Video Transcript
How many of the eight haploid
nuclei inside the embryo sac, female gametophyte, are not involved in double
fertilization?
This question is asking us about
the process of double fertilization in a group of plants called angiosperms, which
are commonly known as flowering plants. To understand this process, we
first need to discuss the eight haploid nuclei that are found within the embryo sac
in the ovule of an angiosperm. The egg cell is the female gamete
in an angiosperm which contains a haploid nucleus. This and the seven other haploid
nuclei in the embryo sac are formed from large diploid cells called spore mother
cells through a special type of cell division called meiosis, which occurs within
each ovule. This meiotic division of the spore
mother cell produces four haploid cells called megaspores, three of which
degenerate, while the fourth grows into a structure called the embryo sac.
The haploid megaspore within the
embryo sac then divides by mitosis three times to produce the eight haploid nuclei
that are mentioned in the question. Two of these haploid nuclei, which
are now called the polar nuclei, move to the center of the embryo sac, forming one
large central cell. The other six nuclei move to
opposite ends of the embryo sac and become independent cells. The three cells furthest from the
micropyle are called antipodal cells. The cell shown closest to the
micropyle at the other end of the embryo sac grows and develops into an egg cell,
while the two cells either side of it develop into cells called synergids.
Now, we are ready to discuss which
of these eight haploid nuclei are involved in double fertilization. It is called double fertilization
as it involves two major fertilization events. One of the two haploid male sperm
cells fuses with the haploid egg cell to produce a diploid zygote. The zygote then divides by mitosis
several times to form an embryo. The synergid nuclei are theorized
to aid this fertilization event but are not directly involved in fertilization
itself.
The other haploid male sperm
nucleus fuses with the two haploid polar nuclei in the center of the embryo sac,
which forms the triploid endosperm nucleus. Of the eight haploid nuclei, three,
the two polar nuclei and the egg cell nucleus, are involved in double
fertilization. You can identify these nuclei as
the orange dots within the ovule. However, the question is asking
about the number of haploid nuclei that are not involved in double fertilization,
which are the three antipodal cells and two synergid cells shown in the ovule as the
pink dots.
Therefore, the number of haploid
nuclei in the embryo sac that are not involved in double fertilization is five.
