Edexcel IGCSE Biology 复习笔记 2.7.1 Diffusion in Gas Exchange

Edexcel IGCSE Biology 复习笔记 2.7.1 Diffusion in Gas Exchange

Diffusion in Gas Exchange


  • 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 movement




Diffusion across the cell membrane




  • Diffusion is the process by which gas exchange occurs
  • Single celled organisms, such as amoeba, can exchange gases sufficiently by simple diffusion through the cell membrane






Gas exchange in single-celled organisms (such as amoeba) occurs through diffusion




  • Multicellular organisms (such as plants and animals), however, have exchange surfaces and organ systems that maximise the exchange of materials
    • Gills are the gas exchange organs in fish, lungs in humans and leaves or roots in plants


  • These organs increase the efficiency of exchange in a number of ways:
    • Having a large surface area to increase the rate of transport
    • short diffusion distance for substances to move across. This short distance is created because the barrier that separates two regions is as thin as possible


  • In addition, animals have:
    • Gas exchange surfaces that are well ventilated to maintain steep concentration gradients