Sexual Reproduction, Development and Growth in Humans and Animals
Sexual reproduction combines genetic material from two parents to produce offspring with new combinations of features, unlike asexual reproduction which produces genetically identical offspring. This chapter covers the human male and female reproductive systems, how gametes are formed, the menstrual cycle and its hormonal control, fertilisation and development of the foetus, twins, and the pattern of growth in humans and other animals.
Because this chapter spans multiple systems and stages, it helps to study it as a sequence of events, gamete formation, the cycle, fertilisation, development and growth, rather than as separate unconnected facts.
The menstrual cycle and its controlling hormones are examined closely and in detail, so a clear timeline linking each hormone to the event it triggers is essential rather than memorising the hormones as an isolated list. A mark scheme for this topic often rewards precise terms such as 'ovulation', 'implantation' and 'gestation' used in the correct order, rather than a general description.
The menstrual cycle is coordinated by four hormones working together: follicle-stimulating hormone (FSH) causes a follicle in the ovary to mature and release oestrogen, oestrogen thickens the uterus lining and triggers a surge of luteinising hormone (LH) that causes ovulation, and progesterone from the remains of the follicle then maintains the thickened lining until either pregnancy or its own decline triggers menstruation.
Development and growth are examined beyond humans too, since many animals do not simply get larger but pass through distinct body forms called metamorphosis; insects such as grasshoppers undergo incomplete metamorphosis with no pupal stage, while insects such as butterflies undergo complete metamorphosis through an egg, larva, pupa and adult, and this contrast is a common comparison question.
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Content standards in this chapter
- 15.1 The Reproductive System of Humans
- 15.2 Gametogenesis in Humans
- 15.3 The Menstrual Cycle
- 15.4 Development of Human Foetus
- 15.5 Formation of Twins
- 15.6 Health Issues Related to the Human Reproductive System
- 15.7 Growth in Humans and Animals
Key concepts
- 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.
- Metamorphosis in animals
- Some animals develop through metamorphosis rather than simply growing larger: insects such as grasshoppers show incomplete metamorphosis, passing through egg, nymph and adult stages that resemble each other, while insects such as butterflies show complete metamorphosis, passing through egg, larva, pupa and adult stages that look very different from one another. Both types of metamorphosis are controlled by hormones released from glands in the insect's head and thorax, similar in principle to how phytohormones control plant growth.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- 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.
Common mistakes
What students write: Saying ovulation happens during menstruation.
What earns the mark: Ovulation occurs around day 14, in the middle of the cycle; menstruation is the shedding of the lining at the start. This mistake often costs marks in a fill-in-the-blank timeline question, since the two events sit at opposite ends of the cycle.
What students write: Writing that identical twins come from two eggs.
What earns the mark: Identical twins come from one fertilised egg that splits; two eggs give non-identical twins.
What students write: Confusing the roles of the ovary and the uterus.
What earns the mark: The ovary produces and releases eggs; the uterus is where the foetus develops.
What students write: Calling growth simply an increase in size.
What earns the mark: Growth is a permanent increase in size and dry mass, not just temporary swelling.
What students write: Confusing the roles of FSH and LH.
What earns the mark: FSH mainly causes a follicle to mature and produce oestrogen; LH causes the mature follicle to release its egg at ovulation, triggered by the oestrogen surge. Remembering the order 'FSH matures, LH releases' is a simple way to keep the two hormones apart.
What students write: Saying maternal and foetal blood mix in the placenta.
What earns the mark: Maternal and foetal blood never mix directly in the placenta; substances cross between the two blood supplies by diffusion across a thin membrane. This separation is exactly why maternal illnesses do not automatically pass every substance directly into the foetus's own bloodstream.
What students write: Mixing up complete and incomplete metamorphosis.
What earns the mark: Incomplete metamorphosis (e.g. grasshopper) has an egg, nymph and adult stage with no pupa; complete metamorphosis (e.g. butterfly) has an egg, larva, pupa and adult stage with a distinct pupal stage. A quick way to tell them apart is to check whether a pupal or resting stage is mentioned in the question.
What students write: Drawing the human growth curve as a straight line.
What earns the mark: The human growth curve is S-shaped (sigmoid), rising slowly in early childhood, rapidly during adolescence, and levelling off in adulthood.
Study this chapter
Structures in this chapter
Frequently asked questions
What happens during the menstrual cycle?
What is the difference between identical and non-identical twins?
How is growth defined in biology?
What roles do FSH, LH, oestrogen and progesterone play in the menstrual cycle?
What is the difference between complete and incomplete metamorphosis?
Source:SRC-DSKP-EN, SRC-FORMAT
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