Form 4 · Worked answers

Sexual Reproduction, Development and Growth in Humans and Animals, worked answers

Fully worked answers for Sexual Reproduction, Development and Growth in Humans and Animals, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Describing the events of the menstrual cycle and the role of its hormones.
Explaining the difference between identical and non-identical twins.
Interpreting a human growth curve, including naming its phases.
Explaining the roles of FSH, LH, oestrogen and progesterone across one menstrual cycle.
Describing how the placenta supports the exchange of substances between mother and foetus.
Comparing complete and incomplete metamorphosis using named animal examples.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

The diagram shows the human male reproductive system. (a) Name the structure in which sperm are produced. (b) State the function of the sperm duct. (c) Explain the role of the fluid added by the seminal vesicle and prostate gland.

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(a) Sperm are produced in the testis, specifically in its seminiferous tubules. (b) The sperm duct (vas deferens) carries sperm from the epididymis towards the urethra during ejaculation. (c) The seminal vesicle and prostate gland add a fluid that nourishes the sperm and provides a medium in which they can swim; sperm together with this fluid form semen. The fluid supplies sugars for energy and helps the sperm remain active.

testisseminiferous tubulessperm ductsemennourishes

Compare spermatogenesis and oogenesis by stating two differences between them.

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Spermatogenesis produces four functional sperm from each parent cell, while oogenesis produces only one functional ovum (the other products form polar bodies that break down). Spermatogenesis occurs continuously from puberty in the testes, while oogenesis begins before birth and usually releases one ovum per menstrual cycle from the ovaries. Both processes involve meiosis, so the gametes produced are haploid.

four spermone ovumpolar bodiescontinuousmeiosishaploid

Describe the roles of FSH and LH in the menstrual cycle.

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Follicle-stimulating hormone (FSH) is secreted by the pituitary gland and stimulates a follicle in the ovary to mature; the developing follicle secretes oestrogen. As oestrogen rises it thickens the endometrium and, once it reaches a high level, it triggers a surge of luteinising hormone (LH). The LH surge causes ovulation, the release of the mature ovum from the ovary at about day 14. LH also causes the empty follicle to become the corpus luteum, which then secretes progesterone. A simple way to keep them apart is that FSH matures the follicle and LH releases the ovum.

FSHfollicleoestrogenLH surgeovulationcorpus luteum

Explain how the placenta allows exchange of substances between the mother and the foetus, and state why their blood does not mix.

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The placenta brings the mother's blood and the foetus's blood very close together, separated only by a thin barrier. Oxygen, glucose, amino acids and antibodies diffuse from the mother's blood, where they are at higher concentration, across the barrier to the foetus. Carbon dioxide and urea diffuse the other way, from the foetus to the mother, to be removed. The two blood supplies do not mix directly; keeping them separate allows the mother and foetus to have different blood groups and prevents the mother's higher blood pressure from damaging the foetal vessels. Substances cross by diffusion across the thin membrane.

thin barrierdiffusionoxygen and glucosecarbon dioxide and ureado not mixumbilical cord

Explain the difference between identical and non-identical twins.

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Identical (monozygotic) twins form when a single ovum is fertilised by a single sperm and the resulting zygote splits into two separate embryos; because they come from one zygote they have identical genes and are always the same sex. Non-identical (dizygotic) twins form when two ova are released and each is fertilised by a different sperm; they are genetically as different as ordinary siblings and can be the same or different sexes.

one zygote splitsidentical genessame sextwo ovatwo spermlike siblings

Define growth and describe the shape of the human growth curve.

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Growth is a permanent and irreversible increase in the size and dry mass of an organism, brought about by cell division, cell enlargement and cell differentiation. When size or mass is plotted against age, the human growth curve is sigmoid (S-shaped): growth is rapid in infancy, slower and steadier through childhood, rapid again during the adolescent growth spurt, and then levels off in adulthood when the adult size is reached. The curve is therefore not a straight line but rises steeply in two phases with a plateau at the end.

permanentdry masscell divisionsigmoidgrowth spurtlevels off

Compare complete and incomplete metamorphosis, giving one named example of each.

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Complete metamorphosis has four stages: egg, larva, pupa and adult, and the larva looks completely different from the adult, as in a butterfly. Incomplete metamorphosis has three stages: egg, nymph and adult, with no pupal stage, and the nymph already resembles a small adult, as in a grasshopper. The key difference is the presence of a pupal stage and a larva that is unlike the adult in complete metamorphosis, compared with a nymph similar to the adult and no pupa in incomplete metamorphosis.

egg larva pupa adultbutterflyegg nymph adultgrasshopperpupal stagenymph resembles adult

Explain why an insect grows in a series of steps rather than smoothly.

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An insect has a rigid exoskeleton (cuticle) that cannot stretch, so while it is in place the insect cannot increase in size. To grow, the insect sheds the old exoskeleton in a process called moulting (ecdysis). Immediately afterwards, before the new cuticle hardens, the body expands quickly. Growth therefore happens in sudden increases at each moult, giving a step-like (staircase) growth curve rather than a smooth one.

rigid exoskeletoncannot stretchmoultingecdysisexpandsstep-like curve

Phrasing that earns marks

Reproductive systems: The male reproductive system produces sperm in the testes and delivers them through the sperm duct, urethra and penis during copulation; the female reproductive system produces eggs in the ovaries, allows fertilisation usually in the oviduct, and its uterus supports and nourishes the developing foetus until birth. Knowing the correct name and function of each structure, rather than only its position, is essential for labelling questions on this topic.
Gametogenesis: Sperm are produced continuously by spermatogenesis in the testes from puberty onwards, while eggs are produced by oogenesis in the ovaries, with one egg usually maturing and being released per cycle; both processes rely on meiosis to halve the chromosome number so the resulting zygote has the normal number after fertilisation.
Menstrual cycle: In a menstrual cycle lasting roughly 28 days, the uterus lining thickens in preparation for a possible pregnancy, an egg is released from an ovary around the midpoint of the cycle in a process called ovulation, and if the egg is not fertilised within about 24 hours the thickened lining breaks down and is shed as menstrual bleeding. Because the cycle length can vary between individuals, exam questions usually specify 'approximately 28 days' rather than treating it as a fixed number.
Fertilisation and development: A sperm nucleus fuses with an egg nucleus in the oviduct to form a zygote, which divides repeatedly as it travels to the uterus, implants in the thickened lining, and develops through an embryo stage into a foetus, gaining organs and tissues over roughly nine months of gestation.
Twins: Identical (monozygotic) twins form when a single fertilised egg splits into two separate embryos early in development, giving them the same genes and always the same sex; non-identical (dizygotic) twins form when two separate eggs are released and fertilised by two different sperm in the same cycle, making them genetically no more alike than ordinary siblings. Because identical twins share all their genes, differences between them in later life are usually put down to environment rather than heredity.
Growth: Growth is a permanent and irreversible increase in the size and dry mass of an organism, achieved through cell division, cell enlargement and cell differentiation; in humans it follows an S-shaped (sigmoid) curve, with distinct phases of slow, rapid and then levelling growth corresponding to infancy, adolescence and adulthood. Growth rate itself can be calculated and plotted, which is why a growth curve is usually paired with a rate-of-growth graph in past exam-style questions.
Hormonal control of the menstrual cycle: Follicle-stimulating hormone (FSH) from the pituitary gland stimulates a follicle in the ovary to develop and to secrete oestrogen, which thickens the uterus lining and, at a high enough level, triggers a sharp surge of luteinising hormone (LH) that causes ovulation; progesterone, secreted afterwards by the remains of the follicle, maintains the uterus lining until it either supports a pregnancy or declines, allowing menstruation to begin. This four-hormone sequence is often tested by giving a graph of hormone levels and asking a student to identify which hormone corresponds to which line and phase.
The placenta in development: The placenta is an organ that forms from both maternal and foetal tissue and attaches the foetus to the uterus wall through the umbilical cord; it allows oxygen, glucose and other nutrients to diffuse from the mother's blood to the foetus and allows carbon dioxide and urea to diffuse the other way, while keeping the two blood supplies separate rather than letting them mix directly. This barrier also protects the foetus from many, though not all, harmful substances and pathogens circulating in the mother's blood.

Frequently asked questions

What happens during the menstrual cycle?

In a roughly 28-day cycle, the uterus lining first thickens ready for a possible pregnancy. Around day 14, ovulation releases a mature egg from an ovary. If the egg is not fertilised, the thickened lining breaks down and is shed as menstruation, and the cycle begins again. Hormones from the ovaries and pituitary gland control these events. Being able to place ovulation, fertilisation and menstruation correctly on a single timeline is a frequently tested skill.

What is the difference between identical and non-identical twins?

Identical twins form when a single fertilised egg (zygote) splits into two, so both have the same genes and are the same sex. Non-identical (fraternal) twins form when two separate eggs are released and fertilised by two different sperm, so they are genetically as different as ordinary siblings and can be different sexes.

How is growth defined in biology?

Growth is a permanent and irreversible increase in the size and dry mass of an organism. In humans it follows a sigmoid (S-shaped) curve: slow at first, rapid during infancy and adolescence, then levelling off in adulthood. It results from cell division, cell enlargement and cell differentiation. A growth curve question often also asks for the phase in which growth rate is fastest, usually adolescence in humans.