Human ReproductionClass 12 Biology NCERT Solutions
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Q1EXERCISES
Fill in the blanks:
(a)
Humans reproduce ______ (asexually/sexually)
(b)
Humans are ______ (oviparous, viviparous, ovoviviparous)
(c)
Fertilisation is ______ in humans (external/internal)
(d)
Male and female gametes are ______ (diploid/haploid)
(e) Zygote is ______ (diploid/haploid)
(f) The process of release of ovum from a mature follicle is called ______
(g) Ovulation is induced by a hormone called ______
(h) The fusion of male and female gametes is called ______
(i) Fertilisation takes place in ______
(j) Zygote divides to form ______ which is implanted in uterus.
(k) The structure which provides vascular connection between foetus and uterus is called ______
Solution
(a) Humans reproduce sexually.
(b) Humans are viviparous (give birth to live young).
(c) Fertilisation is internal in humans.
(d) Male and female gametes are haploid.
(e) Zygote is diploid.
(f) The process of release of ovum from a mature follicle is called ovulation.
(g) Ovulation is induced by a hormone called Luteinising Hormone (LH).
(h) The fusion of male and female gametes is called fertilisation.
(i) Fertilisation takes place in the ampullary region of the fallopian tube.
(j) Zygote divides to form a blastocyst, which is implanted in the uterus.
(k) The structure which provides vascular connection between foetus and uterus is called the placenta.
Q2EXERCISES
Draw a labelled diagram of male reproductive system.
Solution
To draw a labeled diagram of the male reproductive system, refer to Figure 2.1(a) (Diagrammatic sectional view of male pelvis showing reproductive system) and Figure 2.1(b) (Diagrammatic view of male reproductive system) in the textbook.
Key structures to be labeled include:
- Testis
- Scrotum
- Epididymis
- Vas deferens
- Seminal vesicle
- Prostate gland
- Bulbourethral gland
- Urethra
- Penis
- Glans penis
- Foreskin
- Urinary bladder
- Ejaculatory duct
Q3EXERCISES
Draw a labelled diagram of female reproductive system.
Solution
To draw a labeled diagram of the female reproductive system, refer to Figure 2.3(a) (Diagrammatic sectional view of female pelvis showing reproductive system) and Figure 2.3(b) (Diagrammatic sectional view of the female reproductive system) in the textbook.
Key structures to be labeled include:
- Ovary
- Oviduct (Fallopian tube) with its parts: Fimbriae, Infundibulum, Ampulla, Isthmus
- Uterus with its layers: Perimetrium, Myometrium, Endometrium
- Cervix
- Vagina
- External genitalia (Mons pubis, Labia majora, Labia minora, Clitoris)
Q4EXERCISES
Write two major functions each of testis and ovary.
Solution
Two major functions of the testis:
- Spermatogenesis: It is the primary male sex organ responsible for the production of male gametes, i.e., sperms.
- Hormone Production: The Leydig cells of the testes synthesise and secrete testicular hormones called androgens, with testosterone being the principal androgen. These hormones are responsible for the development of secondary sexual characters and regulation of spermatogenesis.
Two major functions of the ovary:
- Oogenesis: It is the primary female sex organ responsible for the production of the female gamete, i.e., ovum.
- Hormone Production: The ovary produces several steroid hormones (ovarian hormones) like estrogen and progesterone. These hormones regulate the menstrual cycle, support pregnancy, and are responsible for the development of female secondary sexual characters.
Q5EXERCISES
Describe the structure of a seminiferous tubule.
Solution
A seminiferous tubule is a highly coiled structure located within the testicular lobules of the testis. It is the site of sperm production.
The structure of a seminiferous tubule is as follows (refer to Figure 2.2 in the textbook):
- Lining: Each tubule is lined on its inside by two types of cells:
- Male germ cells (Spermatogonia): These are the immature germ cells that undergo meiotic divisions to eventually form sperms.
- Sertoli cells: These are large, supportive cells that provide shape to the tubule and nutrition to the developing germ cells. They also secrete factors that help in the process of spermiogenesis.
- Interstitial Spaces: The regions outside the seminiferous tubules are called interstitial spaces. These spaces contain:
- Leydig cells (Interstitial cells): These cells synthesise and secrete testicular hormones called androgens.
- Blood vessels: These supply blood to the testicular tissue.
- Immunologically competent cells: These are also present in the interstitial spaces.
Q6EXERCISES
What is spermatogenesis? Briefly describe the process of spermatogenesis.
Solution
Spermatogenesis is the process of production of sperms from immature male germ cells (spermatogonia) in the testes. This process begins at puberty.
The process of spermatogenesis involves the following stages:
- Multiplication Phase: The spermatogonia, present on the inside wall of seminiferous tubules, are diploid () and multiply by mitotic division to increase in number.
- Growth Phase: Some of the spermatogonia, called primary spermatocytes, periodically grow in size and prepare for meiosis.
- Maturation Phase:
- Each primary spermatocyte (diploid, 46 chromosomes) undergoes the first meiotic division (reduction division) to form two equal, haploid () cells called secondary spermatocytes (23 chromosomes each).
- Each secondary spermatocyte then undergoes the second meiotic division to produce two equal, haploid spermatids (). Therefore, one primary spermatocyte produces four haploid spermatids.
- Spermiogenesis: The non-motile spermatids are transformed into motile spermatozoa (sperms) through this process. The sperm head becomes embedded in the Sertoli cells, and finally, the sperms are released from the seminiferous tubules by a process called spermiation.
Q7EXERCISES
Name the hormones involved in regulation of spermatogenesis.
Solution
The hormones involved in the regulation of spermatogenesis are:
- Gonadotropin-releasing hormone (GnRH): Secreted by the hypothalamus, it stimulates the anterior pituitary gland.
- Luteinising Hormone (LH): Secreted by the anterior pituitary, it acts on the Leydig cells and stimulates the synthesis and secretion of androgens.
- Follicle-Stimulating Hormone (FSH): Secreted by the anterior pituitary, it acts on the Sertoli cells and stimulates the secretion of factors that help in the process of spermiogenesis.
- Androgens (e.g., Testosterone): Secreted by the Leydig cells, they stimulate the process of spermatogenesis.
Q8EXERCISES
Define spermiogenesis and spermiation.
Solution
Spermiogenesis: It is the process by which the haploid, non-motile spermatids are transformed into functional, motile spermatozoa (sperms). This involves structural changes like the formation of a head, middle piece, and tail.
Spermiation: It is the process by which mature spermatozoa (sperms) are finally released from the Sertoli cells into the lumen of the seminiferous tubules.
Q9EXERCISES
Draw a labelled diagram of sperm.
Solution
To draw a labeled diagram of a sperm, refer to Figure 2.6 (Structure of a sperm) in the textbook.
A sperm is a microscopic structure composed of four main parts. The key structures to be labeled are:
- Head: Contains the elongated haploid nucleus and is covered by a cap-like structure called the acrosome.
- Neck: A short region connecting the head and the middle piece.
- Middle Piece: Contains numerous mitochondria, which provide energy for the movement of the tail.
- Tail: A long, flagellar structure that facilitates sperm motility.
The whole body of the sperm is enveloped by a plasma membrane.
Q10EXERCISES
What are the major components of seminal plasma?
Solution
Seminal plasma is the fluid part of semen, which is a mixture of secretions from the male accessory glands. The major components of seminal plasma are:
- Fructose: Provides energy for sperm motility.
- Calcium: Plays a role in sperm activation.
- Certain enzymes: Have various functions, including liquefaction of the ejaculate.
These secretions are contributed by the paired seminal vesicles, the prostate gland, and the paired bulbourethral glands.
Q11EXERCISES
What are the major functions of male accessory ducts and glands?
Solution
Functions of Male Accessory Ducts:
The male accessory ducts include the rete testis, vasa efferentia, epididymis, and vas deferens. Their major functions are:
- Transport of Sperms: They form a pathway to transport sperms from the testes to the outside through the urethra.
- Storage of Sperms: The epididymis and vas deferens temporarily store the sperms.
- Maturation of Sperms: Secretions of the epididymis and vas deferens are essential for the maturation and motility of sperms.
Functions of Male Accessory Glands:
The male accessory glands include the seminal vesicles, prostate gland, and bulbourethral glands. Their major functions are:
- Formation of Seminal Plasma: They secrete seminal plasma, which mixes with sperms to form semen. This fluid medium facilitates sperm transport.
- Nourishment: Seminal plasma is rich in fructose, which provides energy and nutrition to the sperms.
- Lubrication: The secretions of the bulbourethral glands help in the lubrication of the penis during coitus.
Q12EXERCISES
What is oogenesis? Give a brief account of oogenesis.
Solution
Oogenesis is the process of formation of a mature female gamete (ovum). It is markedly different from spermatogenesis as it is initiated during the embryonic development stage in a female fetus.
A brief account of oogenesis is as follows:
- Fetal Stage: Oogenesis begins in the fetal ovary, where a couple of million gamete mother cells (oogonia) are formed. These cells start meiotic division and get arrested in Prophase-I. At this stage, they are called primary oocytes. No more oogonia are formed after birth.
- Childhood to Puberty: A large number of these primary oocytes degenerate. At puberty, only 60,000-80,000 primary follicles are left in each ovary. The primary oocyte is surrounded by granulosa cells, forming a primary follicle.
- After Puberty:
- The primary follicles develop into secondary and then tertiary follicles. The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. This is an unequal division, resulting in the formation of a large, haploid secondary oocyte and a tiny first polar body.
- The tertiary follicle matures into the Graafian follicle. The secondary oocyte is released from the ovary during ovulation.
- The second meiotic division of the secondary oocyte occurs only after the entry of a sperm during fertilization. This division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid).
Q13EXERCISES
Draw a labelled diagram of a section through ovary.
Solution
To draw a labeled diagram of a section through the ovary, refer to Figure 2.7 (Diagrammatic Section view of ovary) in the textbook.
Key structures to be labeled showing the different stages of follicular development are:
- Blood vessels
- Primary follicle
- Secondary follicle
- Tertiary follicle (showing antrum)
- Graafian follicle (mature follicle)
- Ovum (being released during ovulation)
- Corpus luteum (formed from the ruptured follicle)
- Corpus albicans (regressing corpus luteum)
Q14EXERCISES
Draw a labelled diagram of a Graafian follicle?
Solution
A Graafian follicle is the mature ovarian follicle. A labeled diagram should show the following parts (refer to Figure 2.7 in the textbook for a representation within the ovary):
- Theca externa: The outermost fibrous layer.
- Theca interna: The inner layer of cells that produce hormones.
- Granulosa cells: Cells lining the follicle and surrounding the oocyte.
- Antrum: A large, fluid-filled cavity.
- Secondary oocyte: The large haploid cell that will be ovulated.
- Zona pellucida: A glycoprotein layer surrounding the secondary oocyte.
- Corona radiata: A layer of granulosa cells immediately surrounding the zona pellucida.
Q15EXERCISES
Name the functions of the following:
(a)
Corpus luteum
(b)
Endometrium
(c)
Acrosome
(d)
Sperm tail
(e) Fimbriae
Solution
(a) Corpus luteum: It is formed from the remnants of the Graafian follicle after ovulation. Its primary function is to secrete large amounts of the hormone progesterone, which is essential for the maintenance of the endometrium, making it suitable for implantation and pregnancy.
(b) Endometrium: It is the inner glandular layer of the uterine wall. Its functions are:
* It undergoes cyclical changes during the menstrual cycle to prepare for pregnancy.
* It provides the site for the implantation of the blastocyst.
* It nourishes the developing embryo in the early stages of pregnancy.
(c) Acrosome: It is a cap-like structure covering the anterior portion of the sperm head. It is filled with enzymes (like hyaluronidase) that help the sperm to penetrate the zona pellucida and plasma membrane of the ovum during fertilization.
(d) Sperm tail: It is a long, flagellar structure that facilitates sperm motility. The whip-like movements of the tail propel the sperm through the female reproductive tract to reach the ovum for fertilization.
(e) Fimbriae: These are the finger-like projections at the edges of the infundibulum (the funnel-shaped part of the fallopian tube close to the ovary). Their function is to help in the collection of the ovum after it is released from the ovary during ovulation.
Q16EXERCISES
Identify True/False statements. Correct each false statement to make it true.
(a)
Androgens are produced by Sertoli cells. (True/False)
(b)
Spermatozoa get nutrition from Sertoli cells. (True/False)
(c)
Leydig cells are found in ovary. (True/False)
(d)
Leydig cells synthesise androgens. (True/False)
(e) Oogenesis takes place in corpus luteum. (True/False)
(f) Menstrual cycle ceases during pregnancy. (True/False)
(g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience. (True/False)
Solution
(a) False.
Correct statement: Androgens are produced by Leydig cells (or interstitial cells).
(b) True.
(c) False.
Correct statement: Leydig cells are found in the testis (in the interstitial spaces outside the seminiferous tubules).
(d) True.
(e) False.
Correct statement: Oogenesis takes place in the ovary, within the ovarian follicles.
(f) True.
(g) True.
Q17EXERCISES
What is menstrual cycle? Which hormones regulate menstrual cycle?
Solution
Menstrual Cycle: The reproductive cycle in female primates (e.g., monkeys, apes, and human beings) is called the menstrual cycle. It is a series of cyclical events that start at puberty (menarche) and repeat at an average interval of about 28/29 days. The cycle involves changes in the ovary and the uterus, culminating in either menstruation or the establishment of pregnancy. One ovum is typically released during the middle of each cycle.
Hormones that regulate the menstrual cycle:
The menstrual cycle is regulated by a complex interplay of hormones from the hypothalamus, pituitary gland, and ovaries.
- Hypothalamic Hormone:
- Gonadotropin-releasing hormone (GnRH): Stimulates the pituitary gland.
- Pituitary Hormones (Gonadotropins):
- Follicle-Stimulating Hormone (FSH): Stimulates the growth and development of ovarian follicles (follicular phase) and the secretion of estrogens.
- Luteinising Hormone (LH): A rapid secretion of LH (LH surge) around the 14th day induces ovulation. It also promotes the development of the corpus luteum.
- Ovarian Hormones:
- Estrogens: Secreted by the growing follicles, they cause the proliferation of the endometrium (uterine lining).
- Progesterone: Secreted by the corpus luteum, it is essential for the maintenance of the endometrium for implantation.
Q18EXERCISES
What is parturition? Which hormones are involved in induction of parturition?
Solution
Parturition: Parturition is the process of childbirth, which involves the expulsion or delivery of the fully developed foetus from the mother's uterus at the end of the gestation period (about 9 months in humans).
Parturition is induced by a complex neuroendocrine mechanism. The hormones involved in its induction are:
- Cortisol: Produced by the fetal adrenal gland, it is believed to play a role in initiating the process.
- Estrogens: The estrogen-progesterone ratio increases towards the end of pregnancy. Estrogens increase the sensitivity of the uterine muscles to oxytocin.
- Oxytocin: The signals for parturition originate from the fully developed foetus and the placenta, which induce mild uterine contractions (foetal ejection reflex). This triggers the release of oxytocin from the maternal posterior pituitary. Oxytocin acts on the uterine muscle, causing stronger and stronger contractions, which leads to the expulsion of the baby.
- Relaxin: Secreted by the ovary in the later phase of pregnancy, it helps to relax the pelvic ligaments and softens the cervix to facilitate childbirth, although its primary role in induction is less direct than oxytocin.
Q19EXERCISES
In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?
Solution
Blaming women for giving birth to daughters is scientifically incorrect and unjust. The sex of a baby is determined by the type of sex chromosome contributed by the male gamete (sperm) at the time of fertilization, not by the female gamete (ovum).
The chromosomal pattern for sex determination in humans is as follows:
- Females have two X chromosomes (XX). Therefore, all the haploid gametes (ova) they produce contain only the X sex chromosome.
- Males have one X and one Y chromosome (XY). They produce two types of sperms in equal proportions: 50% of sperms carry the X chromosome, and the other 50% carry the Y chromosome.
During fertilization:
- If a sperm carrying the X chromosome fertilizes the ovum (X), the resulting zygote will have the XX combination and will develop into a female baby.
- If a sperm carrying the Y chromosome fertilizes the ovum (X), the resulting zygote will have the XY combination and will develop into a male baby.
Therefore, the sex of the child depends entirely on whether the father's X-sperm or Y-sperm fertilizes the ovum. The mother has no genetic control over the sex of the child. It is the father who determines the sex of the baby.
Q20EXERCISES
How many eggs are released by a human ovary in a month? How many eggs do you think would have been released if the mother gave birth to identical twins? Would your answer change if the twins born were fraternal?
Solution
-
Eggs released in a month: Typically, a human ovary releases one egg (ovum) during each menstrual cycle, which occurs approximately once a month.
-
Identical (Monozygotic) Twins: If a mother gave birth to identical twins, only one egg would have been released. Identical twins are formed when a single fertilized egg (zygote) splits into two separate embryos early in development. Both embryos are genetically identical.
-
Fraternal (Dizygotic) Twins: Yes, the answer would change if the twins born were fraternal. Fraternal twins are formed when two separate eggs are released by the ovary (or ovaries) in the same cycle, and each egg is fertilized by a separate sperm. This results in two zygotes that develop simultaneously. Fraternal twins are genetically no more similar than regular siblings.
Q21EXERCISES
How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?
Solution
Dogs, unlike humans, are polyovulatory, meaning they typically release multiple eggs during each reproductive cycle. Each puppy in a litter develops from a separate fertilized egg.
Therefore, if a female dog gave birth to 6 puppies, it means that at least six eggs were released by her ovaries and each was successfully fertilized by a separate sperm.