Biology (0610) | Sexual reproduction in humans

Welcome to the World of Human Reproduction!

Hello Biologists! This chapter is all about how humans reproduce sexually—a fascinating and vital process that ensures the continuation of our species. Don't worry if some of the terms seem complicated; we will break down the structures, functions, and hormonal controls step-by-step.

Understanding human reproduction is essential not just for your exams, but also for understanding health, growth, and development. Let's get started!

1. Structures of the Reproductive Systems (16.4.1 & 16.4.2)

1.1 The Male Reproductive System

The main goal of the male system is to produce sperm (the male gamete) and deliver them for fertilisation.

• Testes: The primary organs. Their function is to produce sperm and the male hormone testosterone.
• Scrotum: A sac containing the testes. It keeps the testes at a temperature slightly lower than body temperature, which is essential for healthy sperm production.
• Sperm Ducts (Vas deferens): Tubes that carry sperm from the testes/epididymis towards the urethra.
• Prostate Gland: A gland that produces fluid (part of semen) that nourishes and protects the sperm.
• Urethra: A tube passing through the penis that carries both urine and semen out of the body (though never at the same time).
• Penis: The organ used to introduce sperm into the female reproductive tract during sexual intercourse.

1.2 The Female Reproductive System

The female system's goal is to produce egg cells (the female gamete), receive sperm, provide a site for fertilisation, and support the developing fetus.

• Ovaries: The primary organs. They produce egg cells (ova) and the female hormones oestrogen and progesterone.
• Oviducts (Fallopian tubes): Tubes connecting the ovaries to the uterus. This is the usual site of fertilisation.
• Uterus (Womb): A muscular, pear-shaped organ where the embryo implants and the fetus develops during pregnancy.
• Cervix: The narrow ring of muscle and tissue at the base of the uterus, separating it from the vagina.
• Vagina: A muscular tube that receives the penis during sexual intercourse and forms the birth canal during delivery.

Analogy: Think of the uterus as a luxurious, cushioned nest waiting for the fertilised egg to arrive and implant!

2. Gametes and Fertilisation (16.4.3 - 16.4.6)

2.1 What is Fertilisation? (Core)

Fertilisation is the process involving the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell). This forms a single cell called a zygote.

Important note: The nuclei of gametes are haploid (contain one set of chromosomes), while the nucleus of the resulting zygote is diploid (contains two sets of chromosomes).

2.2 Adaptive Features of Gametes (Core)

The Sperm (Male Gamete)

Sperm are adapted for moving quickly and penetrating the egg.

• Flagellum (Tail): Provides motility (movement) so the sperm can swim towards the egg in the oviduct.
• Mitochondria: Clustered in the middle section to provide the large amounts of energy (ATP from respiration) needed for the tail to move.
• Acrosome: A cap at the head containing enzymes that digest the outer layers of the egg cell, allowing the sperm nucleus to enter for fertilisation.

The Egg Cell (Female Gamete)

The egg cell is adapted to sustain the zygote after fertilisation.

• Large Size: It is much larger than the sperm, providing space.
• Energy Stores (Yolk/Cytoplasm): Contains large food/energy stores to provide nutrients for the embryo in its very early stages before implantation.
• Jelly Coat (Zona pellucida): This layer changes its structure immediately after fertilisation by one sperm, preventing any other sperm from entering (blocking polyspermy).

2.3 Comparing Male and Female Gametes (Core)

Feature	Sperm (Male Gamete)	Egg (Female Gamete)
Size	Very small	Very large
Structure	Head, mid-section (mitochondria), tail/flagellum	Spherical, large cytoplasm, jelly coat
Motility	Motile (can swim)	Non-motile (cannot swim)
Numbers	Millions produced daily	Usually one released per menstrual cycle

3. Development of the Fetus (16.4.7 - 16.4.10)

3.1 Early Development and Implantation (Core)

After fertilisation, the zygote immediately begins dividing by mitosis.

• The zygote forms a ball of cells called the embryo.
• The embryo travels down the oviduct to the uterus.
• The embryo implants itself into the thickened, bloody lining of the uterus (endometrium).

3.2 Supporting the Fetus

Once implanted, structures develop to support the growing fetus during the 9 months of pregnancy.

Placenta and Umbilical Cord (Core & Supplement)

The placenta is an organ rich in blood vessels that develops in the uterus wall. It acts as the interface between the mother's blood supply and the fetus's blood supply.

The umbilical cord connects the fetus to the placenta.

The function of the placenta is exchange:

• Exchange of Dissolved Nutrients: Glucose, amino acids, vitamins move from the mother's blood to the fetal blood.
• Exchange of Gases: Oxygen moves from the mother's blood to the fetal blood; carbon dioxide moves the other way.
• Exchange of Excretory Products: Urea moves from the fetal blood to the mother's blood for excretion via the mother's kidneys.

Did you know? The mother's blood and the fetus's blood never actually mix in the placenta. They are separated by thin walls, allowing efficient diffusion of materials.

Risks of the Placenta (Supplement)

Unfortunately, the placenta is not a perfect barrier.

• Some pathogens (like the viruses causing rubella or HIV) can pass across the placenta and affect the fetus.
• Toxins (like alcohol, nicotine, or drugs) can also pass across the placenta and negatively affect fetal development.

Amniotic Sac and Fluid (Core)

• Amniotic Sac: A membrane surrounding the fetus.
• Amniotic Fluid: The fluid inside the sac. Its main function is to cushion and protect the fetus against mechanical shock (bumps or knocks). It also provides a stable temperature environment.

4. Sex Hormones and the Menstrual Cycle (16.5)

4.1 Roles of Sex Hormones in Puberty (Core)

Puberty is the stage of development when secondary sexual characteristics appear due to hormones.

• Testosterone (Male hormone): Responsible for the development and regulation of secondary sexual characteristics in males (e.g., beard growth, deepening of voice, broader shoulders, sperm production).
• Oestrogen (Female hormone): Responsible for the development and regulation of secondary sexual characteristics in females (e.g., breast development, widening of hips, start of menstrual cycle).

4.2 The Menstrual Cycle (Core)

The menstrual cycle typically lasts about 28 days and involves changes in the ovaries (egg release) and changes in the lining of the uterus (preparation for potential implantation).

In simple terms:

1. Days 1–5 (Menstruation): The thickened uterine lining is shed (the period).
2. Days 5–14 (Lining repairs): The lining of the uterus thickens again in preparation for an embryo. An egg matures in the ovary.
3. Day 14 (Ovulation): The mature egg is released from the ovary (this is when fertilisation is most likely).
4. Days 14–28 (Lining maintained): The uterine lining is kept thick and spongy by hormones. If no fertilisation occurs, the cycle returns to Day 1.

4.3 Hormonal Control (Supplement)

The menstrual cycle and pregnancy are tightly controlled by four key hormones:

• Follicle Stimulating Hormone (FSH): Stimulates the development of an egg follicle in the ovary.
• Luteinising Hormone (LH): Triggers ovulation (release of the egg) around Day 14.
• Oestrogen: Stimulates the repair and thickening of the uterus lining. Produced initially by the developing follicle and later by the corpus luteum (in the ovary).
• Progesterone: Maintains the thick, spongy lining of the uterus throughout the second half of the cycle and throughout pregnancy (it stops the lining from being shed). Produced by the corpus luteum, and later, the placenta.

5. Sexually Transmitted Infections (STIs) (16.6)

5.1 Defining STIs

A Sexually Transmitted Infection (STI) is an infection caused by a pathogen (virus, bacterium, fungus, or protoctist) that is transmitted through sexual contact.

5.2 HIV and AIDS (Core)

• Human Immunodeficiency Virus (HIV): This is the specific pathogen (a virus) that causes an STI.
• Acquired Immunodeficiency Syndrome (AIDS): This is the condition or disease that results from a severe HIV infection. AIDS occurs when the immune system becomes so weak that the body can no longer fight off common infections.

5.3 Transmission Methods of HIV (Core)

HIV is mainly transmitted through the exchange of specific body fluids:

1. Unprotected sexual intercourse (vaginal, anal, or oral).
2. Sharing contaminated needles (e.g., among drug users).
3. Blood transfusions (if the blood is not screened).
4. From an infected mother to her child during pregnancy, birth, or breast-feeding.

5.4 Controlling the Spread of STIs (Core)

The spread of STIs, including HIV, can be controlled effectively through:

• Abstinence: Avoiding sexual contact.
• Using Barrier Methods: The correct and consistent use of condoms during sexual intercourse prevents the exchange of body fluids.
• Screening: Regular testing for STIs and treating those who are infected early.
• Limiting Partners: Reducing the number of sexual partners.
• Education: Increasing public awareness about transmission routes and prevention methods.