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Class 12 VBQs Biology Human Reproduction

Very Short Answer Type Questions

Question. How is the entry of only one sperm and not many ensured into an ovum during fertilization in humans? 
Answer :
 During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that blocks the entry of additional sperms. 

Question. What stimulates pituitary to release the hormone responsible for parturition ? Name the hormone. 
Answer :
 Foetal ejection reflex that originates from the fully developed foetus and placenta is responsible for parturition. The hormone is oxytocin. 

Question. Explain the function of umbilical cord.
Answer :
 Umbilical cord connects placenta with the foetus. It helps in the transport of nutrients, respiratory gases and metabolic wastes to and fro between mother and foetus. 

Short Answer Type Questions

Question. State the role of Oxytocin in parturition. What triggers its release from the pituitary ?
Answer :
Oxytocin acts on uterine muscle, and cause stronger uterine contraction, leading to expulsion of the foetus/baby out of uterus. 
The signals from fully formed foetus and the placenta which induce mild uterine contraction/ foetal ejection reflex triggers release of oxytocin.

Question. When and where do chorionic villi appear in humans ? State their function.
Answer :
Chorionic villi are finger like projections that arise from the trophoblast layer that develops in the blastocyst after it has undergone implantation in the uterus. The chorionic villi and uterine tissue get interdigitated with each other to form a structural and functional unit between embryo (foetus) of maternal body, called as placenta. Placenta is connected with the embryo by umbilical cord.
It helps to transport substances to and from the embryo. 

Question. (i) Where do the signals for parturition originate from in humans ?
(ii) Why is it important to feed the newborn babies on colostrum ? 
Answer :
(i) From the fully developed foetus / placenta /foetal ejection reflex. 
(ii) It contains antibodies (IgA), provides immunity to the child.
Detailed Answer :
(i) The signals for parturition in humans originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex.
This causes the release of oxytocin from maternal pituitary.
(ii) The milk that comes out of the mammary glands during initial period of lactation and after parturition is called colostrum. It contains several antibodies, nutrients and is low in fat contents. It provides immunity to the child.
Therefore, it is important to feed the new born babies colostrum. Breast feeding is also recommended by doctors for bringing up a healthy baby.

Question. Differentiate between morula and blastocyst.
Answer :
Morula is a mulberry like solid mass of 8-16 cells called blastomeres, formed by cleavage of zygote, while the blastocyst is a hollow sphere of 64 cells formed by the rearrangement of blastomeres of
morula. It has a cavity called blastocoel and an inner cell mass within. It also has an outer envelop of cells called the trophoblast.

Question. Where does fertilization occur in humans ? Explain the events that occur during this process.
Answer :
Fertilization in humans occurs at ampullaryisthmic junction of the fallopian tube. Secretions of acrosome of sperms help it to enter the ovum through zona pellucida and plasma membrane. It induces meiosis II to form haploid ovum (oocyte/ ootid). The fusion of sperm and ovum to form zygote is called fertilization. 

Question. State any three functions of placenta in human female.
Answer :
(i) Supply oxygen to the embryo/nutrients to the embryo.
(ii) Removes carbon-dioxide/removes waste material produced by the embryo.
(iii) Produces hormones like hCG—human
Chorionic Gonadotropin/hPL—human
Placental Lactogen/ estrogen/progesterone. 

Question. After implantation, interdigitation of maternal and foetal tissues takes place. Identify the tissues involved and justify their role.
Answer :
After implantation, interdigitation of maternal and foetal tissues results in the formation of structural and functional unit between embryo and maternal body called placenta. 
It facilitates supply of oxygen and nutrients to the embryo, removal of carbon dioxide and excretory material. Also acts as endocrine tissue and produces hormones like hCG, hPL, estrogen, progesterone, relaxin.

Question. Describe the embryonic development of a zygote upto its implantation in humans.
Answer :
Zygote moves through isthmus and undergoes cleavage (forming morula), morula continues to divide and transform into blastocyst (as it moves further into uterus), Blastomeres in the blastocyst are arranged into an outer layer trophoblast, and inner cell mass, the trophoblast layer gets attached to endometrium, uterine cells divide and cover the blastocyst. 

Long Answer Type Questions

Question. (i) Briefly explain the events of fertilization and implantation in an adult human female.
(ii) Comment on the role of placenta as an endocrine gland.
Answer : (i) Fertilization :
(a) Sperm comes in contact and enters the secondary oocyte.
(b) Activates/induces secondary oocyte to complete meiosis II, leads to formation of ovum/ootid.
(c) The haploid nucleus of sperm and ovum fuse to form a diploid zygote completing the process of fertilization. 
Implantation :
(a) Trophoblast layer of blastocyst attaches to the endometrium (of the uterus).
(b) The uterine cells divide rapidly and cover the blastocyst.
(c) The blastocyst becomes embedded in the endometrium and the implantation is completed. 
(ii) (a) hCG (human chorionic gonadotropin)
(b) hPL (human placental lactogen)
(c) estrogen
(d) progesterone 
Detailed Answer :
(ii) The placenta acts as an endocrine gland because it produces many hormone like human chorionic- Gonadotropin (hCG), human placental Lactogen (hPL), Oestrogen, progesterone and estrogen. In the later phase of pregnancy, a hormone called as relaxin is also secreted by the ovary. hCG, hPL and relaxin hormone are produced in women only during pregnancy.

Question. (i) Describe the techniques that can help a healthy married woman who is unable to produce viable ova but wants to bear a child.
(ii) Draw a labelled diagram of a human ovum.
(iii) How is polyspermy prevented in humans ?
Answer :
(i) GIFT / Gamete Intra Fallopian transfer – Transfer of an ovum collected from a donor, into the fallopian tube of a healthy married female, who does not produce viable eggs, but can provide suitable environment for fertilisation.

Class 12 VBQs Biology Human Reproduction

(iii) During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum, induces changes in the membrane and block the entry of additional sperms.

Question. (i) Where in the fallopian tube does fertilization occur in humans ? Describe the development of a fertilized ovum upto implantation.
(ii) How is polyspermy prevented in humans ?
Answer :
(i) Ampullary region / ampullary isthmic junction. 
(a) Cleavage occurs in zygote to form 2 – 4 – 8 – 16 daughter cells / upto 16 daughter cells called blastomeres.
(b) 8 – 16 / 16 blastomeres stage called morula.
(c) Morula continues to divide and transform blastocycst (as it moves further into uterus).
(d) Blastomeres in the blastocyst are arranged into an outer layer called trophoblast which gets attached to endometrium.
(e) Inner group of cells called inner cell mass get differentiated into embryo.
(f) Uterine cells divide rapidly and covers blastocyst / implantation. 
(ii) When a sperm comes in contact with a zona pellucida layer of ovum, it induces changes in membrane to block entry of additional sperms. 

Question. Where does fertilization occur in humans ? Explain the events that occur during this process.
Answer :
(i) Ampullary Isthmic junction in fallopian tube / fallopian tube.
(ii) The sperms come in contact with zona pellucida.
(iii) Induces change in the membrane.
(iv) Blocks entry of other sperms / ensures only one sperm fertilizes the ovum / prevents polyspermy.
(v) The secretion of acrosome helps the sperm to enter the cytoplasm.
(vi) Entry of sperm induces completion of second meiotic division forming ovum and 2nd polar body.
(vii) The haploid nucleus of sperm and ovum fuse.
(viii) Formation of diploid zygote, fertilisation completed.

Question. (i) Name the hormones secreted and write their functions :
(a) By corpus luteum and placenta (any two).
(b) During Follicular phase and parturition.
(ii) Name the stages in a human female where :
(a) Corpus luteum and placenta co-exist.
(b) Corpus luteum temporarily ceases to exist.
Answer :
(i) (a) Corpus luteum : progesterone, essential for maintenance of the endometrium.
Placenta : hCG / human chorionic gonadotropin, produce during pregnancy / stimulates and maintains the corpus luteum / to secrete progesterone / growth of mammary glands.
HPL / human placental lactogen, produced during pregnancy.
Estrogen, maintenance of pregnancy / supporting foetal growth / metabolic changes in mother (Any two)
(b) Follicular phase : LH / FSH, stimulates follicular development / secretion of estrogen by growing follicles.
Parturition : oxytocin causes stronger uterine contraction / relaxin, secreted during (later stage) the pregnancy / softens symphysis pubis.
(ii) (a) Pregnancy / gestation.
(b) Menstruation / proliferative phase / ovulatory phase / follicular phase.

Question. The following is the illustration of the sequence of ovarian events (a – i) in a human female.

Class 12 VBQs Biology Human Reproduction

(i) Identify the figure that illustrates ovulation and mention the stage of oogenesis it represents.
(ii) Name the ovarian hormone and the pituitary hormone that have caused the above mentioned event.
(iii) Explain the changes that occur in the uterus simultaneously in anticipation.
(iv) Write the difference between ‘c‘ and ‘h’.
(v) Draw a labelled sketch of the structure of a human ovum prior to fertilization.
Answer :
(i) f is the secondary oocyte. 
(ii) estrogen, LH 
(iii) Endometrium proliferate (glands become corkscrew shaped) highly vascularised, high regeneration anticipating implantation of the fertilized ovum.
(iv) ‘c‘ is developing follicle while ‘h‘ is regressing corpus luteum.

Class 12 VBQs Biology Human Reproduction

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