Transport In Cells

• small organisms like bacteria have high SA:V, so they can rely on diffusion across their surface to exchange everything they need.
• Large organisms that is anything big enough to with your naked eye.

  • have a low SA:V, so it can't rely on diffusion.
  • But they have specialised exchange surface that increase the SA:V (extra surface area on the inside).

  
• Diffusion distances


  As organisms get larger, the distance that molecules have to diffuse to, get from the outside of their body, to the inside of their body increases massively.

  • diffusion will be slower in larger organisms.

  
• Large organisms have exchange surfaces / substances, since they can't rely on diffusion.


  Exchange surfaces - get substances in and out of their bodies.

  • Alveoli / lungs to allow oxygen and carbon dioxide to diffuse in and out of the body / bloodstream.
  • Villi / small intestines allow nutrients like glucose, amino acids, mineral ions. etc. to be absorbed from the small intestines into the bloodstream.
  • Roots and leaves to exchange substances with the environment.

  
• Large organisms have transport systems / substances, as the can't rely on diffusion.


  Transport systems - transport substances to the parts of their bodies that need them.

  • Circulatory system transports molecules from the exchange surfaces around the body, so their molecules only have to diffuse a short distance to get into the cells.
  • Phloem & xylem tissues to transport substances it gets from the exchange substances, around the plant.

  
• Specialised exchange surfaces


  A part of an organism over which substances are exchanged with their environment.

  Examples:

  • Alveoli (in the lungs) - exchange oxygen and carbon dioxide.
  • villi - absorb nutrients: e.g glucose & amino acids.
  • root hair cells - absorb water & mineral ions from the soil.
  • leaves - absorb CO2.

  
• Features of specialised exchange surfaces:

  • Large surface area:
  • lots of molecules can diffuse across at the same time, the rate of diffusion is higher.
  • Exchange surfaces are thin, for a short diffusion distance, so diffusion can take place more quickly.
  • Exchange surfaces are permeable to the substances that they need to exchange:
  • surfaces allow substances to diffuse across.
  • Good blood supply helps maintain concentration gradient.
  • Good supply of external material helps maintain the concentration gradient.