8 min readPathway sheet1-mark MCQ on the ploidy of the zygote and the PEN, a 2 or 3-mark question on why it is called double fertilisation, and part of a 5-mark answer; also a NEET favourite

Double fertilisation

One male gamete fuses with the egg (syngamy) to form the diploid zygote. The other fuses with two polar nuclei (triple fusion) to form the triploid PEN.

SyngamyMale gamete (n) + egg (n) → zygote (2n), which becomes the embryo

Triple fusionMale gamete (n) + two polar nuclei (n + n) → primary endosperm nucleus, PEN (3n)

Double fertilisationSyngamy and triple fusion in the same embryo sac; unique to flowering plants

Primary endosperm cell (PEC)The central cell after triple fusion; it develops into the endosperm

Events of double fertilisation

1
The pollen tube enters a synergid through the filiform apparatus and releases two male gametes into its cytoplasm.
2★
Syngamy: one male gamete (n) moves to the egg (n) and fuses with its nucleus → zygote (2n).
3★
Triple fusion: the other male gamete (n) moves to the two polar nuclei (n + n) in the central cell. It fuses with them → primary endosperm nucleus, PEN (3n).
4
The central cell is now the primary endosperm cell (PEC); it develops into the endosperm.
5
The zygote develops into the embryo.

★ key step

The two fusions compared

Feature	Syngamy	Triple fusion
Nuclei that fuse	One male gamete + egg	One male gamete + two polar nuclei
Number of haploid nuclei	Two	Three
Product	Zygote (2n)	Primary endosperm nucleus (3n)
Develops into	Embryo	Endosperm

Worked example: ploidy and chromosome numbers

Problem

The leaf cells of a flowering plant have 24 chromosomes. How many chromosomes are there in its male gamete, egg, zygote and endosperm?

Working

Leaf cells are diploid: 2n = 24, so n = 24 ÷ 2 = 12.
Male gamete and egg are haploid: n = 12 each.
Zygote = male gamete + egg = 12 + 12 = 24 (2n).
Endosperm comes from the PEN = male gamete + two polar nuclei = 12 + 12 + 12 = 36 (3n).

Answer

Male gamete 12, egg 12, zygote 24, endosperm 36.

How to draw an embryo sac at fertilisation

Draw the embryo sac with the micropylar end at the bottom and the chalazal end at the top.
Draw a pollen tube entering through the micropyle into one synergid; mark the filiform apparatus.
Show one male gamete next to the egg with an arrow; label it syngamy.
Show the second male gamete next to the two polar nuclei with an arrow; label it triple fusion.
Draw three antipodals at the chalazal end. Title: Double fertilisation in the embryo sac.

In real life

The white part of a rice grain, the flour from wheat and the white kernel of coconut are all triploid endosperm made by triple fusion. So much of our daily food comes from this second fusion.

Exam angle

2 marks: “Why is fertilisation in angiosperms called double fertilisation?” Two fusions happen in one embryo sac: syngamy (male gamete + egg → zygote, 2n) and triple fusion (male gamete + two polar nuclei → PEN, 3n).

1 marks: “The leaf cells of a plant have 18 chromosomes. How many chromosomes are in its endosperm cells?” n = 9, so the endosperm (3n) has 27.

3 marks: “Describe double fertilisation. What do its two products develop into?” Pollen tube enters a synergid and releases two male gametes; syngamy gives the diploid zygote; triple fusion gives the triploid PEN; zygote → embryo; PEC → endosperm.

Remember it

Syngamy: 2 nuclei → 2n zygote. Triple fusion: 3 nuclei → 3n PEN.