Ploidy of Structures: Zygote, Endosperm and Embryo Sac Cells
Biology · Sexual Reproduction in Flowering Plants · NEET
In a flowering plant, the egg, synergids, antipodals and each polar nucleus are haploid (n). After double fertilisation the zygote becomes diploid (2n) and the primary endosperm cell (which forms the endosperm) becomes triploid (3n). Memory hook: "Egg + sperm = 2n zygote; two polar nuclei + one sperm = 3n endosperm." NEET asks this almost every year, so if you know n=x, then zygote=2x and endosperm=3x.
Every cell of the mature embryo sac is haploid (n). After double fertilisation, syngamy makes the diploid (2n) zygote, and triple fusion makes the triploid (3n) endosperm.
Your doubts, answered
Is the central cell haploid or diploid before fertilisation?
Before fertilisation the central cell holds two polar nuclei, and each polar nucleus is haploid (n). People call the central cell '2n' because it contains two n nuclei, but each nucleus itself is n. Only after the second male gamete (n) fuses with both polar nuclei does the primary endosperm nucleus become triploid (3n). So: two n polar nuclei + one n sperm = 3n.
Why is endosperm triploid (3n) and not diploid?
Endosperm comes from triple fusion, which joins three haploid nuclei: two polar nuclei (n + n) and one male gamete (n). That gives n + n + n = 3n. The zygote, by contrast, comes from syngamy of just two nuclei (egg n + sperm n = 2n). Endosperm has one extra nucleus in the fusion, so it is triploid.
What is the ploidy of the egg, synergids and antipodal cells?
All seven cells of the mature embryo sac are haploid (n) before fertilisation: 1 egg, 2 synergids, 3 antipodals, and the central cell with 2 polar nuclei (each n). This is because the embryo sac develops from the functional megaspore (which is haploid, formed by meiosis) through mitotic divisions that do not change ploidy.
If diploid number 2n = 36, what is the chromosome number in the endosperm?
First find n: if 2n = 36, then n = 18. Endosperm is triploid, so endosperm = 3n = 3 x 18 = 54. Zygote would be 2n = 36. This is a very common NEET calculation, so always convert to n first, then multiply by 2 for zygote and by 3 for endosperm.
Is the primary endosperm nucleus (PEN) the same as endosperm ploidy?
Yes. The primary endosperm nucleus (PEN) formed by triple fusion is triploid (3n). The central cell then becomes the primary endosperm cell (also 3n), which divides to form the triploid endosperm tissue. So PEN, primary endosperm cell and endosperm are all 3n.
⚠️ The NEET trap ✗ Students mark the central cell (or polar nuclei) as triploid or the endosperm as diploid. ✓ Each polar nucleus is haploid (n); the endosperm becomes triploid (3n) only AFTER triple fusion with the male gamete. The zygote is diploid (2n). 🧠 Ploidy comes from HOW MANY nuclei fuse: egg + 1 sperm = 2n zygote, but 2 polar + 1 sperm = 3n endosperm.
Real NEET questions
2026
If the diploid chromosome number of a typical angiosperm is 36, what would be the chromosome number in its endosperm?
A · 18
B · 36
C · 54 ✓
D · 72
Solution: Endosperm is triploid (3n). If 2n = 36 then n = 18, so endosperm = 3n = 3 x 18 = 54. (Zygote here would be 2n = 36.)
2026
Which one of the following is a triploid cell?
A · Synergid
B · Central cell
C · Zygote
D · Primary endosperm cell ✓
Solution: Syngamy gives the diploid (2n) zygote; triple fusion of the second male gamete with the two polar nuclei gives the triploid (3n) primary endosperm nucleus, and the central cell then becomes the triploid primary endosperm cell. Synergid is haploid, zygote is diploid, so the only triploid cell is the primary endosperm cell.
2023
In angiosperm, the haploid, diploid and triploid structures of a fertilized embryo sac sequentially are:
A · Synergids, Primary endosperm nucleus and zygote
B · Antipodals, synergids, and primary endosperm nucleus
C · Synergids, Zygote and Primary endosperm nucleus ✓
D · Synergids, antipodals and Polar nuclei
Solution: After fertilisation the synergids stay haploid (n), the egg fused with a male gamete gives the diploid (2n) zygote, and triple fusion gives the triploid (3n) primary endosperm nucleus. So haploid to diploid to triploid = Synergids, Zygote, Primary endosperm nucleus.
Solved Sexual Reproduction in Flowering Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The zygote is diploid (2n). It forms by syngamy, the fusion of the haploid egg (n) with one haploid male gamete (n): n + n = 2n.
What is the ploidy of the endosperm in angiosperms?
The endosperm is triploid (3n). It develops from the primary endosperm nucleus, which forms by triple fusion of two haploid polar nuclei and one haploid male gamete: n + n + n = 3n.
Are all cells of the embryo sac haploid before fertilisation?
Yes. All seven cells (egg, 2 synergids, 3 antipodals and the central cell with its 2 polar nuclei) are haploid (n) because the embryo sac develops by mitosis from the haploid functional megaspore.
What is the ploidy of the nucellus and integuments of the ovule?
The nucellus and integuments are part of the mother sporophyte, so they are diploid (2n). This is why perisperm (persistent nucellus) is diploid, unlike the triploid endosperm.
Does the ploidy of the perisperm differ from the endosperm?
Yes. Perisperm is the leftover nucellus and is diploid (2n, sporophytic tissue), while endosperm is triploid (3n, from triple fusion). NEET often contrasts these two.
