Biology B15 of Cambridge IGCSE Co-ordinated Sciences 0654 compares asexual and sexual strategies before following reproduction in flowering plants and humans. Official sections B15.1 to B15.5 cover flower and gamete structure, pollination, seed germination, fertilisation, puberty, the menstrual cycle and sexually transmitted infections. Placental exchange, detailed menstrual hormones, birth, contraception and assisted reproduction are outside this boundary.
Asexual reproduction
Asexual reproduction produces genetically identical offspring from one parent. There is no fusion of gamete nuclei. Genetically identical descendants are clones, although a later mutation can introduce a difference.
Examples can include binary fission in microorganisms, runners or bulbs in plants and other cases shown in diagrams or unfamiliar information. Identify the defining evidence: one parent, no gamete fusion and genetically identical offspring.
In a stable wild environment, rapid reproduction and retention of a successful genotype can increase population quickly. One organism can reproduce without finding a mate. However, low genetic variation means a new disease or environmental change may affect many individuals similarly.
Sexual reproduction
Sexual reproduction involves fusion of nuclei from two gametes to form a zygote and produces offspring genetically different from one another. Fertilisation is specifically the fusion of gamete nuclei.
Gamete nuclei are haploid, containing one set of chromosomes. The zygote nucleus is diploid, containing two sets after fusion.
In a wild population, genetic variation increases the chance that some individuals survive environmental change or disease. Sexual reproduction can require more time and energy, and individuals may need to find a mate or depend on pollination.
Do not describe sexual reproduction as automatically producing better individuals. Variation produces differences; whether a feature is advantageous depends on the environment.
Insect-pollinated flower structure
Sepals protect the flower when it is a bud. Petals can attract insect pollinators through colour, scent or nectar guides.
The stamens are male structures. Each has a filament supporting an anther. Anthers produce and release pollen grains containing male nuclei.
The carpel is the female structure. Its stigma receives pollen, the style holds the stigma and provides a route toward the ovary, and the ovary contains ovules. An ovule contains a female nucleus.
Drawings should show positions and connections rather than decorative shading. Label lines should touch the intended structure and should not cross unnecessarily.
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Pollination is transfer of pollen grains from an anther to a stigma. It occurs before fertilisation.
Insect-pollinated flowers commonly have conspicuous petals, scent and nectar. Anthers and stigmas are held where visiting insects contact them. Pollen grains tend to be larger and sticky or spiky, so they attach to an animal and stigma.
Wind-pollinated flowers usually have small or inconspicuous petals and no need for scent or nectar. Anthers are exposed and loosely attached so wind shakes out pollen. Stigmas are large, feathery and exposed to intercept airborne grains. Pollen is produced in large quantities and tends to be small, light and smooth.
Every feature should be linked to transfer: attraction, attachment, release, transport or capture.
Fertilisation in flowering plants
After compatible pollen lands on a stigma, a pollen tube grows down through the style. The male nucleus travels through the tube. The tube enters an ovule, and the pollen nucleus fuses with a nucleus in the ovule. This nuclear fusion is fertilisation.
Pollination places pollen on the stigma; fertilisation occurs later in an ovule. Details of endosperm production and subsequent development are not required in this outcome.
Seed germination
Germination requires water, oxygen and a suitable temperature. Water rehydrates tissues, activates metabolism and supports enzyme action. Oxygen is needed for aerobic respiration to release energy. A suitable temperature permits enzyme-controlled reactions to proceed at an effective rate.
Light is not in the general required list for this investigation. Some particular species may respond to light, but do not replace the official three conditions.
An investigation changes one condition while keeping seed type, number, age, duration and other conditions comparable. A control must isolate the missing water, oxygen or suitable-temperature factor. Detailed setup and evaluation belong to the practical hub.
Male reproductive system
Testes produce sperm and testosterone. The scrotum holds the testes outside the abdominal cavity at a temperature suitable for sperm production. Sperm ducts carry sperm from the testes. The prostate gland adds fluid to form semen. The urethra carries semen through the penis, and the penis transfers semen into the vagina.
In diagrams, trace a continuous sperm route. Do not confuse a sperm duct with a ureter, which belongs to the urinary system.
Female reproductive system
Ovaries produce egg cells and oestrogen. Oviducts carry egg cells and are the usual site of fertilisation. The uterus supports embryo and fetal development. The cervix is the muscular opening between uterus and vagina. The vagina receives the penis and semen and forms the passage from uterus to outside.
Fertilisation fuses the sperm nucleus with the egg-cell nucleus, normally in an oviduct. Do not confuse this nuclear fusion with transfer of semen into the vagina.
Gamete adaptations and comparison
A sperm cell is small and motile. Its flagellum propels it, mitochondria release energy for movement and acrosome enzymes help it penetrate the egg's outer layers.
An egg cell is large and non-motile. It contains energy stores supporting early development. Its jelly coat changes after fertilisation, preventing entry of additional sperm.
Males produce very large numbers of small sperm; females release far fewer large egg cells. Both gamete nuclei are haploid even though cell size, structure, motility and number differ.
Puberty and sex hormones
Testosterone and oestrogen regulate development of secondary sexual characteristics during puberty. Testosterone is associated with male secondary characteristics, including deeper voice and facial hair. Oestrogen is associated with female secondary characteristics, including breast development and widening hips.
These hormones act throughout the body through target tissues. Puberty changes vary among individuals and do not occur at an identical age or rate.
The menstrual cycle
The menstrual cycle includes changes in ovaries and the uterus lining. Menstruation sheds the lining near the start of a cycle. A follicle develops in an ovary, ovulation releases an egg cell and the uterine lining thickens in preparation for a possible pregnancy. If pregnancy does not occur, the lining later breaks down and another cycle begins. Knowledge of the hormones controlling this cycle is not required in B15.
Sexually transmitted infections and HIV
A sexually transmitted infection, or STI, is an infection transmitted through sexual contact. Human immunodeficiency virus, HIV, is a pathogen that causes an STI. HIV infection may lead to AIDS.
HIV can be transmitted through unprotected sexual contact, infected blood, shared contaminated needles and from mother to child during pregnancy, birth or breast-feeding. It is not spread by ordinary social contact such as sharing a room or touching intact skin.
STI spread is controlled by barrier protection such as condoms, reducing numbers of sexual partners, testing and treatment where available, screening blood, using sterile needles and avoiding sharing injecting equipment. Education and informed consent support safer choices.
Match each measure to a route. Condoms reduce exchange of infected body fluids during sex; screening protects transfusions; sterile single-use equipment prevents blood transfer.
Worked application: connect plant and human fertilisation
In a wind-pollinated plant, exposed anthers release many small light pollen grains and a feathery stigma intercepts them. Pollination ends when pollen reaches the stigma. A pollen nucleus then reaches an ovule and fuses with a nucleus there, completing fertilisation. In humans, a sperm's flagellum supports movement, mitochondria release energy and acrosome enzymes help penetration; its haploid nucleus fuses with the egg's haploid nucleus in an oviduct. Both examples require nuclear fusion, but pollen transfer itself and semen transfer itself are not fertilisation.
The decisive evidence in both cases is fusion of haploid nuclei to form a diploid zygote, not simply movement of the male gamete or its carrier.
Common misconceptions and corrections
Saying asexual reproduction uses two parents. It uses one parent.
Saying asexual offspring can never differ. Mutation can introduce variation later.
Calling any rapid population sexual reproduction. Look for gamete-nucleus fusion.
Saying clone uniformity is always advantageous. Shared susceptibility is a major risk.
Defining sexual reproduction without a zygote. Gamete nuclei fuse to form one.
Saying sexual offspring are genetically identical. They are genetically different from one another.
Calling a gamete nucleus diploid. It is haploid.
Calling a zygote haploid. Its nucleus is diploid.
Saying variation guarantees survival. It changes the probability that some individuals suit conditions.
Calling a filament the pollen-producing part. The anther produces pollen.
Calling the stigma the ovule container. The ovary contains ovules.
Calling petals the fertilisation site. They commonly attract pollinators.
Defining pollination as nuclear fusion. It is pollen transfer from anther to stigma.
Calling fertilisation pollen transfer. It is fusion of nuclei in an ovule.
Saying wind-pollinated flowers need large scented petals. Wind does not require attraction.
Saying wind pollen is large and sticky. It is generally small, light and smooth.
Saying insect pollen is produced mainly to float. It is sticky or spiky for attachment.
Placing fertilisation on the stigma. It occurs after pollen-tube entry into an ovule.
Adding endosperm-development detail as required. It is explicitly excluded.
Saying all seeds require light to germinate. The general required factors are water, oxygen and suitable temperature.
Saying water supplies germination energy. Respiration releases energy; water enables metabolism.
Calling the sperm duct the ureter. They carry different materials.
Saying the prostate makes sperm. Testes make sperm; prostate adds fluid.
Placing human fertilisation in the uterus. It usually occurs in an oviduct.
Saying sperm mitochondria perform propulsion directly. They release energy; the flagellum propels.
Saying the acrosome stores energy. It contains enzymes.
Saying the egg jelly coat never changes. It changes at fertilisation.
Saying sperm and egg are equal in size and number. They differ greatly.
Pairing testosterone with the ovaries. Testes produce it.
Pairing oestrogen with the testes. Ovaries produce it.
Saying HIV and AIDS are identical terms. HIV infection may lead to AIDS.
Saying HIV spreads through ordinary touch. Transmission requires relevant infected fluids or mother-child routes.
Saying condoms screen transfused blood. Match each control to its route.
Importing contraception, assisted reproduction and birth into Biology B15. They are not in the current official outcome list.
Assessment guidance
Use nuclear fusion as the boundary between sexual and asexual reproduction and discuss advantages and disadvantages for wild populations. For flowers, pair every structure or adaptation with protection, attraction, release, transfer, capture or fertilisation, and keep pollination before fertilisation. Human answers should trace the named organs, compare gamete size, structure, motility and number, and place fertilisation in the oviduct. Describe the menstrual cycle through ovarian and uterine-lining changes without importing hormone control. Define HIV as a pathogen causing an STI and match each prevention measure to sexual, blood or mother-child transmission.
Retrieval practice
Compare asexual and sexual reproduction for wild populations. Draw and label insect- and wind-pollinated flowers, sequence pollination to fertilisation and design three germination controls. Then trace male and female reproductive routes, compare gametes, describe ovarian and uterine changes through the menstrual cycle without naming controlling hormones, and match HIV controls to transmission routes.
Theory and practical ownership
This theory note owns reproduction definitions, wild-population comparisons, flower and human anatomy, fertilisation, germination requirements, puberty, menstrual-cycle changes and STI transmission. The dedicated Co-ordinated Sciences practical series owns flower observation, germination apparatus, controls, safe biological handling, measurement, tables, graphs and evaluation.
