CIE IGCSE Biology: 复习笔记:16.1.4 Sexual Reproduction in Humans

CIE IGCSE Biology: 复习笔记:16.1.4 Sexual Reproduction in Humans

The Male Reproduction System

The male reproductive system

 

Male reproductive structures and their function:

The Female Reproduction System

The female reproductive system

 

Female reproductive structures and their function:

Gametes & Fertilisation

Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)

  • It occurs in the oviducts
  • Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo

Comparing sperm and egg cells

 

 

Adaptations of Gametes

Comparison of Male & Female Gametes

 

Pregnancy: Growth & Development of the Fetus

 

 

  • After fertilisation in the oviduct, the zygote travels towards the uterus
  • This takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo
  • In the uterus, the embryo embeds itself in the thick lining (implantation) and continues to grow and develop
  • The gestation period for humans is 9 months
  • Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining
  • After this point the organs are all in place, the placenta has formed and the embryo is now called a fetus
  • The remaining gestation time is used by the fetus to grow bigger in size

 

The fetus in the uterus

 

  • The fetus is surrounded by an amniotic sac which contains amniotic fluid (made from the mother’s blood plasma)
  • This protects the fetus during development by cushioning it from bumps to the mother’s abdomen
  • The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products

The Placenta & Umbilical Cord: Extended

 

 

  • During the gestation period the fetus develops and grows by gaining the glucose, amino acids, fatswater and oxygen it needs from the mother’s blood
  • The bloods run opposite each other, never mixing, in the placenta
  • The fetus’s blood connects to and from the placenta by the umbilical cord
  • The mother’s blood also absorbs the waste from the fetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the fetus’s blood so that they do not build up to dangerous levels
  • Movement of all molecules across the placenta occurs by diffusion due to difference in concentration gradients
  • The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion
  • The placenta acts as a barrier to prevent toxins and pathogens getting into the fetus’s blood
  • Not all toxin molecules or pathogenic organisms (such as viruses, eg rubella) are stopped from passing through the placenta (this usually depends on the size of the molecule)
  • This is why pregnant women are advised not to smoke during pregnancy as molecules like nicotine can pass across the placenta
  • After the baby has been born, the umbilical cord is cut – this does not hurt as there are no nerves in it, just two blood vessels
  • It is tied off to prevent bleeding and shrivels up and falls off after a few days leaving the belly button behind
  • The placenta detaches from the uterus wall shortly after birth and is pushed out due to contractions in the muscular wall of the uterus - known as the afterbirth

 

The placenta

Exam Tip

It is worth learning at least two specific substances that move in either direction across the placenta – this is a common exam question and non-specific answers such as ‘waste products’ and ‘nutrients’ will not get any marks!

转载自savemyexams

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