VBQs Class 12 Biology Human Reproduction
Very Short Answer Type Questions
Question. Name the embryonic stage that gets implanted in the uterine wall of human female.
Answer : Blastocyst.
Question. Mention the function of trophoblast in human embryo.
Answer : It helps in implantation and leads to pregnancy.
Question. Explain the function of myometrium and endometrium in human females.
Answer : (i) The myometrium exhibits strong contraction during parturition.
(ii) Endometrium prepares uterus for pregnancy by undergoing cyclic changes.
Question. During fertilization hundreds of sperms are in the vicinity of an egg cell; but only one sperm enters the ovum. How is this achieved.
Answer : During fertilization when a sperm comes in contact with the egg it induces changes in zona pellucida of ovum and this blocks the entry of any other sperm.
This ensures the entry of only one sperm.
Short Answer Type Questions
Question. Explain the events that follow upto fertilization when the sperms come in contact with the ovum in the fallopian tube of a human female.
Answer : The secretion of the acrosome help the sperm to enter into the cytoplasm of ovum through zona pellucida and the plasma membrane, this induces the completion of second meiotic division of the secondary oocyte, forming second polar body and a haploid ovum, soon the haploid nucleus of the sperm and the ovum fuse together to form a diploid zygote.
Question. Name the stage of the human embryo that gets implanted in the uterus and draw its labelled diagram.
Answer : Blastula or Blastocyst (32-celled stage).
Question. Write the function of each one of the following :
(i) (Oviducal) Fimbriae
Answer : (i) Collection of ovum released by ovary.
(ii) Causes uterine contraction for parturition / promotes milk ejection.
Detailed Answer :
(i) Help in collection of ovum from ovary after ovulation.
(ii) Oxytocin cause uterine contraction for parturition. It also promotes lactation and milk ejection from mammary glands.
Question. What happens to corpus luteum in human female if the ovum is
(ii) not fertilized?
Answer : (i) Corpus luteum continues to secrete progesterone to maintain pregnancy, it persists and produces progesterone.
(ii) It disintegrates and changes into corpus albicans.
Question. Draw a labelled diagram of the embryonic stage that gets implanted in the human uterus. State the functions of the two parts labelled.
(i) Trophoblast – helps in implantation / attachment to endometrium / attachment to uterus.
(ii) Inner cell mass – gets differentiated into an embryo.
Question. Describe the process of parturition in humans.
Answer : (i) Signals originate from the fully developed foetus and placenta.
(ii) Induce mild uterine contractions (foetal ejection reflex).
(iii) Triggers release of oxytocin (from maternal pituitary).
(iv) Oxytocin acts on uterine muscles and causes stronger uterine contractions.
(v) Stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contraction.
(vi) Expel the baby from the uterus.
Detailed Answer :
(i) Parturition is the process of giving birth to a baby.
(ii) The physical activities like uterine and abdominal contractions, dilation of cervix for passage of baby are involved in this process which are collectively called as labour and the discomfort or agony caused by it is known as labour pains.
(iii) Parturition is controlled by a complex neuroendocrine mechanism.
(iv) Signals originate from fully formed foetus and placenta which induce mild uterine contractions called foetal ejection reflex. This causes the release of oxytocin from maternal pituitary.
(v) Oxytocin causes stronger uterine muscle contraction which in turn stimulate further secretion of Oxytocin. This process is continued leading ultimately to expulsion of baby out of the uterus through the birth canal.
(vi) After parturition, the umbilical cord is tied and cut off. The placenta is separated from the
uterus and is expelled out due to series of strong uterine contractions.
Question. (i) How is placenta formed in the human female ?
(ii) Name any two hormones which are secreted by it and are also present in a non–pregnant woman.
Answer : (i) After implantation, the chorionic villi that appear on trophoblast, interdigitate with the uterine tissue, jointly form placenta.
(ii) Estrogen, progesterone.
Long Answer Type Questions
Question. (a) Draw a diagram of the adult human female reproductive system and label the different:
(i) parts of fallopian tube
(ii) layers of uterus wall
(b) Explain the events during fertilization of an ovum in humans.
Answer : (a)
(b) When a sperm comes in contact with Zona pellucida of ovum this induces changes in the membrane that blocks entry of additional sperms secretions of acrosome helps sperm to enter into the cytoplasm of ovum through zona pellucida and plasma membrane, this induces completion of meiotic division of secondary oocyte and formation of a haploid ootid / ovum (and a second polar body), haploid nucleus of ovum fuses with sperm nucleus to form diploid zygote (fertilization occurs).
Question. Arrange the terms given below in their order of occurrence describing their structure and function in the early development of the human embryo: Implantation; cleavage; inner cells mass; trophoblast; blastomeres; endometrium; morula; blastocyst.
Answer : Correct order = cleavage, blastomeres, morula, blastocyst, trophoblast, inner cell mass, endometrium, implantation.
Cleavage : Mitotic division starts as the zygote moves through the oviduct towards the uterus and form (2/4/8/16) daughter cells called blastomeres. Embryo with 8 to 16 blastomeres is called a morula.
Morula continues to divide and transforms into a blastocyst. Blastomeres in blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The trophoblast layer then gets attached to the endometrium and inner cell mass differentiates as the embryo. Ultimately blastocyst becomes embedded in the endometrium of uterus. This is called Implantation.
Question. Mention the site of fertilisation of a human ovum.
List the events that follow in sequence until the implantation of the blastocyst.
Answer : The site of fertilisation is the ampullary isthmic junction (fallopian tube).
Fertilization (Zygote formation), → Cleavage, → 8-16 Blastomeres stage – Morula → Continuously divides and transforms into Blastocyst (as it moves into uterus) → Blastomeres in the Blastocyst arrange to form outer layer trophoblast and inner cell mass → Trophoblast attaches to endometrium and inner cell mass differentiates as embryo → After attachment of blastocyst the uterine cells cover it → Embed blastocyst in the endometrium – implantation.
Question. During the reproductive cycle of a human female, when, where and how does a placenta develop?
What is the function of placenta during pregnancy and embryo development ?
Answer : After implantation uterus, chorionic villi and uterine tissue become interdigitated (physically fused).
Placenta facilitates supply of oxygen / nutrients to the embryo.
Removal of carbon dioxide / waste material / excretory material produced by the embryo.
Production of hCG / hPL / estrogens / progesterone.
Detailed Answer :
After implantation, the cells of inner cell mass of blastocyst differentiate to form the embryo. The trophoblast differentiates into two layers, the outer layer secretes enzymes to dissolve the endometrium of uterus. The inner layer grows out as finger like projections called chorionic villi into the uterine stroma. They are surrounded by the uterine tissue and maternal blood vessels. The chorionic villi and the uterine tissue become interdigitated to form the structural functional unit called placenta.
The placenta secretes hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens and progesterone that are necessary to maintain pregnancy.
Placenta also facilitates supply of oxygen and provides nutrients to the embryo through umbilical cord.
(i) Identify the parts labelled 1, 2 and 3 in the diagram given.
(ii) Draw a labelled diagram of a human blastocyst.
(iii) What is parturition and how is it induced at the end of pregnancy in a human female ?
Answer : (i) 1. Placental Villi
2. Umbilical cord (with its vessels)
(iii) Process of delivery of the foetus (at the end of pregnancy)
The signals for parturition originate from the fully developed foetus and placenta which induce mild uterine contractions / called foetal ejection reflex, this triggers release of oxytocin from the maternal pituitary which acts on uterine muscle and causes stronger uterine contraction which stimulates further secretion of oxytocin, the stimulatory reflex
between uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions leading to parturition.
Question. The following is the illustration of the sequence of ovarian events “a” to “i” in a human female :
(i) Identify the figure that illustrates corpus luteum and name the pituitary hormone that influences its formation.
(ii) Specify the endocrine function of corpus luteum. How does it influence the uterus ? Why is it essential?
(iii) What is the difference between “d” and “e” ?
(iv) Draw a neat labelled sketch of Graafian follicle.
Answer : (i) ‘g’ Luteinising hormone (LH).
(ii) Produces the hormone progesterone causes proliferation of the endometrium which gets highly vascularised. It is essential for the implantation of the fertilized ovum and to maintain the same during pregnancy.
(iii) “d” is the developing tertiary follicle.“e” is the Graafian follicle.
Question. ‘Parturition is induced by a complex Neuro endocrine mechanism’. Justify
Answer : Parturition is a process where vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus.
The signals for parturition originate from the fully developed fetus and the placenta which induce mild uterine contractions called foetal ejection reflex.
This triggers release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscles and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin.
The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions.
This leads to expulsion of the baby out of the uterus through the birth canal – parturition, after the infant is delivered; the placenta is also expelled out of the uterus.