Embryo Sac in Angiosperms: Structure, Development & Diagram | NEET
Embryo Sac in Angiosperms: Structure, Development and Functions
The embryo sac is the female gametophyte of flowering plants (angiosperms). It develops inside the ovule and plays an essential role in sexual reproduction and double fertilization.
For NEET Biology, the structure and development of the embryo sac are important topics from sexual reproduction in flowering plants. This article explains the embryo sac in a simple, NCERT-focused way.
A typical mature angiosperm embryo sac is commonly described as 7-celled and 8-nucleate.
What is an Embryo Sac?
The embryo sac is the female gametophyte of an angiosperm. It develops from a functional megaspore inside the ovule.
The mature embryo sac contains the structures required for fertilization, including the egg apparatus, central cell, and antipodal cells.
Development of Embryo Sac
The formation of the female gametophyte from the functional megaspore is called megagametogenesis.
In the common type of embryo sac development, a single functional megaspore undergoes three successive mitotic divisions. These divisions produce eight nuclei.
The nuclei then become organized within the embryo sac to form the mature female gametophyte.
7-Celled and 8-Nucleate Embryo Sac
A typical mature embryo sac has seven cells but eight nuclei. This is a very important point for NEET and other Biology examinations.
The seven cells are organized into three major groups:
- Egg apparatus at the micropylar end
- Two polar nuclei in the central cell
- Three antipodal cells at the chalazal end
Structure of Mature Embryo Sac
1. Egg Apparatus
The egg apparatus is located at the micropylar end of the embryo sac. It consists of:
- One egg cell
- Two synergids
Therefore, the egg apparatus contains three cells.
2. Synergids
Two synergids are present near the egg cell. They are associated with the entry and guidance of the pollen tube toward the female gametophyte.
Each synergid contains a specialized thickening called the filiform apparatus near its micropylar region.
3. Egg Cell
The egg cell is the female gamete. It is located between the two synergids at the micropylar end of the embryo sac.
4. Central Cell and Polar Nuclei
The large central cell contains two polar nuclei in the typical mature embryo sac. These nuclei may fuse to form the secondary nucleus before or during the process associated with fertilization.
5. Antipodal Cells
Three antipodal cells are present at the chalazal end of the embryo sac.
Micropylar and Chalazal Ends
| Region | Important Structures |
|---|---|
| Micropylar end | Two synergids and one egg cell |
| Central region | Large central cell with two polar nuclei |
| Chalazal end | Three antipodal cells |
Why is the Embryo Sac 7-Celled but 8-Nucleate?
This is one of the most frequently tested facts. The mature embryo sac has seven cells because the two polar nuclei are located within the same central cell.
The arrangement can be summarized as:
- 3 cells at the micropylar end = 1 egg + 2 synergids
- 1 large central cell = 2 polar nuclei
- 3 cells at the chalazal end = 3 antipodals
Therefore:
3 + 2 + 3 = 8 nuclei
Role of Embryo Sac in Double Fertilization
The embryo sac provides the female reproductive structures involved in fertilization. One male gamete fuses with the egg to form the zygote. The other male gamete fuses with the polar nuclei-containing central cell, leading to the formation of the primary endosperm nucleus.
The occurrence of these two fusion events is known as double fertilization, a characteristic feature of angiosperms.
Embryo Sac: NEET Important Points
- The embryo sac is the female gametophyte of angiosperms.
- It develops inside the ovule.
- A typical mature embryo sac is 7-celled and 8-nucleate.
- The egg apparatus contains one egg and two synergids.
- The egg apparatus is located at the micropylar end.
- Three antipodal cells occur at the chalazal end.
- The central cell contains two polar nuclei in the typical mature embryo sac.
- Synergids possess a filiform apparatus.
- The embryo sac participates in double fertilization.
Micropylar = Egg + 2 Synergids
Centre = 2 Polar Nuclei
Chalazal = 3 Antipodals
Embryo Sac vs Ovule
| Feature | Embryo Sac | Ovule |
|---|---|---|
| Meaning | Female gametophyte | Structure within the ovary that contains the developing female gametophyte |
| Location | Inside the ovule | Attached to the placenta of the ovary |
| Role | Contains the egg apparatus and central cell involved in fertilization | Provides the structure in which the female gametophyte develops |
Frequently Asked Questions
What is the embryo sac?
The embryo sac is the female gametophyte of flowering plants and develops inside the ovule.
Why is the embryo sac called 7-celled and 8-nucleate?
A typical mature embryo sac contains seven cells but eight nuclei because the central cell contains two polar nuclei.
How many synergids are present in an embryo sac?
Two synergids are present in the egg apparatus at the micropylar end.
Where are antipodal cells located?
Three antipodal cells are generally located at the chalazal end of the mature embryo sac.
What is the function of the filiform apparatus?
The filiform apparatus is present in synergids and is associated with the uptake and transport of substances and with pollen tube guidance.
Conclusion
The embryo sac is a key reproductive structure in flowering plants. For NEET preparation, remember its development, the 7-celled and 8-nucleate organization, egg apparatus, polar nuclei, antipodal cells, and its role in double fertilization.
Related NEET Biology Topics
- Sexual Reproduction in Flowering Plants
- Microsporogenesis and Pollen Development
- Megasporogenesis
- Double Fertilization in Angiosperms
- Structure of Ovule
- Plant Reproduction
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