IB DP Biology: Reproduction: sexual and asexual, human reproductive systems and flowering plants (DP Biology)

This study guide covers the first part of section D3.1 (Reproduction), statements D3.1.1 to D3.1.12, in the IB Biology guide, first assessment 2025, for DP Biology. Everything here is SL and HL. The later statements of D3.1 (puberty, gametogenesis, pregnancy and hormone replacement therapy) are taught elsewhere, and statements D3.1.13 onwards include HL content.

Use it with the revision notes and the practice questions. The IB DP Biology hub, the printable checklist and the free 10-minute diagnostics help you find weak spots.

What this unit covers

Statement	What you must be able to do
D3.1.1	Compare sexual and asexual reproduction, including the advantage of each
D3.1.2	Explain how meiosis and fusion of gametes work in the sexual life cycle
D3.1.3	Explain the differences between male and female gametes and strategies
D3.1.4	Draw and annotate the male and female human reproductive systems
D3.1.5	Describe the ovary and uterus cycles and how hormones, with feedback, control them
D3.1.6	Describe fertilization in humans
D3.1.7	Explain how hormones are used in IVF
D3.1.8	Describe sexual reproduction in flowering plants
D3.1.9	Draw and annotate an insect-pollinated flower
D3.1.10	Describe methods that promote cross-pollination
D3.1.11	Explain self-incompatibility and why inbreeding is harmful
D3.1.12	Distinguish dispersal from pollination; explain germination

D3.1.1 Sexual and asexual reproduction

Asexual reproduction involves one parent and no gametes. Offspring form by mitosis, so each one has the same genes as its parent and as its siblings. Examples are binary fission in bacteria, budding in yeast and runners in strawberry plants.

Sexual reproduction involves two gametes, usually from two parents, produced by meiosis. Offspring are genetically different from both parents and from each other.

Feature	Asexual	Sexual
Parents	One	Usually two
Cell division	Mitosis	Meiosis, then fusion of gametes
Offspring	Genetically identical	Genetically varied
Speed and cost	Fast, no need to find a mate	Slower, costs energy to find a mate
Best when	The environment is stable	The environment is changing

The guide asks for the relative advantages. Asexual reproduction lets a well-adapted individual produce many offspring that carry the same successful genotype. Sexual reproduction produces new gene combinations, so some offspring may be adapted to a changed environment, for example a new disease or a warmer climate.

D3.1.2 Meiosis and fusion of gametes in the life cycle

A sexual life cycle alternates between haploid cells (n) and diploid cells (2n). Meiosis halves the chromosome number to make haploid gametes. Fusion of gametes, also called fertilization, restores the diploid number in the zygote.

Meiosis breaks up the combinations of alleles that a parent carries. Fusion of gametes then brings together alleles from two different parents in new combinations. Without meiosis, chromosome number would double every generation.

Worked example. Human body cells have 46 chromosomes. Each gamete has 23. The zygote has 23 + 23 = 46 chromosomes, so the diploid number is restored.

D3.1.3 Male and female sexes

The key difference is that the male gamete has to move towards the female gamete. A sperm cell is therefore small, motile and carries little food. An egg cell is much larger, does not move by itself and carries food reserves.

Two consequences follow:

• Males make very large numbers of small gametes. In humans, millions of sperm can be released at once.
• Females make far fewer, larger gametes, and many female animals invest heavily in the offspring after fertilization, for example through pregnancy or egg protection.

Worked example (magnification). A drawing of a sperm head is 36 mm long. Its actual length is 4.5 µm.

magnification = image size ÷ actual size = 36 000 µm ÷ 4.5 µm = ×8000

D3.1.4 Human reproductive systems

You should be able to draw both systems and annotate names and functions.

Male structure	Function
Testis	Makes sperm and testosterone
Scrotum	Holds testes outside the body, slightly cooler than body temperature
Epididymis	Stores sperm while they mature
Sperm duct (vas deferens)	Carries sperm towards the urethra
Seminal vesicle and prostate gland	Add fluid that nourishes and carries sperm
Urethra	Carries semen (and urine) through the penis
Penis	Delivers semen into the vagina

Female structure	Function
Ovary	Contains follicles; makes eggs, oestradiol and progesterone
Oviduct (fallopian tube)	Collects the egg; usual site of fertilization; cilia move it along
Uterus	Muscular organ where an embryo implants and develops
Endometrium	Lining of the uterus that thickens each cycle
Cervix	Narrow opening between uterus and vagina
Vagina	Receives semen; birth canal

D3.1.5 Ovarian and uterine cycles

The ovarian cycle (events in the ovary) and the uterine cycle (events in the endometrium) together make the menstrual cycle. Typical length is about 28 days, but it varies between people.

Hormones involved: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland, and oestradiol and progesterone from the ovary.

1. Early follicular phase. FSH rises and stimulates follicle growth. Menstruation (loss of the endometrium) happens at the start.
2. Follicle growth. Follicle cells secrete oestradiol. Oestradiol repairs and thickens the endometrium. At low levels, oestradiol exerts negative feedback on FSH and LH secretion.
3. Oestradiol peak. When oestradiol is high for long enough, it switches to positive feedback. The pituitary releases a surge of LH, with a smaller rise in FSH.
4. Ovulation. The LH surge triggers release of the egg from the follicle.
5. Luteal phase. The empty follicle becomes the corpus luteum, which secretes progesterone and some oestradiol. Progesterone maintains the thick, vascular endometrium and inhibits FSH and LH (negative feedback).
6. End of cycle. If no embryo implants, the corpus luteum breaks down, progesterone and oestradiol fall, the endometrium is shed, and FSH rises again.

Worked example (data). Endometrium thickness is 2 mm on day 5 and 10 mm on day 25.

percentage change = (10 − 2) ÷ 2 × 100 = +400%

Worked example (interpreting). LH is 8 units on day 12 and 60 units on day 14, a rise of (60 − 8) ÷ 8 × 100 = 650%. Day 14 is the surge, so ovulation follows soon after. The surge is explained by positive feedback from high oestradiol.

D3.1.6 Fertilization in humans

Fertilization usually takes place in the oviduct. The sequence is:

1. The sperm cell membrane fuses with the egg cell membrane.
2. The sperm nucleus enters the egg. The tail and mitochondria of the sperm are destroyed, so mitochondria are inherited from the egg.
3. The nuclear membranes around the sperm and egg nuclei dissolve.
4. The chromosomes condense. All of them, 23 from each parent, take part in one joint mitosis.
5. Mitosis produces two diploid nuclei, each with 46 chromosomes, in a two-cell embryo.

D3.1.7 Hormones in IVF

In vitro fertilization (IVF) means fertilization outside the body. The guide’s key idea is that normal hormone secretion is suspended and artificial doses induce superovulation, so several eggs mature at once.

A typical sequence is:

1. Drugs suppress the person’s own pituitary secretion, which stops the natural cycle.
2. Injections of FSH (often with LH) stimulate many follicles to grow.
3. A further hormone dose triggers final egg maturation, and the eggs are collected.
4. Eggs and sperm are mixed in a dish, and fertilized eggs divide in culture.
5. One or more embryos are placed in the uterus. Progesterone may be given to support the endometrium.

Collecting several eggs raises the chance of at least one healthy embryo. Superovulation can cause ovarian over-stimulation and multiple pregnancies. Protocols differ between clinics.

Worked example. If 12 eggs are collected and 9 fertilize, the fertilization rate is 9 ÷ 12 × 100 = 75%.

D3.1.8 Flowering plant reproduction

Flowering plants reproduce sexually, even when one flower has both male and female parts (hermaphrodite). Gametes are made by meiosis.

• Male side. Cells in the anther undergo meiosis and develop into pollen grains. Each grain contains the male gametes (sperm nuclei) inside a protective wall.
• Female side. Inside each ovule in the ovary, meiosis and later divisions produce the egg cell.
• Pollination is the transfer of pollen from an anther to a stigma.
• Pollen tube. The pollen grain germinates on the stigma and a tube grows down the style to the ovule, carrying the male nucleus to the egg.
• Fertilization. A male gamete fuses with the egg to form a zygote, which divides by mitosis to form the embryo inside the seed.

D3.1.9 An insect-pollinated flower

Draw one flower in section and label:

Structure	Function
Petals	Large, coloured; attract insects; may have nectar guides
Nectary	Makes sugary nectar as a reward
Sepals	Protect the flower bud
Anther	Makes pollen; held on a filament
Filament	Holds the anther where insects will touch it
Stigma	Sticky or feathery surface that catches pollen
Style	Supports the stigma; the pollen tube grows down it
Ovary and ovules	Ovules contain the eggs and become seeds

Pollen grains of insect-pollinated flowers are sticky or spiny so they cling to insect bodies.

D3.1.10 Promoting cross-pollination

Cross-pollination moves pollen between different plants. Methods you need:

• Different maturation times. Anthers and stigmas ripen at different times, so a flower cannot pollinate itself.
• Separate male and female flowers on the same plant.
• Separate male and female plants.
• Animals or wind carry pollen between plants. Animal-pollinated flowers have the features in D3.1.9. Wind-pollinated flowers usually have small, dull petals, large feathery stigmas and anthers hanging outside the flower.

D3.1.11 Self-incompatibility

Self-pollination leads to inbreeding, which lowers genetic diversity and often reduces vigour. Many species have genetic self-incompatibility. If pollen carries an allele that matches one in the stigma tissue, pollen germination or pollen-tube growth is blocked. Only gametes from a different plant can fuse.

Worked example. A stigma belongs to a plant with alleles S1 and S2. Pollen from a plant with S1 and S3 lands on it. Half the pollen carries S1 and is blocked. The half carrying S3 can grow, so 50% of that pollen can fertilize. Pollen from a plant with S3 and S4 would all be accepted.

D3.1.12 Seed dispersal and germination

Pollination moves pollen between flowers. Seed dispersal moves seeds away from the parent plant, by wind, water, animals or explosive fruit, reducing competition with the parent.

Germination needs water, oxygen and a suitable temperature. The sequence is:

1. The seed absorbs water and swells.
2. The embryo releases a plant hormone, gibberellin, which switches on genes for digestive enzymes.
3. Amylase breaks down stored starch into sugars. Protease and lipase act on other reserves.
4. Sugars move to the growing embryo for respiration and new cell walls.
5. The radicle (root) and plumule (shoot) grow until leaves can photosynthesise.

Worked example (data). Dry mass of a seed falls from 52 mg to 38 mg over 6 days.

change = (38 − 52) ÷ 52 × 100 = −26.9%, and rate = 14 ÷ 6 = 2.3 mg per day.

The mass falls because stored food is used in respiration and lost as carbon dioxide before photosynthesis begins.

Common errors

• Writing that meiosis happens in fertilization. Meiosis makes gametes; fertilization is their fusion.
• Describing LH as controlling the whole cycle. Name the feedback type at each stage.
• Confusing pollination with fertilization or with dispersal.
• Saying that hermaphrodite plants reproduce asexually. They still use gametes.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, statements D3.1.1 to D3.1.12.