Human Reproduction Class 12th Biology CBSE Solution

Class 12th Biology CBSE Solution

Exercises
Question 1.

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 _____________


Answer:

(a) sexually


Explanation: Sexual reproduction – is a process by which a new individual from two parents are formed.


(b) viviparous


Explanation: Viviparous – are those types of animals which reproduce, develop, nourishes and give birth to live young ones.


(c) internal


Explanation: Internal Fertilization - is defined as the union of an egg cell with a sperm during sexual reproduction which occurs inside the body of a parent.


(d) haploid


Explanation: Haploid gametes – human reproduce sexually which involves joining together of haploid gamete cells from each parent with half the normal number of chromosomes.


(e) diploid


Explanation: Diploid – human reproduces sexually, which involves joining of cells containing half the normal number of chromosomes from each parent to make a new cell containing exact number of chromosomes from both parents, these cells from each parent then combine to form the zygote which are also called diploid gametes.


(f) ovulation_


Explanation: Ovulation is a phase in women when the egg moves from the ovaries into fallopian tubes to attain fertilization stage.


(g) luteinizing hormone_


Explanation: Luteinizing hormone - is a hormone which is secreted by the anterior pituitary gland, whose function is to stimulate ovulation in females and synthesizes androgen in males.


(h) fertilization___


Explanation: Fertilization – is a process in which two gametes unite keeping the somatic chromosome number intact with the initiation of developing a new individual.


(i) fallopian tube_


Explanation: Fallopian tube – are tubes which carries the egg from ovary to uterus. They are two in numbers for each ovaries.


(j) blastocyst


Explanation: Blastocyst – during the early development of mammals astructure is formed called blatocyst, which possesses an inner cell mass (ICM) that forms the embryo.


(k) placenta


Explanation: Placenta – is an organ which connects mother and the fetus internally, it connets to the uterine wall to allow uptake of nutrients, thermo-regulation, waste elimination and gas exchange through mother’s blood, it also fights internal infection and produces hormones which supports the pregnancy. The unbillical cord develops from placenta.



Question 2.

Draw a labelled diagram of male reproductive system.


Answer:

Diagrammatic representation of male reproductive system:


It consist of Testis, scrotum, epididymis, Vas deferens or sperm duct, seminal vesicles, prostrate gland, penis.



Question 3.

Draw a labelled diagram of female reproductive system.


Answer:

Diagrammatic representation of female reproductive system:


The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region.



Question 4.

Write two major functions each of testis and ovary.


Answer:

Two major functions of testis are:

•Testis produces male gametes called spermatozoa by the process called spermatogenesis.


•Testosterone are the male sex hormones secreted by the seminiferous tubules of the leydig cells, this hormone is responsible for the development of secondary male characteristics.


Two functions of ovary are:


•Ovary produces the female gametes called ova by the process of oogenesis.


•Estrogens are the female sex hormones secreted by the growing Graffian follicles and this hormone is responsible for the development of secondary female characteristics.



Question 5.

Describe the structure of a seminiferous tubule.


Answer:

The seminiferous tubule is a highly coiled structure, located in testicle lobules, where the production of sperms occurs and the process is called as spermatogenesis. Each seminiferous tubule is lined by germinal epithelium, on the inner side it is lined by two types of cells called i) spermatogonia – male germ cells that produce primary spermatocytes by meiotic divisions forms secondary spermatocytes which again undergoes meiotic division to form spermatids, that metamorphoses into male gametes called spermatozoa. ii) Sertoli cells – are also called as nurse cells of the testis as they provide nourishment to the germ cells. They are large and polygonal also called as interstitial or leydig cells, these cells secrete the male hormone called testosterone.

Diagrammatic representation of a seminiferous tubule as below:




Question 6.

What is spermatogenesis? Briefly describe the process of spermatogenesis.


Answer:

The production of sperms from immature germ cells in males occurs by a process called spermatogenesis. The process occurs in testis (seminiferous tubule). In the process, a diploid spermatogonium (male germ cell) increases in size to form a diploid primary spermatocyte, which in its turn undergoes first meiotic division (reductional division ) to form two equal haploid secondary spermatocytes. Now each secondary spermatocytes undergoes second meiotic division to form two equal haploid spermatids, therefore, a diploid spermatogonium produces four haploid spermatids, these spermatids are transformed into spermatozoa (sperm). by the process of spermatogenesis.

Flow chart presentation of the process of spermatogenesis as below:




Question 7.

Name the hormones involved in regulation of spermatogenesis.


Answer:

The hormones involved in regulation of spermatogenesis are :

i) Follicle stimulating hormone(FSH) and ii) Luteinizing hormone (LT). Both the hormones are secreted by gonadotropin releasing hormones from the hypothalamus, they regulate the process of spermatogenesis. FSH acts on the sertoli cells (nurse cells) and LT acts on leydig cells to stimulate the process of soermatogenesis.



Question 8.

Define spermiogenesis and spermiation.


Answer:

Spermiogenesis - is a process in which spermatids are transformed into mature sperms/spermatozoa.

Spermiation – is a process in which the mature spermatozoa are released into the lumen of seminiferous tubules from the sertoli cells.



Question 9.

Draw a labelled diagram of sperm.


Answer:

Spermatozoon is a mature male gamete or reproductive cell. A sperm cell comprises of a round or cylindrical nucleated cell. a short neck, and a thin motile tail. The structure of sperm is very important for its motility and function in sexual reproduction. The nucleus contains half of the genetic information and fuses with the ovum (female gamete) to form a zygote. A sperm cell determines the sex of its offspring in mammals, if it bears Y chromosome, then male offspring will be the result and if X chromosome the female offspring.

Diagrammatic representation of a sperm:




Question 10.

What are the major components of seminal plasma?


Answer:

The semen produced in males are composed of i) sperms and ii) seminal plasma. The major components of the seminal plasma are mucus, spermatozoa and various secretions of accessory glands. it is also rich in fructose, calcium, ascorbic acid and certain enzymes. Its function is to provide nourishment and protection to sperms.



Question 11.

What are the major functions of male accessory ducts and glands?


Answer:

The male accessory ducts comprises of : i) vasa efferentia, ii) epididymis, iii) vas deferens and iv) rete testis. These ducts play an important role in the transport and temporary storage of sperms. Whereas, the male accessory glands comprises of : i) seminal vesicles, ii) prostrate glands and iii) bulbourethral glands. Their functions are to secrete fluids that lubricate the reproductive system and sperms. The sperms remain dispersed in the fluid and therefore transportation into the female body becomes smoother. The fluid is rich in ascorbic acid, fructose and certain enzymes. Hence they provide nutrients to sperms and keep them activated.



Question 12.

What is oogenesis? Give a brief account of oogenesis.


Answer:

The formation of ovum from oogonia in females takes place by a process called oogenesis, which occurs in the ovary. During the process a diploid oogonium or egg mother cells increases in size and transforms into a diploid primary oocyte, the later then undergoes meiosis I or reductional division to form two unequal haploid cells. The smaller cell is called “first polar body” and the larger cell is called “secondary oocyte”. The secondary oocyte undergoes meiosis II or equational division to form a secondary polar body and an ovum.

Flow chart presentation of oogenesis :




Question 13.

Draw a labelled diagram of a section through ovary.


Answer:

Diagrammatic representation of a section through ovary (ovum – egg ) - the female gonad.


Explanation : Ovary is the female gonad, either of the sex glands in the female in which ovaare produced. Ovaries are in pairs, oval organs about 3cm long, 2cm wide and 1 cm thick, situated each one on either side of the uterus, usually near the lateral pelvic wall adjacent to the anterior superior iliac spine. Their major functions are i) ovulation and ii) production of hormones = estrogen and progesterone.



Question 14.

Draw a labelled diagram of a Graafian follicle?


Answer:

A labelled diagrammatic representation of Graffian follicle.



Explanation : Graffian follicle – a small sac, embedded in the ovary which encloses the ovum.



Question 15.

Name the functions of the following:

(a) Corpus luteum (b) Endometrium

(c) Acrosome (d) Sperm tail

(e) Fimbriae


Answer:

a) Corpus luteum – is the outcome of ruptured Graffian follicle, it secretes progesterone hormone during menstrual cycle in luteal phase. Excessive secretion of progesterone prevents ovulation by inhibiting secretions of FSH and LT. it also allows endometrium to proliferate and prepare itself for implantation.


b) Endometrium – is the innermost wall/linig of the uterus which is rich in glands and it undergoes cyclic changes during the phases of menstrual cycle to prepare itself for implantation of the embryo.


c) Acrosome – is a cap-like structure which is present in the anterior part of the sperm, containing hyaluronidase enzyme, that hydrolyses the outer membrane of the egg, hence helping the sperm to penetrate the egg at the time of fertilization.


d) Sperm tail – is the longest region of the sperm that facilitates the movement of the sperm inside the female reproductive tract.


e) Fimbriae – are finger like projections present at the end of ovary in the fallopian tube, these helps in the collection of ovum by beating their cilia (after ovulation).



Question 16.

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)


Answer:

(a) Androgens are produced by Sertoli cells – False


Explanation – Leydig cells of seminiferous tubules present in the testis produces androgens.


(b) Spermatozoa get nutrition from Sertoli cells – True


Explanation – Sertoli cells provide nutrition to the germ cells (Spermatozoa)


(c) Leydig cells are found in ovary – False.


Explanation - Leydig cells are found in the seminiferous tubules of the testis.


(d) Leydig cells synthesise androgens – True


Explanation – Androgens are synthesized by Leydig cells.


(e) Oogenesis takes place in corpus luteum – False


Explanation – Oogenesis takes place in the ovary


(f) Menstrual cycle ceases during pregnancy – True


Explanation– As the uterine lining doesn’t shed because it has housed a growing embryo.


(g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience – True


Explanation– The cavity between labia minora is the vestibule, mostly occupied by the opening of the vagina – the vaginal introitus.


Question 17.

What is menstrual cycle? Which hormones regulate menstrual cycle?


Answer:

In primates, a cyclic physiological changes oocurs inside the female reproductive tract, termed as menstrual cycle. The whole cycle takes 28 days approximately to complete. The end of the cycle occurs with the breakdown of uterine endothelium wall that releases in the form of blood and mucus through the vagina and is termed as menses. Various hormones play their role in menstrual cycle namely follicle stimulating hormone (FSH), luteinizing hormone (LT), estrogen and progesterone. The level of FSH and LT increases during the follicular phase and are secreted by anterior pituitary gland. FSH stimulates the conversion of primary follicle into Graafian follicle under the influence of RH (releasing hormone) from the hypothalamus. Increase in LH level gradually leads to the growth of follicle and secretion of estrogen, then estrogen inhibits secretion of FSH but stimulates LH and also causes the thickening of uterine endometrium. High level of LH causes Graafian follicle to rupture and release ovum into the fallopian tube. This ruptured graafian follicle converts to corpus luteum and secretes progesterone hormone during the luteal phase. Maintenance and preparation of endometrium for the implantation of embryo is done by progesterone and increased level of this hormone in blood decreases the secretion of LH and FSH, thus inhibiting further ovulation.



Question 18.

What is parturition? Which hormones are involved in induction of parturition?


Answer:

The process of giving birth to a baby (child birth) when the development of foetus completes in the mother’s womb is called parturition. It is a neuroendocrine mechanism. Two major hormones involved in the process are:

i) oxytocin – which leads to contract the smooth muscles of myometrium of the uterus and directs the full term foetus towards birth canal. i.e. fetal ejection reflux.


ii) Relaxin hormone causes relaxation of the pelvic ligaments and prepares the uterus for child birth.



Question 19.

In our society the women are often blamed for giving birth to daughters.

Can you explain why this is not correct?


Answer:

Human being contain 23 pairs of chromosomes, human males having 22 pairs of autosomes and containone or two types of sex chromosome. i. e. either X or Y. Whereas human females have 22 pairs of autosomes and only one type of sex chromosome i.e, the X sex chromosome. Hence the sex of an individual is determined by the type of male gamete ( X or Y) fuses with female sex gamete (X). If the fertilizing sperm contains sex chromosome X then baby girl will be born and if the fertilizing sper contains Y sex chromosome the baby boy will be born. Hence it is wrong and incorrect to blame a woman for giving birth to daughters.



Question 20.

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?


Answer:

When two babies are born or produced in succession then it is called twins. Generally every month single egg is released at a time from each of the ovary. But if two eggs are released from each of the ovary at the same time and fertilized by separate sperms, gives rise to fraternal or non identical twins. Thus the young ones developed will have separate genes. On the other hand, when a single egg separates into early blastomeres from first zygotic cleavage, and the young ones produced will have same genetic makeup, hence called identical twins.



Question 21.

How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?


Answer:

Dogs and rodents are polyovulatory species i. e. produces more than one egg at a single time. In these species, during ovulation more than one ovum is released from their ovary, hence six eggs released from the female dog’s ovary produces six puppies.


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