The Life Cycle – Maharashtra Board Class 10 Solutions for Science and Technology (English Medium)
- The two main methods of reproduction are asexual reproduction and sexual reproduction.
- The functional unit in a plant’s sexual reproduction is flower.
- The male reproductive part of a flower is stamen.
- When the transfer of pollen from an anther to the stigma occurs in the same flower, the process is called self-pollination.
- Variations give rise to variety and diversity.
- Hydra uses regenerative cells for reproduction in the process of budding.
- Yeast reproduces by budding.
- During unfavourable conditions multiple type of fission is seen in Amoeba.
- A basic process in reproduction is the creation of a DNA copy.
- Reproduction is necessary to maintain the number of individuals of a species.
- Amoeba : Simple binary fission : : Paramoecium : Transverse binary fission.
- Planaria : Regeneration : : Rhizopus :: Spore formation.
- Root, stem, leaf : Vegetative propagation : : Flower : Sexual reproduction.
- Asexual reproduction : Similarity : : Sexual reproduction: Variety and diversity.
- Binary fission in Amoeba
- Regeneration in Hydra
- Vegetative propagation
Reproduction is the process in which living organisms produce new individuals of the same species from the existing individuals. Reproduction is necessary to maintain the number of individuals of a species and to prevent their extinction. There are two modes of reproduction – asexual reproduction and sexual reproduction.
When new plants are produced from the vegetative parts of the plant such as roots, stem, leaves and buds, it is called vegetative propagation. Example: New plants of potato are produced from the buds (eyes) on the potato.
All plants produced by vegetative propagation are similar to their parent as they are produced from a single parent. Plants produced by vegetative propagation grow and bear flowers and fruits faster than those produced from seeds.
The process of transfer of pollen grains from the anther to the stigma of a flower is called pollination. There are two types of pollination – self-pollination and cross-pollination.
Functions of vagina:
- The vagina provides the route for menstrual blood to leave the body during menstruation.
- It provides a pathway for the sperms at the time of fertilisation.
- It provides a pathway for the baby to come out of the mother’s body at the time of childbirth.
Functions of stigma:
- The stigma is the receptive organ of a carpel which is the female reproductive part of a flower. It receives the pollen grains during pollination.
- The pollen grains stick to the stigma and germinate there.
Functions of ovaries:
- Ovaries develop and release eggs into the oviduct; these eggs are immature in the female’s body since puberty.
- The eggs are released from the ovaries by the process of ovulation.
- The ovaries secrete a hormone oestrogen which brings about changes in girls during puberty.
Functions of the seminal vesicle and prostate glands:
- The seminal vesicle and prostate glands produce ejaculatory fluid.
- This fluid helps in the transport of sperms.
- It also provides nutrition to the sperms.
Functions of the uterus:
- The uterus is a muscular organ with the ability to expand and contract. So, it can accommodate a growing foetus inside it during pregnancy.
- During labour, the baby is pushed out of the mother’s womb because of the forceful contractions of the uterus.
Advantages of sexual reproduction over asexual reproduction:
- In sexual reproduction, two parents are involved in reproduction. So, there is genetic mixing and variations in the offspring produced by sexual reproduction.
- The processes of fertilisation and meiosis take place during sexual reproduction and result in more variations.
- Variations give rise to variety and diversity. They enable individuals to adapt and survive in the changing environment in a better way, thereby preventing their extinction.
A mature woman releases one egg per month during her menstrual cycle. This egg passes through the oviduct to the uterus. On its way, if it gets fertilised, then it forms a foetus. If the egg does not get fertilised, it is thrown out of the body through the vagina along with blood and mucous in the form of a flow. This discharge is called menstruation which lasts for three to five days.
- Sexual reproduction takes place only in flowering plants as the flower is the functional unit of sexual reproduction in plants.
- In a flower, the ovules present inside the ovary produce the female germ cell or the egg and the anthers contain pollen grains with male gametes.
- Sexual reproduction involves three processes – pollination, fertilisation and germination.
- During the process of pollination, the pollen grains are transferred from the anther to the stigma of the flower.
- After pollination, a pollen grain germinates and a pollen tube grows from it, which travels through the style to reach the ovary.
6. Each pollen tube contains two male gametes, one gamete fuses with the egg cell to form the zygote and the other fuses with the secondary nucleus in the embryo sac to form the endosperm. This process is called double fertilisation.
7. The zygote develops into an embryo, and the endosperm serves as a nutritive tissue for the growing embryo. This embryo later grows into a new plant. The ovule develops into a seed, and the ovary develops into a fruit.
8. After maturity, the seed undergoes germination and gives rise to a new plant.
In unicellular organisms, reproduction takes place asexually by binary fission, multiple fission and budding.
1. Binary fission:
- This method is employed mostly by prokaryotes, some protozoa and some organelles within eukaryotic cells.
- Binary fission is the division of the parent cell.
- When the organism matures, it grows in size. Its nucleus duplicates. The cytoplasm divides in the middle.
- A single parent organism gives rise to two identical daughter organisms.
- Examples: Amoeba, Paramoecium
2. Multiple fission:
- During unfavourable conditions, the amoeba withdraws its pseudopodia and forms a cyst.
- Inside the cyst, the nucleus divides into many nuclei and gives rise to several daughter cells.
- The cyst bursts to release the daughter cells during unfavourable conditions.
- Examples: Amoeba, Plasmodium
- A small outgrowth called bud arises on the parent body.
- The bud grows, the nucleus of the parental cell divides and one daughter nucleus migrates into the bud.
- The bud increases in size, breaks off from the parent body and develops into a new individual.
- Example: Yeas
Large family size generally affects individual life as well as community life.
Disadvantages of large family size:
- Economic pressure
- Mother’s poor health
- Poor housing
- Children neglected at home
- Lack of better educational facilities
- Insufficient medical care
- The process of transfer of pollen grains from the anther to the stigma of a flower is called pollination.
- Pollination is of two types – self-pollination and cross-pollination.
- When pollen grains are transferred from the anther to the stigma of the same flower or another flower of the same plant, it is called self-pollination.
4. When pollen grains are transferred from the anther to the stigma of the flower of another plant of the same species, it is called cross-pollination.
5. Pollination occurs with the help of external agents. The agents of pollination are wind, water, animals and insects.
- Regeneration is the ability of an organism to reconstruct its entire body from isolated body cells.
- The capacity to regenerate is seen in some animals such as Planaria and Hydra.
- In such animals, regeneration is carried out by specialised cells. These cells proliferate and make large number of cells which later develop into various cell types and tissues. This gives rise to new organisms.
- Regeneration occurs only if the animal body gets cut into pieces randomly. Hence, it is a kind of repairing process and not a reproductive process.
Germination of seeds
- The growth of a seed into a seedling is called germination.
- Germination begins soon after the process of fertilisation.
- The zygote formed divides several times to form an embryo within the ovule.
- The ovule develops into a seed and the ovary develops into a fruit.
- The radicle comes out through the micropyle and forms the root. Meanwhile, the plumule comes out and forms the shoot.
- Multiple fission is a type of asexual reproduction seen in Amoeba.
- During unfavourable conditions, Amoeba withdraws its pseudopodia and becomes round. It secretes a hard covering around it called cyst.
- The nucleus divides repeatedly into many nuclei inside the cyst. This is followed by the division of the cytoplasm to form many daughter cells.
- When conditions are favourable, the cyst bursts and daughter cells are released.
Importance of variation
- Variations occur because of sexual reproduction.
- As there is fusion of two different germ cells, the new offspring produced have characteristics which are different from their parents.
- Variations give rise to variety and diversity.
- Due to variations, organisms can adapt better and survive in the changing environment, thereby preventing the complete extinction of any species.
- Evolution continues because of these variations and adaptations.