# Reproduction in Humans, Sex Hormones and STIs

> Biology · CIE IGCSE 0610
> Source: https://www.owlsprep.com/study/cie-0610-u11-reproduction-in-humans-sex-hormones/

This guide covers Core and Extended content for CIE IGCSE Biology 0610 sections 16.4-16.6: the human reproductive systems, gametes and fertilisation, the developing fetus and placenta, puberty hormones, the menstrual cycle, and sexually transmitted infections, aligned strictly to the current syllabus.

**Prerequisites:** [Asexual and sexual reproduction, and reproduction in plants (CIE 0610 Unit 11)](https://www.owlsprep.com/study/cie-0610-u11-asexual-and-sexual-reproduction-reproduction/)

## Learning objectives

- Identify the parts of the male and female reproductive systems and state their functions (16.4)
- Describe fertilisation, the adaptive features of sperm and egg cells, and compare the two gametes (16.4)
- Describe how a zygote forms an embryo that implants, and state the functions of the umbilical cord, placenta, amniotic sac and amniotic fluid (16.4)
- Explain the exchange of materials across the placenta between the blood of the mother and the fetus (Extended, 16.4)
- Describe the roles of testosterone and oestrogen at puberty and describe the menstrual cycle (16.5)
- Explain the roles of FSH, LH, oestrogen and progesterone in the menstrual cycle (Extended, 16.5)
- Describe the transmission of HIV and explain how the spread of STIs is controlled (16.6)

## 1. The Male and Female Reproductive Systems (Core)

In humans, sexual reproduction depends on two reproductive systems that produce and bring together the gametes (sex cells). You need to be able to identify each labelled part on a diagram and state its function.

| Male part | Function |
| --- | --- |
| Testes | Produce sperm (the male gametes) and the hormone testosterone |
| Scrotum | Sac that holds the testes outside the main body, keeping them slightly cooler than body temperature, which is best for sperm production |
| Sperm ducts | Carry sperm from the testes to the urethra |
| Prostate gland | Adds fluid, containing nutrients, to the sperm to make semen |
| Urethra | Tube that carries semen (and, at other times, urine) out of the body through the penis |
| Penis | Passes semen into the vagina during sexual intercourse |

| Female part | Function |
| --- | --- |
| Ovaries | Contain and release egg cells (the female gametes) and produce the hormones oestrogen and progesterone |
| Oviducts | Carry the egg cell towards the uterus; fertilisation usually happens here |
| Uterus | Muscular organ in which the embryo implants and the fetus develops; its lining thickens ready to receive an embryo |
| Cervix | Ring of muscle at the base of the uterus that separates it from the vagina |
| Vagina | Receives the penis during intercourse and forms the birth canal |

**Worked example:** State the function of (a) the testes and (b) the oviducts.

1. (a) The testes produce sperm (the male gametes) and also produce the hormone testosterone.
2. (b) The oviducts carry the egg cell from the ovary towards the uterus, and are the usual site of fertilisation.

## 2. Gametes: Sperm and Egg Cells (Core)

**Fertilisation** — Fertilisation is the fusion of the nuclei of a male gamete (sperm) and a female gamete (egg cell) to form a zygote.

Sperm and egg cells are each adapted for their role in fertilisation.

- **Adaptations of the sperm:** a **flagellum** (tail) that allows the sperm to swim to the egg cell; many **mitochondria** that release the energy (from respiration) needed for swimming; **enzymes in the acrosome** (a structure at the tip of the sperm head) that digest the egg cell's outer layers so the sperm can penetrate the egg and the sperm nucleus can fuse with the egg nucleus.
- **Adaptations of the egg cell:** **energy stores** (food stores) in its large cytoplasm to nourish the zygote and early embryo; a **jelly coat** that changes straight after fertilisation to stop any more sperm entering.

| Feature | Sperm (male gamete) | Egg cell (female gamete) |
| --- | --- | --- |
| Size | Much smaller | Much larger |
| Structure | Flagellum (tail), little cytoplasm, many mitochondria, an acrosome containing enzymes | No tail, large amount of cytoplasm with energy stores, surrounded by a jelly coat |
| Motility | Motile - swims using its flagellum | Non-motile - moved along the oviduct by the beating of cilia |
| Numbers | Made in very large numbers | Very few made; usually only one released each cycle |

> **Depth limit**
>
> For 0610 the acrosome adaptation goes only as far as: the sperm's acrosome contains enzymes that digest the egg cell's outer layers, so the sperm can enter and its nucleus can fuse with the egg nucleus. The detailed biochemistry of the acrosome reaction is A-Level content and is not required.

**Worked example:** Give two ways in which a sperm cell is adapted for its function, and explain how each adaptation helps.

1. 1. The flagellum (tail) allows the sperm to swim towards and reach the egg cell in the oviduct.
2. 2. The many mitochondria release energy from respiration to power the movement of the flagellum. (The enzymes in the acrosome, which digest the egg cell's outer layers so the sperm can enter, would also be accepted.)

## 3. Fertilisation, Implantation and the Developing Fetus (Core + Extended)

After a sperm nucleus fuses with an egg-cell nucleus in the oviduct, a **zygote** is formed. In early development the zygote divides to form an **embryo**, a ball of cells, which travels to the uterus and **implants** into the thickened lining of the uterus. The embryo then grows into a **fetus**.

As the fetus develops, several structures support and protect it. You need to identify each on a diagram and state its function:

| Structure | Function |
| --- | --- |
| Placenta | Organ where materials are exchanged between the mother's blood and the fetus's blood |
| Umbilical cord | Connects the fetus to the placenta and carries the fetus's blood to and from the placenta |
| Amniotic sac | Membrane that encloses the fetus and holds the amniotic fluid |
| Amniotic fluid | Fluid that surrounds and cushions the fetus, protecting it from mechanical shock (knocks and bumps) |

> **Extended Only (Papers 2/4): exchange at the placenta**
>
> In the placenta the blood of the mother and the blood of the fetus are brought very close together but do **not** mix. Dissolved materials are exchanged by diffusion across the placenta: **oxygen and dissolved nutrients** (e.g. glucose) pass from the mother's blood to the fetus's blood, while **carbon dioxide and other excretory products** (e.g. urea) pass from the fetus's blood to the mother's blood. These materials are carried between the fetus and the placenta through the umbilical cord.

> **Extended Only: pathogens and toxins**
>
> Some **pathogens** (for example HIV and the rubella virus) and some **toxins** (for example nicotine and alcohol from the mother) can pass across the placenta and harm the developing fetus.

**Worked example:** a) (Core) State the function of the amniotic fluid. b) (Extended) Name one substance that passes from the mother's blood to the fetus's blood, and one that passes in the opposite direction, at the placenta.

1. Part a (Core): The amniotic fluid surrounds and cushions the fetus, protecting it from mechanical shock (bumps and knocks).
2. Part b (Extended): Oxygen (or a nutrient such as glucose) passes from the mother's blood to the fetus's blood; carbon dioxide (or urea) passes from the fetus's blood to the mother's blood. The two blood supplies do not mix.

## 4. Puberty: Roles of Sex Hormones (Core)

Puberty is the stage of development when the reproductive system becomes functional and secondary sexual characteristics develop, controlled by sex hormones released from the gonads (testes and ovaries).

**Testosterone** — Male sex hormone released by the testes that controls the development and regulation of male secondary sexual characteristics during puberty.

*Example:* Male secondary sexual characteristics include deepening of the voice, growth of facial and body hair, and increased muscle mass.

Oestrogen is the main female sex hormone released by the ovaries. It controls the development and regulation of female secondary sexual characteristics, such as breast development, widening of the hips, and growth of body hair, and it starts the menstrual cycle.

**Worked example:** State two effects of testosterone during male puberty, and one effect of oestrogen during female puberty other than starting the menstrual cycle.

1. Step 1: Two male secondary sexual characteristics controlled by testosterone, e.g. deepening of the voice and growth of facial hair (also: increased muscle mass, growth of body hair).
2. Step 2: One female secondary sexual characteristic controlled by oestrogen, e.g. breast development (also: widening of the hips, growth of body hair).

## 5. The Menstrual Cycle (Core + Extended)

The menstrual cycle (about 28 days) is described in terms of changes in the **ovaries** and in the **lining of the uterus**. Core content covers oestrogen and progesterone; Extended content adds FSH and LH from the pituitary gland.

**Progesterone** — A hormone released by the ovary after ovulation that maintains the thickened lining of the uterus in the second half of the cycle, ready for a possible pregnancy.

**Core key points:** the cycle starts with menstruation (days 1-5), when the uterus lining breaks down and is lost. From about days 6-14 the lining is rebuilt as oestrogen levels rise and an egg matures in an ovary. **Ovulation** (release of the egg) occurs around day 14. After ovulation, progesterone maintains the thick lining; if the egg is not fertilised, progesterone levels fall and the lining breaks down, restarting the cycle.

> **Extended Only (Papers 2/4): hormonal control**
>
> Two additional hormones from the pituitary gland control the cycle. **FSH (follicle stimulating hormone)** stimulates an egg to mature in the ovary and stimulates the ovary to release oestrogen. **LH (luteinising hormone)** triggers ovulation. Oestrogen and progesterone from the ovary act on the uterus lining and also influence FSH and LH release.

**Worked example:** a) (Core) State the role of progesterone in the menstrual cycle. b) (Extended) Name the hormone that triggers ovulation and its site of production.

1. Part a (Core): Progesterone maintains the thickened lining of the uterus in the second half of the cycle, ready for the implantation of an embryo if the egg is fertilised.
2. Part b (Extended): Luteinising hormone (LH) triggers ovulation, and is produced by the pituitary gland.

## 6. Sexually Transmitted Infections and Their Control (Core)

**Sexually transmitted infection (STI)** — An STI is an infection that is transmitted through sexual contact.

*Example:* HIV (human immunodeficiency virus) is a pathogen that causes an STI. HIV infection may lead to AIDS.

**HIV** is a virus, so it is a pathogen. It is passed on when infected body fluids are exchanged, mainly **blood, semen, vaginal fluids and breast milk**. Common methods of transmission of HIV are unprotected sexual intercourse, sharing unsterilised needles, and from an infected mother to her baby during pregnancy, birth or breastfeeding. HIV is **not** spread by ordinary social ('casual') contact such as shaking hands, hugging or sharing cutlery.

The **spread of STIs is controlled** by measures such as:

- Using a **condom** during sexual intercourse, which acts as a barrier to the infected body fluids that carry pathogens
- **Not sharing needles** or other equipment that could carry infected blood
- **Screening / testing** for infection before unprotected sex with a new partner, and treating those who are infected (for example with antiviral drugs to lower the amount of virus)
- Limiting the number of sexual partners, or abstaining from sexual intercourse
- Education about how STIs are spread and prevented

**Worked example:** Explain why HIV cannot be transmitted by sharing a drinking glass with an infected person, and state one way the spread of HIV can be reduced.

1. Step 1: HIV is only transmitted through the exchange of infected body fluids such as blood, semen, vaginal fluids or breast milk. Sharing a glass only involves contact with saliva, which does not carry enough virus to cause infection, so casual contact does not spread HIV.
2. Step 2: One valid control method, e.g. using a condom during intercourse (a barrier to infected fluids); other acceptable answers include not sharing needles, screening/testing, or limiting the number of partners.

## Common pitfalls

- **Wrong:** Writing that the mother's and fetus's blood mix in the placenta
  - Why it fails: The two blood supplies are brought close together for exchange by diffusion, but they never mix
  - Correct: State that materials are exchanged across the placenta between the two blood supplies, which do not mix
- **Wrong:** Confusing the adaptations of the sperm and the egg cell
  - Why it fails: Each gamete is adapted differently for its role in fertilisation
  - Correct: Link sperm adaptations (flagellum, many mitochondria, enzymes in the acrosome) to reaching and entering the egg; link egg adaptations (energy stores, jelly coat) to nourishing the zygote and stopping extra sperm entering
- **Wrong:** Confusing oestrogen and progesterone roles in the menstrual cycle
  - Why it fails: Both are ovarian hormones but act at different stages of the cycle
  - Correct: Remember oestrogen builds up the uterus lining in the first half, and progesterone maintains it in the second half
- **Wrong:** Mixing up FSH and LH roles (Extended)
  - Why it fails: Both are pituitary hormones but have distinct functions
  - Correct: Link FSH to its full name (Follicle Stimulating Hormone), so it matures an egg follicle; LH triggers ovulation
- **Wrong:** Claiming HIV spreads by casual contact such as hugging or sharing cutlery
  - Why it fails: HIV is only passed on through the exchange of infected body fluids
  - Correct: List only blood, semen, vaginal fluids and breast milk (e.g. via unprotected sex, shared needles, or mother to baby) as valid HIV transmission routes

## Cheatsheet

| Concept | Core Key Facts | Extended Only Key Facts |
| --- | --- | --- |
| Reproductive systems | Male: testes, scrotum, sperm ducts, prostate gland, urethra, penis. Female: ovaries, oviducts, uterus, cervix, vagina - know each function | No extra content |
| Gametes | Sperm: flagellum, many mitochondria, enzymes in the acrosome. Egg: energy stores, jelly coat. Compare size, structure, motility, numbers | No extra content |
| Fertilisation & fetus | Fertilisation = sperm + egg nuclei fuse to form a zygote; zygote to embryo (ball of cells) implants in uterus lining; know functions of umbilical cord, placenta, amniotic sac, amniotic fluid | Placenta exchanges nutrients, gases and excretory products between mother's and fetus's blood (which do NOT mix); some pathogens/toxins cross the placenta |
| Puberty hormones | Testosterone (male) and oestrogen (female) control secondary sexual characteristics | No extra content |
| Menstrual cycle | Oestrogen builds the uterus lining (first half); progesterone maintains it (second half); ovulation ~day 14 | FSH (pituitary): matures egg + stimulates oestrogen; LH (pituitary): triggers ovulation |
| STIs | STI = infection spread by sexual contact; HIV is a virus (pathogen) that may lead to AIDS; spread of STIs controlled by condoms, not sharing needles, screening/treatment, limiting partners | No extra content |

## What's next

Now that you have covered the human reproductive systems, gametes, the placenta and fetus, sex hormones, the menstrual cycle and STIs for CIE IGCSE Biology 0610, revisit reproduction in plants to compare gametes and fertilisation across organisms. Then work through structured response practice questions, as reproduction is frequently tested in both Paper 3 (Core) and Paper 4 (Extended). Finally, move on to inheritance (chromosomes, genes and monohybrid crosses) to see how genetic information from the gametes is passed to offspring. Always separate Core and Extended content in your revision so you focus on the right depth for your paper tier.

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-0610-u11-reproduction-in-humans-sex-hormones/