CBSE Class 10 Science How Do Organisms Reproduce Notes Set 04

How do Organisms Reproduce?

QUICK RECAP

Reproduction is the production of new organisms from the existing organism of a species that ensures continuity of population of that species.

Types of reproduction : Living organisms reproduce by two ways-asexual reproduction and sexual reproduction.

Asexual reproduction : It involves the production of an offspring from a single parent without the fusion of gametes.

Basic features of asexual reproduction :

• Only one organism is involved; different sexes are not involved.
• All the cell divisions during this type of reproduction are either mitotic or amitotic.
• New individuals produced are genetically identical to the parents.
• It is a rapid mode of multiplication.
• No gametes are formed and hence no fertilisation takes place.

Sexual Reproduction : It may be defined as the production of offspring (new individuals) by the fusion of two gametes (usually one from male parent and the other from female parent) to form a diploid zygote which develops into a mature organism.

Asexual Reproduction Methods

• Fission: A fully grown parent (unicellular organism) divides into two or more daughter cells.
  • Binary: Division of parent body into two equal and identical daughter cells. Occurs under favourable conditions. E.g., Amoeba, Paramecium, etc.
  • Multiple: Many individuals are formed from a single parent. Occurs under unfavourable conditions. E.g., Amoeba, Plasmodium, Monocystis, etc.
• Fragmentation: Parent body breaks into two or more fragments and each fragment develops into a new organism, e.g., sponges, sea anemones, starfish, etc.
• Regeneration: Formation of whole organism from a small body fragment or repair of the damaged cells, tissues or replacement of damaged parts of body. E.g., Hydra, Planaria, lizard, etc.
• Budding: Formation of new individual from small outgrowth called bud, on the parent body, e.g., Hydra, yeast, etc.
• Vegetative propagation: Formation of new plant from the parts like stem, root and leaves of an existing plant.
  • Natural methods:
    • Propagation by roots, e.g., Dalbergia, Dahlia, sweet potato, etc.
    • Propagation by stems: Rhizome – Banana, ginger; Bulb – Onion, garlic; Tuber – Potato, artichoke; Runner – Oxalis, Cyanodon; Corm – Colocasia.
    • Propagation by buds on leaves, e.g., Bryophyllum, Begonia, etc.
  • Artificial methods:
    • Cutting: Cutting of small pieces of stem, root or leaf having a bud to grow new plant when put in soil, e.g., rose, sugarcane, etc.
    • Layering: A branch of plant is lowered and buried into soil to induce adventitious root formation and then cut from parent plant, e.g., jasmine, Hibiscus, etc.
    • Grafting: Method of obtaining superior quality plant from two different plants by taking root system of one plant (stock) and shoot system of another plant (scion), e.g., mango, apple, rubber, etc.
• Spore formation: Spore is microscopic unicellular reproductive structure that germinates to produce new individuals after detaching from the parent body, e.g., Rhizopus, Penicillium.
• Micropropagation/ Tissue culture: Production of large number of plants by cell or tissue culture technique, e.g., commercial production of orchids, carnation, Chrysanthemum, etc.

Sexual Reproduction in Flowering Plants

Advantages of sexual reproduction over asexual reproduction are : (i) Fusion of male and female gametes coming from male and female organisms. Since the fusing gametes come from two different and sexually distinct individuals, the offsprings exhibit diversity of characters, and (ii) Meiosis during gametogenesis provides opportunities for new combination of genes. It plays a prominent role in the origin of new species and leads to variation required for evolution.

The reproductive part of angiosperms is called flower.

• Sepals are green while petals are coloured. They form the non-essential parts of flower.
• Stamens and carpels are the essential reproductive parts of the flower.
• Stamen (androecium) is the male reproductive part of the plant made up of filament and anther. Anther produces the pollen grains that contain male gametes.
• Carpel (gynoecium) is the female reproductive part made up of stigma, style and ovary. Stigma is responsible for receiving pollen during pollination. Style is the middle part of carpel. It is an elongated tubular structure which connects stigma with ovary. The basal swollen part of carpel is ovary. Each ovule has an embryo sac that bears a haploid egg (female gamete).
• The process of transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower is known as pollination. It is of two types - self pollination that takes place in same flower (bisexual) or between two flowers of the same plant and cross pollination which takes place between two flowers of different plants of same species. Insects (entomophily), wind (anemophily), water (hydrophily), birds (ornithophily), etc., are the various pollinating agents.

Double Fertilisation: After pollination, pollen grain on the stigma germinates and grows a pollen tube downwards through style to the ovary. Two non-motile male gametes are formed inside the tube during its growth through the style. After reaching the ovary, pollen tube enters the ovule through the micropyle. The tip of the tube finally pierces the micropylar end of the embryo sac and ruptures releasing two male gametes into the embryo sac. During the act of fertilisation, one male gamete fuses with the egg to form the diploid zygote. The process is called syngamy (or generative fertilisation). The diploid zygote finally develops into an embryo. The other male gamete fuses with the two polar nuclei to form the triploid \( (3n) \) primary endosperm nucleus. The process is called triple fusion (or vegetative fertilisation). This mechanism involving two acts of fertilisation in an embryo sac is called double fertilisation.

The ovary of flower develops into a fruit. Ovule gets converted into a seed on maturation. Seeds germinate under suitable conditions such as optimum moisture, air and warmth, etc., into a seedling and eventually into a new plant.

Sexual Reproduction in Human Beings

Human beings are unisexual and there is a distinct sexual dimorphism. The reproductive systems of males and females consist of many organs which are distinguishable into primary and secondary sex organs. The primary sex organs are gonads, which produce gametes (or sex cells) and secrete sex hormones. The secondary sex organs include the genital ducts and glands which help in transportation of gametes and reproductive process.

• The period of sexual maturity during adolescence when reproductive organs become functional is called puberty.
• In males : Enlargement of penis and scrotum; Broadening of the shoulders and increased muscle development; Enlargement of larynx and thickening of vocal cords producing deepening of the voice; Growth of pubic hair and extra hair on the face, in the armpits and on the chest.
• In females : Growth of breasts and external genitalia (vulva); Growth of pubic hair and extra hair in armpits; Hair on beard, moustache and chest are lacking; Broadening of pelvis; Initiation of menstruation and ovulation.

Menstrual cycle : Sexual cycle begins in a female when she attains puberty. It occurs under the control of sex hormones and repeats every 28 days. It is marked by bleeding or menstrual flow. A single egg is released from either of the two ovaries and the inner wall of uterus, endometrium, becomes ready to receive it after fertilisation. If the ovum gets fertilised by the sperm, a zygote is formed and an embryo starts developing. Till the birth of the child, neither any egg is released from ovary nor menstruation takes place.

• In the absence of fertilisation, the egg is expelled through vagina along with the bleeding caused by the breaking of endometrium. This is called menstruation. Menstruation usually occurs 14 days after ovulation and lasts for 3-5 days.
• The first menstruation in a female's life is called menarche. The sexual cycle in a woman continues upto the age of 45 to 55 years. The ovaries do not release any further egg after that and this stage is called menopause.

Fertilisation : Fertilisation is internal in human beings. The sperms produced in testes of male are transferred into the vagina of female during the intercourse. The ovum released from ovary during ovulation is swept into the adjacent Fallopian tube where fertilisation takes place. Only one sperm can enter into the ovum. Fusion of nucleus of sperm and nucleus of ovum marks the completion of fertilisation. It results in the formation of single-celled zygote.

Post-fertilisation changes : The zygote then undergoes repeated mitotic divisions to form a blastula on the 7th day that descends into uterus and gets implanted. Specialised tissue of fetus and uterine wall of the mother form a structure called placenta. It is a mechanical and physiological connection between an embryo and uterine wall of the mother through which nutrients and other useful substances enter fetus from mother's blood and waste products are expelled into mother's blood.

Reproductive Systems

Male Reproductive System:

• Testes: One pair, present outside the abdominal cavity within scrotal sacs which keep the testicular temperature \( 2^{\circ} - 3^{\circ} \text{C} \) lower than that of normal body temperature. It is the site of production of male gametes called sperms and also secretes a hormone called testosterone.
• Vas deferens: Long, tubular structure present in pair that carries sperms to the seminal vesicles that temporarily store sperms. Vas deferens along with seminal vesicle opens into ejaculatory duct which leads to urethra.
• Urethra: Common pathway for the flow of urine and semen, arises from the urinary bladder and is joined by ejaculatory duct. Runs through the penis and opens outside through male genital pore.
• Penis: Long, thick muscular organ having erectile tissue. Tip of penis is called glans penis (soft and highly sensitive), covered by foreskin. It is a copulatory organ which transfers semen into vagina of female during sexual intercourse.
• Accessory reproductive glands:
  • Seminal vesicles: One pair, sac like, present between rectum and urinary bladder and secrete mucus and alkaline watery fluid that contains fructose (source of energy for sperms) which helps in the movement of sperms.
  • Prostate gland: Single, large gland which surrounds urethra, secretes milky fluid into urethra that helps sperms to swim.
  • Bulbourethral or Cowper’s glands: One pair, very small, present at both sides of urethra, secrete mucus to lubricate the end of penis and secrete alkaline fluid that neutralises the acids from the urine.

Female Reproductive System:

• Ovaries: Paired, almond shaped, present in the lower abdominal cavity near kidneys. They are primary sex organs and perform two functions – production of female gametes (eggs or ova) and secretion of female sex hormones (estrogen and progesterone). One mature ovum is released from either of the ovary (ovulation) at the interval of every 28 days during reproductive phase of a woman.
• Fallopian tubes (Oviducts): Paired tubes originating from uterus, extend upto the ovary of their respective sides. Its terminal part is funnel-shaped with finger-like projections called fimbriae which direct the released ovum into the tube.
• Uterus: Single, inverted pear-shaped, highly muscular, hollow structure. Development of fetus occurs inside it, hence called womb. The narrower lower part is called cervix, which leads into vagina.
• Vagina: About 7-10 cm long tube where penis discharges sperms. It acts as the passage for menstrual flow and also acts as birth canal during parturition (child birth).

Gestation and Reproductive Health

The complete development of fetus, from the initial stage of conception till the birth of the young one, is called gestation. Gestation is completed in about 280 days or 40 weeks. Gestation is followed by parturition (birth of child). Parturition starts with contraction of uterine wall under the influence of oxytocin hormone.

Population control : Continuous increase in population may cause scarcity of food and other resources. Regulation of conception is required. Preventive measures to avoid pregnancy are called contraception.

• Barrier methods : Condoms or Nirodh (for males), Femidom, Diaphragm or Cervical cap (for females). They also protect from STDs.
• Chemical methods : Spermicide creams and jellies. Oral contraceptive pills prevent ovulation.
• Intrauterine contraceptive devices (IUCDs) : Such as Copper T (CuT), which prevent implantation in uterus.
• Natural methods : Rhythm method and coitus interruptus.
• Surgical methods (Sterilisation) : Vasectomy (for males) and Tubectomy (for females).

Sexually Transmitted Diseases (STDs): Infectious diseases spread by sexual contact.

• Gonorrhoea is caused by a bacterium Neisseria gonorrhoeae. Symptoms include inflammation of the urinogenital tract and burning sensation during urination.
• Syphilis is caused by bacterium Treponema pallidum. It causes lesions on mucous membranes.
• AIDS (Acquired Immuno-Deficiency Syndrome) is caused by HIV virus. It weakens the body's immune system and is currently incurable.

Healthy Pregnancy : Malnutrition and stress during pregnancy are major causes of ill health. Expecting mothers should avoid cigarettes, alcohol, and drugs as they cause harmful effects on the fetus. Proper spacing between pregnancies is essential for the mother's health.