16.4 Sexual reproduction in humans
- Syllabus
- 0610–2026–2027
- Topic
- 16.4
- Level
- —
Learning objectives
16.4.1Male reproductive system parts• Identify on diagrams and state the functions of the following parts of the male reproductive system: testes, scrotum, sperm ducts, prostate gland, urethra and penis16.4.2Female reproductive system parts• Identify on diagrams and state the functions of the following parts of the female reproductive system: ovaries, oviducts, uterus, cervix and vagina16.4.3Fertilisation as the fusion• Describe fertilisation as the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell)16.4.4Adaptive features of sperm• Explain the adaptive features of sperm, limited to: flagellum, mitochondria and enzymes in the acrosome16.4.5Adaptive features of egg cells• Explain the adaptive features of egg cells, limited to: energy stores and the jelly coat that changes at fertilisation16.4.6Male and female gametes in terms• Compare male and female gametes in terms of: size, structure, motility and numbers16.4.7In early development, the zygote forms• State that in early development, the zygote forms an embryo which is a ball of cells that implants into the lining of the uterus16.4.8Fetus development structures• Identify on diagrams and state the functions of the following in the development of the fetus: umbilical cord, placenta, amniotic sac and amniotic fluid16.4.9Function of the placenta and umbilical• 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 fetus16.4.10Some pathogens and toxins can pass• State that some pathogens and toxins can pass across the placenta and affect the fetus
Identify the male reproductive system
Trace the sperm pathway from its site of production to the outside: testis → sperm duct → urethra → penis.
| Part | Function and recognition cue |
|---|---|
| testes | produce sperm and testosterone; lie inside the scrotum |
| scrotum | holds the testes outside the body at a cooler temperature suitable for sperm production |
| sperm ducts | carry sperm from the testes towards the urethra |
| prostate gland | adds fluid to sperm to form semen |
| urethra | carries semen and urine through the penis, at different times |
| penis | delivers semen into the vagina |
The sperm duct transports sperm only; the urethra is the shared exit passage for semen and urine.
Identify the female reproductive system
| Part | Function and recognition cue |
|---|---|
| ovaries | produce and release egg cells and secrete female sex hormones |
| oviducts | carry an egg towards the uterus; usual site of fertilisation |
| uterus | muscular organ where the embryo implants and the fetus develops |
| cervix | ring of muscle at the lower end of the uterus; dilates during birth |
| vagina | receives the penis and semen; birth canal |
Follow the egg route: ovary → oviduct → uterus. Fertilisation normally occurs in an oviduct; implantation occurs in the uterus lining.
The ovary produces egg cells, the oviduct is the fertilisation site, and the uterus is the implantation and development site.
Define fertilisation in humans
Fertilisation is the fusion of the nucleus of a sperm with the nucleus of an egg cell.
The two haploid gamete nuclei unite in an oviduct to form the diploid nucleus of a zygote.
Fertilisation is nuclear fusion, not ejaculation, sperm reaching the egg, or implantation in the uterus.
Explain sperm-cell adaptations
| Feature | How it helps fertilisation |
|---|---|
| flagellum | propels the sperm towards the egg |
| many mitochondria | release energy by aerobic respiration for movement |
| acrosome | contains enzymes that digest the egg's jelly coat so the sperm can enter |
The adaptations form a sequence: movement reaches the egg, respiration supplies that movement, and acrosome enzymes enable penetration of the jelly coat.
The flagellum moves the cell but does not release energy; mitochondria release energy but do not directly propel it; the acrosome acts at the egg surface.
Explain egg-cell adaptations
| Feature | How it helps |
|---|---|
| energy stores in the cytoplasm | supply materials and energy for early cell divisions before implantation |
| jelly coat | changes after fertilisation to prevent additional sperm entering |
The egg's energy stores support early development; they do not power active movement. The jelly coat blocks further sperm after one has fertilised the egg.
Compare human gametes
| Feature | Sperm cell | Egg cell |
|---|---|
| size | very small | much larger |
| structure | flagellum, many mitochondria, acrosome, little cytoplasm | large cytoplasm with energy stores and a jelly coat |
| motility | motile; swims using its flagellum | non-motile |
| numbers | produced and released in very large numbers | usually one released per menstrual cycle |
Many small motile sperm increase the chance that one reaches an egg; the larger egg invests cytoplasm and stored materials in early development.
Only one sperm nucleus normally fertilises one egg; large sperm numbers increase probability, not the number that fuse with the egg.
Describe early human development
After fertilisation, the zygote divides repeatedly by mitosis to form an embryo, a ball of cells. The embryo travels along the oviduct and implants into the lining of the uterus. Further growth and development produce a fetus.
| Stage | Key event |
|---|---|
| zygote | single diploid cell formed at fertilisation |
| embryo | ball of cells formed by repeated mitosis; implants in uterus lining |
| fetus | later developing organism with recognisable body structures |
Implantation is attachment of the embryo to the uterus lining; it is not fertilisation and does not occur in the oviduct.
Identify structures supporting the fetus
| Structure | Function |
|---|---|
| placenta | exchange surface between maternal and fetal blood systems |
| umbilical cord | connects fetus to placenta and contains fetal blood vessels |
| amniotic sac | membrane enclosing the fetus and amniotic fluid |
| amniotic fluid | cushions against mechanical shock, supports movement and helps maintain a stable temperature |
Maternal and fetal blood normally remain separate. Substances cross the placenta between them; the two blood supplies do not mix directly.
Explain placental and umbilical exchange
The placenta is an exchange surface between maternal blood and fetal blood, while the umbilical cord carries fetal blood to and from the placenta.
| Direction | Substances | Fetal vessel |
|---|---|---|
| mother → fetus | oxygen and dissolved nutrients such as glucose and amino acids | umbilical vein carries enriched blood to fetus |
| fetus → mother | carbon dioxide and excretory products such as urea | umbilical arteries carry blood to placenta |
A large surface area, thin exchange barrier and good blood supplies maintain short diffusion distance and concentration gradients.
The umbilical vein is named by flow towards the fetal heart, not by oxygen content; here it carries more oxygen than the umbilical arteries.
Recognise placental transfer risks
The placenta is selective but not a complete barrier: some pathogens and toxins can cross it and affect the fetus.
| Example | Possible consequence |
|---|---|
| rubella virus and some other pathogens | infection or damage to the developing fetus |
| nicotine, alcohol and other toxins | disrupted growth or development; smoking also reduces oxygen delivery |
The placenta keeps maternal and fetal blood separate, but separation does not mean that every harmful substance or pathogen is blocked.