CIE iGCSE Biology-16.4 Sexual reproduction in humans- Study Notes- New Syllabus
CIE iGCSE Biology-16.4 Sexual reproduction in humans- Study Notes – New syllabus
CIE iGCSE Biology-16.4 Sexual reproduction in humans- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Identify on diagrams and state the functions of the following parts of the male reproductive system: testes, scrotum, sperm ducts, prostate gland, urethra and penis
- Identify on diagrams and state the functions of the following parts of the female reproductive system: ovaries, oviducts, uterus, cervix and vagina
- Describe fertilisation as the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell)
- Explain the adaptive features of sperm, limited to: flagellum, mitochondria and enzymes in the acrosome
- Explain the adaptive features of egg cells, limited to: energy stores and the jelly coat that changes at fertilisation
- Compare male and female gametes in terms of: size, structure, motility and numbers
- State that in early development, the zygote forms an embryo which is a ball of cells that implants into the lining of the uterus
- Identify on diagrams and state the functions of the following in the development of the fetus: umbilical cord, placenta, amniotic sac and amniotic fluid
Supplement
- Describe the function of the placenta and umbilical cord in relation to the exchange of dissolved nutrients, gases and excretory products between the blood of the mother and the blood of the fetus
- State that some pathogens and toxins can pass across the placenta and affect the fetus
Male Reproductive System: Structures and Functions
Structure | Function |
---|---|
Testes | – Produce sperm cells (male gametes) – Produce the hormone testosterone |
Scrotum | – A sac of skin that holds the testes outside the body – Keeps the testes at a slightly lower temperature (needed for sperm production) |
Sperm Ducts (Vas deferens) | – Carry sperm from the testes to the urethra |
Prostate Gland | – Produces part of the seminal fluid that mixes with sperm to form semen – Helps sperm swim and survive |
Urethra | – Tube that carries sperm (during ejaculation) or urine (during urination) out of the body through the penis (never both at the same time) |
Penis | – Transfers sperm into the vagina during sexual intercourse – Contains erectile tissue and the urethra |
Testes = sperm + testosterone
Urethra = carries either sperm or urine
Prostate adds fluid → forms semen
Female Reproductive System: Structures and Functions
Structure | Function |
---|---|
Ovaries | – Produce egg cells (ova – female gametes) – Secrete female sex hormones like oestrogen and progesterone |
Oviducts (Fallopian tubes) | – Transport the egg from the ovary to the uterus – Fertilisation usually occurs here |
Uterus (Womb) | – Muscular organ where a fertilised egg implants – Supports the development of the foetus during pregnancy |
Cervix | – Narrow opening between the uterus and vagina – Keeps the foetus in place during pregnancy and dilates during childbirth |
Vagina | – Receives the penis during sexual intercourse – Acts as the birth canal during delivery of the baby – Passageway for menstrual flow |
Ovaries = eggs + hormones
Fertilisation → oviduct
Uterus = foetus development
Vagina = receives sperm + birth canal
Fertilisation in Humans

Definition:
Fertilisation is the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell).
📘 Key Points:
- Occurs in the oviduct (fallopian tube) of the female.
- Involves the joining of genetic material from both parents.
- The sperm swims toward the egg, and one sperm nucleus enters the egg.
- This fusion creates a zygote, which is diploid (has two sets of chromosomes – one from each parent).
🔁 After Fertilisation:
Stage | Event |
---|---|
Zygote | A fertilised egg cell |
Cell division | The zygote divides to form an embryo |
Implantation | The embryo implants in the uterus lining |
Pregnancy begins | The embryo grows into a foetus |
Adaptive Features of Sperm Cells
Sperm cells are highly specialized for their role in fertilisation. Here are three key adaptations:
1️⃣ Flagellum (Tail)
- Structure: Long, whip-like tail
- Function: Helps the sperm swim towards the egg through the female reproductive tract
- Benefit: Increases the chance of reaching the egg for fertilisation
2️⃣ Mitochondria
- Location: Packed in the midpiece of the sperm
- Function: Provide energy (ATP) for movement
- Benefit: Enables constant tail movement over long distances
3️⃣ Enzymes in the Acrosome
- Structure: A cap-like structure at the front of the head
- Function: Contains digestive enzymes
- Benefit: Helps the sperm break through the protective layers around the egg (zona pellucida), allowing fertilisation
🗒️ Note:
Flagellum = swims
Mitochondria = energy for swimming
Acrosome enzymes = break into egg
Adaptive Features of Egg Cells
Egg cells are specialized to support fertilisation and early development. Two key features are:
1️⃣ Energy Stores
- Structure: Egg cytoplasm is large and rich in nutrients
- Function: Provides energy and raw materials for the early cell divisions and development of the embryo
- Benefit: Supports the embryo until it implants in the uterus and connects to the mother’s blood supply
2️⃣ Jelly Coat (Zona Pellucida)
- Structure: A gel-like protective outer layer surrounding the egg
- Function:
– Protects the egg from physical damage
– Attracts and binds sperm
– After the first sperm enters, the jelly coat hardens and changes to prevent entry of other sperm (prevents polyspermy) - Benefit: Ensures that only one sperm fertilises the egg, keeping the correct number of chromosomes
🗒️ Note:
Energy store = supports early embryo
Jelly coat = protects & blocks extra sperm
Prevents polyspermy = correct chromosome number
Comparison of Male and Female Gametes
Feature | Male Gamete (Sperm) | Female Gamete (Egg/Ovum) |
---|---|---|
Size | Very small | Much larger than sperm |
Structure | – Has a tail (flagellum) for movement – Contains a small head with nucleus and acrosome – Midpiece with mitochondria | – Round in shape – Large cytoplasm with nutrient reserves – Surrounded by jelly coat |
Motility | Motile – swims using its flagellum | Non-motile – does not move on its own |
Numbers Produced | Millions made continuously after puberty | One egg released per month (in humans) |
📘 Summary:
- Sperm: Many, small, mobile, designed for speed and delivery
- Egg: Few, large, nutrient-rich, designed for nourishment and support
Early Development of the Zygote
✅ Key Statement:
In early development, the zygote forms an embryo, which is a ball of cells that implants into the lining of the uterus.
📘 Key Points:
- Zygote: The single diploid cell formed when a sperm nucleus fuses with an egg nucleus (fertilisation).
- The zygote undergoes mitotic cell division to form a ball of cells.
- This developing structure is called an embryo.
- The embryo travels down the oviduct to the uterus.
- It implants (attaches) into the uterine lining where it will continue to grow and develop.
Zygote → embryo = ball of cells Implants into uterus lining Implantation = start of pregnancy
Structures in Fetal Development – Functions and Identification
Structure | Function |
---|---|
Umbilical Cord | – Connects the fetus to the placenta – Contains blood vessels that carry oxygen and nutrients to the fetus and remove carbon dioxide and wastes |
Placenta | – Exchange organ between mother and fetus – Transfers oxygen, nutrients (like glucose, amino acids) from mother to fetus – Removes waste products (CO₂, urea) from fetus to mother – Also produces hormones to maintain pregnancy |
Amniotic Sac | – Thin membrane that surrounds the fetus – Holds the amniotic fluid inside |
Amniotic Fluid | – Protects the fetus by cushioning it from shocks – Allows fetus to move freely for proper muscular and skeletal development – Maintains a constant temperature around the fetus |
Umbilical cord = transport link
Placenta = exchange organ + hormones
Amniotic sac = membrane around fetus
Amniotic fluid = protection + movement
Function of the Placenta and Umbilical Cord
✅ Placenta – Exchange Organ
The placenta is a temporary organ that forms during pregnancy and attaches to the wall of the uterus.
🔄 Functions:
What Is Exchanged | From/To | Purpose |
---|---|---|
Oxygen | From mother → to fetus | Needed for aerobic respiration |
Nutrients (e.g., glucose, amino acids, vitamins, minerals) | From mother → to fetus | Used for growth and development |
Carbon dioxide | From fetus → to mother | Waste gas from respiration |
Urea and other waste | From fetus → to mother | Removed through mother’s excretory system |
Antibodies (optional) | From mother → to fetus | Provide passive immunity |
The placenta prevents direct blood mixing – materials pass by diffusion across a thin membrane.
🔗 Umbilical Cord – The Connection
The umbilical cord connects the fetus to the placenta and contains two arteries and one vein:
Vessel | Direction | Carries |
---|---|---|
Umbilical artery | Fetus → placenta | Deoxygenated blood + waste (CO₂, urea) |
Umbilical vein | Placenta → fetus | Oxygenated blood + nutrients |
Placenta = exchange center (O₂, food in — CO₂, urea out)
Umbilical cord = transport link No blood mixing, exchange by diffusion
Pathogens and Toxins Crossing the Placenta
Key Statement:
Some pathogens (harmful microorganisms) and toxins (poisonous substances) can pass across the placenta and negatively affect the developing fetus.
🧬 Understanding the Placenta’s Role:
- The placenta allows the exchange of useful substances (oxygen, nutrients) and removal of waste.
- It also acts as a barrier to many harmful substances.
- However, some pathogens and toxins are small enough or specially adapted to cross this barrier.
🦠 Examples of Pathogens:
Pathogen | Effect on Fetus |
---|---|
Rubella virus | Can cause blindness, deafness, heart defects |
HIV | May be passed on, causing HIV in the baby |
Zika virus | Linked to microcephaly (small head, brain defects) |
☠️ Examples of Toxins:
Toxin | Source | Effect on Fetus |
---|---|---|
Alcohol | Drinking alcohol | Can cause fetal alcohol syndrome (FAS) – low IQ, facial deformities |
Nicotine | Smoking | May cause low birth weight, premature birth |
Drugs | Recreational or certain medications | Can lead to addiction, developmental delays |
❗ Why This Matters:
- The developing fetus is very sensitive to its environment.
- Damage in early development may lead to permanent defects or miscarriage.
- Pregnant women are advised to avoid infections, smoking, alcohol, and certain medications.
🗒️Note:
Some viruses (e.g. rubella, HIV) and toxins (alcohol, nicotine) can cross placenta → May cause birth defects, low weight, miscarriage → Placenta = partial barrier, not full protection