Sexual Reproduction, Development and Growth in Humans and Animals, practice questions

B, Sperm are produced in the seminiferous tubules of the testis; the epididymis only stores them.

C, An ovum is normally fertilised in the oviduct (fallopian tube) before the zygote travels to the uterus.

C, A surge of luteinising hormone (LH) causes the mature follicle to release its ovum at ovulation.

C, Ovulation occurs near the middle of the cycle, at about day 14.

B, A thin placental barrier keeps the two blood supplies separate while allowing diffusion between them.

B, Identical (monozygotic) twins arise from one zygote that splits, so they share identical genes.

B, Growth is a permanent and irreversible increase in size and dry mass, not temporary swelling.

B, Human growth follows a sigmoid curve, rapid in infancy and adolescence and levelling off in adulthood.

C, A butterfly passes through egg, larva, pupa and adult, which is complete metamorphosis.

B, The rigid exoskeleton cannot stretch, so growth happens in sudden steps after each moult.

• (a) FSH and LH are secreted by the pituitary gland.
• (b) Oestrogen repairs and thickens the endometrium (uterus lining) after menstruation.
• Oestrogen also rises to a high level and triggers the surge of LH that causes ovulation.
• (c) If the ovum is not fertilised, the corpus luteum breaks down.
• Progesterone then falls because the corpus luteum no longer secretes it.
• The thickened endometrium can no longer be maintained, so it breaks down and is shed as menstruation.

• (a) The placenta, connected to the foetus by the umbilical cord, attaches the foetus to the uterus wall.
• (b) Oxygen and glucose (also amino acids or antibodies) pass from the mother to the foetus.
• Carbon dioxide (or urea) passes from the foetus to the mother.
• These substances cross by diffusion, and the two blood supplies do not mix.
• (c) The amniotic fluid cushions the foetus and absorbs shocks.
• It also supports the foetus and keeps its temperature steady.

• (a) Growth is a permanent and irreversible increase in the size and dry mass of an organism.
• (b) An insect has a rigid exoskeleton that cannot stretch, so it cannot grow while the cuticle is in place.
• It sheds the exoskeleton by moulting, then expands quickly before the new cuticle hardens, giving a step-like curve.
• (c) Complete metamorphosis has a pupal stage while incomplete metamorphosis does not.
• In complete metamorphosis the larva is very different from the adult, while in incomplete metamorphosis the nymph resembles the adult.
• Examples are the butterfly (complete) and the grasshopper (incomplete).

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.

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.

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.

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.

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 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.

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 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.

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.