Reproductive System

Reproduction takes place in two fundamentally different ways, and the difference between them is one of the most commonly asked exam points. In asexual reproduction a single parent produces offspring without any fusion of gametes, which lets an organism rapidly produce many offspring without committing time and resources to courtship, finding a mate, and mating; in sexual reproduction two parents contribute special sex cells called gametes, whose fusion produces offspring that carry a new mixture of characters and so show variation.

• The Starfish can regenerate an entire body from a fragment of the original body.
• Sexual reproduction is the rule in humans and forms the bulk of this chapter.

In sexual reproduction a new individual begins when two haploid gametes fuse to form a single diploid zygote, and understanding the chromosome arithmetic here is essential for the exam. A gamete is made by a special reduction division called meiosis, so it carries only half the normal chromosome number — in humans each gamete has 23 chromosomes — and when a sperm fertilises an ovum the resulting zygote restores the full diploid number of 46 chromosomes, half from each parent.

• Meiosis produces cells that are genetically distinct from each other, which is why sexually produced offspring are never exact copies of their parents.
• Fertilization is the fusion of two such distinctive cells (a sperm and an ovum).

The basic relationship to memorise is simply sperm + ovum → zygote, the zygote being the first cell of the new individual; from this single diploid cell, repeated mitosis builds the embryo and eventually the complete body.

Human reproduction is carried out by special sex organs, and the first fact to fix is the name and product of those organs in each sex. Gonads are sex organs that produce gametes, and they also secrete the sex hormones.

• Female gonads are the ovaries, which produce eggs (ova) and female sex hormones.

The sperm is a highly specialised cell built for one job — to reach and fuse with the ovum — and its three regions are a favourite diagram-label question. The head carries the nucleus capped by the acrosome; the middle piece is packed with mitochondria that supply energy; and the long tail, a flagellum, propels the cell forward.

• Once sperm form they move into the epididymis, where they mature and are stored.

The male system is built to manufacture sperm, mix it with nourishing fluids and deposit it into the female tract, and its organs fall neatly into three working groups. The paired testes sit outside the body in a sac called the scrotum, whose external position keeps them cooler than core body temperature, a condition the sperm need to form properly.

• Second category — ejaculatory-fluid-producing glands: these glands secrete fluids that nourish and transport sperm, forming semen — the seminal vesicles, prostate glands, and the vas deferens.
• Final category — copulation and deposition of spermatozoa: these organs deliver and deposit sperm into the female tract — the penis, urethra, vas deferens, and Cowper's gland.

• Exam pointer: an important male sex hormone is androgen, particularly testosterone — secreted by the testes, it drives sperm production and male secondary sexual characters.

The female system is designed not only to produce eggs but also to receive sperm, shelter a developing baby and nourish it until birth. The human female reproductive system contains three main parts — the vagina, the uterus, and the ovaries — together with the tubes that link them.

• The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes.
• Each ovary contains hundreds of egg cells or ova.
• Approximately every 28 days, the pituitary gland releases a hormone that stimulates some of the ova to develop and grow.
• One ovum is released and it passes through the fallopian tube into the uterus.

• Hormone pathway: pituitary gland releases hormone → stimulates ova to develop and grow.

From puberty (about 13–14 years of age) until menopause, a woman's body runs a repeating monthly cycle, lasting on average 28 days, that prepares the uterus for a possible pregnancy. The lining of the uterus, the endometrium, and unfertilised ova are shed each cycle through menstruation.

• If the ovum is fertilised by sperm, it attaches to the endometrium and the fetus develops.
• A fertilised egg = zygote.

If fertilisation does occur the cycle stops and menstruation does not happen, because the embryo embeds in the endometrium; otherwise the whole 28-day cycle simply repeats.

When sperm are deposited in the vagina they swim up into the fallopian tube, and it is here that fertilisation occurs. Fertilisation of the egg takes place in the fallopian tube — not in the uterus; the developing embryo then becomes attached to the uterus (implantation in the uterus).

• The connection between the developing embryo and the mother is called the placenta.
• The placenta supplies food, hormones, and dissolved oxygen to the embryo from the mother's blood vessels.
• The birth of a child is technically called parturition.

Human pregnancy, or gestation, lasts about 280 days — roughly 40 weeks or nine months — and ends with the muscular contractions of the uterus, the act of birth being technically called parturition.

The external genital region of the female has its own set of diagram labels worth memorising. The female external genitals are collectively known as the vulva.

Reproduction is controlled by a small set of chemical messengers, the hormones, and questions on which gland makes which hormone are common. The master gland is the pituitary, which releases hormones that switch the ovaries and testes on, while the gonads themselves make the sex hormones.

Some infections spread mainly through sexual contact and are grouped together as sexually transmitted diseases. STDs can affect the sex partners, the fetus, and newborn infants, and they are grouped into three categories.

• These produce inflammation of the urethra, epididymis, cervix, or oviducts.
• Gonorrhoea and chlamydia are the most common STDs in this category.
• Both diseases can be treated and cured with antibiotics, once diagnosed.
• Antibiotics = medicines that kill bacteria and cure bacterial infections.

• These produce sores on the external genitalia.
• Genital herpes is the most common disease in this class; its symptoms can be treated by antiviral drugs, but the infection cannot be cured.
• Syphilis is a bacterially caused infection and, if left untreated, can cause serious symptoms and death — however, it is curable with antibiotics.
• Examples: Genital Herpes, Syphilis.
• Key points: antiviral drugs help manage symptoms; antibiotics cure syphilis (a bacterial disease); early detection and treatment are important.

• These viral diseases affect organ systems other than the reproductive system.
• AIDS and hepatitis B are in this category; both can be spread by sexual contact or blood.
• Infectious individuals may appear symptom-free for years after infection.
• Examples of viral STDs: AIDS, Hepatitis B. Modes of spread: sexual contact, blood.

Separating intercourse from pregnancy uses methods blocking one of the three stages of reproduction:

• 1. Release and transport of gametes
• 2. Fertilization
• 3. Implantation

Flowering plants reproduce sexually too, and the flower is their reproductive organ. The reproductive parts of the flower are the stamen and carpel.

The male gametes of a plant are carried inside pollen grains, which form deep within the anther. Pollen grains contain the male gametophyte (microgametophyte) phase of the plant.

Once pollen is formed it must reach the female part of a flower, a transfer with its own name and agents. Pollination is the transfer of pollen from the anther to the female stigma, accomplished by a variety of methods.

After pollination the pollen grain does not fertilise directly but grows a tube down to the ovule. Once pollination is accomplished, the pollen tube grows through the stigma and style toward the ovules in the ovary.

The final stage of plant reproduction turns the fertilised flower into a fruit and seed. After fertilization, the ovary wall develops into a fruit.

• Note: seeds germinate, and the embryo grows into the next-generation sporophyte.