NCERT Exemplar Solutions Class 10 Science Chapter 7

Sort animal from microbe and plant. The single animal in the list gives the game away, because animals like a dog reproduce sexually.

Concept used. Asexual reproduction uses one parent and no gametes and gives offspring identical to the parent. Vegetative propagation, budding and binary fission are all asexual.

1. Banana (i). New banana plants sprout from the parent's underground stem, so this is vegetative and asexual.
2. Yeast (iii). A bud grows on the parent yeast cell and breaks off as a new cell. Budding is asexual.
3. Amoeba (iv). The single cell splits into two by binary fission, an asexual method.
4. Dog (ii). A dog produces young by mating, with male and female gametes fusing. That is sexual, so it is left out.

Why this matters. Recognising that many crop plants are propagated asexually explains why a whole banana field can be a single clone, which is useful but risky if a disease strikes.

Option (b): the asexual reproducers are banana, yeast and Amoeba.

Match gamete to the exact part. Each option must name the precise gamete-making structure, not the whole organ.

Concept used. The anther is the pollen-making tip of the stamen, and the ovary is the swollen base of the pistil that contains the ovules with the egg cells.

1. Male side. Pollen, which carries the male gamete, is made in the anther.
2. Female side. The egg, the female gamete, sits inside an ovule, and ovules are housed in the ovary.
3. Reject the rest. Filament, stigma and style only support or receive, they do not make gametes, so only "anther and ovary" fits.

Why this matters. Knowing exactly where each gamete forms is the key to following the path of fertilisation, where pollen from the anther must reach the egg inside the ovary.

Option (c): anther (male gamete) and ovary (female gamete).

Order by cause and effect. Each stage is the cause of the next, so arranging them by which must happen first gives the answer at once.

Concept used. Pollination delivers pollen; fertilisation fuses gametes into a zygote; the zygote becomes the embryo; germination of the seed gives the seedling.

1. First, pollination. Without pollen reaching the stigma, nothing else can begin.
2. Then fertilisation. The pollen's male gamete fuses with the egg to make a zygote.
3. Then the embryo. The zygote divides and grows into the embryo packed inside the seed.
4. Finally the seedling. When the seed germinates, the embryo grows out as a young seedling. So the order is option (c).

Why this matters. Getting this sequence right helps students explain why an unpollinated flower never sets seed and why a seed is really a ready-made baby plant waiting to grow.

Option (c): the correct sequence is pollination, fertilisation, embryo, seedling.

Where does the DNA come from? Similarity comes from a single, unmixed source of DNA, so look for the two statements that point to that.

Concept used. Clones are alike because all their DNA is copied from one parent with no input from a second parent and no shuffling through gametes.

1. One parent (i). With a single parent, there is only one set of genes to inherit, so the offspring match the parent.
2. No gametes (ii). Gametes are where two parents' genes mix. With no gametes, no such mixing happens, so no new combinations appear.
3. Timing is a distractor. When asexual reproduction occurs relative to sexual reproduction (iii, iv) has nothing to do with how similar the offspring are.

Why this matters. This explains why a cutting from a mango tree gives fruit just like the parent, a fact gardeners rely on to keep a prized variety unchanged.

Option (a): statements (i) and (ii) explain the high similarity.

Find the unit of inheritance. Among the cell parts listed, only one is the recognised carrier of inherited information.

Concept used. Genes, made of DNA, are the units of heredity. They sit on chromosomes in the nucleus and are copied and passed on during reproduction.

1. Genes (d). Each gene is a stretch of DNA that decides a trait and is handed down to the next generation.
2. Cytoplasm (a). It is the jelly that holds the cell parts but is not the store of heredity.
3. Ribosomes and golgi (b, c). These build and package proteins; they do not carry the inherited code.
4. So the only correct answer is genes.

Why this matters. Pinning heredity to genes sets up the next chapter on Heredity, where students learn how dominant and recessive genes decide the traits a child shows.

Option (d): inherited characters are stored in the genes.

Copying is faithful, not flawless. The truth lies between the extremes, so the "both" option is the natural choice.

Concept used. Inheritance passes on DNA copies that keep most features the same (similarity) but include minor copying changes and gene mixing (variation).

1. Source of similarity. Genes are copied accurately, so most traits look like the parents'.
2. Source of variation. Tiny copying errors and the reshuffling of genes in sexual reproduction create small differences.
3. Conclusion. Both effects happen together, so offspring carry similarities and variations at once.

Why this matters. Variation lets a species adapt to changing surroundings, while similarity keeps the species recognisable, a partnership that is central to survival.

Option (c): characters show both similarities and variations.

Test each option against all three. A common feature must be true for Amoeba, Spirogyra and Yeast together.

Concept used. Binary fission, fragmentation and budding are all asexual methods, so the reproductive habit is what unites these three very different organisms.

1. Asexual (a). Amoeba uses fission, Spirogyra uses fragmentation, Yeast uses budding. All asexual, so this is shared.
2. Unicellular (b). Amoeba and Yeast are unicellular, but Spirogyra is multicellular, so this is not common.
3. Only sexual (c) and multicellular (d). Both are false for at least one of the three, so they are rejected.

Why this matters. It shows that a single reproductive strategy, asexual reproduction, is used across unrelated groups, from animals to algae to fungi.

Option (a): the shared feature is asexual reproduction.

Match the body to the method. A long, thread-like body suggests breaking into pieces, which is fragmentation.

Concept used. Fragmentation is an asexual method in which a multicellular body splits into fragments, each able to grow into a new individual.

1. The body type. Spirogyra is a filament of many cells, so it can break across its length.
2. The method. The filament breaks into smaller bits, and each bit grows into a new filament.
3. Reject the rest. Splitting one cell into two (b) is binary fission, into many (c) is multiple fission, and budding (d) is for yeast, none of which describes Spirogyra.

Why this matters. Fragmentation lets a single filament quickly fill a pond with copies of itself, which is why Spirogyra can bloom rapidly in still water.

Option (a): asexual reproduction in Spirogyra is by fragmentation.

Count the daughters. The word "several cells" rules out the methods that give only two cells or a single bud.

Concept used. Multiple fission produces many daughter cells from one parent cell in a single round, unlike binary fission which gives two.

1. The clue. The question says the cell divides into several cells, pointing to many offspring at once.
2. Multiple fission (d). Plasmodium first makes many nuclei, then splits into many cells together. This matches the clue.
3. Reject the rest. Budding (a) gives one outgrowth, reduction division (b) is meiosis, and binary fission (c) gives only two cells.

Why this matters. This burst of many parasites at once is exactly what causes the sudden fever cycles of malaria, linking the biology to a real disease.

Option (d): the method in Plasmodium is multiple fission.

Build the chain from the start. Gametes are the starting point, so any sequence that does not begin with them is wrong.

Concept used. Reproduction proceeds gamete → zygote (by fusion) → embryo (by growth) → seedling (by germination).

1. Gametes first. Pollen and egg cells are produced before anything can fuse.
2. Zygote next. Fusion of the two gametes gives the zygote.
3. Embryo then seedling. The zygote grows into an embryo inside the seed, which germinates into a seedling. Only option (a) follows this order.

Why this matters. This chain links the microscopic gamete to the visible seedling, helping students see reproduction as one connected story rather than separate facts.

Option (a): the correct order is gametes, zygote, embryo, seedling.

Halve then double back. The trick to constant chromosome number is to halve at gamete formation and restore at fusion.

Concept used. Meiosis during gamete formation reduces the chromosome number to half. Fertilisation then combines two halves into one full set in the offspring.

1. Before gametes. Body cells carry the full chromosome number, for example 2n.
2. During gamete formation. Meiosis halves this to n in each gamete.
3. At fertilisation. Two gametes (n + n) fuse to give 2n again in the zygote, so the species number stays fixed.

Why this matters. If the number were not halved before fusion, it would double every generation. This control is what keeps every member of a species with the same chromosome count.

Option (b): halving during gamete formation, restored at fertilisation, keeps the number constant.

Name the fungal thread. Fungi are built from threads, and the correct biological name for them is hyphae.

Concept used. Hyphae are the tubular filaments that make up a fungus. Reproductive hyphae bear sporangia, the spore-producing cases, at their tips.

1. The structure. Rhizopus consists of fine tubular threads.
2. The name. These threads are called hyphae, not filaments, rhizoids or roots.
3. The function. The upright hyphae carry sporangia at their tips, releasing spores for reproduction.

Why this matters. Recognising hyphae and sporangia explains how a fuzzy mould spreads so fast over food, releasing thousands of spores into the air.

Option (b): the spore-bearing tubular threads of Rhizopus are hyphae.

Vegetative means non-flower parts. The word "vegetative" excludes the reproductive parts, so flowers and fruits cannot be the source.

Concept used. Vegetative propagation uses the vegetative organs, stem, root and leaf, to make new plants asexually, giving clones of the parent.

1. Stem. Potato, sugarcane and ginger sprout new plants from the stem.
2. Root. Sweet potato and dahlia grow from roots.
3. Leaf. Bryophyllum grows tiny plantlets along its leaf margins. So stem, roots and leaves together is the correct set.

Why this matters. Because the new plants are clones, a desirable variety, such as a seedless fruit, can be multiplied exactly through vegetative propagation.

Option (b): new plants form from stem, roots and leaves.

Separate cause from structure. The factors that cause spread are about numbers and conditions, not about the shape of the mould's parts.

Concept used. Rapid growth of a mould depends on a high spore count and a favourable environment, namely moisture and nutrients.

1. Many spores (i). The more spores released, the more new colonies can form, speeding the spread.
2. Moisture and nutrients (ii). Moist, nutrient-rich bread lets spores germinate and hyphae grow quickly.
3. Structures (iii, iv). Hyphae and sporangia are how the mould is built and how it makes spores, but they are not the driving reasons for fast spread.

Why this matters. This explains why bread kept in a warm, damp place goes mouldy in days, while dry, refrigerated bread stays fresh much longer.

Option (c): statements (i) and (ii) explain the rapid spread.

Trace the tube's journey. The tube starts where the pollen sits and ends where the egg is, so its length is fixed by those two points.

Concept used. The pollen tube carries the male gamete from the stigma, through the style, to the ovule inside the ovary.

1. Start point. The pollen grain rests on the upper surface of the stigma.
2. End point. The tube must reach the ovule, deep inside the ovary.
3. The length. So the tube is as long as the distance from the pollen on the stigma to the ovule, which is exactly option (b).

Why this matters. In flowers with a long style, the pollen tube has to grow much longer, which is one reason some pollen grains succeed and others do not in the race to the egg.

Option (b): tube length depends on the stigma-to-ovule distance.

Test each claim for exceptions. A statement with the word "always" or "all" often fails, so check those first.

Concept used. Flowers are reproductive organs that turn into fruits after fertilisation, but they can be bisexual or unisexual and occur only in flowering plants.

1. (ii) true. The flower is indeed the plant's sexual reproductive organ.
2. (iv) true. After fertilisation the ovary ripens into a fruit.
3. (i) false. Flowers are not always bisexual; unisexual flowers exist.
4. (iii) false. Only flowering plants make flowers, not all plant groups. So the true pair is (ii) and (iv).

Why this matters. Knowing that the ovary becomes the fruit explains why a fruit always carries the seeds, the next generation, safely inside it.

Option (d): only statements (ii) and (iv) are true.

One organ only. Unisexual means a single sex organ, so any statement claiming both organs is automatically false.

Concept used. Unisexual flowers carry only stamens or only a pistil. With the sexes on separate flowers, cross pollination is needed, and only the pistil-bearing flower can make a fruit.

1. (ii) true. A unisexual flower has either stamen or pistil.
2. (iii) true. Pollen from a male flower must reach a female flower, so cross pollination happens.
3. (iv) true. Without a pistil and ovary, a male-only flower cannot set fruit.
4. (i) false. Having both organs would make it bisexual, not unisexual. So the correct set is (ii), (iii) and (iv).

Why this matters. This is why in crops like papaya only the female trees bear fruit, and growers must keep some male trees nearby for pollination.

Option (b): statements (ii), (iii) and (iv) are correct.

Clones are the odd one out. Three statements describe sexual reproduction correctly; the "clone" claim belongs to asexual reproduction.

Concept used. Sexual reproduction joins two different gametes through compulsory fertilisation to form a zygote, and the resulting offspring are genetically varied, not identical.

1. (i) true. Two gamete types, male and female, are required.
2. (ii) and (iii) true. Their fusion is compulsory and always produces a zygote.
3. (iv) false. Mixing two parents' genes gives varied offspring, not clones. So the correct set is (i), (ii) and (iii).

Why this matters. The variation produced by sexual reproduction is what lets a species adapt and survive changing conditions, the very advantage clones lack.

Option (c): statements (i), (ii) and (iii) are correct.

Read the seed by position. Top means shoot, bottom means root, and the large stored-food part is the cotyledon.

Concept used. In a germinating dicot seed the plumule (shoot tip), cotyledons (food store) and radicle (root tip) are arranged from top to bottom.

1. A (top). The upper part that becomes the shoot is the plumule.
2. B (middle). The thick seed leaf that feeds the embryo is the cotyledon.
3. C (bottom). The lower part that grows down into soil as the root is the radicle.
4. Reading A, B, C in order gives plumule, cotyledon, radicle, which is option (c).

Why this matters. Spotting these parts explains how a seed already holds a tiny complete plant, with its shoot, root and food packed and ready to germinate.

Option (c): A plumule, B cotyledon, C radicle.

Two same-species parents. Variation needs two parents, but they must belong to the same species for reproduction to work at all.

Concept used. Sexual reproduction blends genetic material from two parents of one species, producing offspring with fresh combinations of genes and hence variation.

1. Two parents. Each parent contributes one gamete, so the offspring gets two sets of genes.
2. Same species. The parents must be of the same species, since members of different species generally cannot interbreed.
3. Result. The two gene sets combine in new ways, giving variation among the offspring, which is option (b).

Why this matters. This variation is the toolkit of evolution: it gives a population many slightly different individuals, some of which may cope better when conditions change.

Option (b): genetic material from two parents of the same species causes variation.

Individual versus species. An organism can live without reproducing, so the purpose of reproduction must be about the species, not the individual.

Concept used. Life processes keep an individual alive, but reproduction exists to create the next generation and so continue the species.

1. Not for staying alive. An organism survives by nutrition, respiration and the other life processes, not by reproducing.
2. The real role. Reproduction produces offspring that carry the species forward.
3. The conclusion. So reproduction is essential to continue the species generation after generation, option (d).

Why this matters. This is why a species can survive long after every one of its original members has died, each generation handing life on to the next.

Option (d): reproduction keeps the species going across generations.

General growth versus puberty change. Separate everyday growth from the special changes that only appear at sexual maturity.

Concept used. Sexual maturation at puberty causes specific changes, such as a deepening voice in boys, driven by sex hormones. Height and weight gain are part of general growth.

1. Cracking voice (c). The larynx grows at puberty, so the voice deepens and cracks. This is a sexual-maturation change.
2. Height and weight (b, d). These rise steadily through childhood as general growth, not just at puberty.
3. Milk teeth (a). Losing milk teeth happens in early childhood, well before puberty. So the answer is (c).

Why this matters. Knowing which changes mark puberty helps young students understand their own bodies and see these changes as a normal, healthy part of growing up.

Option (c): cracking of voice marks sexual maturation in boys.

Find the reproductive signal. Among the puberty changes, only one directly shows the reproductive system has started working.

Concept used. Menstruation is the monthly shedding of the uterine lining and begins when the ovaries start releasing eggs, marking the reproductive phase.

1. Menstruation (d). Its onset shows the ovaries are now releasing eggs, so the reproductive phase has begun.
2. Body growth (a) and hair pattern (b). These are general or secondary puberty changes, not the reproductive marker.
3. Voice change (c). This is more marked in boys and is not the signal of the female reproductive phase. So the answer is menstruation.

Why this matters. Understanding menstruation as a normal sign of maturity helps remove the stigma around it and supports good menstrual health among students.

Option (d): menstruation marks the onset of the reproductive phase.

Why outside the body? The position of the testes makes sense only when you think about the temperature sperm formation needs.

Concept used. The testes produce sperm, and this process works best a few degrees below body temperature, which is why the scrotum holds them outside the abdomen.

1. The need. Sperm form properly only at a temperature lower than the core body temperature.
2. The solution. The scrotum, hanging outside the body, keeps the testes cooler than the inside of the body.
3. The result. This cooler environment supports healthy sperm formation, so option (b) is the key reason.

Why this matters. This explains a real fertility concern: anything that overheats the testes, such as very tight clothing, can lower sperm production.

Option (b): the scrotum aids sperm formation by keeping the testes cool.

List what the testes truly do. First name the genuine testis functions, then the two that remain are the answer.

Concept used. The testes have two jobs at puberty: making sperm and secreting testosterone. Placenta formation and estrogen secretion are female or pregnancy roles, not testis roles.

1. Real functions. The testes form germ cells (i) and secrete testosterone (ii).
2. Placenta (iii). It develops in the uterus during pregnancy, so it is not a testis function.
3. Estrogen (iv). This is the female hormone from the ovaries, not the testes. So (iii) and (iv) are the non-functions, option (c).

Why this matters. Sorting male from female reproductive roles keeps the two systems clear, which helps when comparing the testes and ovaries side by side.

Option (c): statements (iii) and (iv) are not functions of the testes.

Follow the sperm, not the urine. Pick the tube made for sperm (vas deferens) and avoid the urine tube (ureter).

Concept used. Sperm travel from the testis through the vas deferens to the urethra, which is the shared exit. The ureter belongs to the urinary system.

1. Start at the testis. This is where sperm are made.
2. Through the vas deferens. This tube carries sperm away from the testis.
3. Out through the urethra. The urethra is the final passage to the outside. So the correct order is option (a).

Why this matters. The urethra in males carries both urine and sperm at different times, a neat example of one tube serving two systems, which is a favourite exam point.

Option (a): testis, vas deferens, urethra.

Group the STDs together. Three of the four are classic STDs, so the odd one out, spread mainly another way, is the answer.

Concept used. STDs pass through sexual contact. Syphilis, gonorrhoea and HIV-AIDS are STDs; hepatitis A spreads chiefly through contaminated food and water.

1. Syphilis and gonorrhoea (a, d). Bacterial STDs spread by sexual contact.
2. HIV-AIDS (c). A viral STD spread by sexual contact and infected blood.
3. Hepatitis (b). The common form spreads mainly through contaminated food and water, so it is the one that is not chiefly an STD.

Why this matters. Correctly grouping diseases by how they spread is the first step in choosing the right way to prevent each one.

Option (b): hepatitis is the disease not chiefly transmitted sexually.

Pistil decides the fruit. Fruit comes from the ovary in the pistil, so as long as the pistil works, fruit can form even without the flower's own stamens.

Concept used. Removing stamens (emasculation) prevents self pollination, but the intact pistil can still receive pollen from another flower, leading to fertilisation and fruit.

1. What was removed. Only the stamens were taken out, so the flower lost its own pollen supply.
2. What remained. The pistil, with its stigma, style and ovary, is still in place.
3. What happened. Pollen from a nearby flower reached the stigma, fertilised the egg, and the ovary grew into a fruit.

Why this matters. Plant breeders deliberately remove stamens to force cross pollination between two chosen parents, which is how new and improved crop varieties are bred.

Cross pollination reached the intact pistil, fertilisation occurred, and the ovary grew into a fruit despite the stamens being removed.

One cell, one body. For a single-celled creature, dividing the cell literally means making a new organism.

Concept used. In a unicellular organism the cell is the whole body, so cell division produces new complete individuals, which is reproduction.

1. The body is one cell. For Amoeba or bacteria, the single cell is the entire organism.
2. Division makes two. When the cell divides, two daughter cells form.
3. Each is an organism. Both daughter cells are full new individuals, so the division counts as reproduction.

Why this matters. This is why a few bacteria can become millions overnight: each division doubles the number, a simple but very fast way to reproduce.

Yes, since each division of the single cell yields two complete new individuals, cell division is reproduction in unicellular organisms.

Same DNA, same look. Similarity in clones comes straight from sharing identical DNA with one parent.

Concept used. A clone is genetically identical to its single parent. Asexual reproduction copies the parent's DNA exactly, with no second parent to add variation.

1. Define a clone. It is an offspring that is a near-exact genetic copy of one parent.
2. One parent only. Asexual reproduction uses a single parent, so there is no second set of genes to mix in.
3. Exact DNA copy. The offspring get faithful copies of the parent's DNA, so they resemble it and each other very closely.

Why this matters. This is why a field grown from one banana plant's suckers is a single clone, all plants alike, which is handy for uniform crops but risky if one disease can attack them all.

A clone is a genetically identical copy of one parent; asexual offspring look alike because they inherit exact copies of that parent's DNA.

Halve, then add back. Constancy of chromosome number is a simple balance: cut to half before fusion, return to full after fusion.

Concept used. Meiosis reduces the chromosome number to half in the gametes, and fertilisation combines two halves to give the full number again in the offspring.

1. Parent cells. Carry the full number, 2n.
2. Gamete formation. Meiosis halves it to n in each gamete.
3. Fertilisation. The two gametes (n + n) fuse to make 2n in the zygote, matching the parents.
4. So the offspring grow with the same chromosome number as their parents.

Why this matters. Without this halving step, chromosome numbers would double each generation, so this control is what keeps a species genetically stable over time.

Halving by meiosis in the gametes, then doubling back by fusion at fertilisation, keeps the chromosome number the same in offspring and parents.

No food, no growth. Multiplication needs energy, and energy comes from food, so the difference between the two liquids is the food supply.

Concept used. Yeast obtains energy by breaking down sugar. Without a food source it cannot power the growth and division needed to reproduce.

1. Energy is needed. Reproducing yeast must spend energy to grow and bud.
2. Sugar provides it. In sugar solution, yeast ferments the sugar to release energy, so it multiplies.
3. Water lacks it. Plain water has no nutrients, so the yeast has no energy source and fails to multiply.

Why this matters. This is exactly why bakers add sugar to activate yeast for bread dough, giving the yeast the food it needs to grow and produce the gas that makes bread rise.

Sugar supplies energy for yeast to grow and multiply, while plain water provides none, so colonies multiply only in sugar solution.

Moisture is the deciding factor. Both slices have food, so the one extra thing the moist slice offers, water, is what allows the mould to thrive.

Concept used. The hyphae of Rhizopus need moisture to grow. Nutrients alone are not enough; without water the spores cannot develop into a spreading mould.

1. Both slices have nutrients. Bread provides food for the mould in either case.
2. Only the moist slice has water. Moisture lets the spores germinate and the hyphae spread.
3. So growth differs. The moist slice supports profuse growth, while the dry slice, lacking water, supports almost none.

Why this matters. This is why bread is kept dry and refrigerated to slow mould: removing moisture takes away the one condition the mould most needs to grow.

Mould needs moisture for hyphal growth, so it grows profusely on the moist slice, which has both water and nutrients, but barely on the dry slice.

Two parents, many gamete mixes. Variation has two roots: combining two parents' DNA, and the different gene mixes carried by different gametes.

Concept used. Sexual reproduction blends genes from two parents, and because each gamete carries a different combination of genes, the offspring turn out varied.

1. Reason one: two parents. The male and female parents have different characters, so their DNA combines to give new mixtures in the offspring.
2. Reason two: different gametes. Each gamete carries a different combination of the parent's genes, so siblings differ too.
3. Result. Together these give the variation seen among the progeny.

Why this matters. This variation is the raw material on which natural selection works, letting a species slowly adapt to its surroundings over generations.

Variation arises because (1) DNA comes from two different parents and (2) different gametes carry different gene combinations.

Each half rebuilds the rest. Planaria's regeneration means a half is not a dead end; it grows back into a whole worm.

Concept used. In regeneration, specialised cells of Planaria divide and rebuild the missing body parts, so each fragment can become a complete organism.

1. The cut. A vertical cut gives two halves, each missing the other side.
2. The regrowth. Each half regenerates the lost side, the new region being the shaded part shown in D and E.
3. The result. Both halves become full, independent Planaria, so the answer is yes, two individuals form.

Why this matters. This remarkable power makes Planaria a favourite model for scientists studying how bodies repair and regrow tissue, with hopes of using such ideas in medicine.

Yes, each half regenerates its missing region (shaded in D and E) and grows into a complete individual, giving two Planaria.

Break the size myth. Two of the three answers are firm "no"s, because chromosome number is not tied to size or to reproductive ease.

Concept used. Chromosome number is a species-specific value with no fixed link to body size or reproduction, but since DNA is the chief part of a chromosome, more chromosomes do carry more DNA.

1. (a) Size. No clear link exists; large and small organisms can have many or few chromosomes.
2. (b) Reproduction. The process is common across species and does not get easier with fewer chromosomes.
3. (c) DNA content. Chromosomes are mostly DNA, so a cell with more chromosomes has more DNA, which justifies the statement.

Why this matters. This stops students from assuming "more complex means more chromosomes", a myth, and focuses attention on DNA as the real carrier of information.

(a) No link to size; (b) reproduction does not depend on chromosome number; (c) yes, more chromosomes mean more DNA, as DNA is the main part of a chromosome.

Equal gametes, doubled zygote. Both gametes share the same number, and fertilisation adds them, so the zygote count is simply double.

Concept used. Gametes are haploid (n) and both sexes of a species carry the same n. Fertilisation fuses two gametes, giving the diploid (2n) zygote.

1. Female gamete. It must match the male gamete of the species, so it also has 24 chromosomes.
2. Fusion. The two gametes, 24 and 24, fuse at fertilisation.
3. Zygote. The total is 24 + 24 = 48 chromosomes in the zygote.

Why this matters. This simple sum shows the halve-and-restore rule in action, confirming that the offspring (from the zygote) will carry the full species number, 48 in tobacco.

Female gamete: 24 chromosomes; zygote: 24 + 24 = 48 chromosomes.

No delivery, no fusion. Pollination is the delivery step; without it, the male gamete can never reach the egg to fuse.

Concept used. Pollination transfers pollen (with the male gamete) to the stigma. Fertilisation, the fusion of male and female gametes, depends on this transfer happening first.

1. What fertilisation needs. The male gamete must reach and fuse with the egg.
2. What pollination does. It carries pollen, and so the male gamete, to the stigma where the journey to the egg begins.
3. Without pollination. The male gamete is never delivered, so there is nothing to fuse with the egg, and fertilisation fails.

Why this matters. This is why farmers worry about a fall in pollinators like bees: fewer pollinators mean less pollination, and so less fertilisation, fruit and seed.

Without pollination the male gamete never reaches the stigma, so it cannot fuse with the egg and fertilisation cannot take place.

Growth is all mitosis. From zygote to adult, the body grows only by mitosis, which never alters the chromosome number.

Concept used. Mitosis produces daughter cells with the same chromosome number as the parent cell. Since all body growth uses mitosis, the number stays constant from zygote to adult.

1. Zygote. Has the full species chromosome number after fertilisation.
2. Embryonal cells. Form when the zygote divides by mitosis, so they have the same number.
3. Adult. Built by repeated mitotic divisions, so every body cell keeps the same number. Hence the constancy across all three stages.

Why this matters. This shows why a person's every body cell, from infancy to old age, carries the same 46 chromosomes, all traced back to one zygote.

Yes, the number is constant because the zygote, embryonal cells and adult all arise through mitotic divisions, which preserve the chromosome number.

Follow the egg's address. The zygote forms exactly where the egg is, which is inside the ovule within the ovary.

Concept used. The egg lies in the ovule, the ovule lies in the ovary, and fertilisation, the fusion that makes the zygote, happens right there inside the ovule.

1. The egg's place. The egg is inside the ovule.
2. The ovule's place. The ovule is inside the ovary.
3. The zygote's place. Since the zygote forms from the egg, it too lies inside the ovule, within the ovary.

Why this matters. Knowing the zygote sits in the ovule explains why the ovule becomes the seed and the ovary becomes the fruit after fertilisation.

The zygote lies inside the ovule, which is present in the ovary of the flower.

Steady numbers, steady traits. Reproduction both replaces lost members and keeps the species' characters consistent, which is what stability means.

Concept used. Reproduction transfers DNA to the next generation. Consistent copying preserves the species' identity, while minor variation lets it adapt, keeping the population stable.

1. Replacement. New offspring keep the population from dying out as older members pass away.
2. Consistency. DNA copying is faithful, so the species keeps its characters across generations.
3. Adaptability. The minor variations that arise help the species survive changing conditions, adding to its stability.

Why this matters. This is why a species can persist for thousands of years even as individuals come and go, the population staying stable through continual reproduction.

Reproduction hands on DNA with consistency and slight variation, replacing members and helping adaptation, which keeps the species population stable.

Whole body versus reproductive readiness. General growth changes the whole body's size, while sexual maturation prepares it specifically for reproduction.

Concept used. General growth is steady body development through childhood, whereas sexual maturation is the hormone-driven set of changes at puberty that make reproduction possible.

1. General growth. Includes height increase, weight gain and change in body shape and size, occurring throughout childhood.
2. Sexual maturation. Includes puberty changes such as cracking of voice, new hair patterns and breast development in females.
3. The difference. Growth is about overall size; maturation is about becoming able to reproduce.

Why this matters. Telling these apart helps students understand which body changes are simply growing up and which signal that the reproductive system is maturing.

General growth increases overall body size and shape; sexual maturation is the specific puberty changes that prepare the body for reproduction.

Track the sperm and its helpers. Follow the sperm from testis to exit, then name the glands that feed and carry it on the way.

Concept used. Sperm travel testis to vas deferens to urethra. The seminal vesicle and prostate gland add nourishing fluid that also helps transport the sperm out of the body.

1. From the testis. Sperm produced in the testis enter the vas deferens.
2. Through the urethra. The vas deferens leads the sperm into the urethra, the final passage out.
3. Gland help. The seminal vesicle and prostate secrete fluids that nourish the sperm and ease their movement, forming semen.

Why this matters. Without these gland secretions, sperm would lack the fluid and energy they need to swim toward the egg, so the glands are vital for successful fertilisation.

Sperm move testis → vas deferens → urethra; the seminal vesicle and prostate gland secrete fluids that nourish and transport the sperm.

Lining built, then shed. The uterus prepares for a baby every month; when none comes, it clears the preparation away.

Concept used. The uterine lining thickens each cycle to receive an embryo. Without fertilisation, this lining is shed as menstruation.

1. Preparation. The uterus develops a thick, blood rich lining each month.
2. No embryo. If fertilisation does not happen, no embryo implants, so the lining is not used.
3. Shedding. The lining breaks down and leaves the body as blood and mucus through the vagina, the menstrual flow.

Why this matters. Understanding menstruation as the shedding of an unused lining helps students see it as a normal, healthy monthly process rather than something to fear.

The thick, spongy uterine lining breaks down and is shed through the vagina as blood and mucus (menstruation) when fertilisation does not occur.

From lining to lifeline. Once the embryo implants, the uterus builds the placenta, the embryo's lifeline to the mother.

Concept used. After implantation, the uterine wall thickens and a placenta forms, linking the embryo to the mother's blood to supply nutrients and oxygen.

1. Wall thickens. The uterine wall grows thicker and gains a rich blood supply.
2. Placenta forms. A special tissue, the placenta, develops to connect the embryo to the uterine wall.
3. Support begins. Through the placenta, the embryo receives nutrients and oxygen from the mother's blood.

Why this matters. The placenta is what allows a growing baby to be fed and to breathe inside the womb, making safe development through pregnancy possible.

After implantation, the uterine wall thickens and a placenta develops, connecting the embryo to the mother and supplying it with nutrients and oxygen.

One barrier, two benefits. A mechanical barrier blocks both sperm and germs, so it serves contraception and infection control at the same time.

Concept used. A mechanical barrier such as a condom physically prevents sperm from meeting the egg and also blocks the exchange of fluids that spread infections.

1. Avoids pregnancy. By stopping sperm from reaching the egg, the barrier prevents fertilisation.
2. Prevents infection. By blocking fluid exchange, it stops the spread of sexually transmitted infections.
3. So it serves two purposes. It is both a contraceptive and a protection against disease.

Why this matters. For students learning about reproductive health, this shows a simple method that protects both against unplanned pregnancy and against serious infections.

Mechanical barriers prevent pregnancy by stopping sperm reaching the egg and prevent the spread of infections during the sexual act.

Match part to function. Each of the four functions points to one specific organ of the female reproductive system.

Concept used. The ovary produces eggs, the oviduct is the site of fertilisation, the uterus is the site of implantation, and the vagina is the passage through which sperms enter.

1. Ovary (a). Produces and releases the egg each month.
2. Oviduct (b). Carries the egg, and is where the sperm meets and fertilises it.
3. Uterus (c). Receives the young embryo, which implants and grows in its wall.
4. Vagina (d). The passage through which sperms enter the female body.

Why this matters. Labelling this system correctly helps students follow the whole path from egg release to pregnancy, a core idea in human reproduction.

(a) Ovary (egg production); (b) Oviduct (fertilisation); (c) Uterus (implantation); (d) Vagina (sperm entry).

Half to full, X versus X or Y. The ratio is one half to one whole, and the genetic difference lies in the sex chromosome the sperm carries.

Concept used. The egg is haploid (n) and the zygote is diploid (2n), giving a 1:2 ratio. The egg always has an X chromosome, while the sperm may carry an X or a Y.

1. The ratio. Egg has n chromosomes, zygote has 2n, so egg to zygote is 1 : 2 .
2. The egg's sex chromosome. An egg always carries an X chromosome.
3. The sperm's sex chromosome. A sperm carries either an X or a Y chromosome, which is how it differs genetically from the egg.

Why this matters. This sets up the genetics of sex determination studied in the next chapter, where an X-bearing sperm gives a girl and a Y-bearing sperm gives a boy.

Egg to zygote ratio is 1:2; the sperm carries either X or Y, while the egg always carries X.

One parent, no gametes. The common thread is that each method needs just one parent and skips gametes, which is the definition of asexual reproduction.

Concept used. Asexual reproduction uses one parent without gametes. In Planaria, regeneration relies on specialised cells that divide and rebuild lost parts so that each fragment becomes a whole animal.

1. Why all three are asexual. Budding (Hydra, yeast), fragmentation (Spirogyra) and regeneration (Planaria) each use a single parent and no fusion of gametes.
2. Planaria is cut. The worm's body is divided into pieces by cutting or breaking.
3. Cells rebuild. Special cells in each piece multiply quickly and form the missing head, tail or sides.
4. Whole worms form. Each piece grows into a complete, separate Planaria, as the diagram shows.

Why this matters. Planaria's regeneration is studied by scientists hoping to understand how lost tissues might one day be regrown in humans, which would transform medicine.

All three are asexual because they use one parent and no gametes; in Planaria each cut piece regenerates the lost parts and grows into a complete worm.

Compare, then explain variation. First contrast the two modes on parents and gametes, then trace variation to the fusion of two different gametes.

Concept used. Asexual reproduction (one parent, no gametes) gives clones; sexual reproduction (two parents, fusing gametes) gives variation because the two gametes carry different DNA.

1. Parents. Asexual uses one parent; sexual uses two.
2. Gametes and fertilisation. Asexual makes no gametes and has no fertilisation; sexual makes gametes that fuse at fertilisation.
3. Source of variation. The two fusing gametes have the same chromosome number but different DNA, so the offspring inherit new gene combinations, leading to variation.

Why this matters. The variation from sexual reproduction is the reason a species can adapt over time, while clones from asexual reproduction stay the same and risk being wiped out together by one threat.

Two differences: number of parents (one vs two) and gametes/fertilisation (absent vs present). Variation arises because two gametes with different DNA fuse, giving new gene combinations.

Transfer versus fusion. Pollination is just the transfer of pollen; fertilisation is the actual fusion of gametes that follows.

Concept used. Pollination moves pollen from anther to stigma. Fertilisation, the fusion of gametes inside the ovule, gives the zygote, the first cell of the new plant.

1. Pollination. Pollen grains are transferred from the anther to the stigma.
2. Fertilisation. The male gamete fuses with the egg, forming a zygote.
3. Site and product. Fertilisation happens in the ovule, and its product is the zygote.
4. Pollen tube. The pollen grain on the stigma grows a tube down the style into the ovule, delivering the male gamete to the egg, as the diagram shows.

Why this matters. Distinguishing the transfer step (pollination) from the fusion step (fertilisation) clears a very common confusion and explains why one cannot happen without the other.

Pollination = pollen transfer anther to stigma; fertilisation = fusion of gametes. Site: ovule; product: zygote. The pollen tube grows from the stigma into the ovule.

Before and after fusion. A gamete is the sex cell before fusion; the zygote is the single cell that results after fusion.

Concept used. Gametes are haploid sex cells that carry parental DNA. Their fusion at fertilisation forms the diploid zygote, the starting cell of the new organism.

1. Gamete. A sex cell, male or female, carrying half the chromosomes and the parent's characters in its DNA.
2. Zygote. The cell formed when two gametes fuse, carrying the full chromosome number.
3. Their roles. Gametes deliver the two parents' characters; the zygote combines them and divides to form an embryo and then a new individual.

Why this matters. This pair of terms is the heart of sexual reproduction: the gamete is the messenger of heredity, and the zygote is where two lines of heredity meet to start a new life.

Gamete: a haploid sex cell carrying parental characters; zygote: the diploid cell from their fusion. Gametes bring parents' DNA; the zygote grows into a new individual.

Four rings from outside in. A flower's whorls go from protective (calyx) to showy (corolla) to reproductive (androecium and gynoecium).

Concept used. The flower has four whorls, and the gamete-producing organs are the anther of the stamen (male) and the ovary of the pistil (female).

1. Calyx. The outer ring of sepals that protect the bud.
2. Corolla. The ring of petals that attract pollinators.
3. Androecium and gynoecium. The stamens (male) and the pistil (female) are the reproductive whorls.
4. Gamete organs. Male gametes form in the anther/stamen, female gametes in the pistil/ovary/ovule.

Why this matters. Identifying the whorls and the gamete organs is the foundation for understanding how pollination and fertilisation move pollen from the anther to the ovule.

Four whorls: calyx, corolla, androecium, gynoecium. Male gametes form in the anther/stamen; female gametes in the pistil/ovary/ovule.

The embryo's lifeline. The placenta is the bridge that lets the mother feed, oxygenate and clean up after the embryo.

Concept used. The placenta connects the embryo to the uterine wall and, through its villi, exchanges nutrients, oxygen and wastes between the mother's blood and the embryo.

1. What it is. A special tissue linking the embryo to the uterine wall.
2. Its villi. Finger-like projections that increase the surface area for exchange.
3. Supply. Nutrients and oxygen pass from the mother to the embryo through it.
4. Removal. Wastes from the embryo pass into the mother's blood through it.

Why this matters. The placenta is why a baby can grow safely inside the womb without breathing or eating on its own, drawing everything it needs from the mother.

The placenta is the tissue connecting embryo and uterine wall; through its villi it supplies nutrients and oxygen to the embryo and removes the embryo's wastes into the mother's blood.

Block the meeting or the egg. Contraceptives either keep sperm and egg apart or stop the egg from being released at all.

Concept used. Contraceptive methods are mechanical, chemical, device based or surgical. Pills, a chemical method, alter hormones so that the ovary does not release an egg.

1. Mechanical. Barriers such as condoms stop sperm reaching the egg.
2. Chemical. Pills change the hormonal balance.
3. Devices and surgery. The copper-T or loop sits in the uterus, while surgical methods block the ducts.
4. Pill in detail. By changing the hormonal balance, the pill prevents the release of the egg, so there is no egg to fertilise and pregnancy is prevented.

Why this matters. Understanding how each method works lets students appreciate why choices differ in how reliably and how safely they prevent pregnancy, a key part of reproductive health education.

Methods: mechanical, chemical (pills), devices (copper-T) and surgical. Pills change the hormonal balance to stop egg release, preventing fertilisation.

Sperm goes up, egg comes down. They meet in the oviduct, and because only one egg is released a month, fertilisation is limited to that window.

Concept used. Fertilisation is the fusion of sperm and egg in the oviduct. One egg is released per month, so the timing of possible fertilisation is tied to that monthly release.

1. Sperm travel. Sperm enter through the vagina and swim upward.
2. Egg travels. The egg released by the ovary reaches the oviduct.
3. They meet. The sperm fuses with the egg in the oviduct, which is fertilisation.
4. Once a month. Since only one egg is released each month, fertilisation can occur only around that time.

Why this matters. This timing explains the idea of a fertile window each month and underlies how the menstrual cycle relates to the chance of pregnancy.

Fertilisation is the fusion of sperm and egg in the oviduct; as only one egg is released monthly, fertilisation can occur only around that time, so about once a month.

Survival is one thing, continuity another. Staying alive is the job of life processes; reproduction's job is to keep the species going.

Concept used. Life processes keep an individual alive, while reproduction passes DNA to new generations with constancy and variation, which stabilises the species.

1. Energy for survival. An organism stays alive through nutrition and respiration, not through reproduction.
2. Cost of reproduction. Reproduction needs energy and does not benefit the individual's own survival.
3. DNA handed on. Reproduction transfers genetic material to the next generation.
4. Constancy and variation. The DNA is copied faithfully but with some variation, giving the species both identity and the ability to adapt, hence stability.

Why this matters. This reframes reproduction as a service to the species: individuals come and go, but reproduction ensures the species endures and adapts.

Survival is powered by life processes, not reproduction. Reproduction transfers DNA with consistency and variation to new generations, stabilising the species rather than the individual.

Infectious through contact, blocked by barriers. STDs spread by sexual contact and are best prevented by stopping the exchange of body fluids.

Concept used. STDs are infections passed during sexual contact, of bacterial or viral origin, and a mechanical barrier such as a condom blocks their transmission.

1. What they are. Infectious diseases transmitted during sexual contact.
2. Their causes. Some are bacterial (syphilis, gonorrhoea) and some viral (HIV-AIDS).
3. Prevention. A mechanical barrier like a condom prevents the exchange of fluids, stopping the spread of infection.

Why this matters. Teaching how STDs spread and how to prevent them empowers students to make safe choices and reduces the spread of serious infections in society.

STDs are bacterial or viral infections spread during sexual contact; using a mechanical barrier like a condom prevents their transmission.