughout the life cycle of Angiosperms, which involves alternation of generations between a diploid sporophyte and a haploid gametophyte. The ovule contains structures from both generations.
- Sporophyte Generation (\(2n\)): The main plant body is the sporophyte. Tissues like the nucellus and integuments of the ovule belong to the maternal sporophyte generation and are diploid.
- Gametophyte Generation (\(n\)): The embryo sac (female gametophyte) within the ovule is haploid. It arises from a haploid megaspore.
- Megasporogenesis: A diploid megaspore mother cell (\(2n\)) undergoes meiosis to produce four haploid (\(n\)) megaspores. Usually, only one functional megaspore survives.
- Megagametogenesis: The functional megaspore (\(n\)) undergoes mitotic divisions to form the multicellular embryo sac (female gametophyte), which typically contains seven cells and eight nuclei: the egg cell (\(n\)), two synergids (\(n\)), three antipodal cells (\(n\)), and a central cell with two polar nuclei (\(n\) + \(n\), which often fuse before fertilization to form a diploid or secondary nucleus, though the question refers to the *cells* which are derived from the initial n nuclei). However, the cells (synergids, egg, antipodal) are derived directly from the \(n\) megaspore nuclei.
- Fertilization: Double fertilization occurs in Angiosperms, involving fusion of one male gamete (\(n\)) with the egg cell (\(n\)) to form the diploid zygote (\(2n\)), and fusion of the other male gamete (\(n\)) with the central cell's polar nuclei (often \(2n\)) to form the triploid primary endosperm nucleus (\(3n\)).
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