Reproduction in Humans | ICSE Class 8 Biology Notes

This note covers human reproduction, male and female reproductive organs, reproductive cells, the joining of these cells, development before birth, and monthly changes in the womb.

What is sexual reproduction in humans?

Reproduction is the process by which living organisms produce new individuals of their own kind. It helps a species, a kind of organism, continue across generations. Reproduction is not necessary for maintaining the life of an individual, but it produces the next generation.

Humans reproduce by sexual reproduction, in which male and female reproductive cells join. These specialised cells are called gametes, or germ-cells. The male gamete is a sperm; the female gamete is an egg. Both contribute to forming a new individual.

Definition: Fertilisation is the fusion, or joining, of a male gamete with a female gamete. The fertilised egg formed by this union is called a zygote.

How are reproduction and growth different?

Growth makes an individual's body larger. Reproduction creates another individual. An increase in a person's height is therefore different from the production of a new individual through the joining of reproductive cells.

Human bodies have specialised organs for producing and transferring gametes. An organ is a body structure made of tissues that perform particular functions; tissues are organised groups of specialised cells. Reproductive organs work together as a reproductive system.

The two systems have different roles. The male system produces sperms and delivers them. The female system produces eggs and provides the place where a new individual develops before birth. Connecting each organ with its function explains how the systems work together.

Offspring, the new individuals produced by parents, may closely resemble their parents. Resemblance does not mean that offspring are exact copies. Sexual reproduction combines genetic material, the material carrying inherited information, from two individuals. This helps explain both similarity and differences between parents and offspring.

What do the testes and scrotum do?

The testes are the male organs in which sperms form. A single one is called a testis. They lie in the scrotum, outside the abdominal cavity, the internal space in the abdomen. The scrotum is the pouch surrounding the testes.

Why is their position important?

Sperm formation requires a temperature lower than normal body temperature. The position of the testes in the scrotum is connected with this requirement. Their location is therefore part of the explanation of their reproductive function, rather than merely a label to memorise.

Each sperm is a tiny reproductive cell consisting mainly of genetic material and a long tail. The tail helps it move towards the female germ-cell. The sperm itself is a cell; the testis is the organ that produces such cells.

The testes also secrete testosterone, a hormone involved in regulating sperm formation and bringing about changes in appearance in boys at puberty. A hormone is a chemical substance used for control and coordination in the body.

Puberty is the period during which reproductive tissues mature. It occurs during adolescence, the transition towards adulthood. These changes happen gradually over months and years. They do not all occur at the same time in one person or at an exact age in everyone.

Note: The testes produce sperms; the scrotum surrounds the testes. These names describe different structures. The lower temperature needed for sperm formation explains why the testes are positioned outside the abdominal cavity.

To connect structure with function, remember three links: testes with sperm formation, scrotum with the position of the testes, and testosterone with the regulation of sperm formation and changes at puberty. The hormone is a chemical substance, whereas a sperm is a reproductive cell.

How does the male reproductive system transport sperms?

The male reproductive system includes sperm-producing organs and structures that deliver sperms. The vas deferens, or sperm duct, carries sperms from the testis. It joins a tube coming from the urinary bladder, the organ that holds urine.

The urethra forms a common passage for sperms and urine in males. It passes through the penis, the organ involved in transferring sperms during sexual intercourse. Sexual intercourse is the sexual act through which sperms can enter the vagina, the female passage receiving them.

How do glands help sperm transport?

A gland is a structure that produces and releases a substance called a secretion. The prostate gland and seminal vesicles add secretions along the sperm transport pathway. These secretions provide a fluid that makes sperm transport easier and supplies nutrition.

The testes and these glands therefore have different functions. Sperms are formed in the testes. The prostate gland and seminal vesicles add secretions around those sperms. Producing sperm cells and producing the fluid that supports their transport are distinct activities.

1. Sperms form in the testes, which are situated in the scrotum.
2. The vas deferens carries the sperms away from the testes.
3. Secretions from glands such as the prostate and seminal vesicles add fluid, easing transport and providing nutrition.
4. The urethra provides the passage through the penis for delivery of the sperms.

The side-view drawing labels the testis within the scrotum, the vas deferens, seminal vesicle, prostate gland, penis and urethra. The bladder is drawn above the prostate gland, and a ureter, a tube carrying urine towards the bladder, is also labelled.

See Fig. 7.10 in your NCERT textbook

A ureter is a tube carrying urine towards the bladder. Do not confuse it with the urethra shown passing through the penis. Follow the vas deferens from the testis when identifying the sperm pathway in the drawing.

What are the roles of the ovaries and oviducts?

The ovaries are the female organs that produce eggs. One is called an ovary. The ovaries also produce hormones. Thus, like the testes, they have a role in producing reproductive cells and a role in chemical coordination.

At birth, the ovaries already contain thousands of immature eggs. At puberty, some of these start maturing. Egg cells are therefore present before they begin to mature for reproduction. The presence of immature eggs and the maturation of eggs describe different stages.

Where does an egg go?

An egg is released from one of the ovaries each month. It is carried towards the womb through a thin tube called an oviduct, also known as a fallopian tube. These two names refer to the same structure.

The uterus, or womb, is the elastic, bag-like organ into which the two oviducts lead. The ovary, oviduct and uterus should be distinguished: the ovary supplies the egg, the oviduct carries it, and the uterus provides the place for later development.

Fertilisation occurs in the oviduct when a sperm and egg fuse there. The egg's route and the sperm's route can therefore meet in this tube. The egg is not produced in the uterus, and fertilisation and later development do not occur at the same site.

Note: Ovary and oviduct are different terms. The ovary produces the female reproductive cell; the oviduct is the tube carrying that cell towards the uterus and is the site of fertilisation.

When tracing the egg's route, begin at an ovary, move along the oviduct, and then identify the uterus. This order connects the organs by function and avoids treating all parts of the female reproductive system as interchangeable.

How do the uterus, cervix and vagina work together?

The uterus provides the place where an embryo develops. An embryo is the early developing individual formed as the zygote divides into a ball of cells. The uterus has a lining that prepares to receive and nourish this developing embryo.

The uterus opens into the vagina through the cervix, its narrow lower region. The vagina is the passage through which sperms enter during sexual intercourse. It is also the passage through which material from the uterine lining leaves during menstruation.

Menstruation is the roughly monthly breakdown and discharge of the uterine lining as blood and mucus when the egg is not fertilised. Mucus is a slippery secretion. The details of this sequence are considered after fertilisation and development.

What changes in the uterus?

The uterine lining thickens and becomes richly supplied with blood in preparation for receiving an embryo. If an embryo develops, it attaches to this lining. This attachment is called implantation. The embryo continues growing and developing organs there.

The drawing shows an ovary on each side, with curved oviducts leading towards the central uterus. The labelled cervix lies below the uterus, and the vagina continues downwards from it. One tube is labelled “Oviduct or Fallopian tube”.

See Fig. 7.11 in your NCERT textbook

Use the drawing to distinguish a tube from a chamber and an opening. The oviduct carries the egg. The uterus receives the embryo. The cervix connects the uterus with the vagina. Each name belongs to a particular part with a particular role.

Fertilisation is the joining of reproductive cells; implantation is attachment of the developing embryo to the uterine lining. Knowing the order and location of these events explains why both the oviduct and the uterus are essential parts of the reproductive sequence.

How can the reproductive organs and their functions be compared?

Compare organs by asking whether they produce cells, carry cells, add fluid, or support development. This keeps the structure, the named body part, separate from its function, the work it performs. Similar-sounding names do not mean that two structures have the same function.

Which male structures perform each task?

Which female structures perform each task?

The testes and ovaries can be compared because both produce gametes and hormones. The gametes they produce differ: testes produce sperms, while ovaries produce eggs. Neither the sperm nor the egg should be confused with the much larger organ that produces it.

The sperm duct and oviduct are both involved in transport, but they carry different reproductive cells. The oviduct also provides the site where fertilisation occurs. A comparison should identify the shared transport role while preserving this difference.

The uterus has a further role in receiving and supporting the developing embryo. Do not assign this role to the ovary because both are female reproductive organs. Trace each step to its correct organ rather than giving one general function for the entire system.

How does fertilisation produce a zygote?

Fertilisation connects the male and female contributions to reproduction. Sperms enter through the vaginal passage during sexual intercourse. They travel upwards towards the oviduct, where they may encounter the egg. The presence of sperms does not by itself mean that fertilisation has occurred.

What is the sequence before the first cell forms?

1. An egg is released from an ovary and is carried into the oviduct towards the uterus.
2. Sperms enter the female reproductive system through the vaginal passage during intercourse.
3. The sperms move upwards through the reproductive passages and reach the oviduct, where they may encounter the egg.
4. A sperm fuses with the egg during fertilisation, producing the fertilised egg called a zygote.

The egg's release and the entry of sperms are the two routes leading to a possible meeting; this list does not prescribe a fixed timing between those routes. The defining event is fusion of the gametes, not their movement towards each other.

The zygote is the fertilised egg from which development begins. It is different from either gamete considered separately. The sperm and egg are the parental reproductive cells; the zygote is the cell produced by their union.

Fertilisation occurs inside the female body, in the oviduct. Later, the developing embryo becomes attached to the uterine lining. This separates the site of fertilisation from the site of implantation and continued growth.

Note: Reaching the oviduct and fertilising the egg are different events. A sperm may encounter the egg there; fertilisation is the actual fusion that produces a zygote.

To explain fertilisation clearly, identify the two cells, state where they meet, name their fusion, and give the name of the resulting cell. This sequence links the reproductive organs with the beginning of a new individual without confusing fertilisation with development.

How does the embryo develop before birth?

After fertilisation, the zygote divides and forms an embryo. Cell division means that a cell produces further cells. The embryo becomes implanted in the uterine lining, where it continues growing and developing organs. The developing stage with organs is called a foetus.

What sequence connects fertilisation and birth?

1. The fertilised egg, or zygote, begins to divide into further cells.
2. A ball of cells forms and is called an embryo.
3. The embryo becomes implanted in the lining of the uterus.
4. The implanted embryo continues to grow and develop organs, becoming a foetus.
5. Development inside the mother's body takes approximately nine months, followed by birth through rhythmic contractions of the uterine muscles.

Contractions are the tightening actions of muscles. Rhythmic contractions occur repeatedly. The uterus therefore supports development before birth and also has a muscular role in the process of birth.

Draw labelled boxes for sperm and egg joining in the oviduct, zygote, dividing embryo, implantation in the uterine lining, developing foetus, and birth. Connect the boxes with arrows showing the order. Label the arrows as the direction of the sequence.

How does the placenta support development?

The placenta is a special tissue through which the embryo receives nutrition from the mother's blood. It forms a disc embedded in the uterine wall. Villi, small projections on the embryo's side of the placenta, are surrounded by blood spaces on the mother's side.

This arrangement provides a large surface area for glucose, a sugar used as food, and oxygen to pass from the mother to the embryo. The developing embryo also produces waste substances. These can pass through the placenta into the mother's blood for removal.

Thus, placental exchange involves movement in both directions: useful substances pass towards the developing embryo, while its waste substances pass towards the mother's blood. The placenta's large surface area supports this exchange.

The terms zygote, embryo and foetus refer to stages of development, while uterus and placenta name structures supporting development. Keeping these categories separate helps explain both what is developing and how its surroundings support that development.

What happens when the egg is not fertilised?

If the egg is not fertilised, it lives for about one day. Meanwhile, the uterus has prepared its lining for a possible embryo. The lining has become thick and spongy and has a rich blood supply to support development if fertilisation occurs.

When the egg is not fertilised, this prepared lining is no longer needed for an embryo. It slowly breaks down and comes out through the vagina as blood and mucus. This process is called menstruation.

How are egg release and uterine changes connected?

1. An ovary releases an egg as part of the monthly reproductive sequence.
2. The uterus prepares a thick, spongy lining to receive and nourish a possible embryo.
3. If the egg is not fertilised, it lives for about one day, and the prepared uterine lining is no longer needed for an embryo.
4. The lining slowly breaks down and leaves through the vagina as blood and mucus.

This cycle takes place roughly every month. Menstruation usually lasts for about two to eight days. “Roughly”, “usually” and “about” matter: the description is not a promise of identical timing or duration in every person.

Menstrual discharge comes from the breakdown of the uterine lining. It should not be described simply as the egg leaving the body. The egg and the lining are different structures, and the visible discharge includes blood and mucus from the prepared lining.

Note: The uterus prepares its lining before the outcome of fertilisation is known. If an embryo develops, the lining supports it. If the egg is not fertilised, the lining breaks down instead.

This explains menstruation as part of the reproductive sequence: egg release, preparation of the uterus, and a different outcome depending on fertilisation. The same organs are involved, but the fate of the uterine lining differs from its role during development of an implanted embryo.

Glossary

• Reproduction — The process by which living organisms produce new individuals of their own kind.
• Gamete — A specialised reproductive cell that joins another reproductive cell during sexual reproduction.
• Sperm — The male reproductive cell, formed in the testes and possessing a long tail.
• Egg — The female reproductive cell produced in an ovary and carried into an oviduct.
• Testes — Male reproductive organs that form sperms and secrete the hormone testosterone.
• Scrotum — The pouch surrounding the testes outside the abdominal cavity, where conditions support sperm formation.
• Vas deferens — The sperm duct carrying sperms from a testis towards the urethra.
• Oviduct — The fallopian tube carrying an egg towards the uterus and providing the site of fertilisation.
• Uterus — The elastic, bag-like organ in whose lining an embryo implants and develops.
• Fertilisation — The fusion of a sperm and an egg to form a zygote.
• Zygote — The fertilised egg that begins dividing to form a developing embryo.
• Implantation — The attachment of the developing embryo to the lining of the uterus.
• Foetus — The developing stage reached as the embryo grows and develops its organs.
• Placenta — Special tissue through which substances pass between the mother's blood and the developing embryo.
• Menstruation — The roughly monthly breakdown and discharge of the uterine lining when the egg is not fertilised.

Common errors and misconceptions

• Misconception: Reproduction keeps an individual alive. Correct: Reproduction produces new individuals and supports continuation across generations; it is not required to maintain an individual's life.
• Misconception: The prostate gland produces sperms. Correct: Sperms form in the testes. The prostate gland adds secretions that help transport and nourish them.
• Misconception: The scrotum and testes are the same structure. Correct: The scrotum surrounds the testes, which are the sperm-producing organs.
• Misconception: Eggs are formed in the uterus. Correct: Eggs are produced in the ovaries and carried along the oviducts towards the uterus.
• Misconception: Fertilisation and implantation are the same event. Correct: Fertilisation joins gametes in the oviduct; implantation attaches the embryo to the uterine lining.
• Misconception: The placenta supplies nutrients but has no role in waste transfer. Correct: Waste substances from the embryo can also pass through the placenta into the mother's blood.
• Misconception: Menstruation has an identical duration in everyone. Correct: It usually lasts for about two to eight days, and the cycle occurs roughly every month.

Exam-style questions with model answers

Q1. In human sexual reproduction, name the male and female gametes and state the organ that produces each. [2 marks]

1. The male gametes are sperms, which are formed in the testes.
2. The female gametes are eggs, which are produced in the ovaries.

Q2. State where the testes are located, explain how this location supports sperm formation, and name their hormone with one reproductive function. [3 marks]

1. The testes lie in the scrotum, outside the abdominal cavity. The scrotum is the pouch surrounding these sperm-producing organs.
2. Sperm formation requires a temperature lower than normal body temperature, explaining why the testes are situated outside the abdominal cavity.
3. The testes secrete testosterone, a hormone that helps regulate sperm formation.

Q3. Trace sperm transport in humans by stating the roles of the testes, vas deferens, prostate gland and seminal vesicles, and urethra. Give four points. [4 marks]

1. The testes form the sperm cells that enter the male reproductive transport pathway.
2. The vas deferens, or sperm duct, carries these sperms from the testes towards the urethra.
3. The prostate gland and seminal vesicles add secretions that provide nutrition and make the transport of sperms easier.
4. The urethra forms the passage through the penis by which sperms are delivered; in males it is also a passage for urine.

Q4. Identify the roles of these five female reproductive structures: ovaries, oviducts, uterus, cervix and vagina. Include the locations of fertilisation and implantation. [5 marks]

1. The ovaries produce eggs, the female reproductive cells. They also produce hormones, so their functions include both gamete production and chemical coordination.
2. The oviducts carry eggs from the ovaries towards the uterus. Fertilisation occurs in an oviduct when a sperm fuses with an egg.
3. The uterus receives the developing embryo in its lining. Implantation occurs there, followed by continued growth and development of the embryo.
4. The cervix is the narrow lower region of the uterus through which it opens into the vagina.
5. The vagina receives sperms during sexual intercourse and also provides a passage for blood and mucus during menstruation.

Q5. Put these stages in developmental order and explain each: implantation, zygote, foetus, embryo. Begin with the result of fertilisation. [4 marks]

1. The zygote is the fertilised egg formed when a sperm and an egg fuse in the oviduct.
2. The zygote divides to form a ball of cells called an embryo, beginning the development of the new individual.
3. Implantation follows as the embryo attaches to the lining of the uterus, where it continues to grow.
4. The growing embryo develops organs and becomes a foetus, the later developing stage within the uterus.

Q6. Explain placental support of the embryo in five points covering its position, villi and blood spaces, exchange surface, supply of useful substances, and removal of wastes. [5 marks]

1. The placenta is a special tissue forming a disc embedded in the uterine wall. It supports exchange between the mother and developing embryo.
2. Villi, small projections on the embryo's side of the placenta, are surrounded by blood spaces on the mother's side.
3. The arrangement of villi and blood spaces provides a large surface area across which substances can pass.
4. Glucose and oxygen pass from the mother to the embryo through the placenta, helping support the needs of the developing individual.
5. Waste substances generated by the developing embryo can pass through the placenta into the mother's blood for removal.

Q7. Explain why menstruation occurs when an egg is not fertilised. Include the preparation of the uterus, the fate of the lining and the route of discharge. [3 marks]

1. The uterus prepares for a possible embryo by making its lining thick, spongy and richly supplied with blood.
2. If the egg is not fertilised, the prepared lining is no longer needed for an embryo and slowly breaks down.
3. The broken-down lining leaves through the vagina as blood and mucus. This process is called menstruation and occurs roughly every month.

Q8. Use these descriptions: an unfertilised egg lives for about one day; menstruation usually lasts for about two to eight days; the cycle occurs roughly every month; development inside the mother takes approximately nine months. Explain the qualified time description for each event. [4 marks]

1. An egg that is not fertilised lives for about one day; “about” preserves the approximate nature of this duration.
2. Menstruation usually lasts for about two to eight days, rather than having one identical duration in every person.
3. The menstrual cycle takes place roughly every month, which describes its approximate frequency.
4. Development of the child inside the mother's body takes approximately nine months before birth.

Key takeaways

• Human sexual reproduction involves male and female gametes joining during fertilisation to produce a zygote.
• The testes produce sperms, while their position in the scrotum supports the lower temperature required for sperm formation.
• The vas deferens carries sperms, and gland secretions provide fluid that eases transport and supplies nutrition.
• The ovaries produce eggs; the oviducts carry them towards the uterus and provide the site of fertilisation.
• The embryo implants in the uterine lining, continues growing and develops organs to become a foetus.
• The placenta allows glucose and oxygen to pass towards the embryo and its waste substances towards the mother's blood.
• If the egg is not fertilised, the uterine lining breaks down and leaves through the vagina during menstruation.
• Reproductive timing includes qualifications: menstruation occurs roughly monthly, and development before birth takes approximately nine months.

Test yourself

Why are the testes located outside the abdominal cavity?

Sperm formation requires a temperature lower than normal body temperature, which explains the testes' position in the scrotum.

What is the difference between a sperm and a testis?

A sperm is a male reproductive cell; a testis is an organ that produces sperms.

What other name is used for an oviduct?

An oviduct is also called a fallopian tube; it carries an egg towards the uterus.

Where do fertilisation and implantation occur?

Fertilisation occurs in the oviduct, while implantation attaches the embryo to the uterine lining.

Which structure connects the uterus with the vagina?

The cervix forms the narrow lower region through which the uterus opens into the vagina.

What does the placenta allow to pass towards the embryo?

Glucose and oxygen pass from the mother to the embryo through the placenta.

What happens to the prepared uterine lining if the egg is not fertilised?

The lining slowly breaks down and leaves through the vagina as blood and mucus during menstruation.

What brings about birth after development inside the mother?

The child is born through rhythmic contractions of the muscles in the uterus.