CBSE Class 10 Science Chapter 8 How Do Organisms Reproduce Notes

Introduction

Reproduction

Reproduction is the process by which all organisms multiply in number and increase their population.

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Asexual reproduction

Asexual reproduction is a method of reproduction that involves only one organism. A single organism reproduces two or multiple organisms on its own. This is seen in all unicellular organisms, some multicellular organisms and a few plants.

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Sexual reproduction

The mode of reproduction that involves two individuals; one male and one female. They produce sex cells or gametes which fuse to form a new organism.

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Asexual Reproduction

Fission

• Fission is an asexual reproduction that is common in most of the unicellular organisms.
• When the fission results in two daughter cells, it is binary fission (e.g. paramecium).
• When fission results in many daughter cells, it is called multiple fission (e.g. Plasmodium).
• Planes of fission may be different for different organisms.

 

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Budding

• Budding is a type of asexual reproduction in which a small cyst-like structure is formed on the parent’s body, which gives rise to a new individual.
• Bud may remain attached to the parent (yeast) or may separate and become a new individual (hydra).

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Regeneration and fragmentation

• Regeneration is the process of growing back the lost organ or body part by the organism (e.g. lizard).
• Fragmentation is the process by which an organism gets fragmented into smaller pieces and each piece grows into a whole new organism.
• E.g. Planaria, Hydra

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Spore formation

Organisms such as fungi make spores that can grow into complete new individuals when dispersed from their fruiting body.

Vegetative propagation

• This is a type of asexual reproduction seen in plants.
• The vegetative part of the plant, like leaves, stem, roots, gives rise to a new plant.
• Vegetative propagation can be artificial or natural.
• Natural vegetative propagation happens through leaves (e.g. bryophyllum), stem (e.g. turmeric, ginger), runners/stolon (e.g.grass runners, strawberry), bulbs ( e.g. onion, lily), etc.
• Artificial methods include cutting, grafting, layering and plant tissue culture.

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Sexual Reproduction

Types of Cell division

Two types of cell division seen in eukaryotic organisms:

Mitosis

• Takes place in somatic cells
• Maintains the chromosome number
• Produces two, diploid daughter cells
• Required for asexual reproduction, development and growth, cell replacement and  regeneration

Meiosis

• Takes place in sex cells
• Reduces the number of chromosomes by half
• Produces four haploid daughter cells
• Required for sexual reproduction, i.e gamete formation

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The Reproductive System

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Male reproductive system

• The main reproductive organ in males is a pair of testes.
• They produce the male sex cells called sperms and also produce male sex hormone testosterone.

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Male main reproductive organs

• The main reproductive organ in males is a pair of testes.
• They are present in scrotal sacs outside the body and contain seminiferous tubules as the structural and functional unit.
• Male sex cells, sperms, are produced by seminiferous tubules and mature in the epididymis.
• Leydig cells or interstitial cells present in between the seminiferous tubules secrete hormone testosterone.

Male accessory reproductive organs

• Several accessory reproductive organs that aid in the reproductive process.
• The prostate gland and the seminal vesicles are glands of reproductive system which make semen and nourish the sperms.
• Penis, having urethra passing through it, is called copulatory organ.

Male Ducts

• In males, the vas deferens and the urethra are the main ducts.
• A single vas deferens carries sperms from respective testis up to urethra.
• Urethra acts as a common passage for semen and urine.

Female reproductive system

The human female reproductive system consists of a pair of ovaries, a pair of fallopian tubes/oviducts and the accessory organs such as the uterus and the vagina.

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Female main reproductive organ

• The main reproductive organ in a female is a pair of ovaries.
• They produce the female sex cells called eggs or ova and also produce female sex hormones called estrogen and progesterone.

Female accessory reproductive organ

• Uterus and vagina are the accessory reproductive organs in human females.
• The uterus is the site of fetal development and vagina receives sperms from the male.

Menstrual Cycle

Menstruation

• Menstruation is the cyclic event of the release of the ovum from the ovary and its removal from the body when fertilization does not happen.
• During menstruation, the blood-rich endometrium of the uterus also breaks down while the ovum is being removed from the body.
• Two pituitary hormones, LH and FSH and two ovarian hormones, estrogen and progesterone, all have their roles in menstruation.
• In humans, the cycle repeats every 28 days.

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Fertilization

Human reproduction

Humans reproduce sexually. The male produces sperms and the female produces eggs. When the sperm fuses with the egg, it forms a zygote that gives rise to a new progeny.

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Contraceptive Methods

Reproductive health

Reproductive health deals with the prevention of STDs and unwanted pregnancy. Understanding the reproductive system is also a part of reproductive health awareness.

Contraceptives

• Contraceptives are devices that prevent unwanted pregnancy and help avoid STDs.
• Contraceptives can be of various types such as mechanical barriers, hormonal/chemical methods, surgical methods, etc.

Coitus Interruptus

• It is a very unreliable contraceptive method where the coitus is stopped before the male ejaculates inside the female reproductive tracts.

Rhythm Method

• Another unreliable method of contraception where coitus is avoided when the female is fertile and the chances of fertilization are very high.

Condoms

• One of the most effective methods of contraception.
• A mechanical barrier that stops the semen from entering the female tract preventing pregnancy.
• It also avoids the possibility of contracting STDs.

Diaphragms

• Diaphragms are barriers that can be added inside the female reproductive tracts.
• They stop the entry of semen inside the female tract and thus prevent pregnancy.

Contraceptive Pills

• Contraceptive pills are chemical methods of contraception.
• They change the level of hormones in the body that prevents the release of the ovum from the ovaries.

Emergency Pill

• Emergency pills are those pills which can be taken after coitus to avoid pregnancy.
• They quickly change the level of hormones in the body and prevent a successful implantation even if the egg gets fertilized.

IUD

• IUD stands for Intrauterine Device.
• They can be used for a couple of years.
• It is a device that is inserted into the uterus, changing its shape and preventing successful implantation of the zygote.

Sterilization

• Sterilization is a surgical method of going permanently sterile.
• This can be done in both males and females.
• In males, it is called vasectomy and in females, it is called tubal ligation.

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Reproduction in Plants

Sexual reproduction in flowering plants

• Sexual reproduction in plants happens through flowers.
• Essential whorls of the flowers such as androecium and the gynoecium help in the sexual reproduction of plants.

Non-essential parts of flowers

• The typical structure of flower contains essential whorls and non-essential whorls.
• Sepals and Petals are called non-essential whorls as they do not directly take part in reproduction.
• Sepals protect the inner delicate whorl during bud condition and also perform photosynthesis if they are green in colour.
• Petals, when they are coloured, attract insects for pollination.

Essential whorls of flowers

• Androecium and gynoecium are called as essential/reproductive whorls of a flower.
• Androecium produces pollen grains containing male gametes and gynoecium produces ovules which are female gametes.
• Bisexual flowers contain both the whorls while unisexual flowers contain either of them.
• Each individual member of androecium is called a stamen and consists of anther and filament.
• Anther produces haploid pollen grains.
• Each individual member of gynoecium is called pistil and consists of stigma, style and ovary.
  

Pollination

The process of transfer of pollen grains from anthers to the stigma of a flower is known as pollination.

• It is required for fertilization.
• Pollination has two types, self-pollination (autogamy) and cross-pollination (allogamy).
• In self-pollination, transfer of pollen grains takes place from anthers to the stigma of the same flower or another flower of the same plant.
• In cross-pollination, pollens are transferred from anthers to the stigma of another flower.
• Many pollinating agents play their roles in cross-pollination. Examples: water, wind, insects, birds, bats, etc.

Fertilization

Fusion of male and female gametes is known as fertilization.

• In flowering plants after pollination, the pollens germinate on the stigma surface of pistil and generate two male nuclei.
• Ovule has egg cell and two polar nuclei.
• One male nucleus fuses with two polar nuclei and forms triploid endosperm.
• Another male nucleus fuses with the egg cell and forms the zygote that gives rise to the embryo and future plant.
• After fertilization, ovary becomes fruit and ovules turn into seeds. All other parts wither away.