Sexual reproduction in humans

1. Overview

Sexual reproduction in humans involves the fusion of specialized haploid cells called gametes (sperm and egg) to form a diploid zygote. This process ensures genetic variation in offspring and the continuation of the species through complex biological systems designed for gamete production, fertilization, and fetal development.

Key Definitions

• Gamete: A sex cell (sperm or egg) containing half the number of chromosomes (haploid) found in a normal body cell.
• Fertilisation: The fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell) to form a zygote.
• Zygote: The diploid cell formed by the fusion of two gametes.
• Embryo: A ball of cells formed from the repeated division of a zygote.
• Implantation: The process by which an embryo embeds itself into the thickened lining of the uterus.
• Fetus: The stage of development after the embryo, where the main organs and body parts are formed.

---

Core Content

The Male Reproductive System

• Testes: Production of sperm cells and the hormone testosterone.
• Scrotum: A sac that holds the testes outside the body cavity to maintain a temperature slightly lower than body temperature (essential for sperm production).
• Sperm Ducts: Tubes that carry sperm from the testes to the urethra.
• Prostate Gland: Secretes a nutritious fluid (semen) that mixes with sperm to neutralize acidity and provide energy.
• Urethra: A tube that carries both urine and semen out of the body (at different times).
• Penis: Organ used to pass semen into the vagina during sexual intercourse.

The Female Reproductive System

• Ovaries: Contain undeveloped egg cells and produce hormones (estrogen and progesterone).
• Oviducts (Fallopian tubes): The site of fertilisation; they carry the egg toward the uterus using cilia.
• Uterus (Womb): A muscular bag with a soft lining where the fetus develops.
• Cervix: A ring of muscle at the lower end of the uterus that keeps the fetus in place during pregnancy.
• Vagina: A muscular tube that receives the penis during intercourse and serves as the birth canal.

Adaptive Features of Gametes

Feature	Sperm Cell (Male)	Egg Cell (Female)
Size	Very small.	Very large (visible to naked eye).
Structure	Head, mid-piece, and tail (flagellum).	Large cytoplasm and a jelly coat.
Motility	Highly motile (swims using flagellum).	Non-motile (moved by cilia in oviduct).
Numbers	Produced in millions daily.	Released one per month (usually).

Specific Adaptations:

• Sperm:
  • Flagellum: For swimming to the egg.
  • Mitochondria: Located in the mid-piece to provide energy for swimming.
  • Acrosome: A vesicle in the head containing enzymes to digest the jelly coat of the egg.
• Egg:
  • Energy stores: Cytoplasm contains yolk/lipids to nourish the embryo before implantation.
  • Jelly coat: Changes its structure immediately after fertilisation to prevent other sperm from entering.

Early Development and Fetal Support

1. Fertilisation: Nuclei of sperm and egg fuse in the oviduct.
2. Zygote to Embryo: The zygote divides by mitosis to form a ball of cells (embryo).
3. Implantation: The embryo moves down to the uterus and sinks into the lining.
4. Support Structures:
   • Amniotic sac: A membrane that encloses the developing fetus.
   • Amniotic fluid: Protects the fetus against mechanical shock (cushioning).
   • Placenta: Organ that allows for exchange between maternal and fetal blood.
   • Umbilical cord: Connects the fetus to the placenta.

---

Extended Content (Extended Curriculum)

The Placenta and Umbilical Cord

The placenta is highly adapted for exchange, featuring a large surface area (villi) and a thin membrane to minimize diffusion distance.

• From Mother to Fetus (via umbilical vein): Oxygen, glucose, amino acids, mineral ions, vitamins, and water.
• From Fetus to Mother (via umbilical arteries): Carbon dioxide and urea (excretory products).

Important Note: The blood of the mother and fetus never mix. This prevents high-pressure damage to the fetus and protects it from some (but not all) pathogens.

Pathogens and Toxins

The placenta acts as a partial barrier, but some harmful substances can cross:

• Pathogens: Viruses like HIV or Rubella (German measles) can cross and cause birth defects or infection.
• Toxins: Nicotine and alcohol can cross, leading to low birth weight or Fetal Alcohol Syndrome.

---

Key Equations

While there are no mathematical equations for this topic, the "chromosomal equation" of fertilisation is: Haploid (n) + Haploid (n) = Diploid (2n) (23 chromosomes + 23 chromosomes = 46 chromosomes)

---

Common Mistakes to Avoid

• ❌ Wrong: Stating that the mother's blood flows into the fetus.
• ✓ Right: Maternal and fetal blood remain separate; substances move between them via diffusion across the placenta.
• ❌ Wrong: Confusing the "urethra" with the "ureter" (the ureter connects the kidney to the bladder).
• ✓ Right: Use "urethra" for the tube that exits the body.
• ❌ Wrong: Thinking fertilisation happens in the uterus.
• ✓ Right: Fertilisation occurs in the oviduct.

---

Exam Tips

• Command Word - "State": These are quick marks. Be ready to state the function of any part of the system (e.g., "State the function of the scrotum" -> "To keep testes cooler than body temperature").
• Command Word - "Identify": Practice labeling diagrams. Examiners often use "Front View" and "Side View"—ensure you can recognize the prostate gland and sperm duct from both angles.
• Contexts: You may be asked about the impact of "Biological" toxins (like drugs/nicotine) on the fetus. Always link the toxin crossing the placenta to a specific outcome like "reduced oxygen supply" or "low birth weight."
• Structure-Function: When describing the placenta or sperm cell, always link the structure (e.g., mitochondria) to the function (e.g., energy for movement).

---

Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term fertilisation. [2]

(b) State three adaptive features of a human sperm cell that enable it to reach and fertilize an egg cell. [3]

Worked Solution:

(a)

1. Fertilisation is the fusion of the nucleus of a male gamete (sperm) with the nucleus of a female gamete (egg cell). [This defines the term, referencing the key components and process]

How to earn full marks:

• Mention the fusion of nuclei (1 mark)
• Mention both sperm and egg cell (1 mark)

(b)

1. Flagellum provides movement for the sperm to swim towards the egg cell. [This describes how the flagellum helps the sperm]
2. Mitochondria provide energy for movement via respiration. [This explains the role of mitochondria in providing energy]
3. Enzymes in the acrosome digest the jelly coat of the egg cell. [This explains how enzymes in the acrosome help the sperm]

How to earn full marks:

• State the flagellum and its function (1 mark)
• State the mitochondria and its function (1 mark)
• State the acrosome and its function (1 mark)

Common Pitfall: Students often forget to mention the function of each adaptive feature. Simply stating "flagellum," "mitochondria," or "acrosome" without explaining their roles in fertilization will not earn full marks. Make sure you link the structure to its specific function.

Exam-Style Question 2 — Extended Response [9 marks]

Question:

The diagram shows a cross-section of the human female reproductive system.

(a) Identify the parts labelled in the diagram. [5]

(b) Describe the changes that occur in the uterus lining during the menstrual cycle and explain how these changes are controlled hormonally. [4]

Worked Solution:

(a)

1. Ovary [Correctly identifies the ovary]
2. Oviduct (Fallopian tube) [Correctly identifies the oviduct]
3. Uterus [Correctly identifies the uterus]
4. Cervix [Correctly identifies the cervix]
5. Vagina [Correctly identifies the vagina]

How to earn full marks:

• 1 mark for each correct identification.

(b)

1. The uterus lining (endometrium) thickens during the first half of the cycle. [Describes the thickening of the lining]
2. This is due to increasing levels of oestrogen. [Links oestrogen to the thickening]
3. After ovulation, if fertilisation does not occur, the uterus lining breaks down. [Describes the breakdown of the lining]
4. This breakdown is due to decreasing levels of progesterone. [Links progesterone to the breakdown]

How to earn full marks:

• Mention the thickening of the lining (1 mark)
• Link thickening to oestrogen (1 mark)
• Mention the breakdown of the lining (1 mark)
• Link breakdown to progesterone (1 mark)

Common Pitfall: When describing the hormonal control of the menstrual cycle, students sometimes confuse the roles of oestrogen and progesterone. Remember that oestrogen primarily causes the thickening of the uterine lining, while progesterone maintains the lining and its decrease leads to breakdown if fertilization doesn't occur.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

(a) State two differences between male and female gametes in humans. [2]

(b) During pregnancy, the fetus develops within the uterus. State the function of the:

(i) placenta [2] (ii) amniotic fluid [2]

Worked Solution:

(a)

1. Sperm are much smaller than egg cells. [Correctly states the size difference]
2. Sperm are motile, while egg cells are non-motile. [Correctly states the motility difference]

How to earn full marks:

• 1 mark for each correct difference.

(b)(i)

1. The placenta allows the exchange of dissolved nutrients, gases, and excretory products between the mother's blood and the fetus's blood. [Correctly states the function of the placenta]

How to earn full marks:

• Mention exchange of nutrients, gases and waste (2 marks)

(b)(ii)

1. Amniotic fluid cushions and protects the fetus from mechanical shock. [Correctly states the function of amniotic fluid]

How to earn full marks:

• Mention cushioning and protection (2 marks)

Common Pitfall: For the placenta's function, be sure to mention the exchange of materials. It's not enough to simply say the placenta provides nutrients; you need to specify that it facilitates the transfer of nutrients (and waste) between mother and fetus.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

A researcher is studying the effect of different chemicals on sperm motility. He collects sperm samples from healthy donors and divides them into four groups. Each group is exposed to a different concentration of a pesticide, measured in parts per million (ppm). After 30 minutes, he measures the percentage of motile sperm in each sample. The results are shown in the table.

Pesticide Concentration (ppm)	Percentage of Motile Sperm (%)
0 (Control)	85
10	78
20	65
30	42

(a) Describe the trend shown in the table. [2]

(b) Explain how the pesticide might affect sperm motility at the cellular level. [4]

(c) Some pathogens can cross the placenta and affect the fetus. State two examples of such pathogens. [2]

(d) Suggest one ethical consideration relevant to research involving human sperm samples. [2]

Worked Solution:

(a)

1. As the pesticide concentration increases, the percentage of motile sperm decreases. [Describes the inverse relationship between concentration and motility]
2. The motility decreases from 85% at 0 ppm to 42% at 30 ppm. [Provides specific data to support the description]

How to earn full marks:

• State the inverse relationship (1 mark)
• Include specific data points from the table (1 mark)

(b)

1. The pesticide may damage the mitochondria within the sperm. [Identifies the target organelle]
2. This would reduce ATP production. [Links damage to reduced ATP production]
3. With less ATP, the flagellum has less energy to move. [Links reduced ATP to reduced flagellum movement]
4. Therefore, the sperm's motility is reduced. [Concludes that motility is reduced]

How to earn full marks:

• Mention damage to mitochondria (1 mark)
• Mention reduced ATP production (1 mark)
• Link ATP to flagellum movement (1 mark)
• Conclude with reduced motility (1 mark)

(c)

1. Rubella virus [Correctly states a pathogen]
2. HIV (Human Immunodeficiency Virus) [Correctly states a pathogen]

How to earn full marks:

• 1 mark for each correct pathogen. Accept any valid pathogen.

(d)

1. Informed consent must be obtained from the sperm donors. [States the ethical consideration of informed consent]
2. This ensures they are aware of the research purpose and potential risks. [Explains the importance of informed consent]

How to earn full marks:

• Mention informed consent (1 mark)
• Explain why it's important (1 mark)

Common Pitfall: When explaining the effect of the pesticide on sperm motility, many students only focus on the flagellum. While the flagellum is directly responsible for movement, remember to link it back to the mitochondria and ATP production. The flagellum needs energy to function, and the mitochondria are the powerhouses providing that energy.