BIOLOGY | Unit 2 : Animal Physiology | Reproduction in humans

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Reproduction in Humans Answer Key

Syllabus Practice Edexcel IGCSE Biology Pages 126 – 129

Pages 126–127: Q1 - Q6

Pages 128–129: Q7 - Q10

Question 1 (Skills: Critical Thinking)

Which of the following describes sex cells (gametes)?

• A: diploid cells produced by meiosis
• B: diploid cells produced by mitosis
• C: haploid cells produced by meiosis
• D: haploid cells produced by mitosis

Correct Answer: C

Syllabus Reasoning: Sex cells (gametes) must be haploid (containing half the normal set of chromosomes, n = 23 in humans) so that when fertilisation occurs, the resulting zygote will have the full diploid set (2n = 46). Meiosis is the special reduction division that produces haploid gametes.

Question 2 (Skills: Critical Thinking)

Which of the following is not a function of the ovaries?

• A: the secretion of progesterone
• B: the production of eggs
• C: the secretion of oestrogen
• D: the site of fertilisation

Correct Answer: D

Syllabus Reasoning: The ovaries produce oocytes (eggs) and secrete sex hormones (oestrogen and progesterone). However, fertilisation (the fusion of the sperm and egg nuclei) does not occur in the ovaries; it takes place in the oviduct (Fallopian tube).

Question 3 (Skills: Critical Thinking)

Which of the following organs produce(s) the hormone progesterone?

1. pituitary gland  •  2. ovary  •  3. uterus  •  4. placenta

• A: 1 and 2
• B: 2 and 3
• C: 3 and 4
• D: 2 and 4

Correct Answer: D (2 and 4)

Syllabus Reasoning: Progesterone is produced in the ovary by the corpus luteum (the yellow body formed from the ruptured follicle after ovulation). During pregnancy, the placenta develops and takes over the secretion of progesterone to maintain the thick, nutrient-rich lining of the uterus.

Question 4 (Skills: Critical Thinking)

A woman's first day of menstruation was on 1st June. Assuming she has a 28-day menstrual cycle, when was she most likely to ovulate?

• A: 7 June
• B: 10 June
• C: 14 June
• D: 21 June

Correct Answer: C (14 June)

Syllabus Reasoning: In a typical 28-day menstrual cycle, ovulation (the release of a mature egg from an ovary) occurs precisely in the middle of the cycle, on Day 14. Day 1 is the first day of menstruation (1st June), making June 14th the predicted day of ovulation.

Question 5 (Skills: Analysis)

The Fetus in the Uterus

Fetus in Uterus Model (Before Birth)

a) Name parts A to F on the diagram.

Anatomical Labels:

• A: Umbilical cord
• B: Placenta
• C: Amniotic fluid
• D: Amnion (or amniotic sac)
• E: Cervix (neck of the uterus)
• F: Muscular uterine wall (uterus wall)

b) What is the function of A during pregnancy?

Function of the Umbilical Cord (A):

The umbilical cord links the fetus's blood system to the placenta.

• The umbilical artery transports deoxygenated blood and metabolic waste (e.g. carbon dioxide and urea) from the fetus back to the placenta.
• The umbilical vein delivers oxygenated, nutrient-rich blood (carrying glucose, amino acids, and antibodies) from the placenta to the growing fetus.

c) What must happen to D and E just before birth?

Events Just Before Birth:

• The amnion (D) must rupture (commonly referred to as "water breaking"), releasing the lubricating amniotic fluid (C) out through the vagina.
• The cervix (E) must dilate (widen and stretch) to approximately 10 cm to allow the baby's head to pass through from the uterus into the vagina.

d) What must E and F do during birth?

Mechanics of Childbirth:

• The cervix (E) must remain fully dilated and flexible to facilitate childbirth.
• The muscular uterine wall (F) must undergo strong, coordinated, and regular contractions (stimulated by the hormone oxytocin) to push the baby downwards through the cervix and vagina.

Question 6 (Skills: Critical Thinking)

Reproductive Methods in Hydra

Hydra can reproduce in two ways: Method A shows sex organs (testes and ovary) on the body, and Method B shows a "young Hydra bud" growing out of the parent.

a) Which of the two methods shows asexual reproduction? Give a reason for your answer.

Asexual Pathway: Method B

Method B represents asexual reproduction (specifically by budding).

Reason: It involves only one parent. A small outgrowth (bud) develops on the parent's body through mitotic cell division and eventually detaches to live independently, without the fusion of gametes or involvement of reproductive organs.

b) Explain why organisms produced asexually are genetically identical to each other and to the organism that produced them.

Genetic Identity:

Asexual reproduction relies entirely on mitosis (cell division that produces genetically identical cells). Since there is no fusion of gametes (fertilisation) and no introduction of genetic material from another parent, the offspring inherit an exact copy of the parent's DNA, making them clones.

c) When the surroundings do not change for long periods, Hydra reproduces mainly asexually. When the conditions change, Hydra begins to reproduce sexually. How does this pattern help Hydra to survive?

Evolutionary Strategy:

• During Stable Conditions (Asexual): This is highly advantageous because it is rapid and energy-efficient. Since the parent organism is already well-adapted to the stable environment, producing genetically identical offspring ensures they will also be well-suited to survive in those exact conditions.
• During Changing Conditions (Sexual): Sexual reproduction involves meiosis and fertilisation, which introduces genetic variation among the offspring. This variation ensures that if a new disease, predator, or climate shift occurs, some offspring may possess genetic traits or alleles that allow them to survive, preventing the entire population from being wiped out.

Question 7 (Skills: Analysis & Interpretation)

Anatomy & Cycles of Female Reproduction

Female Reproductive System

a) Which letter represents:

• i) the site of production of oestrogen and progesterone: B (Ovary)
• ii) the site where fertilisation usually occurs: A (Oviduct / Fallopian Tube)
• iii) the structure that must dilate when birth commences: D (Cervix)
• iv) the structure that releases ova: B (Ovary)

b) The graph shows changes in the thickness of the uterus lining over 100 days.

i) Name the hormone that causes the thickening of the uterine lining.

Oestrogen & Progesterone:

The rebuilding and initial thickening of the uterine lining is caused by oestrogen. The subsequent maintenance and further thickening of this lining is caused by progesterone. Both hormones work together during the menstrual cycle.

ii) Use the graph to determine the duration of this woman's menstrual cycle. Explain your answer.

Calculation: ~28 days

The cycle length is calculated by measuring the time between consecutive drops in the graph (which represent menstruation).

• The first drop ends on approximately Day 4.
• The second drop ends on approximately Day 32.
• The third drop ends on approximately Day 60.
• The intervals between these drops are: $32 - 4 = 28$ days, and $60 - 32 = 28$ days. Therefore, the average cycle duration is 28 days.

iii) From the graph, deduce the approximate day on which fertilisation leading to pregnancy took place. Explain how you arrived at your answer.

Estimated Day of Fertilisation: Day 74

The normal menstrual cycle continues up to the third shedding phase, which ends around Day 60. The next ovulation is expected around Day 74 ($60 + 14$).

Following this phase, the uterine lining does not shed at Day 88 as expected. Instead, it remains continuously thickened through to Day 100. This tells us that fertilisation occurred around Day 74, and the resulting embryo successfully implanted in the uterine wall.

iv) Why must the uterus lining remain thickened throughout pregnancy?

Importance of the Endometrium:

The thick, vascularised lining of the uterus is essential to keep the placenta anchored securely. The placenta supplies the growing fetus with oxygen, glucose, amino acids, and antibodies from the mother's blood, and removes waste products (carbon dioxide and urea). If this lining sheds, it would lead to miscarriage.

Question 8 (Skills: Research & Synthesis)

Sperm Count Decline & Environmental Oestrogens

"Carry out an Internet search to find out the evidence that human sperm counts have decreased due to environmental oestrogens and other pollutants."

Summary of Scientific Findings:

1. Evidence of Sperm Count Decline:

Since the landmark 1992 study by Carlsen et al. and more recent global meta-analyses, evidence indicates a 50–60% decline in average sperm counts among men worldwide over the last 50 years. This downward trend is most pronounced in industrialized regions.

2. Role of Environmental Oestrogens & EDCs:

Endocrine Disruptor Chemicals (EDCs) are synthetic substances that mimic natural hormones. Environmental oestrogens (xenoestrogens) enter our water systems through:

• Agricultural pesticide runoff (e.g. atrazine).
• Industrial plasticizers (e.g., Bisphenol A [BPA] and phthalates).
• Sewerage runoff containing synthetic contraceptive residues (ethinylestradiol) that wastewater treatment plants cannot fully filter out.

3. Biological Mechanism of Impairment:

When these chemicals are absorbed into the body, they bind to oestrogen receptors on cells in the male reproductive tract. This interferes with the hormonal pathways that regulate spermatogenesis (the production of sperm in the testes). It can also cause cellular damage through oxidative stress, reducing both sperm count and sperm motility.

Question 9 (Skills: Analysis & Reasoning)

Hormonal Interactions in the Menstrual Cycle

Ovarian Hormone Profiling (Days 0 - 28)

a) Identify the two hormones produced by the ovary, which are shown by the lines A and B on the graph.

• Line A: Oestrogen (produced by the growing follicle, peaks just before ovulation on Day 14).
• Line B: Progesterone (produced by the corpus luteum, peaks during the luteal phase around Day 21).

b) Name the structure C (released at ovulation).

Structure C: Egg Cell (Ovum / Oocyte)

The cell released from the mature follicle during ovulation on Day 14 is the egg cell (also referred to as an ovum or secondary oocyte).

c) What is the purpose of the thickening of the uterus lining at D?

Purpose of Endometrial Thickening:

The thickening of the uterine lining prepares the womb for potential implantation. It ensures there is a blood-rich, nutrient-dense tissue layer ready to receive and support a fertilised egg (embryo), allowing it to grow and develop.

d) When is sexual intercourse most likely to result in pregnancy: at day 6, 10, 13, 20 or 23?

Optimal Fertilisation Day: Day 13

Intercourse on Day 13 is most likely to result in pregnancy.

Why? Ovulation occurs on Day 14. Sperm cells can survive inside the female reproductive tract for up to 3 to 5 days, so having intercourse on Day 13 ensures viable sperm are present in the oviduct to fertilise the egg as soon as it is released.

e) Why is it important that the level of progesterone remains high in the blood of a woman during pregnancy? How does her body achieve this:

i) just after she becomes pregnant?
ii) later on in pregnancy?

Progesterone Maintenance Mechanisms:

Why it must remain high: High progesterone levels are essential to prevent the uterine lining from shedding (preventing menstruation) and to inhibit the release of FSH and LH from the pituitary gland, which stops further egg development during pregnancy.

• i) Just after becoming pregnant: The newly formed embryo secretes the hormone hCG, which signals the corpus luteum in the ovary to continue producing progesterone.
• ii) Later on in pregnancy: After about 12 weeks of development, the placenta becomes fully functional and takes over the production of progesterone, secreting it at high levels for the remainder of the pregnancy.

Question 10 (Biology Only - Skills: Synthesis)

Hormones of the Menstrual Cycle Table

Comprehensive Hormone Matrix:

Hormone Name	Where It Is Produced	Functions in Menstrual Cycle
FSH (Follicle Stimulating)	Pituitary Gland	1. Stimulates follicles to mature in the ovaries. 2. Stimulates ovaries to secrete oestrogen.
Oestrogen	Ovaries (by growing follicle)	1. Rebuilds and thickens the uterine lining. 2. Inhibits further release of FSH. 3. Triggers a surge in LH release.
LH (Luteinising Hormone)	Pituitary Gland	1. Triggers ovulation (release of egg on Day 14). 2. Stimulates the ruptured follicle to form the corpus luteum.
Progesterone	Ovaries (corpus luteum) and the Placenta	1. Maintains and further thickens the uterine lining. 2. Inhibits the release of both FSH and LH (preventing further ovulation).

🎯 Exam Marking Points

Gametes vs Zygotes: Always state that gametes are haploid (n) and zygotes are diploid (2n).

Site of Fertilisation: Ensure you write oviduct or Fallopian tube. Do not write ovary or uterus.

Twin endocrine sources: Progesterone is made by both the ovaries and the placenta.

"Waters Breaking": This refers to the amnion (D) rupturing to release amniotic fluid (C).

Mitosis vs Meiosis: Asexual reproduction relies entirely on mitosis, meaning offspring are genetically identical to the parent.

IGCSE Class A Study Resource • Edexcel Biology Revision Guide • Developed for Year 10 Students

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