Chapter 8 How do Organisms Reproduce

Banana, yeast and amoeba reproduce by asexually. Banana reproduce by vegetative propagation; yeast reproduces by budding and; Amoeba reproduces by binary fission.

In a flower, anther produces male gametes and ovary produces female gametes.

In flowering plants, the male and female gametes fuse and form zygote. This is called fertilisation. The zygote rapidly grows and develops into embryo and then seed.

The off springs formed by asexual reproduction method have greater similarity among themselves because in asexual reproduction, only one parent is involved without forming gametes.

The characteristics that are transmitted from parents to offspring are present in genes. Genes are present on chromosomes.

Characters that are transmitted from parents to offspring during sexual reproduction show both similarities and variations with parents.

Amoeba, yeast and Spirogyra reproduce by asexually. Amoeba reproduce by binary fission, yeast reproduces by budding and Spirogyra reproduces by fragmentation.

In spirogyra algae, the plant body breaks up into smaller fragments. Each fragment grows into a new individual.

The ability of a cell to divide into several cells during reproduction in Plasmodium is called multiple fission.

In flowering plants, the male and female gametes fuse to form zygote. The zygote rapidly grows and develops into embryo and then seed.

The gametes have half number of chromosomes as compared to that normal body cells. Reduction division (meiosis) takes place during gamete formation which halves the number of chromosomes in both male and female gametes. The original chromosome number (as in parent) is restored after fertilisation in sexual reproduction.

In Rhizopus fungus, the fine thread-like structures spread on the whole surface of slice of bread are called hyphae.

Vegetative propagation is the method of asexual reproduction in which new plants are produced from the vegetative parts of the plant like stems, roots and leaves.

The factors responsible for the rapid growth of bread mould on slices of bread are presence of large number of spores in air as well as presence of moisture and nutrients on the slice of bread.

The length of pollen tube depends on the distance between pollen grain on upper surface of stigma and ovule.

Flowers are reproductive part of the plants because they contain sexual reproductive organs. These organs produce gametes which take part in the process of fertilisation and form zygote which give rise to fruits.

Unisexual flowers have either stamen or pistil. They exhibit cross-pollination. Unisexual flowers which have only stamens cannot produce fruits.

During the sexual reproduction in flowering plants, male and female gametes fuse to form zygote. This process is known as fertilisation.

In the given picture, A represents the plumula, B represents the cotyledon and C represents the radicle.

In sexual reproduction, offspring has lot of variation because DNA of both individuals (male and female) get combine. Due to lot of variations, sexual reproduction allows species to change to more advanced forms from one generation to the next and speed up evolution.

Reproduction is one of the important characteristics of living things. The ability of organism to produce young ones of its own kind is called reproduction. It is essential for the continuity of species.

The human body undergoes several changes during adolescence. These changes mark the onset of puberty. The change which is associated with sexual maturation in boys is cracking of voice.

In human females, menstruation indicates the onset of reproductive phase.

The testes are located outside the abdominal cavity in the scrotum because the temperature of scrotum is less than the normal body temperature which is requires for sperm formation.

The function of testes at the stage of puberty are:

(a) To produce germ cells (sperms)

(b) To produce male sex hormone, testosterone.

Testes produces sperms or germ cells. The sperms transfer into epididymis from where vas deferens carries the sperms to another tube called urethra.

AIDS, Syphilis and gonorrhea are examples of sexually transmitted diseases. Hepatitis is not sexually transmitted disease.

Bisexual flower consists both stamens (male reproductive part) and carpel (female reproductive part). If in a bisexual flower stamens are removed artificially and carpel remains intact in the flower then, cross-pollination may occur in this flower which may lead to the formation of fruit.

We consider cell division as a type of reproduction in unicellular organism because it leads to the formation new cells.

The organisms which are produced asexually are genetically identical to the parent and are called clones. The basic event in the reproduction is the creation of DNA copy. The replication of DNA occurs by certain biochemical reactions which synthesize more of genetic material. When the DNA already present in the nucleus of the parent cell is replicated by making more DNA at the time of asexual reproduction then slight variations come in the two copies formed. Due to this the two DNA molecules formed will be similar but not identical.

Gamete formation is the first step in sexual reproduction. The gametes have half number of chromosomes as compared to that normal body cells. Reduction division (meiosis) takes place during gamete formation which halves the number of chromosomes in both male and female gametes. The original chromosome number (as in parent) is restored after fertilisation in sexual reproduction. This explains how, offspring and parents of organisms reproducing sexually have the same number of chromosomes.

Yeast cells fail to multiply in water because water does not provide any nutrition to yeast cells. Whereas in sugar solution, they multiply rapidly because sugar provides nutrition to carry out reproduction.

Moisture is necessary for the growth of bread mould. The bread mould grows profusely on moist slice of bread because it provides both moisture and nutrients for growth. The dry slice of bread provides nutrients but no moisture. So, bread mould does not grow on the dry slice of bread.

Two reasons for the appearance of variations among the progeny formed by sexual reproduction are mentioned below-

(i) The progeny formed from sexual reproduction involves two parents with different sets of characters.

(ii) The genetic material is exchanged between chromosomes before forming of zygote. Deoxyribonucleic acid (DNA) exchange in the chromosome. This results in forming of variation in the progeny.

Simple organisms like hydra and planaria are capable of producing new individual through the process of regeneration.

Number of chromosomes in somatic cells of some organisms:

(a) Number of chromosomes is not related to the size of an organisms. In the given table, we can see that even though Kingfisher is smaller in size than a dog, the number of chromosomes is more in Kingfisher.

(b) Ease of reproduction is not dependent on the number of chromosomes present. It depends on other factors like availability of water, nutrients, suitable mate and favorable environment.

(c) Chromosomes are made up of DNA. If chromosomes are more, this implies DNA will be more.

In tobacco plant, the female gamete have 24 chromosomes. The number of chromosome in the zygote is 48.

The transfer of pollen grains from the anther of a flower to the stigma of the same or another flower is known as pollination. It is done by insects, birds, wind and water. For the process of fertilization, it is necessary that the male gamete reaches the female gamete. It can possible through the process of pollination. Hence, fertilisation cannot take place if pollination does not occur.

Yes, the chromosome number of zygote, embryonal cells and adult of a particular organism is always constant.

The gametes have half number of chromosomes as compared to that normal body cells. Reduction division (meiosis) takes place during gamete formation which halves the number of chromosomes in both male and female gametes. The original chromosome number (as in parent) is restored after fertilisation in sexual reproduction. This process maintains the chromosome number and the amount of DNA in each generation.

After fertilization, zygote is located in the ovary.

Individual organisms of a species get some variations by the process of reproduction which makes them survive adverse environmental conditions (heat, cold etc.). Thus, variations during reproduction gives stability to a species.

General growth and sexual maturation are different from each other. During general growth, one attains physical maturity to become an adult. On the other hand, sexual maturity starts after physical maturity is achieved. During sexual maturity, the body is prepared for physical and psychological responsibilities related to reproduction.

The path of sperm during ejaculation is as follows:

Vas deferens → Seminal Vesicle → Urethra

Glands associated with the male reproductive system and their functions are as follows:

Seminal Vesicles: The secretion of seminal vesicle provides a fluid medium for the movement of sperms.

Prostate glands: The secretion of prostate gland produces a fluid which keeps sperms floating in it and provides nourishment.

In female, ovary produces one egg every month. The uterus also prepares itself to receive a fertilised egg. So, the wall of uterus becomes thick and soft containing lots of blood capillaries. If egg gets fertilised with sperm, then fertilised egg gets attached with uterus wall and gets nourishment from it. In case, egg is not fertilised, then the inner lining of uterus breaks down and comes out in the form of blood and mucus through the vagina. This cycle occurs every month and is called menstruation.

Once the zygote is implanted in the uterine wall, several changes take place in the uterus. Uterus prepares itself to receive and nurture the growing embryo. The uterine lining thickens and is richly supplied with blood to give support and nourishment to the growing embryo.

The benefits of using mechanical barriers during sexual act are as follows-

(i) Avoidance of unwanted pregnancy.

(ii) Prevention of sexually transmitted diseases.

1. Fallopian tube- Site of fertilisation.

2. Ovary- Production of eggs

3. Uterus – Site of implantation

4. Vagina – Entry of sperms

The ratio of chromosome number between and egg and its zygote is 1 : 2. Both sperm and egg contain half number of chromosome i.e., 23 chromosomes. Sperm is genetically different from the egg in the way that it contains either X or Y chromosome whereas an egg always contains an X chromosome.

Budding, fragmentation and regeneration are considered as asexual types of reproduction because in all these methods, only single parent takes part and forms new offspring without using gametes

The following figure shows regeneration in planaria:

The process of getting back a full organism from its body parts is called regeneration. Planaria and hydra are two organisms which can be regenerated fully from their body parts.

When the body of planaria is cut off into several parts, each part regenerates its complementary part to develop into a new individual.

Differences between asexual and sexual reproduction are:

In sexual reproduction, offspring has lot of variation because DNA of both individuals (male and female) get combine. Due to lot of variations, sexual reproduction allows species to change to more advanced forms from one generation to the next and speed up evolution.

Difference between pollination and fertilization:

Ovary is the site of fertilization and embryo is the product of fertilization.

The following figure shows fertilization in plant:

Role of gamete in sexual reproduction: Gamete formation results in halving of number of chromosomes which is necessary to maintain the number of chromosomes in an organism which reproduces sexually.

Role of zygote in sexual reproduction: Zygote formation makes the number of chromosome equal to the normal body cells. After fertilization, zygote divides several times to form an embryo and subsequently a new individual.

The gamete producing organs in the flower are: anther and ovary.

Placenta is a special tissue between developing foetus and uterine wall through which the foetus gets all the requirements from the mother's body. It is a disc-like structure which is embedded in the uterine wall. It contains villi on the embryo’s side of the tissue and blood spaces on the mother’s side. The blood spaces surround the villi. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo. The developing embryo will also generate waste substances which can be removed by transferring them into the mother’s blood through the placenta.

Contraception is the prevention of pregnancy by interfering with the normal process of ovulation, fertilisation and implantation.

There are 3 methods of contraception: (i) Barrier method (ii) Chemical method (iii) Surgical method.

1. Barrier method: In the barrier methods of preventing preganancy, the physical devices such as diaphragm (or cap) and condoms are used. Diaphragm (or Cap) is used by human females which is put over the cervix. Condoms are used by males.

2. Chemical method: In the chemical methods of preventing pregnancy, the females use two types of pills- oral pills and vaginal pills. The oral pills contain hormones which stop the ovaries from releasing ovum into the oviduct. The vaginal pills contain the chemical called spermicides which kill the sperms.

3. Surgical method: Vasectomy and tubectomy

(i) Vasectomy: It is a surgical method to block the gamete transfer. It is done in males. In this method, a small portion of the Vas deferens is removed by surgical operation and both the cut ends are ligated properly. This prevents the sperms from coming out.

(ii) Tubectomy – It is also a surgical method to block gamete transfer in females. In this method, a small portion of the fallopian tube is removed and the cut ends are ligated. This prevents the entry of ovum into the oviducts.

The fusion of male gamete (sperm) with the female gamete (ova) is known as fertilisation. The process of fertilisation takes place in the fallopian tube. As sperm enter into the vagina through the process of copulation; it moves upwards and enter into the oviduct. In the oviduct, ovum fuse with the sperm to form zygote. Numerous sperms reach the fallopian tube but only one sperm is required for fertilization.

The zygote (fertilised egg) get implanted in the inner lining of uterus and undergoes repeated division forming an embryo.

This process occurs once in a month in humans because ovary releases egg (ovulation) once every month.

Each species face many forces that reduce the number of individuals constantly. Some of them are namely struggle for survival, competition

For natural rescores, prediction, natural cycle of ageing and death, any natural calamity etc. All these natural forces reduce the number of

Individuals per species.

Reproduction is the process of production of own kind. It includes production of offspring having both similarity and variations among themselves and from presents. Further, the process of DNA replication and its inheritance to offspring ensure production of own kind only.

Therefore, reproduction not only restore the number of individuals removed from the species by above mentioned natural forces but also maintain heredity of genetic character and introduction of variations, as needed for continuity and stability of species. Without it all of the exciting species will diminish soon life will come to an end.

The diseases which are transmitted from infected person to another person by sexual contact are called Sexually Transmitted Diseases.

Examples: AIDS, Syphilis.

Some of the ways to prevent Sexually transmitted diseases are as follows:

Using condom during sexual intercourse.

Avoiding sexual contact with unknown person.

Maintaining personal hygiene.