How do Organisms Reproduce?
Reproduction is an integral feature of all living beings. The process by which a living being produces its own like is called reproduction.
Importance of Reproduction:
Reproduction is important for each species, because this is the only way for a living being to continue its lineage. Apart from being important for a particular individual, reproduction is also important for the whole ecosystem. Reproduction helps in maintaining a proper balance among various biotic constituents of the ecosystem. Moreover, reproduction also facilitates evolution because variations come through reproduction; over several generations.
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. No meiosis happens during asexual reproduction.
Sexual Reproduction:- When two parents are involved and gamete formation takes; the method is called sexual reproduction. Meiosis happens during gamete formation; which is an important step of sexual reproduction.
Reproduction in Simple Organisms
Multiple Fission:- When conditions become unfavourable, i.e. food, moisture, proper temperature, etc. are not available; this is the preferred mode of reproduction by unicellular organisms. The organism develops a thick coating around itself. This is called cyst. The cyst helps the organism to tide over the bad phase. The nucleus divides into several nuclei and each daughter nucleus is surrounded by a membrane. All metabolic activities stop in the organism, after cyst formation. When favourable conditions return, the cyst dissolves or breaks down; releasing the daughter nuclei. The daughter nuclei; in turn; grow into new individuals. Plasmodium and entamoeba undergo cyst stage, when they are not in the body of their prime host, i.e. humans.
Budding:- Yeast is an example of unicellular organism which reproduces by budding. Hydra is an example of multicellular organism which reproduces by this method.
- Budding in Yeast:- 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:- 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.
Fragmentation:- Reproduction by fragmentation is seen in filamentous algae, e.g. spirogyra. The filament of spirogyra divides into many pieces and each piece develops into a new individual.
Regeneration:- Some simple organisms can regenerate a new individual from a part which has been accidentally cut off. Planaria shows good example of reproduction by regeneration. If a planaria is cut into several pieces; each piece develops into a new individual.
Spore Formation:- Most of the fungi, bryophytes and pteridophytes reproduce by this method. Spores are produced in special spore-bearing organs; called sporangium. When spores mature; the sporangium bursts open to release them.
Advantages of Spore Formation: In fact, 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. Scientists consider spores are precursors of seeds.
Vegetative Propagation: Vegetative propagation is a special case, as it happens in higher plants; which otherwise have the capability to reproduce sexually. When a vegetative part of a flowering plant reproduces a new plant, it is called vegetative propagation. Some examples of vegetative propagation are given below.
Tuber of Potato: 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. Farmers utilize this capability of potatoes to grow potato more quickly; which is not possible by using the seeds of potato.
Modified roots of Carrot and Sweet Potato:- Carrot and sweet potato are examples of modifications of roots; for food storage. These roots can give rise to new plants; when kept under the soil.
Rhizomes of Ginger and Turmeric: Rhizomes are examples of modified underground stems for food storage. These contain nodes, internodes and scaly leaves. When buried under the soil, the rhizomes produce new plants.
Leaf of Bryophyllum:- Leaves of bryophyllum have notches on the margin. If a leaf is put under the soil, small saplings grow from the notches on the leaf margin.
Artificial Vegetative Propagation: Man has used artificial vegetative propagation to grow many plants. This has enabled farmers and horticulturists to grow many plants in shorter duration and has helped them to earn more profit. Artificial vegetative propagation has also helped in developing many new varieties of plants. Stem cutting, layering and grafting are the preferred means of artificial vegetative propagation.
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.
How do Organisms Reproduce?
Sexual Reproduction and Variations:-As discussed earlier, sexual reproduction involves two parents and gamete formation. Gametes are special cells which are formed after meiosis. There are two types of gametes, viz. male and female gametes. The number of chromosomes is haploid in the gametes. When gametes fuse during fertilization, the number of chromosomes becomes diploid. This is important for maintaining the unique identity of a particular species which reproduces by sexual method.
In sexual reproduction, the offspring gets sets of genes from two different individuals. This leads to subtle variation through subsequent generations. These variations accumulate over thousand of generations and finally may give rise to a new species. That is how all complex organisms have evolved from a common ancestor.
DNA Replication:DNA replication is the process by which DNA makes a copy of itself. DNA replication happens during the S – phase (synthesis phase) of the cell cycle. This is important because the daughter cells would need additional copies of the DNA. The process of DNA replication is a foolproof process, yet some alterations do take place. These alterations may lead to some variations in the characters of the daughter cells.
Sexual Reproduction in Flowering Plants:Flower is a modified leaf which bears special organs and plays the role of the reproductive system in plant.
Structure of a typical Flower:A typical flower is composed of four distinct whorls, viz. calyx, corolla, androecium and gynoecium.
Calyx:- The outermost whorl of the flower is called calyx. It is composed of green leaf-like structures; called sepals.
Corolla:- The second whorl of the flower is called corolla. It 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.
Androecium:- This is the third whorl in the flower. It is composed of stamens. Stamen is made of a slender stalk and anthers on top. Anthers produce the pollen-grains. Pollen grains are the male gametes.
Gynoecium:- This whorl is at the centre of the flower. It is composed of a swollen base; called ovary. A slender style stands upright on the ovary. It has a flat top; called stigma. Ovules are inside the ovary. Ovules are the female gametes.
Pollination:- The pollen grains need to be transferred to the stigma so that fertilization can take place. The transfer of pollen grains from anther to the stigma is called pollination. If the pollen grains from the same flower or the same plant are transferred to the stigma; it is called self 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.
Fertilization:- The fusion of male and female gametes is called fertilization. The product of fertilization is called zygote. Zygote undergoes several rounds of mitosis and develops into an embryo. Subsequently, the embryo develops into a new individual.
Fertilization in flowering plant:-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 calyx and corolla wither and fall off and so do the stamens. 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 kick-starts a new life.
How do Organisms Reproduce?
Reproduction in Human Beings
Male Reproductive System:
The male reproductive system in human beings is composed of following parts:-
- Testis:- 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. Apart from that, testis also produces testosterone. Testosterone is also called the male hormone, as it is responsible for developing certain secondary sexual characters in boys.
- 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:-
- Uterus:- 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.
- Fallopian Tubes:- 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.
- Ovary:- There are two ovaries; one near each fallopian tube. Ovary produces the eggs or the female gametes. All the eggs are produces 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.
- Vagina:- 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.
Sexual Dimorphism:- 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.
Changes in Boys during Puberty:- The boys suddenly grow in height dramatically. Voice becomes deep and the Adam’s apple becomes prominent. Shoulders become broad and body becomes muscular. Facial hairs begin to grow. Hairs also grow under the armpit and in the pubic region.
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.
Menstruation:- Menstruation is a trait which is unique to humans and some primates. 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. The whole sequence of events during an ovulation cycle is called menstrual cycle. The bleeding which occurs for few days is called menstruation. The first menstrual flow is called menarche and the last menstrual flow (which happens in the late 40s) is called menopause.
Human beings are different than other animals because they have the power of thinking. They have to obey certain moral values and need to behave sensibly in most of the aspects of life. Beginning of puberty does not mean that a person is psychologically ready for the process of reproduction. For a human being, reproduction involves more than just producing an offspring. As any act of sex has the potential of fertilization, so taking care of contraception becomes important. Moreover, the act of sex also has the potential of creating many sexually transmitted diseases. Examples of STDs are; gonorrhea, Herpes, syphilis, AIDS, Hepatitis B. AIDS and Hepatitis B are incurable till date. Even the curable STDs are potential dangerous; not only physically but also psychologically.