Reproduction in Humans Chapter Notes | Biology Class 8 ICSE PDF Download

Introduction

Reproduction is how living things make new individuals like themselves. In humans, reproduction helps create babies and continue the human species. Humans reproduce sexually, which means a male and a female are needed to produce a new baby. This chapter explains how humans reproduce, the different ways some animals reproduce, and how a baby grows inside the mother's body. It also talks about the changes that happen when children grow into adults.

Asexual Reproduction in Animals

1. Fission

• Fission is the most common way of asexual reproduction in single-celled animals like amoeba and paramecium.
• In binary fission, the parent amoeba divides its nucleus into two parts first.
• Then, the cytoplasm also splits into two, and the two new amoebas separate to live as new individuals.

2. Budding

• In Hydra, small buds grow on the parent's body, which later separate to become new Hydras.
• Budding is a common way of reproduction in Hydra.

3. Regeneration

• Animals like Hydra and starfish can regrow their missing or lost body parts.
• This process of regrowing body parts is called regeneration.
• For example, if Hydra is cut into 2-3 pieces, each piece can regrow into a new Hydra.

Sexual Reproduction in Animals

Most animals, including humans, reproduce by sexual reproduction. In sexual reproduction, two parents are needed: a male and a female. The male produces special cells called sperms. The female produces special cells called eggs. When a sperm and an egg join together, a new individual starts to form.

Human Reproductive System

Humans reproduce through sexual reproduction, which involves a male and a female. The male reproductive system produces sperms, and the female reproductive system produces eggs. When a sperm and an egg join, a new baby starts to form.

Female

• The female reproductive system has ovaries, oviducts, and the uterus.
• Ovaries are two small organs that produce eggs.
• The oviduct, also called the fallopian tube, is a tube that connects the ovary to the uterus.
• The egg travels through the oviduct after it is released from the ovary.
• The uterus is a muscular organ where the baby grows during pregnancy.
• The uterus opens to the outside through a muscular tube called the vagina.
• The vagina is also called the birth canal because the baby comes out through it during birth.
• The urethra, which carries urine, is separate from the vagina.

Male

• The male reproductive system has testes, sperm ducts, and the penis.
• Testes are two small organs present in a sac called the scrotum, located outside the body.
• Testes produce sperms, which are the male reproductive cells.
• Testes are outside the body because sperms need a slightly lower temperature (2–3°C less than the body temperature) to be produced.
• Sperms travel through tubes called sperm ducts to reach the penis.
• The penis is the organ that passes the sperms into the female body during reproduction.
• The urethra in males carries both urine and sperms, but not at the same time.

Semen

• Semen is a mixture of sperms and a milky white fluid produced by special glands in the male body.
• The milky fluid provides food and energy to the sperms so they can swim to the egg.
• During sexual intercourse, around 2–3 ml of semen is released, which contains about 20,000,000 sperms.
• Out of these millions of sperms, only one sperm usually joins with the egg to form a zygote.

Fertilisation

• Fertilisation is the process where a sperm and an egg join together to form a single cell called the zygote.
• In humans, fertilisation happens inside the female’s body in the oviduct.
• During sexual intercourse, the male releases sperms into the female’s vagina.
• The sperms swim through the vagina, uterus, and reach the oviduct.
• Millions of sperms are released, but only one sperm usually joins with the egg.
• When a sperm joins with the egg, it forms a zygote.
• The zygote then starts dividing and grows into an embryo.
• The embryo continues to grow and develop into a baby inside the uterus.

Implantation

• After fertilisation, the zygote starts dividing and forms a small ball of cells called the embryo.
• The embryo moves from the oviduct to the uterus.
• In the uterus, the embryo gets fixed into the wall of the uterus; this process is called implantation.
• After implantation, the embryo continues to grow and develop into a baby over the next nine months.

Pregnancy

• Pregnancy is the period during which the baby grows inside the mother’s uterus.
• It usually lasts for about nine months (around 280 days).
• During pregnancy, the embryo grows into a foetus, and by the end of nine months, it becomes a fully formed baby ready to be born.

Growth

• Growth means the increase in the size and weight of an individual as they develop.
• In humans, growth happens from the time of fertilisation until the individual becomes an adult.
• After fertilisation, the zygote divides and forms an embryo, which grows into a foetus and then a baby.
• After birth, the baby continues to grow into a child, then a teenager, and finally an adult.
• Growth is faster during the early years of life, especially during puberty (teenage years).

Birth

• Gestation is the complete duration of an embryo’s development inside the uterus. 
• In humans, this period is approximately 280 days. 
• During childbirth, the baby is delivered head first through the vagina, aided by strong muscular contractions of the uterus.

Puberty 

• Puberty is the stage during which the reproductive organs in boys and girls become fully functional. 
• For girls, it typically begins around the age of twelve, with breast development as the earliest sign. 
• In boys, puberty generally starts around thirteen years of age, and the initial indication is the growth of the testes. 

A table below outlines the key physical changes that occur in boys and girls during this phase.

Adolescence

• Adolescence is the period of life when a child’s body starts changing into an adult’s body.
• It usually begins around 10–12 years of age for girls and 12–14 years of age for boys.
• Adolescence can last until 18–19 years of age and is also called the teenage years.
• During adolescence, the body goes through many physical, emotional, and mental changes.
• These changes happen because of hormones produced by the endocrine system.

Adulthood

• Adulthood is the stage after adolescence when a person becomes fully grown.
• During adulthood, a person becomes physically and emotionally mature.
• Adults are ready to take up responsibilities like jobs, family, and contributing to society.
• Adulthood is a time of stability in physical growth, but mental and emotional growth continues.