Sexual vs asexual reproduction

• Asexual reproduction produces genetically identical offspring and is advantageous when organisms are already well adapted to a stable environment.

• Sexual reproduction produces new combinations of alleles, increasing genetic variation and improving the chance of adaptation to changing environments.

• In exam answers, link asexual reproduction with continuity and sexual reproduction with variation and change.

• Meiosis and fertilization are the key processes that generate variation in the sexual life cycle.

Meiosis and fertilization in the sexual life cycle

• Meiosis halves the chromosome number, producing haploid gametes.

• Meiosis also breaks up parental allele combinations, creating genetically different gametes.

• Fusion of gametes (fertilization) restores the diploid chromosome number.

• Fertilization creates new allele combinations, so both meiosis and gamete fusion increase variation.

• Always connect sexual reproduction to variation, inheritance, and adaptation by natural selection.

Differences between male and female sexes in sexual reproduction

• The key biological difference is that the male gamete travels to the female gamete.

• Therefore, the sperm is small, motile, and has very little food reserve.

• The egg is large, non-motile, and contains more cytoplasm and nutrient reserves.

• Males usually produce many small gametes; females produce fewer, larger gametes.

• These differences lead to different reproductive strategies in males and females.

Human reproductive systems

• Be able to draw and annotate both the male-typical and female-typical reproductive systems.

• Male system: testes produce sperm and testosterone; scrotum keeps testes cooler; epididymis stores and matures sperm; vas deferens transports sperm; seminal vesicles and prostate gland add fluid to form semen; urethra carries semen out through the penis.

• Female system: ovaries produce eggs and hormones; oviducts / fallopian tubes transport the egg and are the usual site of fertilization; uterus supports embryo development; endometrium thickens for implantation; cervix is the neck of the uterus; vagina receives penis/sperm and acts as the birth canal.

• In labeled diagrams, include both structure names and functions.

This diagram shows the main structures of the human male reproductive system, including the testis, epididymis, vas deferens, prostate, and penis. It is useful for practicing labeling and linking structure to function in IB exam questions. Source

This diagram shows the main structures of the human female reproductive system, including the ovaries, oviducts, uterus, cervix, and vagina. It is ideal for practicing annotated biological drawings and identifying where fertilization and implantation occur. Source

Menstrual cycle: ovarian + uterine cycles

• The menstrual cycle includes both the ovarian cycle and the uterine cycle.

• In the ovarian cycle, FSH stimulates follicle development in the ovary.

• Developing follicles secrete oestradiol.

• Oestradiol repairs and thickens the endometrium after menstruation.

• Moderate/high oestradiol first exerts negative feedback on FSH, reducing stimulation of other follicles.

• High oestradiol just before ovulation causes positive feedback, triggering a surge in LH.

• The LH surge causes ovulation.

• After ovulation, the follicle becomes the corpus luteum.

• The corpus luteum secretes progesterone (and some oestradiol).

• Progesterone maintains the endometrium in preparation for implantation.

• Progesterone and oestradiol exert negative feedback on FSH and LH.

• If pregnancy does not occur, the corpus luteum degenerates, progesterone and oestradiol levels fall, and the endometrium is shed as menstruation.

• Common exam task: explain the cycle using FSH, LH, oestradiol, progesterone, positive feedback, and negative feedback in the correct order.

This image summarizes the follicular phase, ovulation, and luteal phase, alongside changing levels of FSH, LH, oestradiol, and progesterone. It is especially helpful for exam questions asking you to interpret hormone graphs and link them to ovarian and uterine events. Source

Fertilization in humans

• Fertilization involves fusion of the sperm cell membrane with the egg cell membrane.

• The sperm nucleus enters the egg.

• The sperm tail and mitochondria do not contribute to the embryo; they are destroyed.

• The nuclear membranes of the sperm and egg nuclei dissolve.

• The chromosomes from both parents take part in a joint mitosis.

• This produces two diploid nuclei, beginning development of the new organism.

• Remember: inheritance of mitochondria is effectively maternal, because sperm mitochondria do not contribute.

IVF (in vitro fertilization)

• In IVF treatment, hormone secretion is first suppressed/suspended.

• Artificial hormone doses are used to induce superovulation so that multiple eggs mature.

• Eggs are collected from the ovaries.

• Fertilization occurs outside the body in a laboratory.

• The resulting embryo(s) are transferred to the uterus.

• Exam link: IVF uses hormonal control of the menstrual cycle and manipulates it to increase the number of eggs available.

This infographic shows the major stages of IVF, from ovarian stimulation and egg collection to fertilization and embryo transfer. It is useful for understanding how hormones are used to induce superovulation and how fertilization can occur outside the body. Source

Sexual reproduction in flowering plants

• Flowering plant reproduction is sexual reproduction.

• Male gametes form inside pollen grains.

• Female gametes form inside ovules.

• Pollination is transfer of pollen from anther to stigma.

• After pollination, the pollen grain germinates and a pollen tube grows down the style.

• The male nucleus/gametes travel down the pollen tube toward the ovule.

• Fertilization produces a zygote, which develops into an embryo.

• Distinguish clearly between pollination (transfer of pollen) and fertilization (fusion of gametes).

• A species can be hermaphroditic and still reproduce sexually.

Features of an insect-pollinated flower

• Large / brightly coloured petals attract insects.

• Scent and nectar attract pollinators.

• Nectaries produce nectar.

• Anthers are usually positioned so insects brush against them.

• Stigma is sticky to catch pollen.

• Pollen grains are often relatively large, sticky, or spiky so they attach to insects.

• Be able to draw and annotate an insect-pollinated flower with both structure names and functions.

Cross-pollination and self-incompatibility

• Cross-pollination transfers pollen between different plants.

• It increases genetic variation and reduces inbreeding.

• Mechanisms promoting cross-pollination include:

  • Different maturation times of anthers and stigma.

  • Separate male and female flowers on the same plant.

  • Separate male and female plants.

  • Use of animals or wind to move pollen between plants.

• Self-pollination can cause inbreeding, reducing genetic diversity and vigour.

• Self-incompatibility mechanisms prevent male and female gametes from the same plant fusing.

• These mechanisms help maintain genetic diversity within a species.

This diagram shows pollination, where pollen is transferred from one flower to another before fertilization. It helps distinguish pollination from fertilization and supports explanation of how cross-pollination increases genetic variation. Source

Fertilization, seed dispersal, and germination in flowering plants

• After fertilization, the zygote develops into the embryo.

• The ovule develops into a seed.

• Seed dispersal moves seeds away from the parent plant.

• Do not confuse seed dispersal with pollination.

• During germination, the embryo resumes growth.

• Stored food reserves are mobilized to support early growth.

• Germination leads to growth of the embryonic plant into a seedling.

This image shows how pollen reaches the ovule, how fertilization occurs inside the flower, and how the embryo and food store begin to form. It is useful for linking pollination, fertilization, and seed formation in one sequence. Source

Exam traps to avoid

• Do not confuse pollination with fertilization.

• Do not say meiosis alone creates all variation; fertilization also creates new allele combinations.

• Do not forget that the sperm nucleus enters the egg, but the tail and mitochondria are destroyed.

• Do not mix up FSH and LH: FSH stimulates follicle development; LH surge triggers ovulation.

• Do not state that hermaphroditic flowering plants reproduce asexually; they can still reproduce sexually.