CIE IGCSE Biology: 复习笔记:3.1.1 Diffusion

CIE IGCSE Biology: 复习笔记:3.1.1 Diffusion

Diffusion

  • Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration
  • Molecules move down a concentration gradient, as a result of their random movementDiffusion across the cell membrane, IGCSE & GCSE Biology revision notes

Diffusion across the cell membrane

  • For living cells, the principle of the movement down a concentration gradient is the same, but the cell is surrounded by a cell membrane which can restrict the free movement of the molecules
  • The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all
  • The simplest sort of selection is based on the size of the molecules
  • Diffusion helps living organisms to:
    • obtain many of their requirements
    • get rid of many of their waste products
    • carry out gas exchange for respiration

 

Examples of diffusion in living organisms

  • You will need to learn examples of substances that organisms obtain by diffusion
  • Don’t forget that plants require oxygen for respiration at all times, as well as carbon dioxide for photosynthesis when conditions for photosynthesis are right (e.g. enough light and a suitable temperature)

Examples of Diffusion Table

Where does the energy for diffusion come from?

  • All particles move randomly at all times
  • This is known as Brownian motion
  • The energy for diffusion comes from the kinetic energy of this random movement of molecules and ions

Brownian motion

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