CBSE 2025 · Region 6 · Set 1 · Q32 · 5 marks
(a)Explain the structure of a mature embryo sac of a typical flowering plant.(ii)How is triple fusion achieved in these plants ?Describe the changes in the ovary and the uterus as induced by the changes in the level of pituitary and ovarian hormones during menstrual cycle in a human female.
(a)
Explain the structure of a mature embryo sac of a typical flowering plant.
(ii)
How is triple fusion achieved in these plants ?
Describe the changes in the ovary and the uterus as induced by the changes in the level of pituitary and ovarian hormones during menstrual cycle in a human female.
Marking-scheme solution
(a)
Three cells are grouped together at the micropylar end of embryo sac ,constitute
the egg apparatus, the egg apparatus in turn consists of two synergids , one egg cell, the synergids have special cellular thickenings at the micropylar tip called filiform apparatus, three cells are at the chalazal end and are called the antipodals, the large central cell has two polar nuclei, thus a typical angiosperm embryo sac at maturity though $\displaystyle 8$-nucleate is $\displaystyle 7$-celled.
(ii)
One of the two male gametes fuses with the two polar nuclei located in the
central cell to produce a triploid primary endosperm nucleus (PEN).
Phases of Menstrual
cycle
Pituitary
hormones
Ovarian
hormone
Ovary
Uterus
1) Menstrual
phase/day $\displaystyle 1$-$\displaystyle 5$
Low FSH
and LH
Low
Progesterone
and
Estrogen
Follicle start
to develop
Lining of
endometrium
shed and
expelled as
menstrual
flow
2)Follicular/Proliferat
ive phase/day $\displaystyle 6$-$\displaystyle 13$
FSH and
LH rises
Estrogen
rises and
low
progesterone
Follicle
mature
Endometrium
to thicken
3) Ovulatory
phase/day $\displaystyle 14$-$\displaystyle 15$
LH surge
(about 14th
Increase
level of
Graafian
follicle
Uterus lining
further
day)
estrogen and
low
progesterone
ruptures
thickens
4) Luteal phase/day $\displaystyle 16$-$\displaystyle 28$
FSH and
LH level
begins to
decline
High
progesterone
and low
Estrogen
Ruptured
follicle
transformed
into the
corpus
luteum
which
produce
progesterone
and estrogen
Uterus lining
continues to
thicken for
implantation
Sexual Reproduction in Flowering PlantsDouble FertilisationUnderstandlong_answermedium
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CBSE Class 12 Biology past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.