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Chapter 7 How Do Organisms Reproduce?
Reproduction is not a process necessary for the survival of an individual organism, unlike nutrition, respiration, or excretion. However, it is crucial for the continuation of a species. Organisms are noticeable because they exist in large numbers, belonging to distinct species. This large population size is a direct result of reproduction. Organisms reproduce to create new individuals that are similar to themselves, ensuring the survival and perpetuation of their kind across generations.
Do Organisms Create Exact Copies Of Themselves?
Organisms of the same species look similar because they share a similar body design. This similarity in body design relies on similar blueprints or genetic information. In cells, this information is stored in **DNA (Deoxyribonucleic Acid)** molecules, located in the chromosomes within the nucleus. DNA contains the instructions (information source) for making proteins, which ultimately determine the body design and functions.
Therefore, the fundamental event in reproduction is the creation of a **DNA copy**. Cells use biochemical reactions to replicate their DNA. This process creates two copies of the DNA within a cell. To maintain cellular organisation and ensure the new copy can function, the DNA copying is accompanied by the synthesis of additional cellular structures. Effectively, a cell divides, producing two cells, each receiving one DNA copy and its own cellular components.
While the resulting cells are generally similar, they are unlikely to be absolutely identical. Biochemical copying processes are not perfectly accurate, and some **variations** inevitably occur during DNA replication. These variations in the DNA copies mean that the new cells are similar to, but subtly different from, the original cell and each other. This inherent tendency for variation during reproduction forms the basis for evolution.
The Importance Of Variation
Populations of organisms occupy specific environments or **niches** within an ecosystem, utilizing their ability to reproduce. Consistent DNA copying helps maintain the body design features suited for that particular niche, ensuring the stability of the species' population.
However, environments (niches) can change drastically and unpredictably (e.g., temperature fluctuations, water level changes, natural disasters). If a population were perfectly suited to a specific niche and that niche underwent a major change, the entire population could face extinction. But, if some **variations** exist among individuals within the population, some variants might possess traits that allow them to survive in the altered environment. These survivors can then reproduce, and the population will gradually adapt to the new conditions.
For example, a population of bacteria living in cool waters might mostly die if global warming increases water temperatures, but a few heat-resistant variants in that population could survive and multiply, ensuring the species' survival in the warmer water. Thus, variation is not necessarily beneficial for an individual (a drastic variation might even be lethal), but it is crucial for the **survival of the species** over time, allowing populations to adapt to changing environments.
Question 1. What is the importance of DNA copying in reproduction?
Answer:
DNA copying is a basic event in reproduction because DNA contains the genetic information (blueprint) for the body design of an organism. Creating a copy of DNA ensures that the information necessary for constructing and maintaining the body structure and functions is passed on from the parent organism to the new generation. This allows the new individual to develop and resemble its parent.
Question 2. Why is variation beneficial to the species but not necessarily for the individual?
Answer:
Variation is beneficial to the **species** because it increases the genetic diversity within a population. In a changing environment, some individuals with specific variations may possess traits that allow them to survive and reproduce, while others without those variations might perish. This enhances the chances of the species surviving over time by adapting to new conditions. For the **individual**, a variation can sometimes be harmful or even lethal (e.g., a mutation leading to a severe disease). So, not every variation is beneficial at the individual level, but it is the collection of variations within a population that provides resilience to the species.
Modes Of Reproduction Used By Single Organisms (Asexual Reproduction)
Organisms use different modes of reproduction depending on their body design. For single-celled organisms or simple multicellular organisms, methods of reproduction often involve a single parent producing new individuals. This is known as **asexual reproduction**.
Fission
**Fission** is a mode of asexual reproduction used by unicellular organisms, where cell division leads to the creation of new individuals. The parent cell splits to form two or more daughter cells.
- **Binary fission:** The parent cell divides into **two equal halves**. This can happen in any plane in some organisms like **Amoeba**.
In organisms with some body organisation, like **Leishmania** (causing kala-azar), binary fission occurs in a **definite orientation** relative to specific structures (like the whip-like flagellum).
- **Multiple fission:** The parent cell divides into **many daughter cells simultaneously**. This occurs in organisms like the malarial parasite, **Plasmodium**.
Fragmentation
**Fragmentation** is a mode of asexual reproduction seen in simple multicellular organisms, where the body simply breaks up into two or more smaller pieces upon maturation. Each piece then grows into a complete new individual.
Example: **Spirogyra**, a filamentous alga. Its body consists of simple filaments. When mature, the filament breaks into fragments, and each fragment grows into a new Spirogyra filament.
This method is only possible in organisms with relatively simple body organisation where breaking into pieces does not lose any complex tissue or organ necessary for survival and growth.
Regeneration
**Regeneration** is the ability of a fully differentiated organism to regrow lost body parts. In some simple organisms, if the body is cut or broken into many pieces, many of these pieces can grow into complete new organisms. However, this is not the same as reproduction, as most organisms don't naturally reproduce by being cut up.
Example: Simple animals like **Hydra** and **Planaria** can regenerate into complete individuals from body fragments.
Regeneration is carried out by specialized cells that proliferate and differentiate into various cell types and tissues in an organised sequence to form the missing parts or a whole organism. More complex organisms cannot regenerate into new individuals from simple cuts because their complex body design with specialised tissues and organs makes simple cell-by-cell development from a fragment impractical.
Budding
**Budding** is a mode of asexual reproduction where a small outgrowth or **bud** develops on the parent body due to repeated cell division at a specific site. The bud develops into a tiny individual and eventually detaches from the parent to become a new independent organism.
Example: **Hydra** and **Yeast**. In Hydra, a bud forms on the side of the body. In yeast, a bud forms as a small protuberance from the cell wall.
Vegetative Propagation
**Vegetative propagation** is a mode of asexual reproduction in **plants** where new plants grow from vegetative parts of the parent plant, such as roots, stems, or leaves, under appropriate conditions. This is widely used in agriculture and horticulture.
Examples of vegetative propagation methods:
- Growing new plants from **stem cuttings** (e.g., roses, sugarcane, grapes, money plant).
- Growing new plants from **roots** (e.g., sweet potato).
- Growing new plants from **leaves** that have buds in their notches (e.g., Bryophyllum).
Advantages of vegetative propagation:
- Plants raised by vegetative propagation can produce flowers and fruits **earlier** than those grown from seeds.
- It allows propagation of plants that have **lost the capacity to produce viable seeds** (e.g., banana, orange, rose, jasmine).
- All plants produced are **genetically identical** to the parent plant, inheriting all its desirable characteristics.
**Layering** and **grafting** are artificial methods of vegetative propagation used for agricultural purposes.
**Tissue culture** is an advanced technique where new plants are grown from small pieces of plant tissue or cells in a sterile artificial medium containing nutrients and hormones. This method can produce many plants from one parent in disease-free conditions and is used for ornamental plants and crop improvement.
Spore Formation
**Spore formation** is a mode of asexual reproduction common in fungi (like bread mould, Rhizopus) and some bacteria and plants. Specific reproductive parts produce spores, which are small, lightweight reproductive units.
Example: **Rhizopus** (bread mould). The thread-like structures are hyphae. The tiny blob-on-a-stick structures are **sporangia**, which contain numerous **spores**. Spores are reproductive cells.
Spores are covered by thick protective walls that help them survive unfavorable conditions (like dryness). When spores land on a suitable moist surface with nutrients, they germinate and grow into new individuals.
Question 1. How does binary fission differ from multiple fission?
Answer:
| Feature | Binary Fission | Multiple Fission |
|---|---|---|
| Number of daughter cells | Parent cell divides into two daughter cells. | Parent cell divides into many daughter cells simultaneously. |
| Nuclear division | Nucleus divides once, followed by cytoplasmic division. | Nucleus divides repeatedly first, followed by simultaneous cytoplasmic division to form many cells. |
| Conditions | Usually occurs under favourable conditions. | Often occurs under unfavourable conditions (e.g., Plasmodium inside a host) or favourable conditions depending on the organism. |
| Examples | Amoeba, Leishmania, bacteria. | Plasmodium, some algae. |
Question 2. How will an organism be benefited if it reproduces through spores?
Answer:
An organism that reproduces through spores benefits in several ways:
- **Survival:** Spores are often covered by thick protective walls that enable them to survive in unfavourable environmental conditions like extreme heat or dryness.
- **Dispersal:** Spores are typically lightweight and can be easily dispersed over long distances by agents like wind or water, helping the organism colonize new habitats.
- **Large numbers:** Organisms can produce a very large number of spores, increasing the chances that at least some will land in suitable conditions and grow.
Question 3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?
Answer:
More complex organisms (like humans, mammals) cannot give rise to new individuals through regeneration from body parts primarily because of their high degree of **specialisation and complexity**.
- Complex organisms have a complex body design with specialised cells, tissues, organs, and organ systems that perform specific functions and are precisely organised.
- A simple cut or fragmentation would result in pieces that lack the necessary combination of all organ systems required for survival.
- Even if regeneration cells are present, they would need the blueprint and guidance to recreate a complete organism with its intricate organisation, which is a far more complex task than simply regenerating a specific organ or limb (which some complex animals can do to a limited extent). The level of coordination and development required from a fragment is too high for a complex organism.
Question 4. Why is vegetative propagation practised for growing some types of plants?
Answer:
Vegetative propagation is practised for growing some types of plants for several reasons:
- **Earlier Flowering and Fruiting:** Plants grown vegetatively often bear flowers and fruits earlier than those grown from seeds.
- **Propagation of Seedless Plants:** It allows the propagation of plants that have lost the ability to produce viable seeds (e.g., banana, orange, rose, jasmine).
- **Genetic Uniformity:** All plants produced are genetically identical clones of the parent plant, preserving desirable traits (like yield, disease resistance, fruit quality).
- **Faster and More Reliable:** It can be a faster and more reliable method of reproduction compared to growing from seeds for certain plants.
Question 5. Why is DNA copying an essential part of the process of reproduction?
Answer:
DNA contains the genetic blueprint for the body design and functions of an organism. In the process of reproduction, a new individual is created that inherits traits from the parent(s). DNA copying is essential to pass this genetic information from the parent cell(s) to the new daughter cell(s) that will develop into the new organism. This ensures that the new individual has the instructions needed to develop and function similarly to the parent(s), maintaining the characteristics of the species.
Sexual Reproduction
**Sexual reproduction** is a mode of reproduction that involves the participation of **two individuals**, typically a male and a female, to create a new generation. This mode of reproduction is distinct from asexual reproduction, which involves a single parent.
Why The Sexual Mode Of Reproduction?
Sexual reproduction has a significant advantage over asexual reproduction: it generates **greater variation** in the population. While DNA copying mechanisms are relatively precise, they are not perfectly accurate, leading to some variations in the DNA copies. In asexual reproduction, variations accumulate slowly from errors in DNA copying over generations of a single lineage.
In sexual reproduction, genetic material (DNA) from **two different individuals** is combined. Since two individuals from a population would likely have different accumulated patterns of variation, combining their DNA creates **new combinations of variants** in the offspring. Each combination is unique and novel. This process of combining variations from two parents greatly speeds up the generation of genetic diversity compared to the slow accumulation of variations from copying errors in asexual reproduction.
While increased variation is beneficial for the species' adaptation and survival, it poses a challenge for the organism: combining DNA from two parents means each new generation would have twice the amount of DNA as the previous one, potentially disrupting cellular control.
Complex multicellular organisms solve this by producing **specialized germ-cells (gametes)** that contain only **half the number of chromosomes** and half the amount of DNA compared to the non-reproductive body cells. This reduction in chromosome number is achieved by a special type of cell division called **meiosis**. When a male gamete and a female gamete combine during sexual reproduction, the original number of chromosomes and DNA content characteristic of the species is re-established in the resulting zygote.
Gametes can be similar in simple organisms, but in more complex ones, they specialize. One gamete (the **female gamete** or egg) is typically large and contains stored food for the developing embryo. The other gamete (the **male gamete** or sperm) is usually smaller and motile (able to move) to reach the female gamete.
Sexual Reproduction In Flowering Plants
The reproductive parts of flowering plants (angiosperms) are located in the **flower**. A typical flower has sepals, petals, stamens, and a pistil.
- **Stamen:** The **male reproductive part**, consisting of an anther (producing yellowish pollen grains containing male germ-cells) and a filament (stalk).
- **Pistil (or Carpel):** The **female reproductive part**, located in the center, consisting of a swollen **ovary** at the base (containing ovules), a middle **style** (stalk), and a sticky terminal **stigma** (receptive surface for pollen). Each ovule contains an egg cell (female gamete).
Flowers can be **unisexual** (containing either stamens or pistil, e.g., papaya, watermelon) or **bisexual** (containing both stamens and pistil, e.g., Hibiscus, mustard).
The process of sexual reproduction in flowering plants involves:
- **Pollination:** Transfer of pollen grains from the anther to the stigma.
- **Self-pollination:** Pollen is transferred to the stigma of the same flower or another flower on the same plant.
- **Cross-pollination:** Pollen is transferred from one flower to the stigma of another flower on a different plant of the same species. This transfer is achieved by agents like wind, water, or animals (insects, birds, etc.).
- **Fertilisation:** After landing on a suitable stigma, a pollen grain grows a tube (pollen tube) that travels through the style to reach the ovary. The male germ-cell from the pollen grain fuses with the female gamete (egg cell) inside the ovule. This fusion of gametes is called fertilisation, which forms a zygote.
After fertilisation, the zygote develops into an **embryo** within the ovule. The ovule develops a tough protective coat and matures into a **seed**. The ovary grows and ripens to form a **fruit**. Other flower parts (petals, sepals, stamens, style, stigma) usually wither and fall off.
The seed contains the embryo (future plant) and stored food. Under appropriate conditions (water, air, temperature), the embryo in the seed develops into a seedling. This process is called **germination**.
Reproduction In Human Beings
Humans reproduce sexually. The process of sexual maturation occurs during adolescence (teenage years) and is called **puberty**. Puberty is marked by significant physical changes that prepare the body for reproduction. These changes include general changes in body shape and appearance (common to both sexes, like hair growth in armpits and genital area, oily skin, pimples) and sex-specific changes (e.g., breast development and menstruation in girls, facial hair and voice cracking in boys, enlargement of sexual organs).
Puberty begins as the rate of general body growth slows down, allowing the body's resources to be directed towards the maturation of reproductive tissues and organs. These pubertal changes serve as signals of sexual maturity to other individuals and facilitate the process of mating and reproduction (transfer of germ-cells, development of offspring). In mammals like humans, the female body is adapted to carry the developing baby internally and breastfeed it after birth, requiring the maturation of the uterus and breasts.
Male Reproductive System
The human male reproductive system (Fig 7.10 in textbook) is designed to produce male germ-cells (sperms) and deliver them to the site of fertilisation.
Components:
- **Testes:** The primary reproductive organs, located outside the abdominal cavity in a sac called the **scrotum**. Sperm formation requires a lower temperature than normal body temperature, hence their external location. Testes also produce the male sex hormone **testosterone**, which regulates sperm formation and causes the changes of puberty in boys.
- **Vas deferens:** A tube that carries sperms from the testes upwards to unite with a tube coming from the urinary bladder.
- **Urethra:** A common passage for both urine (from the bladder) and sperms.
- **Associated Glands (Prostate gland and Seminal vesicles):** These glands add their secretions to the vas deferens. Their secretions provide a fluid medium (semen) for the sperms, making their transport easier and providing nutrition.
Sperms are tiny structures consisting mainly of genetic material (nucleus) and a tail for motility.
Female Reproductive System
The human female reproductive system (Fig 7.11 in textbook) is designed to produce female germ-cells (eggs), receive sperms, provide a site for fertilisation, and nurture the developing embryo and fetus.
Components:
- **Ovaries:** The primary reproductive organs, producing female germ-cells (eggs) and female sex hormones (oestrogen and progesterone). Girls are born with thousands of immature eggs in the ovaries, which start maturing at puberty. One egg matures and is released by one ovary approximately every month.
- **Oviducts (Fallopian tubes):** Thin tubes that carry the egg from the ovary to the uterus. Fertilisation typically occurs in the oviduct.
- **Uterus (Womb):** A large, elastic bag-like structure where the fertilised egg implants and develops into a fetus. The two oviducts unite here.
- **Cervix:** The narrow lower part of the uterus that opens into the vagina.
- **Vagina:** A tube-like structure that receives sperms during sexual intercourse and serves as the birth canal.
After sexual intercourse, sperms enter through the vagina, travel upwards into the uterus and fallopian tubes, where fertilisation may occur if an egg is present. The fertilised egg (zygote) begins to divide and develops into an embryo. The embryo implants in the richly blood-supplied lining of the uterus, which prepares monthly to receive and nourish a potential embryo.
The embryo receives nutrition and oxygen from the mother's blood through a special tissue called the **placenta**. The placenta is a disc embedded in the uterine wall with villi on the embryo's side surrounded by maternal blood spaces, providing a large surface area for exchange. Waste substances from the embryo are also transferred to the mother's blood through the placenta.
Development of the child (fetus) inside the mother's body takes about nine months. Birth occurs through rhythmic contractions of the uterine muscles, expelling the baby through the vagina.
What Happens When The Egg Is Not Fertilised? (Menstruation)
If the egg released by the ovary is not fertilised by a sperm within about a day, it does not implant. Since the uterus prepares itself monthly with a thickened, spongy lining and rich blood supply to nourish a potential embryo, this lining is no longer needed if fertilisation does not occur.
The uterine lining slowly breaks down and is shed from the body through the vagina as blood and mucous. This monthly process is called **menstruation** or periods. It usually lasts for about 2 to 8 days and occurs roughly every month from puberty until menopause (around age 50).
Reproductive Health
**Reproductive health** refers to physical, mental, and social well-being in all matters relating to the reproductive system at all stages of life. Sexual maturation at puberty does not automatically mean an individual is ready for sexual activity or parenthood.
Concerns regarding reproductive health include:
- **Sexually Transmitted Diseases (STDs):** Infections transmitted through sexual contact. These include bacterial infections (gonorrhoea, syphilis) and viral infections (warts, HIV-AIDS). Using protective methods like condoms can help prevent transmission to some extent.
- **Preventing Unwanted Pregnancy:** Pregnancy makes major demands on a woman's body and mind. Various **contraceptive methods** are used to avoid pregnancy:
- **Mechanical barriers:** Prevent sperm from reaching the egg (e.g., condoms, diaphragms).
- **Hormonal methods:** Change hormonal balance to prevent egg release (e.g., oral pills). These can have side effects.
- **Intrauterine Devices (IUDs):** Devices placed in the uterus (e.g., loop, copper-T) to prevent implantation. Can cause irritation.
- **Surgical methods:** Blocking the vas deferens in males (vasectomy) or fallopian tubes in females (tubectomy) to prevent sperm or egg transfer. These are generally safe for contraception but involve surgery risks.
- **Population Control:** High population growth rates pose challenges to improving living standards. Contraceptive methods play a role in family planning.
- **Ethical Issues:** Concerns like sex-selective abortion (illegal termination of pregnancy based on sex, particularly female fetuses) raise serious ethical and social issues, leading to declining female-male sex ratios in some areas. Prenatal sex determination is legally prohibited.
Responsible decision-making regarding sexual activity, contraception, and family planning is crucial for individual and societal health.
Question 1. How is the process of pollination different from fertilisation?
Answer:
| Feature | Pollination | Fertilisation |
|---|---|---|
| What happens | Transfer of pollen grains (containing male germ-cells) from the anther to the stigma of a flower. | Fusion of the male germ-cell (from pollen) with the female gamete (egg cell in the ovule) to form a zygote. |
| Location | Occurs on the stigma of the flower. | Occurs inside the ovule, within the ovary of the flower. |
| Requirement | Involves external agents (wind, water, animals) for cross-pollination. | Requires the growth of a pollen tube from the pollen grain to reach the ovule. |
| Outcome | Leads to the landing of pollen on the stigma, a prerequisite for fertilisation. | Leads to the formation of a zygote, which develops into an embryo. |
Question 2. What is the role of the seminal vesicles and the prostate gland?
Answer:
Seminal vesicles and the prostate gland are accessory glands in the male reproductive system. Their role is to produce secretions that are added to the sperms as they travel through the vas deferens and urethra. These secretions:
- Provide a **fluid medium** (semen) that facilitates the transport (movement) of sperms.
- Provide **nutrition** to the sperms, keeping them viable and motile.
Question 3. What are the changes seen in girls at the time of puberty?
Answer:
Changes seen in girls at the time of puberty include:
- Increase in breast size and darkening of nipple skin.
- Onset of menstruation (periods).
- Growth of thick hair in the armpits and pubic area.
- Possible appearance of thinner hair on legs and arms.
- Skin may become more oily, potentially leading to pimples.
- Body shape changes, with widening of hips.
- Increase in height and overall growth.
Question 4. How does the embryo get nourishment inside the mother’s body?
Answer:
Inside the mother's body, the embryo gets nourishment through a special tissue called the **placenta**. The placenta is a disc that is embedded in the uterine wall. It contains villi on the embryo's side, which are surrounded by blood spaces on the mother's side. This arrangement provides a large surface area that facilitates the efficient transfer of nutrients (like glucose, amino acids, fatty acids, vitamins, minerals), oxygen, and water from the mother's blood to the embryo's blood by diffusion. Waste substances generated by the developing embryo are also transferred from the embryo's blood into the mother's blood through the placenta for removal.
Question 5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
Answer:
No, if a woman is using a copper-T, it will **not** help in protecting her from sexually transmitted diseases (STDs). A copper-T is an intrauterine device (IUD) that is effective in preventing pregnancy by preventing implantation or affecting sperm movement. However, it does not provide a barrier to prevent the transmission of infections between partners during sexual contact. To protect against STDs, barrier methods like condoms are necessary.
Intext Questions
Page No. 114
Question 1. What is the importance of DNA copying in reproduction?
Answer:
Question 2. Why is variation beneficial to the species but not necessarily for the individual?
Answer:
Page No. 119
Question 1. How does binary fission differ from multiple fission?
Answer:
Question 2. How will an organism be benefited if it reproduces through spores?
Answer:
Question 3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?
Answer:
Question 4. Why is vegetative propagation practised for growing some types of plants?
Answer:
Question 5. Why is DNA copying an essential part of the process of reproduction?
Answer:
Page No. 126
Question 1. How is the process of pollination different from fertilisation?
Answer:
Question 2. What is the role of the seminal vesicles and the prostate gland?
Answer:
Question 3. What are the changes seen in girls at the time of puberty?
Answer:
Question 4. How does the embryo get nourishment inside the mother’s body?
Answer:
Question 5. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
Answer:
Exercises
Question 1. Asexual reproduction takes place through budding in
(a) Amoeba.
(b) Yeast.
(c) Plasmodium.
(d) Leishmania.
Answer:
Question 2. Which of the following is not a part of the female reproductive system in human beings?
(a) Ovary
(b) Uterus
(c) Vas deferens
(d) Fallopian tube
Answer:
Question 3. The anther contains
(a) sepals.
(b) ovules.
(c) pistil.
(d) pollen grains.
Answer:
Question 4. What are the advantages of sexual reproduction over asexual reproduction?
Answer:
Question 5. What are the functions performed by the testis in human beings?
Answer:
Question 6. Why does menstruation occur?
Answer:
Question 7. Draw a labelled diagram of the longitudinal section of a flower.
Answer:
Question 8. What are the different methods of contraception?
Answer:
Question 9. How are the modes for reproduction different in unicellular and multicellular organisms?
Answer:
Question 10. How does reproduction help in providing stability to populations of species?
Answer:
Question 11. What could be the reasons for adopting contraceptive methods?
Answer: