Nutrition, respiration, excretion and transportation are some essential life processes for the survival of every individual. But reproduction is not necessary to maintain the life of an individual. And secondly, when an organism or individual going to create more individual a lot of energy will be spent in the process. Then a question is aroused in our mind …. Why do organisms (both animals and human) reproduce? Or what is the importance of reproduction?
Let us start with the definition of reproduction:
WHAT IS REPRODUCTION?
This is a process through which an individual give birth or create their offspring. The by which plants and animals rise to off spring.
“The production of new organisms from the existing organisms of the same species is known as reproduction.”
IMPORTANCE OF REPRODUCTION:
1. It is essential for the continuation of the species.
2. It ensures the continuation of similar kinds of individual generation after generation.
3. It creates variation in species.
4. New species facilitate evolution of organisms.
MODES OF REPRODUCTION IN HUMAN BEINGS:
There are two modes of reproduction in human beings:
1. Asexual reproduction
2. Sexual reproduction
1. ASEXUAL REPRODUCTION:
Asexual reproduction is a process in which a single cell participates. In this case the organisms do not make zygote; so the offspring is exactly same as their parents.
Asexual reproduction has following types:
As the name suggests the organism breaks into two or more than two parts by simple cell division (mitosis).
I. BINARY FISSION:
The organisms break into two parts. This type of division occurs in unicellular organisms.
Example: amoeba, paramecium and bacteria. Amoeba reproduces by binary fission by dividing its body into two parts. This happens in the favourable conditions. In the ffavourableconditions, when the amoeba cell has reached its maximum size of growth then first the nucleus of amoeba lengthens and divides into two parts. After dividing the nucleus, the cytoplasm of amoeba divides into two parts; one part around each nucleus. In this way, one parent amoeba divides to form two daughter amoebae.
IMPORTANT POINTS TO REMEMBER:
Both daughter amoebae are identically same with each other as well as with the parent amoeba.
II. MULTIPLE FISSION:
In the multiple fission, the parent organism divides or split to form many new organisms at the same time. It happens during unfavorable conditions.
For example: Malaria Parasite
As the unfavourable conditions came the malaria parasite enclose him in a protective wall which is known as a cyst. Inside the cyst, the nucleus of malaria parasite split into many tiny nuclei. These nuclei known as daughter nuclei. Little bits of cytoplasm collect around each daughter nuclei and thin membranes are formed around them. In this way many new daughter cells are formed from a single parent cell within the cyst. As the favourable conditions came, the cyst absorbs water and burst out. All the daughter cells from the cyst came out and start their life.
It is the method of asexual reproduction. This type of reproduction occurs in hydra and yeast.
HYDRA is a simple multicellular animal. In the favorable conditions, a small outgrowth is formal on the site of its body by the repeated mitotic division of its cells. This small outgrowth is known as bud. This bud then grows gradually to form a small hydra by developing a mouth and tentacles and finally, the tiny new hydra detaches itself from the body of parent hydra and lives as a separate organism.
Yeast is a tiny, unicellular non green plant. It is a fungus. A single parent yeast cell is a complete plant.
In yeast, first a bud appears on the outside of the cell wall. The nucleus of parent yeast cell then divides into two parts and one part of the nucleus moves into the bud. Ultimately, the bud separates off from the parent yeast cell and forms a new yeast plant (yeast cell).
IMPORTANT POINT TO REMEMBER:
The budding in yeast, however often takes place so fast that the first buds start forming their own buds and all of them remain attached to the yeast cells forming a chain of yeast cells. After sometimes, all the yeast cells of the chain separate from one another and form individual yeast plant.
It is the most common method of reproduction in fungi and bacteria.
During the growth of a fungus plant, a tiny round, bulb like structure develops at the top of the erect by hyphen. This structure is known as sporangium. The nucleus divide several times within the sporangium and each part of the nucleus along with a bit of cytoplasm develop into a spore. When the sporangium of the plant bursts, the spores spread into the air. When these airborne spores land on soil or food under favorable conditions (like damp and warm conditions), they germinate and produce new fungus plants.
The breaking up of a multicellular organism into two or more plants (on its own). Each of which grows to form a new organism is called the fragmentation.
The organisms like spirogyra and planarian can reproduce by this method.
IMPORTANT POINT TO REMEMBER:
Fragmentation is a special case of regeneration in which a multicellular organism on maturing breaks up naturally to produce two or more daughter organisms.
The planarian grows and matures. After maturing, the body of planarian breaks up into three parts on its own or by accidentally; each of which develops to form a full planarian worm.
The spirogyra which is an alga plant breaks up into two or more fragments on maturing and each fragment grows into a new spirogyra.
IMPORTANT POINT TO REMEMBER:
In fission, unicellular organisms break up to form two or more daughter organisms. Whereas in fragmentation, a multicellular organism breaks up to form two or more daughter organisms.
ADVANTAGES OF ASEXUAL REPRODUCTION:
i) The organism does not have to depend on other organisms for caring out reproduction because a single parent is needed.
ii) It takes less time than sexual reproduction and hence more number of offspring can be produced in shorter time.
iii) The offspring are exact clones of their parents.
DISADVANTAGES OF ASEXUAL REPRODUCTION:
i) As a single parent is involved, so there is negligible chance of variation.
ii) In the most of cases, in simple organisms, the parent generates ceases to exist after asexual reproduction.
iii) It cannot give risen to biodiversity which is important for a healthy ecosystem.
It is a method of asexual reproduction in some higher plants. In this method of reproduction, a new plant develops from the vegetative part of a plant such as a stem, a root, or a leaf without the help of any reproductive organs.
Vegetative production is done in two ways:
1) NATURAL WAY:
I) BY LEAF:
Leaves of phylum notches on the margin. If a leaf is put under the soil, the rhizomes produce new plants.
II) BY STEM:
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 potato can give rise to a new potato plant.
III) BY ROOTS:
Carrot and sweet potato are examples of modifications of roots for food storage. These roots can give rise to new plants when keep under the soil.
2) ARTIFICIAL WAY:
I) BY LAYERING:
In this method, one or more than one branches from the parent plants have buried in the moist soil. After few days, the parts of branches of the parent plant which are buried in soil grow their new one root. When this happens, the branches of the parent plant connecting the newly formed plants are cut off. So that the newly formed plants may grow on their own and develops into mature plants like the parent plant.
Jasmine, strawberry and raspberry are propagated by this method.
II) BY GRAFTING
Grafting is a method in which the cut stems of two different plants one with roots which is fixed in the soil is called stock and other without roots having stemmed (which may have leaves on it) is called scion.
In grafting method, the cut surfaces of the scion and the stock are fitted together and bound tightly with a piece of cloth and covered with polythene sheet(to protect from the infection of bacteria and the loss of water and plant sap from the cut of scion and stock). While joining the scion to the stock, care should be taken to make sure that the cambium of stock because cambium is responsible for the growth. After few days, it will grow into a mature tree.
Sometimes, scions from different varieties of plants are grafted on the same stock to obtain flowers and fruits having different desired characteristics.
Many varieties of mangoes, oranges, lemon, and grapes have been produced by grafting.
ADVANTAGES OF VEGETATION PROPAGATION:
i) The new plants produced will be exactly like the parent plant.
ii) Any desirable features of the parent plant will be replicated in the new plants.
2. SEXUAL REPRODUCTION:
Sexual reproduction is the generation of offspring from two parents. One is known as female parent and other is known as male parent. The male parent produces male gamete known as sperm and the female parent produces female gamete known as egg or ovum.
The union of male and female gamete is known as fertilization. Fertilization results in the formation of zygote. Zygote is a diploid structure.
The zygote develops into embryo and the embryo develops into a baby. This is known as sexual reproduction.
a) THE FEMALE GAMETE:
It is known as egg or ovum or oocyte. These are the one of the largest cells in the female body. They are round in shape and can be visible by naked eye. It has a large amount of cytoplasm (protoplasm and nucleus). The cytoplasm contains Mitochondria, golgi bodies, ribosomes.
The ovum is non mobile from outer. But the nucleus inside the cell is active and internally mobile. The mobility of nucleus is due to the cytoplasm. An ovum or egg cell has a limited life span and once they are released from the ovaries they can stay alive for about 12 to 24 hours. During this type if they fertilized by a sperm then the ovum converts into zygote. But if the ovum remains unfertilized they dissolve in the fallopian tube.
b) THE MALE GAMETE:
It is known as sperm. Sperm is one of the smallest cells in the male body. These are microscopic. They have a head and a tail. The head portion of the sperm contains a small nucleus with a small amount of cytoplasm. The sperm cytoplasm contains mitochondria.
The head portion of the sperm is slightly pointed and surrounded interiorly by an acrosome. The acrosome contains enzymes used for penetrating the female egg. The mid piece has a central filamentous core with many mitochondria. These mitochondria produce energy in the form of ATP for the journey of sperm through the female cervix, uterus and uterine tubes. The tail is also known as Flagellum. The tail executes the lashing movements that propel the spermatocyte.
The sperm cells are extremely active and use their tails to swim against stream of fluid in the oviduct. The sperm cells have a longer life span as compared to the egg cells. They can live 2 to 4 days in a woman’s womb and can also be stored by freezing them to a certain temperature.
DIFFERENCE BETWEEN OVAM AND SPERM:
1. It is round or oval in shape and the largest cell in the woman’s body.
2. It is produced in the ovary of a female human being.
3. Only a single cell is produced during once menstruation cycle.
4. Eggs require warm body temperature for sustenance.
5. Egg cells contain cytoplasm which allows being active internally. After the nucleus is released from the follicular cell. It becomes less mobile.
1. It consists of a head a mid-piece and a tail and it is the smallest cell in the men’s body.
2. It is produced in the testis of a male human being.
3. Millions of cells are released during a single ejaculation.
4. Requires 2-3 degrees lesser than the body temperature for sustenance.
5. Sperm cells are active and externally mobile. They use their tail to swim against the stream of fluid in the oviduct to the fallopian tube. It becomes less mobile.
REPRODUCTION IN FLOWERS
MAIN PARTS OF A FLOWER:
A flower is a most beautiful and fragrant part of the plant. It has following main parts:
The base of a flower to which all parts of a flower are attached is called receptacle.
Through receptacle a flower attach with a branch of the main plant.
The green leaf like parts in the outer most circle of a flower called sepals. All the sepals taken together are called calyx.
It protects the flower in its initial stages when it is in the form of a bud.
It is the colorful parts of a flower. The petals lie inside the sepals. All the petals taken together are called corolla.
i) It attracts insects for pollination.
ii) It protects the reproductive organs which are at the centre of the flower.
It is the male reproductive organ of the plant. The stamen is made up of two parts:
The stalk of stamen is known as filament.
The swollen top of stamen is known as anther. Anther makes the pollen grains and stores them. The pollen grains appear to be yellow powder like substance to us.
Pollen grains contain two male gametes. Each pollen grain contains two male gametes of the plant. These two male gametes are called male nuclei of pollen grain.
A flower usually has a number of stamens in it.
5. CARPEL (PISTIL):
In the centre of a flower, there is a flask shaped organ called carpel or pistil. It is the female reproductive organ of the plant. A carpel is made up of three parts.
PARTS OF CARPEL:
It is the little top part of the carpel. It is little sticky to receive the pollen grains from the anther to stamen.
It is a tube which connects stigma to the ovary.
It is a swollen part at the bottom of a carpel. It makes ovule and stores them. Ovules contain the female gametes. The female gamete which is present in the ovule is called ovum or egg.
IMPORTANT POINTS TO REMEMBER:
i) Some flowers are unisexual: like papaya, watermelon.
ii) Some flowers are bisexual: like hibiscus, mustard.
FORMATION OF SEEDS:
A new seed of the plant is formed when the male gamete present in the pollen grains unites with the female gamete present in the ovule.
THIS HAPPENS IN TWO STEPS:
It is the first step. The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination. It is done by insects, wind, air and water. Pollination occurs in two ways:
I) SELF POLLINATION:
When the pollen grain from the anther of a flower are transferred to the stigma of the same flower or another flower on the same plant. It is called self pollination.
II) CROSS POLLINATION:
When the pollen grain from the anther of a flower are transferred to the stigma of the another flower. It is called cross pollination.
IMPORTANT POINTS TO REMEMBER:
1) Cross pollination is better from the perspective of variants. Many agents help plants in cross pollination.
Examples: insects, animals, air water, wind, etc.
2) Insects are the main pollinators for the plant kingdom.
It is the second step. The fusion of male and female gametes is known as fertilization. The product of fertilization is zygote. The zygote forms the seed and ovary becomes the fruit. The ovary protects the seeds until they are mature and ready to grow into plants.
After entering one of the synergies, the pollen tube releases the two male gametes into the cytoplasm of the synergism. One of the male gametes moves towards the egg cell and fuses with its nucleus thus completing the syngamy. This results in the formation of the diploid cells the zygote.
The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid Primary Endosperm Nucleus (PEN). As this involves the fusion of three haploid nuclei it is termed as Triple Fusion.
Since two types of fusions, syngamy and triple fusion takes place in the embryo sac. The phenomenon is termed double fertilization an even unique to flowering plants.
The central cell after triple fusion becomes primary endosperm cell and develops into the endosperm while the zygote develops into an embryo.
IMPORTANT POINTS TO REMEMBER:
The yellowish powder that often sticks to our hands if we touch the stamen of a flower. Ovary contains ovules and each ovule has an egg cell. The male germ cell produced by the pollen grain fuses with the female gamete present in the ovule. The fusion of the germ cells gives us the zygote which is capable of growing into a new plant.
HOW FERTILIZATION OCCURS:
o We know, fertilization is the second step, first is pollination. Pollination means the transfer of pollens from the anther of one flower to the stigma of another flower. It may be self pollination or it may be cross pollination.
o The pollination is achieved by agents. E.g.: wind, water, insects, animals, etc.
o After the pollen lands on as suitable stigma, it has to reach the female germ cells which are in the ovary. For this a tube grows out of the pollen grain and travel through the style to reach the ovary.
o After fertilization, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is gradually converted into a seed. The ovary grows rapidly and ripens to form a fruit. Meanwhile, the petals, sepals, stamens, styles and stigma may shrivel and fall off.
o The seed contains the embryo which develops into a seeding under appropriate condition. This process is known as germination.
A female gametophyte corresponds to the embryo sac and covers an egg cell, central cell, two synergic cells and several antipodal cells and two polar cells.
Endosperm is a tissue produced in the seeds of the most flowering plants. It surrounds the embryo and provides nutrition in the form of starch. It can also contain oil and protein. This can make endosperm a source of nutrition in human diet.