• 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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