GCSE Biology Unit 2 (4)

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  • Osmosis is just a special case of diffusion
  • Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to an area of low water concentration.
  • A partially permeable membrane is one with very small holes in it. 
  • So small that only tiny molecules such as water can pass through them, and bigger molecules (e.g. glucose) can't.
  • The water molecules actually pass both ways through the membrane during osmosis.
  • This happens because water molecules move about randomly all the time.
  • But because there are water molecules on one side than the other, there's a steady net flow into the region with fewer water molecules.
  • The water acts like it's trying to even up the concentration on either side of…


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