Edexcel IGCSE Biology 复习笔记 2.7.1 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
  • A 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

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