Question
What is the structure of the mature embryo sac, and what is the function of each of its 7 cells and 8 nuclei?
Solution — Step by Step
The mature embryo sac (female gametophyte) of angiosperms is called the Polygonum type (most common). It develops from a single megaspore through three mitotic divisions, producing 8 nuclei arranged in 7 cells:
- 3 cells at the micropylar end (egg apparatus)
- 3 cells at the chalazal end (antipodals)
- 1 large central cell with 2 polar nuclei
graph TD
subgraph "Micropylar End"
A[Egg Cell - 1 nucleus]
B[Synergid 1 - 1 nucleus]
C[Synergid 2 - 1 nucleus]
end
subgraph "Central Region"
D[Central Cell - 2 polar nuclei]
end
subgraph "Chalazal End"
E[Antipodal 1 - 1 nucleus]
F[Antipodal 2 - 1 nucleus]
G[Antipodal 3 - 1 nucleus]
end
B --> A
C --> A
A --> D
D --> ETotal: 7 cells, 8 nuclei (the central cell has 2 nuclei, all others have 1 each).
| Cell Type | Count | Nuclei | Function |
|---|---|---|---|
| Egg cell | 1 | 1 | Fuses with one male gamete to form the zygote (2n) |
| Synergids | 2 | 2 (1 each) | Guide the pollen tube to the egg; contain the filiform apparatus that attracts the pollen tube |
| Central cell | 1 | 2 (polar nuclei) | Fuses with the second male gamete to form primary endosperm nucleus (3n), which develops into endosperm |
| Antipodals | 3 | 3 (1 each) | Nutritive function; degenerate after fertilisation |
During double fertilisation (unique to angiosperms):
- Syngamy: One male gamete + egg cell = zygote (2n)
- Triple fusion: One male gamete + two polar nuclei = primary endosperm nucleus (3n)
Both events happen simultaneously inside the embryo sac. The zygote develops into the embryo; the PEN develops into the endosperm (food for the embryo).
Why This Works
The 7-celled structure is the result of evolutionary optimisation. The egg cell is the actual female gamete. Synergids act as a guidance system (their filiform apparatus secretes chemicals that attract the pollen tube). The central cell ensures endosperm formation for nourishing the embryo. Antipodals provide temporary nutritive support.
This design ensures that fertilisation and nutrition provisioning happen simultaneously — a key advantage of angiosperms over gymnosperms.
NEET asks about embryo sac structure almost every year. The classic question: “How many cells and nuclei does a mature embryo sac have?” Answer: 7 cells, 8 nuclei. Follow-up: “Which cell is the largest?” Answer: the central cell. Memorise the table above.
Alternative Method
Use the mnemonic “SEA-C-AAA” to remember the arrangement from micropylar to chalazal end:
- Synergid, Egg, synergid (Another) = 3 cells at micropylar end
- Central cell = 1 cell in the middle
- Antipodal, Antipodal, Antipodal = 3 cells at chalazal end
Common Mistake
Students write “8 cells and 8 nuclei” instead of “7 cells and 8 nuclei.” The central cell has TWO polar nuclei but is still ONE cell. It is the only binucleate cell in the embryo sac. This is the most frequently tested detail — getting the count wrong costs you the entire mark.