Edexcel IGCSE Biology 复习笔记 3.1.5 Sexual Reproduction in Humans

The male reproductive system

Male Reproductive Structures & their Function Table

The female reproductive system

Female Reproductive Structures & their Function Table

• A gamete is a sex cell (in animals: sperm and ovum; in plants pollen nucleus and ovum) produce by meiosis
• Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells
• This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells
• In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomes
• Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo

Sperm Cell & Egg Cell Table

Comparing sperm and egg cells

Adaptive Features of Gametes Table

Comparison of Gametes Table

• During ejaculation in the male, millions of sperm cells move along the sperm duct (also known as the vas deferens)
• The sperm are suspended in semen (a fluid secreted by the prostate gland)
• The semen passes into the urethra
• During sexual intercourse, semen is ejaculated into the vagina of the female, near the cervix
• The sperm cells then follow a chemical trail and travel through the plug of mucus in the cervix to reach the uterus (the womb)
• The sperm cells then travel into the oviducts
• If a sperm cell meets an egg cell in the oviduct, fertilisation can occur
  • This is most likely to occur 1-2 days after the female has ovulated (released an egg cell from one of her ovaries into an oviduct)

   

• Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)
  • During fertilisation, the head of a sperm cell releases enzymes that digest a path through the protective outer layer of the egg cell, allowing the sperm to pass through the egg cell membrane
  • Once this occurs, the egg cell immediately releases a thick layer of material that prevents any more sperm cells from entering, ensuring only one sperm cell can fertilise the egg cell

   

• When the male and female gametes fuse, they become a zygote (fertilised egg cell)
• This zygote contains the full 46 chromosomes (23 pairs of chromosomes), half of which came from the father and half from the mother
• The zygote divides by mitosis to form two new cells, which then continue to divide like this until an embryo is formed after a few days
• Cell division continues and eventually many of the new cells produced become specialised to perform particular functions and form all the body tissues of the offspring

The process of fertilisation in humans

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