Diffusion, osmosis and active transport

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Diffusion - the net movement of molecules of a as or liquid from a high concentration to an area of low concentration

Factors that affect diffusion -  

  • Temperature - the higher the temperature the more kinetic energy = more collisions = faster rate of diffusion
  • Surface area - greater the surface area the more room for molecules to work upon = faster diffusion rate
  • Diffusion pathway - diffusion effcient over short distances. Surfaces which are thin like the epithelium of the intestines = shorter distance to diffuse = faster diffusion rate
  • Concentration gradient - the greater the difference = faster the rate of diffusion.   e.g. the blood is continually transporting products of digestion away from the intestine wall, ensuring large difference in conc = faster rater of diffusion
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Facilitated Diffusion

Facilitated Diffusion - diffusion which involves an intrinsic carrier protein 

  • When a large molecule wants to gain entry into a cell through the cell membrane e.g. Glucose 
  • For the molecule to pass thought it must be correct/specific shape
  • If it is the correct complementary shape the carrier protein will release it onto the other side.


Speeds up with Kinetic Energy

Passive process but does not require energy from respiration

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Osmosis - the net movement of water from a region of high water potential (  ) to low (  ) through a partially permable membrane 

High (  ) = a high concentration of water = much more movement = osmosis possible

Low (  ) = low concentration = little movement of water = osmosis likey to be low.

Affecting Water potential (  )

  • By dissolving substance in the water
  • MORE dissolved susbtances = the lower the water potential becomes ( Na+ , Cl- , Glucose)

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Active Transport

Active Transport - movement of molecules against a concentration gradient from low to high using ATP energy and a carrier protein


  • for concentrating susbtances e.g  Salt Gland Marine Iguanas
  • Taking up ions from the soil into plant root cells where there are more concentrated.
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