CBSE Class 10 Biology How do Organisms Reproduce Previous Year Questions

Plasmodium reproduces by a process known as multiple fission. Multiple fission is a type of asexual reproduction.

In multiple fission, the nucleus of Plasmodium divides multiple times inside a cyst. The nuclei then surround themselves with small amounts of cytoplasm, forming merozoites, which are released when the cyst bursts.

Spirogyra reproduces by fragmentation under favourable conditions.

This is an asexual type of reproduction. The Spirogyra filament breaks into smaller fragments, and each fragment grows into a new organism.

Pollination allows pollen grains that produce the male germ cell to reach the carpel, which contains the female germ cell (egg).

Thus, fertilisation — which involves the fusion of male and female germ cells — can only occur after pollination. Without pollination, the male gamete cannot reach the female gamete, making fertilisation impossible.

The parts of a bisexual flower that are not directly involved in reproduction are:

1. Sepals (Calyx)
2. Petals (Corolla)
3. Thalamus

Testis — produces sperms (male gametes).
Ovary — produces ova (female gametes / eggs).

Two functions of the ovary:

1. To produce the female gamete / ovum.
2. To secrete female hormones — estrogen and progesterone.

Any animal that lays eggs after internal fertilisation, such as birds, lizards, or other reptiles.

(a)
The reproductive parts of bread mould (Rhizopus) are the sporangia (sporangiospores), which are involved in the production and dispersal of spores for reproduction.

The non-reproductive parts include the mycelium, which consists of hyphae responsible for nutrient absorption and growth. The spores are covered by thick walls that protect them until they come in contact with a moist surface and begin to grow.

(b) Advantages of vegetative propagation:

1. Offspring are genetically identical to the parent, so useful traits can be preserved.
2. It is a rapid and economical method of propagation.
3. Seedless varieties can be obtained through this method.

(a)
The greenish-black powdery mass on a stale piece of bread is due to bread mould Rhizopus, which reproduces by spore formation.

(b)
Hyphae (thread-like structures) are the vegetative part, and tiny bulb-like structures called sporangia are the reproductive parts.

(a)
In Amoeba, splitting of the cell into two during cell division can take place in any plane.

Leishmania has a whip-like structure at one end of the cell (a flagellum). Hence, binary fission occurs in a definite orientation in relation to this flagellum.

(b)
Plasmodium (the malarial parasite) reproduces by the process of multiple fission. Within a cyst, the nucleus of the cell divides multiple times. The nuclei then surround themselves with a small amount of cytoplasm, forming merozoites that are released when the cyst bursts.

The stamen and pistil are the reproductive parts of an angiosperm. They are located in the flower.

Structure of the Stamen (male reproductive part):

The stamen consists of:

1. Anther: A sac-like structure that produces pollen grains containing the male germ cells.
2. Filament: A thin stalk-like structure that supports the anther and holds it in position.

The part labelled as A is the plumule;
B is the cotyledon;
C is the radicle.

Part B (the cotyledon) is considered important during germination because it serves as a food store. It provides nutrients to the growing embryo during germination, before the seedling is able to photosynthesise on its own.

Events after fertilisation in a flowering plant:

1. Fertilisation results in the formation of a zygote.
2. The zygote divides several times to form an embryo.
3. The ovule develops a thick coat and changes into a seed.
4. The ovary grows rapidly and ripens to form the fruit.

(i) The developing embryo gets nutrition from the mother’s blood with the help of the placenta. The placenta contains villi on the embryo’s side that are surrounded by blood spaces on the mother’s side. These villi provide a large surface area that facilitates the passage of nutrition and oxygen to the embryo from the mother. Waste substances produced by the embryo are also removed through the placenta into the mother’s blood.

(ii) Every month, one egg is released by the ovary, and the uterus prepares itself to receive a fertilised egg. As a result, the lining of the uterus becomes thick and spongy to provide nourishment to the embryo after fertilisation.

Puberty is the period of sexual maturity. In this phase, the rate of general body growth begins to slow down, and the reproductive tissue begins to grow and mature.

Two changes common to both boys and girls in early teenage years:

1. Thick hair growth in the armpits and genital area.
2. Skin becomes oily, which may result in pimples (acne).

(a)
(i) Oral contraceptive pills act by changing the hormonal balance of the body so that eggs are not released. As these pills alter hormonal balance, they may cause side effects to the body.

(ii) Copper T is placed in the uterus to prevent pregnancy. It can cause side effects due to irritation of the uterus.

(b)
The condom acts as a mechanical barrier that prevents the meeting of sperm and egg.

The condom is better than other methods because it also prevents the direct contact of body fluids during sexual intercourse, and thus prevents the transmission of sexually transmitted diseases (STDs).

(a)
The path of the male gamete (sperm) in the female reproductive tract:
Vagina → Uterus → Fallopian tube, where fertilisation occurs to form a zygote.

(b)
A zygote has 2 sets of chromosomes (2n = 46 chromosomes in humans), one set inherited from each parent.

(i) Oral pills:
Change the hormonal balance of the body so that eggs are not released, preventing fertilisation.

(ii) Loop / Copper-T:
Placed in the uterus to prevent pregnancy by checking the entry of sperm through the vagina and preventing implantation.

(a) Differences between Binary Fission and Multiple Fission



(b)
A mature Spirogyra filament breaks into smaller pieces, and each fragment develops into a new organism through the process of fragmentation.

The main difference between asexual and sexual reproduction is genetic diversity:
— Asexual reproduction yields genetically identical offspring.
— Sexual reproduction produces genetically diverse offspring.

Species reproducing sexually have better chances of survival due to genetic variation. This variation enables populations to adapt to changing environments, develop disease resistance, and enhance evolutionary potential — advantages that asexually reproducing species lack.

(a) Each piece regenerates into a new complete Planaria through the process of regeneration, carried out by specialised cells.

(b) The Bryophyllum leaf has buds at its notches (leaf margins), which develop into new plants through vegetative propagation.

(c) When sporangia burst, they release spores, which germinate into new mycelium under moist and warm conditions.

In asexual reproduction: Only mitotic divisions are involved, so the chromosome number remains the same in all daughter cells. The DNA (in the chromosomes) is copied and equally divided between the two daughter cells.

In sexual reproduction: Specialised germ cells with only half the number of chromosomes (haploid) are formed through meiosis. When the germ cells from two individuals combine during fertilisation, the original chromosome number is restored in the offspring.

Example: In humans, both father and mother have 46 chromosomes (23 pairs). The sperm has 23 chromosomes and the egg also has 23 chromosomes. When the sperm and egg fuse, the zygote has 46 chromosomes — the same as the parents. Thus, the chromosome number remains constant across generations.

After pollen lands on a suitable stigma, the pollen grain absorbs water and nutrients and produces a pollen tube. The pollen tube grows through the style and reaches the ovary. Its tip contains a tube nucleus and two male gametes.

The advancing pollen tube enters an ovule through the micropyle and reaches the interior of the embryo sac. Here, the tube bursts to release the two male gametes. One male gamete fuses with the egg to form a zygote (syngamy).



The second male gamete fuses with the binucleate central cells, forming the endosperm. Since three haploid nuclei are involved, this is known as triple fusion. Together, syngamy and triple fusion constitute double fertilisation, which is unique to angiosperms.

Pollination: Transfer of pollen grains from the stamen/anther to the stigma of a flower.

Fertilisation: The fusion of the male and female germ cells (gametes) inside the ovule.

Site of fertilisation: Ovary / Ovule.

Product of fertilisation: Zygote.

The carpel is the female reproductive part of a flower. It consists of three parts:

1. Stigma: Receives pollen grains during pollination.
2. Style: A slender tube that provides a passage for the growth of the pollen tube from the stigma to the ovary.
3. Ovary: The site where fertilisation occurs; contains the ovules, each with a female germ cell (egg).

A. Anther: It produces pollen grains containing the male germ cells.

B. Style: It provides the path through which the pollen tube grows and reaches the ovary.

C. Ovary: It contains ovules, each with an egg cell (female gamete). After fertilisation, it develops into the fruit.

Future shoot: Plumule
Future root: Radicle

Function of cotyledon: It stores food for the future plant or embryo, providing nutrients during germination before the seedling can photosynthesise on its own.

Part A — Stigma:
It is the terminal part of the carpel, which may be sticky and helps in receiving the pollen grains from the anther of the stamen during pollination.

Part B — Pollen tube:
The pollen tube grows out of the pollen grain through the style to reach the ovary. It carries male gametes into the embryo sac in the ovule.

Part C — Female germ cell (egg):
It is the female gamete, which fuses with the male gamete to form a diploid cell known as the zygote.

The total number of chromosomes in humans is 46.
In a human male, the two sex chromosomes are X and Y.
In a human female, both sex chromosomes are X and X.

During sexual reproduction, a female gamete (egg) fuses with a male gamete (sperm). Both are haploid (containing 23 chromosomes each). When they fuse, they form a diploid zygote with 46 chromosomes — 23 from the mother and 23 from the father. In this way, an equal genetic contribution from both parents is ensured, and the chromosome number remains constant across generations.

Changes in fertilised egg:

(a) The zygote (fertilised egg) starts dividing repeatedly to form an embryo.
(b) Implantation of the zygote occurs in the inner uterine wall.
(c) The embryo grows with the help of the placenta, which facilitates exchange of nutrients and oxygen, resulting in the development of the child.
(d) The birth of a child occurs as a result of rhythmic contraction of the muscles in the uterus.

When the egg is not fertilised:
The inner lining of the uterus slowly breaks down and comes out through the vagina as blood and mucus. This process is called Menstruation.

Three reasons to justify the use of contraceptive methods:

(a) To prevent unwanted pregnancies.
(b) To control population and birth rate.
(c) To prevent the transfer of sexually transmitted diseases (STDs).

(a)


Steps in binary fission: The nucleus first divides mitotically into two equal halves, followed by the division of the cytoplasm (cytokinesis). The parent cell finally splits into two daughter cells, each with a complete copy of the genetic material.

(b)
Multicellular organisms cannot reproduce by a single cell because they are not made up of a simple random collection of cells. In multicellular organisms, specialised cells are organised into tissues, which are organised into organs. Cell-by-cell division would be impractical for such organisms.

Therefore, multicellular organisms require more complex ways of reproduction such as sexual reproduction, which ensures the proper development and functioning of the organism.

(i) Two modes of asexual reproduction in Hydra:

1. Budding: A bud develops as an outgrowth in Hydra due to repeated cell division at a specific site. When fully mature, the bud detaches from the parent body and develops into a new independent individual.

2. Regeneration: Small cut or broken parts of Hydra’s body grow or regenerate into separate individuals. Regeneration is carried out by specialised cells that proliferate and make a large number of cells to form a complete organism.

(ii) Vegetative propagation is the development of a new plant from the vegetative parts (roots, stems, and leaves) of a plant.

Two advantages:
1. Plants produced through vegetative propagation can bear flowers and fruits earlier than those grown from seeds.
2. It allows the propagation of plants that have lost the capacity to produce seeds (e.g., banana, orange).

Vegetative propagation is the development of a new plant from the vegetative parts (roots, stems, and leaves) of a plant.

Three advantages of Vegetative Propagation:

1. Earlier Flowering and Fruiting: Plants produced through vegetative propagation can bear flowers and fruits earlier than those grown from seeds.

2. Propagation of Seedless Plants: It allows the propagation of plants (e.g., banana, orange) that have lost the capacity to produce seeds.

3. Genetic Similarity: All plants produced are genetically identical to the parent plant and thus retain its desirable characteristics.

Plants grown by vegetative propagation from the given list:
— Jasmine
— Banana

(a) Asexual Reproduction Methods in Different Organisms:

1. Binary Fission in Amoeba: The nucleus first divides mitotically into two, followed by division of the cytoplasm. The cell finally splits into two daughter cells. From one Amoeba parent, two daughter amoebae are formed.

2. Budding in Hydra: A bud develops as an outgrowth due to repeated cell division at one specific site. When fully mature, the bud detaches itself from the parent body and develops into a new, independent individual.

3. Regeneration in Planaria: Small cuts or broken parts of Planaria’s body grow or regenerate into separate individuals. Planaria can be cut into any number of pieces, and each piece grows into a complete organism.

(b) Differences between Sexual and Asexual Reproduction:

(i) Sexual reproduction requires two parents; asexual reproduction requires only one parent.
(ii) Offspring in sexual reproduction are genetically dissimilar from parents; offspring in asexual reproduction are genetically identical to the parent.

(a) During reproduction, genetic information is passed from parents to offspring through DNA. If the DNA copying process is altered, different proteins will be produced, which can lead to changes in body designs.

(b) Fertilisation requires the fusion of the male and female gametes. If pollination does not occur, the male gamete (pollen) is not transferred to the female gamete (egg), so fertilisation cannot take place.

(c) Regeneration and fragmentation can only occur when the organism’s body is made up of relatively similar types of cells. Complex multicellular organisms have highly differentiated tissues and organ systems specialised for particular functions, preventing them from regenerating a complete new individual from a body fragment.

(d) Vegetative propagation is used specifically for plants that do not form viable seeds or produce very few seeds (e.g., banana, pineapple), and for preserving the quality of a particular plant variety. It is not suitable for all plants.

(e) Gametes in sexually reproducing animals are haploid (half the number of chromosomes). During fertilisation, the male and female gametes fuse to form a diploid zygote with the same chromosome number as the parents, ensuring continuity of the chromosome number across generations.

Pollination is the transfer of pollen from the anther (stamen) to the stigma of a flower.

Types of Pollination:

1. Self-Pollination: Pollen is transferred from the anther to the stigma within the same flower or a flower of the same plant.

2. Cross-Pollination: Pollen is transferred from the anther of one flower to the stigma of another flower on a different plant of the same species. This introduces genetic variation.

Two Agents of Pollination: Wind, Water, Insects, Animals (any two).

How pollination leads to fertilisation:
After pollination, a tube grows out of the pollen grain, travelling through the style to reach the female germ cell in the ovary, where fertilisation occurs — the male gamete fuses with the female gamete to form a zygote.

Unisexual Flower: Papaya / Watermelon (or any other)
Bisexual Flower: Hibiscus / Rose (or any other)

Types of Pollination:
1. Self-Pollination: Pollen grains are transferred from the anther to the stigma of the same flower or to a flower of the same plant.
2. Cross-Pollination: Pollen grains are transferred from the anther to the stigma of a flower of a different plant.

Events after pollen lands on a suitable stigma:

1. Pollen Tube Growth: A pollen tube grows out of the pollen grain and travels through the style to reach the ovary.
2. Fertilisation: The male germ cell (from pollen) fuses with the female germ cell (egg) in the ovule to form a zygote.
3. Embryo Formation: The zygote divides several times to form an embryo within the ovule.
4. Seed Development: The ovule develops a tough coat and gradually converts into a seed.

(a) Two reasons why the use of a condom is beneficial for both sexes:

1. Prevents Unwanted Pregnancies and STDs: The use of a condom helps avoid unwanted pregnancies and provides protection against sexually transmitted diseases (STDs).
2. Prevents Transmission of Infections: Condoms act as a barrier that prevents the transmission of infections from one person to another, ensuring safer sexual activity for both.

(b) Oral contraceptives alter the hormonal balance of the body, preventing the release of eggs (ovulation). This reduces the chances of fertilisation and helps avoid pregnancies.

(c) Sex-selective abortion is the procedure of terminating pregnancies based on the sex of the foetus, often done to eliminate female foetuses (female foeticide).

Impact on society: This practice adversely affects the male-to-female sex ratio, leading to a disproportionate number of males compared to females, which causes serious social and demographic imbalances.

(a) Categories of Contraceptive Methods (any two):

1. Mechanical Barrier: Prevents sperm from reaching the egg (e.g., condom).
2. Contraceptive Pills: Alter the hormonal balance to prevent release of eggs.
3. Contraceptive Devices: IUDs placed in the uterus to prevent pregnancy.
4. Surgical Methods: Vasectomy (in men) or tubal ligation (in women).

(b) Sexually transmitted infections:
Bacterial: Gonorrhea, Syphilis
Viral: AIDS (HIV), Warts (HPV)

(c) Two advantages of using a condom:
1. Prevents Unwanted Pregnancies — acts as a contraceptive by blocking sperm from reaching the egg.
2. Prevents Transmission of STDs/STIs — protects both partners from sexually transmitted infections such as HIV and gonorrhea.

(a)
Prostate glands and seminal vesicles add their secretions so that the sperms are in a fluid (semen). This makes their transport easier and also provides nutrition for the sperms.

Testes secrete testosterone, which brings about changes in the appearances of boys at the time of puberty (secondary sexual characteristics) and is essential for sperm production.

(b)
Surgical removal of pregnancies is misused for female foeticide — the illegal sex-selective abortion of female foetuses — which severely disturbs the male-to-female sex ratio.

(c)
Oral contraceptive pills interfere with the hormonal signals that trigger the release of eggs (ovulation). Since eggs are not released, fertilisation cannot take place and conception is prevented.

(a)


Correct parts to be marked:
(i) Fallopian Tube / Oviduct — where tubal ligation is performed.
(ii) Uterus — where CuT (Copper-T) is inserted.
(iii) Vagina — inside which a condom can be placed.

(b)
People prefer the use of condoms because they prevent STDs (Sexually Transmitted Diseases) and provide privacy to the user.

Principle of condom use: Condoms create a mechanical barrier, preventing the meeting of sperms and ovum, thus preventing fertilisation.

(a)

1. Testis: Responsible for the production of sperm (spermatogenesis) and secretion of testosterone, which is essential for the development of male secondary sexual characteristics.

2. Scrotum: Houses and protects the testes and helps regulate their temperature, ensuring an environment cooler than body temperature that is conducive to sperm production.

3. Vas deferens: A duct that transports mature sperm from the epididymis to the urethra during ejaculation.

4. Prostate glands: Secrete prostatic fluid that becomes part of semen, nourishing and protecting the sperm and enhancing their motility and viability.

(b)
The placenta is an organ that develops during pregnancy and forms a connection between the mother and the developing embryo/foetus.

Functions of the placenta:
1. Facilitates exchange of nutrients, oxygen, and waste products between the mother’s bloodstream and the developing foetus.
2. Produces hormones such as progesterone and estrogen, which maintain the pregnancy and support the growth and development of the foetus.
3. Eliminates waste products (urea, uric acid) produced by the foetus, transferring them to the mother’s bloodstream for excretion.

(a) Sperm formation will be adversely affected because it requires a lower temperature than body temperature. The testes need to be in the scrotum (2–3°C cooler) for sperm to develop properly.

(b) The vas deferens is the passage for transfer of sperms, so sperms will not be transferred further and the man will be infertile (this is the principle behind vasectomy).

(c) The prostate and seminal vesicles will not add secretions for nourishment and medium for transport of sperms, making sperms unable to survive or travel effectively.

(d) The unfertilised egg lives for about one day. Then, the thickened lining of the uterus breaks, leading to the discharge of blood and mucus along with the unfertilised egg. This is called menstruation.

(e) Nutrition and oxygen will not be adequately provided to the growing embryo, seriously affecting its growth and development, which could have life-threatening implications for the foetus.

1. Two common signs of sexual maturation in boys and girls (any two):
— Thick hair growth in armpits and genital area.
— Thinner hair on arms, legs, and face.
— More active oil secretion from glands on skin.
— Occurrence of pimples (acne).

OR — Two factors that determine the size of a population:
1. Rate of birth.
2. Rate of death.

2. Reckless female foeticide results in an imbalance in the male-to-female ratio and a decline in the child sex ratio, causing serious social and demographic problems.

3. Oral pills (oral contraceptives) change the hormonal balance of the body to prevent the release of eggs.