CBSE NOTES CLASS 10 SCIENCE CHAPTER 8
Reproduction and Its Importance
Reproduction is an integral feature of all living beings. The process, by which living beings produce individuals that look like themselves, is called reproduction.
Reproduction is important for each species,
- Because this is the only way for a living being to continue its lineage and the survival of species is possible.
- Reproduction is also important for the whole ecosystem and helps in maintaining a proper balance among various biotic constituents of the ecosystem.
- Reproduction also facilitates evolution because variations come through reproduction; over several generations.
DNA Copying, Variation and Evolution
DNA contains the information or blueprint about body designs. The DNA in the cell nucleus is the information source for making proteins. If the information is changed, different proteins will be made. Different proteins will lead to altered body designs.
For organism to look similar, reproduction involves making copies of the blueprints of body design or DNA.
Cells use chemical reactions to build copies of their DNA. For supporting the DNA, organised cellular structure for maintaining life processes, is created and two independent cells are created.
No bio-chemical reaction is absolutely reliable.
Hence, although the DNA copies generated will be similar, but may not be identical to the original.
Some of these variations might be so drastic that the new DNA copy cannot work with the cellular apparatus it inherits. Such a newborn cell will simply die.
However there will be many other variations in the DNA copies that would not lead to such a drastic outcome hence they will survive.
Thus surviving cells are similar to, but subtly different from each other.
This inbuilt tendency for variation during reproduction is the basis for evolution.
The Importance of Consistency & Variation
The consistency of DNA copying during reproduction is important for the maintenance of body design features that allow the organism to use the particular environment or niche.
However, niches can change because of reasons beyond the control of the organisms.
If a population of reproducing organisms were suited to a particular niche and if the niche were drastically altered, the population could be wiped out. However, if some variations were present in a few individuals in these populations, there would be some chance for their survival.
For example, if there were a population of bacteria living in temperate waters, and if the water temperature were to be increased by global warming, most of these bacteria would die, but the few variants resistant to heat would survive and grow further. Variation is thus useful for the survival of species over time, even if it may not be good for individuals.
Types of Reproduction:
There are two main types, viz. asexual and sexual reproduction.
Asexual Reproduction: When a single parent is involved and no gamete formation takes place; the method is called asexual reproduction.
Sexual Reproduction: When two parents are involved and gamete formation takes place; the method is called sexual reproduction.
Reproduction in Simple Organisms
Most of the unicellular organisms reproduce by binary fission. The mother cell divides into two daughter cells; and each daughter cell begins its life like a new individual.
Many bacteria and protozoa simply split into two equal halves during cell division. The division can happen along any plane.
First the nucleus divides and then the supporting cell structure is created and the parent cell is divided in two.
Example - Amoeba
In case of some unicellular organisms which have more organized body structure, binary fission occurs in a definite orientation in relation to these structures.
Example - Leishmania (which cause kala-azar), which have a whip-like structure at one end of the cell.
When conditions become unfavourable some single-celled organisms, such as the malaria parasite, Plasmodium, divide into many daughter cells simultaneously using multiple fission.
Budding in Yeast
Yeast is an example of unicellular organism which reproduces by budding. A small bud grows at any end of the yeast cell. Nucleus gets elongated and a part of it protrudes into the bud. The nucleus then divides into two nuclei. One of the nuclei goes into the bud. The bud grows to certain extent and gets detached from the mother cell.
Budding in Hydra
Hydra is an example of multicellular organism which reproduces by budding. A bud grows anywhere on the main body of hydra. The bud grows to a certain size and gets detached from the mother hydra. This develops further to grow into a new individual.
Simply breaking up of parent organism into smaller pieces upon maturation is called fragmentation. These pieces or fragments grow into new individuals. The spirogyra divides into many pieces or fragments and each piece develops into a new individual.
Fragmentation does not work for multicellular organism with specialized, tissues, organs, systems etc., as these have definite positions in the body. In such a carefully organised situation, cell-by-cell division would be impractical.
Multicellular organisms with specialized tissues, therefore, need to use more complex ways of reproduction. Reproduction in such organisms is the function of a specific cell type that is capable of growing, proliferating and making other cell types under the right circumstances.
Development of new individual from parts of bodies cut off from the parent body in some simple organism is called regeneration.
Examples - Hydra and Planaria.
Regeneration is carried out by specialised cells. These cells proliferate and make large numbers of cells which undergo changes to become various cell types and tissues.
The organized sequence of change leading to formation of different types of cells and tissues from a single cell type is called development.
- Regeneration cannot be called reproduction in true sense, since most organisms would not normally depend on being cut up to be able to reproduce.
This is a mode of reproduction; resembling multiple fission; in which spores are involved. Spores are produced in special spore bearing organs called sporangium or round-shaped blob-on-stick type of structures. When spores mature; the sporangium bursts open to release them.
The spores are covered by thick walls that protect them until they come into contact with moist surface where they can grow.
Example - Bread mould (Rhizopus)
The thread-like structures which develop on the bread are the hyphae of the bread mould on which sporangia are supported.
Most of the fungi, bryophytes and pteridophytes reproduce by this method.
Advantages of Spore Formation
Spores give certain survival benefits to the organisms which reproduce by spores. Spores can be disseminated through air and water or even through some other carriers; like animals. This helps an organism to spread its presence to a wider geographical area. Spores can also remain dormant for a long time, till favourable conditions are found.
This is a mode of reproduction in which parts of some plants like the root, stem and leaves develop into new plants under appropriate conditions. This happens only in plants.
Vegetative Propagation by Cutting
A cutting is separated portion of root, stem or leaf, which is put into the soil. Sugar cane, Roses, Citrus fruits, Grapes, Cocoa, Bougainvillea etc are largely and rapidly propagated by stem cuttings.
Vegetative Propagation by Layering
This is a method of propagating a plant in which a shoot is fastened down to form roots while still attached to the parent plant.
Vegetative Propagation by Grafting
This is a technique whereby tissues of two plants are joined so as to continue their growth together.
The upper part of the combined plant is called the scion while the lower part is called the rootstock.
Vegetative propagation by roots
The roots of some plants develop buds on them e.g., Dalbergia sissoo (Shisham), Guava. Some tuberous roots besides possessing buds also contain sufficient quantities of, food, e.g., Dahlia and Sweet potato, Carrot etc.
Vegetative propagation by stems
The potato tuber is a modified stem. Many notches can be seen on the surface of potato. These are called ‘eyes’ of potato. Each ‘eye’ of a potato can give rise to a new potato plant. Other examples are, Banana, Ginger, Turmeric, Colocasia (Kachalu), Garlic and Onion.
Doob grass and Mint form runners and grows roots at the buds. If the stem is cut off, the new part with roots forms a new plant.
Vegetative propagation by leaf
Some plants produce buds on their leaves e.g., Bryophyllum, Begonia, Streptocarpus, Kalanchoe and Saintpaulia. In Bryophyllum. These buds usually remain dormant, when the leaf is attached with plant. The leaves when in contact with moist soil develop new plantlets along the margins. However in some species of Bryophyllum plantlets develop along the margins of intact leaves.
Advantages of Vegetative Propagation
- Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds.
- Vegetative propagation also makes possible the propagation of plants such as banana, orange, rose and jasmine that have lost the capacity to produce seeds.
- All plants produced are genetically similar enough to the parent plant to have all its characteristics.
Advantages of Asexual Reproduction
- The organism does not have to depend on another organism for carrying out reproduction; because a single parent is needed.
- It takes less time than sexual reproduction and hence more number of offspring can be produced in shorter time.
- The offspring are exact clones of their parent.
- Desirable characteristics can be easily incorporated into plants with artificial vegetative propagation.
Disadvantages of Asexual Reproduction
- As a single parent is involved, so there is negligible chance of variation.
- In most of the cases in simple organisms, the parent generation ceases to exist after asexual reproduction.
- Asexual reproduction cannot give rise to biodiversity which is important for a healthy ecosystem.
Advantages of sexual reproduction
- Since two parents are involved in sexual reproduction, more variations are produced. Thus, it helps in evolution and ensures survival of species in a population.
- The new formed individual has characteristics of both the parents.
In tissue culture, first tissue or cells are removed from the growing tip of a plant and placed in an artificial medium where they divide rapidly to form a small group of cells or callus. The callus is transferred to another medium containing hormones for growth and differentiation, which grows to a new plantlet. The plantlets are then placed in the soil so that they can grow into mature plants. Using tissue culture, many plants can be grown from one parent in disease-free conditions. This technique is commonly used for ornamental plants.
Sexual reproduction involves two parents and gamete formation. Gametes are special cells which are formed after meiosis. They only have single chromosome of each type.
There are two types of gametes, viz. male and female gametes.
When gametes fuse during fertilization, the number of chromosomes becomes diploid or form pairs again.
Why Sexual reproduction offers greater scope for variation than asexual reproduction?
In case of asexual reproduction only one parent is involved. Hence the variation can happen only because of DNA copying errors. However in case of sexual reproduction, two parents are involved, who have different characteristics, accumulated over generations. Since all of these variations are in living individuals, it is assured that they do not have any really bad effects.
Combining variations from two or more individuals would thus create new combinations of variants and may give rise to new species in the long run.
Sexual Reproduction in Flowering Plants
Flower is a modified leaf which bears special organs and plays the role of the reproductive system in plant. The flower may be unisexual (papaya, watermelon) when it contains either stamens or carpels or bisexual (Hibiscus, mustard) when it contains both stamens and carpels.
Structure of a Typical Flower
A typical flower is composed of four distinct parts,
Calyx: The outermost part of the flower is called calyx. It is composed of green leaf-like structures; called sepals.
Corolla: This part of the flower is composed of colourful leaf-like structures; called petals.
Petals are colourful so that insects and birds can be attracted; to assist the flower in pollination.
Stamen: Stamen is the male part of the flower. It is made of a slender stalk called filament and anthers on top. Anthers produce the pollen grains which are male gametes.
Pistil or carpel: This is the female part of the flower. It is composed of an ovary (swollen base), a style (the thin long tube like structure) standing upright on the ovary and stigma (the flat top).
Ovules or eggs or female gametes are inside the ovary.
The transfer of pollen grains from anther to the stigma is called pollination.
Self Pollination: If the pollen grains from the same flower or the same plant are transferred to the stigma; it is called self pollination.
Cross Pollination: If pollen grains from a different plant are transferred to the stigma; it is called cross pollination.
Cross pollination is better; from the perspective of variations. Many agents help plants in cross pollination, e.g. insects, animals, air, water, etc. Insects are the main pollinators for the plant kingdom.
The fusion of male and female gametes is called fertilization. The product of fertilization is called zygote. Zygote undergoes several rounds of mitosis (cell division) and develops into an embryo. Subsequently, the embryo develops into a new individual.
Fertilization in flowering plants
After landing at the stigma, pollen grains absorb moisture and germinate. A pollen grain develops a pollen tube; which penetrates through the tissue of the style and reaches the ovule. Pollen nuclei are transferred through the pollen tube. After fertilization, zygote is formed; which finally develops into the embryo.
Changes in flower after fertilization
The outer parts, namely sepals and petals wither. The male reproductive part stamens also fall off.
The ovary turns into the fruit. The embryo turns into seed. Once the seed becomes mature, fruit dries up so that dispersal of seeds can take place.
Structure of Seed
A seed contains an embryo, some reserve food and is enclosed by a protective covering; called seed coat. The reserve food is stored in the cotyledons. The embryo has two pointed parts. The upper part is called plumule which gives rise to the shoot system. The lower part is called radicle which gives rise to the root system. Cotyledons supply food when the embryo needs it during germination.
Seed germination is the process by which the embryo in the seed develops into a seedling under appropriate conditions.
Seeds are of two types – dicots with two cotyledons (gram, rajma etc) and monocots with one cotyledon (corn, wheat, rice)
Advantages of seed formation
- The seed contains the future plant or embryo which develops into a seedling
- Seeds can get dispersed by wind or animals into new areas, thereby allowing the species to colonize new areas and also protects the species from becoming extinct.
- The seed is protected and can survive unfavourable conditions for a period of time before growing
REPRODUCTION IN HUMAN BEINGS
Male Reproductive System
The male reproductive system in human beings is composed of following parts:
Testes: There is a pair of testes; which lie in a skin pouch; called scrotum.
Scrotum is suspended outside the body; below the abdominal cavity. This helps in maintaining the temperature of testes below the body temperature. This is necessary for optimum sperm production.
Testis primarily serves the function of sperm production.
Sperms are the male gametes.
Testes also produce testosterone. Testosterone is also called the male hormone, as it is responsible for developing certain secondary sexual characters in boys.
Semen is the fluid containing sperms as well as other material like food. Semen is produced and originates from the seminal vesicle, which is located in the pelvis.
Sperms being in a fluid makes their transport easier and this fluid also provides nutrition.
The sperms are tiny bodies that consist of mainly genetic material and a long tail that helps them to move towards the female germ-cell.
Vas Deferens: Vas deferens is the tube which carries sperms to the seminal vesicle.
Seminal Vesicle: This is the place where sperms are stored. Secretions from the seminal vesicle and prostate gland add up to make the semen.
Penis: It is a muscular organ which serves the genitor-urinary functions. The urethra works as the common passage for urine as well as for sperms.
Female Reproductive System
The female reproductive system in human beings is composed of following parts:
Ovary produces the eggs or the female gametes. There are two ovaries one near each fallopian tube. All the eggs are produced by the ovary when the female child is still in the womb. One egg matures in each ovulation cycle and is released from the ovary. The egg is caught by the flimbriae and transferred to the fallopian tube.
One fallopian tube comes out from each side at the top of the uterus. The fallopian tubes end in finger-like structures; called flimbriae. Fertilization happens in the fallopian tube.
This is pear-shaped hollow muscular organ. Uterus is the place where the embryo gets implanted and develops into a newborn baby. The wall of the uterus provides safety and nutrition to the growing foetus.
The cervix (mouth of the uterus) opens into the vagina. Vagina is a muscular tube-like organs; which serves as the passage for the sperms and also as the canal during the child birth.
This is a disc like structure which is embedded in the uterine wall of mother through which the embryo gets nutrition from the mother’s blood with the help of a special tissue called Placenta. It contains villi on the embryo’s side of the tissue. On the mother’s side are blood spaces, which surround the villi which provide 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.
The development of the child inside the mother’s body takes approximately nine months. The child is born as a result of rhythmic contractions of the muscles in the uterus.
The distinct phase in the life of human beings which marks the onset and attainment of sexual maturity, is called puberty. It usually starts between 10 and 14 years for girls and 12 and 16 years for boys. It usually ends at around 18th year of age in girls and at around 19th year of age in boys. Since the years during puberty end in ‘teens’; hence this phase is also called teenage.
Changes in Boys during Puberty
- Broadening of chest and shoulders
- Deepening of voice
- Appearance of facial hair, body hair and pubic hair
- Increase in size of genitals
- Growth gets faster.
Changes in Girls during Puberty
- The voice becomes thin.
- Shoulders and hip become rounded.
- Breasts get enlarged.
- Hairs grow under the armpit and in the pubic region.
- Onset of menstruation
The physical dissimilarities in the male and female of a species which give them different appearances, is called sexual dimorphism.
Secondary Sexual Characters
Features which highlight sexual dimorphism are called secondary sexual characters.
Menstruation is a process in which a sexually mature girl's or a woman's body releases a small amount of blood and tissue through her vagina. During each ovulation cycle, the uterus prepares itself in anticipation of a possible pregnancy. The uterine wall develops an additional lining. When the egg is not fertilized, it gets disintegrated and so does the additional lining in the uterine wall. The fragments of disintegrated tissues are shed; along with blood. This is observed in the form of bleeding through the vagina which can last from 3 to 7 days. This bleeding which occurs for few days is called menstruation.
The whole sequence of events during an ovulation cycle is called menstrual cycle.
Menarche: The first menstrual flow in girls is called menarche.
Menopause: The last menstrual flow in women (which happens in the late 40s) is called menopause.
What could be the reasons for avoiding pregnancy?
Pregnancy will make major demands on the body and the mind of the woman, and if she is not ready for it, her health will be adversely affected. Also there are economic and social responsibilities like expenses for medical conditions and raising of children, attached to pregnancy.
Methods of avoiding unwanted preganancy
The methods to avoid pregnancy are called contraceptive methods and fall in a number of categories.
- Natural method: It involves avoiding the chances of meeting of sperms and ovum. In this method, the sexual act is avoided from day 10th to 17th of the menstrual cycle because during this period, ovulation is expected and therefore, the chances of fertilization are very high.
- Creation of a mechanical barrier so that sperm does not reach the egg. Condoms etc.
- Changing the hormonal balance of the body so that eggs are not released and fertilization cannot occur. These drugs commonly need to be taken orally as pills.
- Since the pills change hormonal balances, they can cause side-effects.
- Insertion devices: Contraceptive devices such as the loop or the copper-T are placed in the uterus to prevent pregnancy.
- They can cause side effects due to irritation of the uterus.
- Surgical methods:
- While surgical methods are safe in the long run, surgery itself can cause infections and other problems if not performed properly.
Vasectomy - If the vas deferens in the male is blocked, sperm transfer will be prevented.
Tubectomy - If the fallopian tube in the female is blocked, the egg will not be able to reach the uterus.
Removal of unwanted pregnancy (MTP – Medical Termination of Pregnancy)
Surgery can also be used for removal of unwanted pregnancies. These may be misused by people who do not want a particular child, as happens in illegal sex-selective abortion of female foetuses, called female foeticide. For a healthy society, the female-male sex ratio must be maintained. Because of reckless female foeticides, girl child sex ratio is declining at an alarming rate in some sections of our society, although pre-natal sex determination has been prohibited by law.
Diseases can be transmitted from person to person during sexual act, since it is a very intimate connection of bodies. These are called STDs (sexually transmitted disease).
Bacterial infections - gonorrhoea and syphilis
Viral infections - warts and HIV-AIDS
It is possible to prevent the transmission of such diseases to some extent, during the sexual act by using a covering called condom. One should also keep the genitals clean.