How do Organisms Reproduce Chapter Notes - Science Class 10

Reproduction is the process by which living organisms produce new individuals similar to themselves. It ensures the continuity of life on Earth.

• Reproduction is significant because it allows for the existence of large numbers of organisms belonging to a single species.
• If there was only one non-reproducing member of a particular kind, it is unlikely that we would have noticed its existence.
• Organisms belonging to the same species are often identified by their similarity in appearance.
• Reproducing organisms create new individuals that closely resemble themselves.

Do Organisms Create Exact Copies of Themselves?

• Organisms have similar body designs because their blueprints for these designs are similar. Reproduction involves making copies of the blueprints for body design.
• The chromosomes in the nucleus of a cell contain DNA molecules, which provide information for inheritance of features from parents to the next generation.
• DNA in the cell nucleus is the information source for making proteins. Different proteins are made if the DNA information is changed, leading to altered body designs.
• Reproduction involves creating a copy of DNA, which is accompanied by the creation of an additional cellular apparatus.
• The two copies of DNA separate and give rise to two similar but not necessarily identical cells.
• This inherent tendency for variation during reproduction is the basis for evolution.

The Importance of Variation

• Organisms occupy specific niches in ecosystems based on their ability to reproduce.
• Consistency in DNA copying during reproduction is important for maintaining body design features that allow organisms to utilize their particular niche.
• Reproduction is linked to the stability of species populations.
• Niches can change due to factors beyond the control of organisms, such as temperature fluctuations, water level variations, or meteorite impacts.
• If a niche is drastically altered, a population of organisms suited to that niche may be wiped out.
• However, if there are variations present in a few individuals within the population, there is a chance for survival.
• For example, if a population of bacteria living in temperate waters experiences a temperature increase due to global warming, most bacteria would die, but a few heat-resistant variants may survive and continue to grow.
• Variation is therefore beneficial for species survival over time.

Types of Reproduction

Modes of Reproduction used by Single Organisms

Asexual Reproduction is a mode of reproduction where offspring are produced without the involvement of gametes or the fusion of genetic material from two parents. There are several modes or methods of asexual reproduction that are observed in different organisms. 

Here are some common modes of asexual reproduction:

1. Fission

Fission reproduction refers to a type of asexual reproduction in which an organism splits into two or more parts, and each part develops into a separate individual. The parent cell divides into daughter cells.

• Binary fission: 2 cells are formed.
  Example: amoeba.
  
  Binary Fission in Amoeba
• Multiple fission: Many cells are formed.
  Example: Plasmodium.

Multiple Fission

2. Fragmentation

• Fragmentation is a type of asexual reproduction where an organism breaks into fragments, and each fragment develops into a new individual. Example: Spirogyra.

Fragmentation in Spirogyra

3. Regeneration

• When an organism is cut or broken into multiple pieces, and each of those pieces has the ability to develop into a complete and functional organism, it is referred to as regeneration.
  Example: Planaria.

4. Budding

• In some organisms, a bud forms as a small outgrowth on the parent body. This bud undergoes development and gradually matures into a miniature individual. 
• Once matured, the bud detaches from the parent body and continues to grow and develop independently, eventually becoming a fully functional new individual. 
• This process is known as budding and is a form of asexual reproduction observed in various organisms.
  Example: Hydra

5. Vegetative Propagation

• Vegetative propagation is a type of asexual reproduction in plants where new individuals are produced from vegetative parts, such as stems, roots, or leaves, without the involvement of seeds or spores.
• Vegetative propagation, such as layering or grafting, is used to grow many agricultural plants like sugarcane, roses, and grapes.
• Plants raised through vegetative propagation can flower and bear fruits earlier than those grown from seeds.
• This method allows the propagation of plants like banana, orange, rose, and jasmine, which cannot produce seeds.
• Vegetative propagation ensures that all the plants produced are genetically similar to the parent plant and possess its characteristics.

Tissue Culture

New plants are grown by using the growing tip of a plant. These growing cells are kept in a culture medium leading to the formation of calli. Callus is then transferred to the hormone medium which causes growth and differentiation.

Examples: ornamental plants, and orchids.

Benefits of tissue culture

• We can grow plants like banana, rose, jasmine etc. that have lost the capacity to produce seeds.
• New plants are genetically similar to their parents.
• Helps in growing seedless fruits.
  

6. Spore Formation

• Spores are small bulb-like structures that are covered by thick walls. Under favourable conditions, they germinate and produce new organisms.
• The thread-like structures on the bread are called hyphae and they belong to the bread mold Rhizopus. Hyphae are not reproductive parts.
• The tiny structures resembling blobs on a stick are called sporangia and they are involved in reproduction.
• Sporangia contain cells, called spores, which can develop into new individual Rhizopus organisms.

Example: Rhizopus

Sexual Reproduction

• Sexual reproduction refers to the process of reproduction in which the fusion of male and female gametes occurs, leading to the formation of genetically diverse offspring with unique combinations of traits.
• The fusion of gametes is called fertilization which results in variation.  This variation is essential for adaptability, evolution, and the survival of species in changing environments.

Why the Sexual Mode of Reproduction?

• The creation of two new cells from one involves copying the DNA and cellular apparatus.
• DNA copying mechanisms are not completely accurate and result in variations in populations of organisms.
• Variations are useful for ensuring the survival of the species.
• If the DNA copying mechanisms were less accurate, many of the resultant copies would not be able to work with the cellular apparatus and would die.
• Each new variation is made in a DNA copy that already has variations accumulated from previous generations.
• Combining variations from different individuals creates new combinations of variants.
• The sexual mode of reproduction incorporates this process of combining DNA from two different individuals.
• This creates the difficulty of each new generation having twice the amount of DNA as the previous generation, which can mess up the control of the cellular apparatus by the DNA.
• One solution to this problem is to have specialized lineages of cells in specialized organs with half the number of chromosomes and DNA content.
• This is achieved through a process called meiosis during sexual reproduction.
• The zygote needs sufficient stores to grow and develop into an organism with highly specialized tissues and organs.
• As the body designs become more complex, the germ cells also specialize, with one containing food stores and the other being motile.
• The motile germ cell is called the male gamete and the germ cell containing the stored food is called the female gamete.
• Differences in male and female reproductive organs and sometimes in the bodies of male and female organisms arise from the need to create these two different types of gametes.

Sexual Reproduction in Flowering Plants

• Flowers, the reproductive organs of plants, are complex structures specifically designed for the purpose of sexual reproduction.
• The reproductive parts of a flower are the stamens and pistil, which contain the germ-cells.
• The petals and sepals of a flower serve possible functions other than reproduction.
• The petals and sepals can attract pollinators such as insects, birds, or bats.
• They can also protect the reproductive parts of the flower.
• The stamen is the male reproductive part and produces pollen grains.
• The pistil is the female reproductive part and is made up of the ovary, style, and stigma.
• The ovary contains ovules, each with an egg cell.

• Fertilization occurs when the male germ-cell from the pollen grain fuses with the female gamete in the ovule.
• Pollen can be transferred from the stamen to the stigma in the same flower (self-pollination) or from one flower to another (cross-pollination).
• Agents such as wind, water, or animals can assist in the transfer of pollen.
• After pollen lands on the stigma, a tube grows from the pollen grain and travels through the style to reach the ovary.
• After fertilization, the zygote divides to form an embryo within the ovule.
• The ovule develops into a seed with a tough coat.
• The ovary grows rapidly and matures into a fruit, while the stigma may wither and fall off.
• Seeds contain the future plant or embryo and can germinate under suitable conditions to form a seedling.
• Seed formation provides advantages for the plant, such as dispersal and the ability to survive unfavorable conditions.

Reproduction in Human Beings

• Humans use a sexual mode of reproduction.
• Sexual maturation: The period of life when the production of germ cells i.e. ova (female) and sperm (male) starts in the body. This period of sexual maturation is called puberty.

• Changes at Puberty

  • Common in male and female
    • Thick hair growth in armpits and genital area.
    • Skin becomes oily, which may result in pimples.
  • In girls
    • Breast size begins to increase.
    • Girls begin to menstruate.
  • In boys
    • Thick hair growth on the face.
    • The voice begins to crack.
• These changes occur slowly over months and years and vary between individuals.
• The resources of the body during growth are mainly directed towards achieving adult size, but as general body growth slows down, reproductive tissues begin to mature.
• Changes during puberty serve as signals of sexual maturation and readiness to participate in the process of mating.

Male Reproductive System

(i) Testes

• A pair of testes are located inside the scrotum which is present outside the abdominal cavity. 
• Scrotum has a relatively lower temperature needed for the production of sperm.
• Male germ cells i.e. sperms are formed here.
• Testes release the male sex hormone (testosterone).

• The function of testes:

  • Regulate the production of sperm.
  • Bring changes at puberty.

• (ii) Vas deferens

  • It passes sperm from the testes up to the urethra.

• (iii) Urethra

  • It is a common passage for both sperm and urine. Its outer covering is called the penis.

• (iv) Associated glands 

  • Seminal vesicles and prostate gland add their secretion to the sperms. This fluid provides nourishment to sperms and makes their transport easy.
  • Sperm along with the secretion of glands form semen.

Female Reproductive System

(i) Ovary

• A pair of ovaries are located on both sides of the abdomen.
• Female germ cells i.e. eggs are produced here.
• At the time of birth of a girl, thousands of immature eggs are present in the ovary.
• At the onset of puberty, some of these eggs start maturing.
• One egg is produced every month by one of the ovaries.

• (ii) Oviduct or Fallopian tube

  • Receives the egg produced by the ovary and transfers it to the uterus.
  • Fertilisation i.e. fusion of gametes takes place here.

• (iii) Uterus

  • It is a bag-like structure where the development of the baby takes place.
  • The uterus opens into the vagina through the cervix.

Fertilisation of Egg

1. When the egg is fertilised

• The fertilized egg called a zygote is planted in the uterus and develops into an embryo.
• The embryo gets nutrition from the mother’s blood with the help of a special tissue called the placenta. It provides a large surface area for the exchange of glucose, oxygen and waste material.
• The period from fertilization up to the birth of the baby is called the gestation period. It is about 9 months.

2. When the egg is not fertilised

• The uterus prepares itself every month to receive fertilized eggs.
• The lining of the uterus becomes thick and spongy, required to support the embryo.
• When fertilisation had not taken place, this lining is not needed any longer.
• This lining breaks and comes out through the vagina as blood and mucus.
• This cycle takes around 28 days every month and is called menstruation.

Reproductive Health

• Reproductive health means total well-being in all aspects of reproduction i.e. physical, emotional, social and behavioural.

• Sexually Transmitted Diseases (STDs)

  • Many diseases can be sexually transmitted such as:
    (i) Bacterial: Gonorrhoea and syphilis
    (ii) Viral: Warts and HIV-AIDS
  • The use of condoms prevents these infections to some extent.
  • Contraception: It is the avoidance of pregnancy, which can be achieved by preventing the fertilisation of ova.

• Methods of contraception

  

  Methods of Contraception

  (i) Physical barrier

  • To prevent the union of egg and sperm.
  • Use of condoms, cervical caps and diaphragm.

• (ii) Chemical methods

  • Use of oral pills
  • These change the hormonal balance of the body so that eggs are not released.
  • May have side effects.

• (iii) Intrauterine contraceptive device (IUCD)

  • Copper-T or loop is placed in the uterus to prevent pregnancy.

• (iv) Surgical methods

  • In males, the vas deferens are blocked to prevent sperm transfer called vasectomy.
  • In females, the fallopian tube is blocked to prevent egg transfer called tubectomy.

Female Foeticide

• The practice of killing a female child inside the womb is called female foeticide.
• For a healthy society, a balanced sex ratio is needed which can be achieved by educating people to avoid malpractices like female foeticide and prenatal sex determination.
• Prenatal sex determination is a legal offence in our country to maintain a balanced sex ratio.

Frequently Asked Questions (FAQs)

Q1. What is asexual reproduction?
Ans. Asexual reproduction is a type of reproduction in which a single organism gives rise to offspring that is genetically identical to itself. This type of reproduction does not involve the fusion of gametes. Some examples of asexual reproduction are budding, binary fission, and spore formation.

Q2. What is the difference between sexual and asexual reproduction?
Ans. Sexual reproduction involves the fusion of two gametes (sex cells) to form a zygote, which develops into a new organism. This type of reproduction leads to genetic variation in offspring. On the other hand, asexual reproduction does not involve the fusion of gametes and leads to offspring that are genetically identical to the parent. Therefore, asexual reproduction does not lead to genetic variation. 

Q3. What are the advantages and disadvantages of sexual reproduction?  
Ans. The advantages of sexual reproduction are that it leads to genetic variation in offspring, which helps in the adaptation of organisms to changing environments. It also helps in the elimination of harmful mutations through the process of natural selection. However, sexual reproduction requires the production of gametes, which is energetically expensive and time-consuming. It also requires the search for a suitable mate, which can be difficult and risky. 

Q4. What are the different types of asexual reproduction?
Ans. The different types of asexual reproduction are: 

• Budding: where a small bud develops on the parent organism and eventually detaches to form a new organism.
• Binary fission: where the parent organism divides into two equal halves, each of which grows into a new organism.
• Spore formation: where the parent organism produces spores that develop into new organisms under favourable conditions.
• Vegetative propagation: where new plants develop from the vegetative parts of the parent plant, such as stems, roots, and leaves.