Sexual Reproduction, Development and Growth in Humans and Animals, revision notes

Overview

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.

The male and female reproductive systems (CS 15.1)

Content Standard 15.1 deals with the structures that make sexual reproduction possible. In the male system the two testes produce sperm and the hormone testosterone.

Sperm pass into the epididymis, where they are stored and matured, travel along the sperm duct (vas deferens), and leave through the urethra during ejaculation. The seminal vesicle and prostate gland add a nourishing fluid on the way, and sperm mixed with this fluid is called semen.

The female system is built around the two ovaries, which produce ova and the hormones oestrogen and progesterone. An ovum released at ovulation is swept into the funnel of the oviduct (fallopian tube), which is the usual site of fertilisation.

A fertilised ovum travels on to the uterus, a muscular organ whose thick lining, the endometrium, receives and nourishes the developing embryo. The neck of the uterus is the cervix, which opens into the vagina.

Labelling questions reward the correct name paired with the correct function, not just the position on a diagram. A reliable way to revise is to trace the path a sperm takes from testis to urethra, and the path an ovum takes from ovary to uterus, naming each structure and stating what it does as you go.

Gametogenesis: how sperm and ova are formed (CS 15.2)

Content Standard 15.2 covers gametogenesis, the formation of gametes by meiosis so that each gamete is haploid. Spermatogenesis takes place in the seminiferous tubules of the testes and runs continuously from puberty onward; one parent cell gives four small, motile sperm.

Oogenesis takes place in the ovaries, begins before birth, and is completed one ovum at a time; one parent cell gives a single large ovum that stores food, together with small polar bodies that break down.

A sperm is specialised for reaching and fertilising the ovum. An acrosome at the tip releases enzymes that digest the ovum's outer layers, a haploid nucleus carries the genetic material, a middle piece packed with mitochondria supplies energy, and a long tail (flagellum) drives the sperm forward.

An ovum is specialised differently: it is large and non-motile, its cytoplasm stores food for the early embryo, and once one sperm has entered, its membrane changes to block any further sperm from fusing with it.

The menstrual cycle and its four hormones (CS 15.3)

The menstrual cycle prepares the uterus for a possible pregnancy over roughly 28 days, counted from the first day of menstruation. During menstruation (about days 1 to 5) the thickened endometrium breaks down and is lost.

The lining is then repaired and thickened again, an ovum matures, and ovulation releases it around day 14. If no fertilisation occurs, the lining is shed again and a new cycle begins.

Four hormones control this sequence, and placing each hormone with the event it triggers matters more than listing them. Follicle-stimulating hormone (FSH) from the pituitary gland makes a follicle in the ovary mature and secrete oestrogen.

Rising oestrogen repairs and thickens the endometrium and, at a high level, triggers a surge of luteinising hormone (LH). The LH surge causes ovulation and turns the empty follicle into the corpus luteum, which secretes progesterone.

Progesterone maintains the thick lining; if the ovum is not fertilised the corpus luteum breaks down, progesterone falls, and menstruation follows.

Fertilisation, the placenta and development of the foetus (CS 15.4)

Fertilisation is the fusion of a sperm nucleus with an ovum nucleus in the oviduct, forming a diploid zygote. The zygote divides repeatedly as it moves to the uterus and embeds itself in the endometrium, a step called implantation.

The ball of cells becomes an embryo and, once its main organs have formed, a foetus, over a gestation of about nine months.

The placenta develops from both foetal and maternal tissue and connects to the foetus through the umbilical cord. Oxygen, glucose, amino acids and antibodies diffuse from the mother's blood to the foetus, while carbon dioxide and urea diffuse the other way.

The two blood supplies run close together but never mix directly, so every substance crosses a thin barrier by diffusion. The foetus is cushioned by amniotic fluid held in the amnion, which absorbs shocks, supports the foetus and keeps its temperature steady.

A frequent examined point is that the placenta keeps the two circulations separate. This is why the mother and foetus can have different blood groups, and why not every substance in the mother's blood automatically reaches the foetus, although some harmful substances and pathogens can still cross.

Formation of twins (CS 15.5)

Twins form in two distinct ways. Identical (monozygotic) twins come from a single ovum fertilised by a single sperm; the early zygote splits into two embryos, so the twins carry the same genes and are always the same sex.

Non-identical (dizygotic) twins come from two ova released in the same cycle, each fertilised by a different sperm, so the twins are genetically no more alike than ordinary siblings and may be different sexes.

Because identical twins share all their genes, any differences that appear between them as they grow up are usually attributed to their environment rather than their heredity, a point examiners often build a question around.

Health issues of the reproductive system (CS 15.6)

Content Standard 15.6 looks at conditions that affect reproductive health. Sexually transmitted infections include the bacterial diseases gonorrhoea and syphilis and the viral infection HIV, which can lead to AIDS.

These spread through unprotected sexual contact and, for HIV, also through infected blood and from mother to foetus. The syllabus treats them at the level of cause, transmission and prevention rather than clinical detail.

Infertility, the inability to conceive, can arise from blocked oviducts, a low sperm count, or a hormonal imbalance that disrupts ovulation. Questions here ask you to link a named cause to its effect on fertilisation or on the menstrual cycle, so a clear grasp of the normal process from the earlier content standards is what makes this section answerable.

Growth in humans and animals (CS 15.7)

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. Plotting size or mass against time for a human gives a sigmoid (S-shaped) curve: growth is rapid in infancy, steadier through childhood, rapid again during the adolescent growth spurt, then levels off in adulthood.

The gradient of this curve, plotted separately, gives a growth-rate curve with peaks in infancy and adolescence.

Animals with an exoskeleton, such as insects and other arthropods, cannot grow steadily because their rigid cuticle will not stretch. They grow in steps: the old exoskeleton is shed in a process called moulting (ecdysis), the body expands quickly before the new cuticle hardens, and the growth curve rises in a staircase pattern rather than smoothly.

Some animals also change body form as they develop, a process called metamorphosis. In complete metamorphosis the animal passes through egg, larva, pupa and adult, with the larva looking completely unlike the adult; a butterfly is the standard example.

In incomplete metamorphosis the animal passes through egg, nymph and adult, with the nymph already resembling a small adult and no pupal stage; a grasshopper or cockroach is the standard example.

Key concepts to master

• 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.

Quick recall checklist

1. Can you define and explain Reproductive systems?
2. Can you define and explain Gametogenesis?
3. Can you define and explain Menstrual cycle?
4. Can you define and explain Fertilisation and development?
5. Can you define and explain Twins?
6. Can you define and explain Growth?
7. Can you define and explain Hormonal control of the menstrual cycle?
8. Can you define and explain The placenta in development?
9. Can you define and explain Metamorphosis in animals?

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