How do Organisms Reproduce?
1. Do Organisms Create Exact Copies?
Organisms look similar because their body designs are similar. The blueprint for these designs is stored in the DNA (Deoxyribonucleic Acid) in the cell nucleus.
- DNA Copying: During reproduction, cells create copies of their DNA.
- Cellular Apparatus: DNA copying is accompanied by the creation of additional cellular apparatus, and then the cell divides.
Importance: Variation is the basis of Evolution. It allows species to survive in drastically changing environments (e.g., bacteria surviving heat waves due to heat-resistant variation).
2. Modes of Reproduction
| Feature | Asexual Reproduction | Sexual Reproduction |
|---|---|---|
| Parents | Single parent involved. | Two parents (Male & Female). |
| Gametes | No gamete formation/fusion. | Gametes fuse (Fertilization). |
| Offspring | Genetically identical (Clones). | Show variations. |
3. Modes of Asexual Reproduction
A. Fission
In unicellular organisms, the parent cell divides into daughter cells.
- Binary Fission: Parent divides into two equal halves. Example: Amoeba (any plane), Leishmania (specific orientation).
- Multiple Fission: Parent divides into many daughter cells simultaneously (usually inside a cyst). Example: Plasmodium (Malarial parasite).
B. Fragmentation
In simple multicellular organisms, the body breaks up into smaller pieces upon maturation. Each fragment grows into a new individual.
Example: Spirogyra.
C. Regeneration
If an organism is cut or broken into pieces, specialized cells proliferate and differentiate to re-grow the missing parts or whole organism.
Example: Hydra and Planaria.
Note: Regeneration is NOT the same as reproduction.
D. Budding
A bud develops as an outgrowth due to repeated cell division at one specific site. It develops into a tiny individual and detaches when mature.
Example: Hydra, Yeast.
E. Vegetative Propagation
New plants are obtained from parts of old plants (stems, roots, leaves) without reproductive organs.
- Natural: Bryophyllum (Leaf notches), Potato (Tuber).
- Artificial: Layering, Grafting, Cutting (Sugarcane, Rose).
- Tissue Culture: Growing plants from cells/tissues in artificial medium (Micropropagation).
F. Spore Formation
Example: Rhizopus (Bread Mould).
Structures called sporangia contain spores. Spores are covered by thick walls to protect them until they find a moist surface to grow.
(Next Section: Sexual Reproduction in Flowering Plants)
4. Sexual Reproduction in Flowering Plants
Sexual reproduction involves the formation and fusion of gametes. In flowering plants (Angiosperms), the reproductive parts are located in the flower.
A. Structure of a Flower
A typical flower consists of four main whorls:
- Sepals (Calyx): Green leaf-like parts that protect the flower in the bud stage.
- Petals (Corolla): Colored parts to attract insects for pollination.
- Stamen (Androecium): The male reproductive part. It produces pollen grains (yellowish powder). It has two parts: Anther (swollen top) and Filament (stalk).
- Pistil / Carpel (Gynoecium): The female reproductive part present in the center. It has three parts:
- Stigma: Sticky terminal part (receives pollen).
- Style: Middle elongated tube.
- Ovary: Swollen bottom part containing ovules. Each ovule has an egg cell (female gamete).
1. Unisexual: Contains either stamens or carpel. (e.g., Papaya, Watermelon).
2. Bisexual: Contains both stamens and carpel. (e.g., Hibiscus, Mustard).
B. Pollination
The transfer of pollen grains from the anther to the stigma of a flower is called Pollination.
| Self-Pollination | Cross-Pollination |
|---|---|
| Transfer of pollen occurs within the same flower. | Transfer of pollen from one flower to another flower. |
| No external agent required. | Agents like wind, water, or animals (insects) are required. |
C. Fertilization
After pollination, the male gamete (in pollen) must reach the female gamete (in ovule).
- Pollen Germination: Pollen grain lands on a suitable stigma and germinates to produce a pollen tube.
- Journey: The pollen tube grows through the style and reaches the ovary.
- Fusion: The male gamete travels down the tube and fuses with the female gamete (egg) in the ovule. This fusion is called Fertilization.
- Result: A Zygote is formed.
D. Post-Fertilization Changes
After fertilization, the flower undergoes significant changes:
- Zygote: Divides several times to form an Embryo within the ovule.
- Ovule: Develops a tough coat and converts into a Seed.
- Ovary: Grows rapidly and ripens to form a Fruit.
- Other Parts: Sepals, petals, stamens, style, and stigma shrivel and fall off.
E. Germination
The seed contains the future plant or embryo. Under appropriate conditions (moisture, air, temp), the embryo develops into a seedling. This process is called Germination.
1. Cotyledon: Food store.
2. Plumule: Future shoot (grows upwards).
3. Radicle: Future root (grows downwards).
(Next Section: Reproduction in Human Beings)
5. Reproduction in Human Beings
Humans use a sexual mode of reproduction. This process begins with the production of germ cells (gametes) in specialized organs.
A. Puberty: The Age of Adolescence
The period during adolescence when the reproductive tissues begin to mature is called Puberty.
Girls specific: Breast size increases; menstruation begins.
Boys specific: Thick facial hair growth; voice begins to crack.
B. Male Reproductive System
The male reproductive system consists of portions which produce the germ-cells and other portions that deliver the germ-cells to the site of fertilization.
- Testes: located outside the abdominal cavity in the scrotum.
- Function 1: Production of sperms.
- Function 2: Secretion of hormone Testosterone (regulates sperm formation and puberty changes).
- Why outside? Sperm formation requires a lower temperature (2-3°C lower) than the normal body temperature.
- Vas deferens: The tube that carries sperm from the testes to the urethra.
- Glands (Prostate & Seminal Vesicles): They add their secretions so that the sperms are now in a fluid (semen). This makes transport easier and provides nutrition to sperms.
- Urethra: It forms a common passage for both sperm and urine.
C. Female Reproductive System
- Ovaries: A pair of ovaries is located in the abdominal cavity. They produce:
- Egg cells (Ova): One egg is released every month by one of the ovaries.
- Hormones: Oestrogen and Progesterone.
- Oviduct (Fallopian Tube): The egg is carried from the ovary to the womb through this tube. Fertilization occurs here.
- Uterus (Womb): A bag-like elastic structure where the two oviducts unite. The baby develops here.
- Cervix & Vagina: The uterus opens into the vagina through the cervix.
D. Fertilization and Development
1. Fertilization: If the sperm enters the vagina during sexual intercourse, it travels up to the fallopian tube. If it meets the egg, fertilization occurs to form a Zygote.
2. Implantation: The zygote divides repeatedly to form an embryo, which gets implanted in the lining of the uterus.
Structure: It is a disc embedded in the uterine wall. It contains villi on the embryo's side and blood spaces on the mother's side.
Function: It provides a large surface area for glucose and oxygen to pass from the mother to the embryo and removes waste generated by the embryo.
Gestation: The development of the child inside the mother’s body takes approximately 9 months.
E. What happens if the egg is NOT fertilized? (Menstruation)
- The ovary releases one egg every month.
- The uterus prepares itself every month by making its lining thick and spongy to nourish the embryo.
- If fertilization does not occur, this lining is not needed. It slowly breaks and comes out through the vagina as blood and mucus.
- This cycle takes place roughly every month and is known as menstruation (2-8 days).
6. Reproductive Health
A. Sexually Transmitted Diseases (STDs)
Diseases transmitted through sexual contact:
- Bacterial: Gonorrhoea and Syphilis.
- Viral: Warts and HIV-AIDS.
B. Contraception (Birth Control)
The prevention of pregnancy is called contraception. Methods include:
| Method | Mechanism | Example |
|---|---|---|
| Mechanical Barrier | Prevents sperm from meeting egg. | Condom (also prevents STDs), Diaphragm. |
| Chemical Method | Changes hormonal balance to prevent egg release. | Oral Pills (OCP). |
| IUCD | Placed in uterus to prevent pregnancy. | Copper-T. |
| Surgical Method | Blocks gamete transfer (Permanent). | Vasectomy (Males), Tubectomy (Females). |
Vasectomy: Vas deferens is blocked to prevent sperm transfer.
Tubectomy: Fallopian tube is blocked to prevent egg transfer.
Foeticide: Killing of a female foetus (illegal in India).
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