Read the given passage and answer the following questions:
The oviducts (fallopian tubes), uterus and vagina constitute the female accessory ducts. Each fallopian tube is about 10-12 cm long and extends from the periphery of each ovary to the uterus, the part closer to the ovary is the funnel-shaped infundibulum. The edges of the infundibulum possess finger-like projections called fimbriae, which help in collection of the ovum after ovulation. The infundibulum leads to a wider part of the oviduct called ampulla. The last part of the oviduct, isthmus has a narrow lumen and it joins the uterus. The uterus is single and it is also called womb. The shape of the uterus is like an inverted pear. It is supported by ligaments attached to the pelvic wall. The uterus opens into vagina through a narrow cervix. The cavity of the cervix is called cervical canal which alongwith vagina forms the birth canal. The wall of the uterus has three layers of tissue. The external thin membranous perimetrium, middle thick layer of smooth muscle, myometrium and inner glandular layer called endometrium that lines the uterine cavity. The endometrium undergoes cyclical changes during menstrual cycle while the myometrium exhibits strong contraction during delivery of the baby.
Given below are two statements: one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A): Not all copulations lead to fertilisation and pregnancy.
Reason (R): During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): Not all copulations lead to fertilisation and pregnancy. — True. A is about why copulation does not always lead to pregnancy. Fertilisation takes place in the ampullary region of the fallopian tube, and it happens only if the ovum and the sperms reach this region at the same time.
Reason (R): During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. — True. R is about what happens once a sperm touches the ovum. The sperm makes changes in the membrane of the ovum that block the entry of more sperms, so only one sperm fertilises the ovum.
Why R does not explain A? R describes an event that takes place only after a sperm has already met the ovum. A depends on an earlier condition: whether the ovum and the sperms arrive in the ampullary region together. Blocking extra sperms explains why an ovum is fertilised by one sperm, not why many copulations give no pregnancy at all. So both statements are correct, but R is not the explanation of A.
NCERT Extract:
Fertilisation and Implantation – “… The ovum released by the ovary is also transported to the ampullary region where fertilisation takes place. Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary region. This is the reason why not all copulations lead to fertilisation and pregnancy. The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum (Figure 2.10) and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum.”