Biology Notes

Reproduction, Fertility and Contraception

These are notes for the 2017 specification on the GCSE Biology topic entitled 'Reproduction, Fertility and Contraception', covering the following content:


Reproduction in Humans

The type of reproduction in humans is called sexual reproduction, meaning two parents are involved. Each parent cell produces special cells called gametes, or sex cells, which fuse in a process called fertilisation to produce a zygote.

Each haploid gamete contains 23 chromosomes, and once the two join, a diploid zygote containing 46 chromosomes is produced.

Diagram

Eventually, this will form a new individual that is different from the parents. The zygote develops internally over the (approximately 9-month long) gestation period until a baby is born.



The Male Reproductive System

The following list explains the functions of various structures in the human male reproductive system:

Diagrams


The Female Reproductive System

The following list explains the functions of various structures in the human female reproductive system:

Diagrams


Egg Cells and Sperm Cells

The gametes are the reproductive cells. In humans, the female produces egg cells (or ova, the singular being ovum) and the male produces sperm cells. Both are adapted for reproduction as they are haploid.


Egg cells are produced in the ovary by meiosis and so are haploid. They have a structure typical of animal cells, and are 0.1mm in diameter with a layer of protective jelly. At birth, approximately 70,000 immature eggs will be present in the ovary, and of these, around 500 will likely develop and be released.

Egg release begins at puberty under the influence of the hormone oestrogen, and one is released each month.

A human egg cell and normal body cell contain a few differences: a yolky cytoplasm, jelly coat, and the fact that it is haploid.

Diagram

Sperm cells are produced in the testes by meiosis and so are haploid. After the initial division the sperm cells differentiate and mature in the testes. They are specialised cells which have a tail, and so are adapted for swimming. The neck or middle piece contains many mitochondria which produce energy during respiration, which allows the tail to beat, propelling the sperm along.

Sperm production begins at puberty under the influence of the hormone testosterone and is a continuous process.

In humans, the sperm is about 30 times smaller than the egg.

The adaptations of a sperm cell to its function are: a tail for swimming, mitochondria for energy production and the fact that it is haploid.

Diagram


Sexual Intercourse

Sexual intercourse is the process by which sperm cells are transferred from the male to the female so that fertilisation can occur. For sperm to leave the male, an erection must be achieved (occurring when blood is pumped into special erectile tissue in the penis). The penis is then placed inside the vagina of the female where a reflex action known as ejaculation occurs in which glands pour fluid into the sperm ducts and urethra.

During ejaculation, muscles around the sperm ducts contract and force the fluid (semen) and the sperm out of the body. The semen is deposited near the cervix.

An ejaculation contains many millions of sperm and has a volume of about 4cm³.



Fertilisation

The sperm swim through the cervix and up through the uterus. If an egg has been released within the previous day, it will be in one of the oviducts. The sperm swim up the oviducts until they reach the egg, although very few actually complete the journey. When one sperm reaches the egg, it penetrates the protective jelly and the head enters the cell. No other sperm cells can enter now. Fertilisation is most likely just after ovulation.

Fertilisation occurs in the oviduct and is where the nucleus of the egg cell fuses with the nucleus of the sperm cell.

Each haploid gamete contains 23 chromosomes. When the two join they form a diploid zygote with 46.

If there is no egg in the oviduct, the sperm remain capable of fertilising an egg for 2-3 days. If an egg is released by the ovary within this time, it may be fertilised. If there are no sperm present in the oviduct when the egg is released, the egg can survive for about one day. If intercourse occurs within this time, fertilisation may occur.


Immediately after fertilisation occurs, the cell is known as a zygote and begins to divide rapidly by mitosis as it moves towards the uterus. When it reaches the uterus, it has become a hollow ball of cells and is called an embryo.

The embryo sinks into the soft lining of the uterus to receive nutrition and this is called implantation. This is really when pregnancy begins. The embryo then differentiates to form a variety of tissues and organs.

Diagram

To recap, some terms and definitions:



Development

The embryo continues to grow and develop and after some weeks it is called a foetus, as it has taken on a more human form. The foetus needs oxygen and dissolved nutrients from the mother, and to enable this to occur, the placenta forms. This is a disc of attached to the lining of the uterus and it has a rich supply of the mother's blood.

The foetus is attached to the placenta by an umbilical cord which contains a umbilical arteries (carrying deoxygenated blood) and an umbilical vein (carrying oxygenated blood) carrying the foetal blood.

Antibodies, oxygen, food, viruses, alcohol and drugs are all substances that pass from mother to foetus. Carbon dioxide and urea pass from foetus to mother.

At the placenta, the mother's blood comes close to the foetal blood to allow exchange of substances, but the two circulations never mix (as mixing blood groups could trigger an immune response causing blood clotting, which may prove fatal to the baby.)

The placenta has the following adaptations:

Diagram

As the foetus increases in size it grows back out into the cavity of the uterus, enclosed in the amnion, a fluid filled protective membrane containing amniotic fluid to cushion it.

The diagram below shows the position of the placenta in relation to the developing foetus:

Diagram

Please pardon my puny handwriting, by the way.


Structures such as the plug of mucus in the cervix help to protect the foetus from:



Sex Hormones and Secondary Sexual Characteristics

Despite being born with a complete set of sex organs, they only become active later in life. Between the ages of around 10-14, the immature eggs start to mature in the ovaries and the testes start to produce sperm during puberty, which prepares the body for reproduction.

Adolescence begins roughly when puberty starts, and finishes roughly when growth stops at around 18, or at the age of legal adulthood. Hormones can both bring about mood changes and increase sexual urges.


The pituitary gland at the base of the brain starts to make hormones which make the sex organs active. The sex organs start to produce sex hormones.

Ovaries start making oestrogen and the testes start making testosterone. These hormones circulate around the body and bring about the development of secondary sexual characteristics (sexually dimorphic characteristics that are not directly involved in reproduction).


Secondary sexual characteristics in males:


Secondary sexual characteristics in females:



Amniocentosis

Amniocentosis is a — rarely carried out — medical procedure, used primarily in prenatal diagnosis of chromosomal abnormalities (genetic defects) and fetal infections, as well as for sex determination.

It may be offered when:



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